UPTEC Q10004

Examensarbete 30 hpMaj 2010

Influence of composition, grain size and manufacture process on the anisotropy of tube materials

Daniel Gullberg

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Influence of composition, grain size and manufactureprocess on the anisotropy of tube materials

Daniel Gullberg

A problem with cold pilgered tubes for OCTG applications is that they can getanisotropic properties with regard to yield strength. One source of anisotropy istexture that is developed during the cold deformation. EBSD measurements havebeen made on several austenitic stainless steels with different deformations to seewhat influence the composition has on the texture formation. The samemeasurements were used to study the influence of grain size on texture formation.The conclusion was that the composition can have an impact on the texture andhence has potential to also affect the anisotropy. The differences in texture cannot beassociated with a specific alloying element, but is rather a synergetic effect. It was alsoconcluded that grain structure has no strong influence on texture formation. Anevaluation of three different tool designs used for cold pilgering was made. Thedesigns evaluated are referred to as design A, B and C. EBSD measurements showedlarge deviations in texture in the middle of the wall compared to close to the surfaceof pilgered OCTG. However, the measurements showed no large differencesbetween the three designs and the texture could not be coupled to the anisotropy.

Tryckt av: Ångströmlaboratoriet, Uppsala Universitet

Sponsor: Sandvik Materials TechnologyISSN: 1401-5773, UPTEC Q10004Examinator: Åsa Kassman RudolphiÄmnesgranskare: Urban WiklundHandledare: Guocai Chai, Katarina Persson, Anders Nyström och Fredrik Meurling

Inverkan av sammansättning, kornstorlek och tillverkningsprocess på anisotropi i rörmaterial

Daniel Gullberg

För att ta upp olja från oljekällor i havsbottnen används stålrör. I takt med att oljetillgångarna i

jordskorpan minskar måste tekniken utvecklas för att kunna komma åt oljan även i tuffa

miljöer. Miljön där oljan finns är ofta kraftigt korrosiv och de mekaniska påfrestningarna på

rören är höga. Det krävs speciella stål som har hög hållfasthet och mycket bra

korrosionsegenskaper. För att få tillräcklig hållfasthet krävs det oftast en kallbearbetning av

rören. Det finns flera olika sätt att åstadkomma kallbearbetningen. De vanligaste sätten är

kalldragning eller stegvalsning. Sandvik AB är ett av de företag som tillverkar rostfria rör för

den här tillämpningen. Sandvik AB stegvalsar sina rör. Ett problem som uppstår vid

stegvalsning men även vid kalldragning är anisotropi. Anisotropi innebär i det här fallet att

hållfastheten (sträckgränsen) blir olika i olika riktningar. Oftast blir sträckgränsen parallellt

med röret högre än vad den blir transversellt i röret, men även andra fall förekommer. Det

finns mycket att vinna på att bli av med anisotropin. För det första så är det lättare att göra

hållfasthetsberäkningar på rören om sträckgränsen är lika i alla riktningar. För det andra så

skulle materialåtgången kunna minskas vilket dels har ekonomiska fördelar och dels praktiska

fördelar eftersom rören skulle bli lättare.

Syfte

Syftet med den här studien är att undersöka vilken inverkan sammansättningen, kornstorleken

och tillverkningsprocessen har på anisotropin. En av källorna till anisotropi är textur i

materialet. För att förstå vad textur är och hur texturen kan påverka anisotropin är det viktigt

att först känna till hur ett metalliskt material är uppbyggt. I den här undersökningen

förekommer endast austenitiska rostfria stål. Dessa stål består av endast en fas där

legeringselementen är inlösta. Stålet är uppbyggt av korn i storleksordningen tiotalet

mikrometer. Varje korn är en kristall, det vill säga atomerna i kristallen är uppradade efter ett

visst mönster som är typiskt för den fasen. Textur är hur de olika kristallerna eller kornen är

orienterade i förhållande till varandra. Varje kristall i sig är anisotropisk och har olika

egenskaper och styrka i olika riktningar. Så om fördelningen av kristallorienteringar inte är

helt slumpvis kommer även materialet bli anisotropt.

Utförda undersökningar och resultat

För att se hur sammansättnigen påverkar texturen och i sin tur anisotropin har några olika

austenitiska stål valts ut där sammansättnigen varierar på ett sådant sätt att det skulle vara

möjligt att dra slutsatser om inverkan av specifika legeringselement. De element som ansågs

mest intressanta var kväve, molybden och koppar. I tillverkningen av rör uppstår texturen vid

stegvalsningen. Stegvalsningsprocessen är komplicerad och för att förenkla försöket

deformerades proverna istället genom dragning i en dragprovningsmaskin. Varje stålsort

deformerades till fem olika deformationsgrader varvid texturmätningar genomfördes.

Texturmätningarna gjordes med EBSD (electron backscatter diffraction), som är en

analysmetod där diffraktionsmönstret från en elektronstråle detekteras och ger information om

kristallorienteringen i den punkt elektronstrålen träffar provet. Resultaten visade att vid

dragningen av proverna vrids kornen så att vissa specifika kristallriktningar linjerar sig

parallellt med dragriktningen. Dessa riktningar benämns <111> och <100>. Två av stålen

visade avvikande texturbildning jämfört med de andra med en mer långsam vridning vilket

resulterade i en svagare textur. Det gick inte att koppla den här effekten till något specifikt

legeringselement utan är snarare en synergieffekt. Eftersom det går att se skillnader i texturen

är det troligt att det även skulle resultera i skillnader i anisotropi.

Ett liknande försök genomfördes för att undersöka vilken inverka kornstorleken kan ha på

anisotropin hos stegvalsade produkter. Genom att värmebehandla material kunde prover med

samma sammansättning fast med olika kornstorlek undersökas. Resultatet blev att det inte

gick att se några skillnader i texturutvecklingen utifrån kornstorleken.

Slutligen gjordes en utvärdering av tre olika valsverktygsdesigner för att se vilken påverkan

valsverktyget har på anisotropin. De tre designerna benämns A, B och C. Design B deformerar

röret mer homogent än de övriga mot slutet av valsningen. Design C reducerar väggen mer i

förhållande till diametern än de övriga designerna i början av valsningen och tvärt om mot

slutet. För alla designerna har röret samma ingående och utgående dimension.

Texturmätningar har gjorts på rören som visar att texturen är annorlunda i mitten av rörväggen

jämfört med nära ytter och inner ytan. Texturen är däremot av samma typ för alla tre

designerna. Även om det går att se små skillnader på bland annat styrkan hos texturen så går

inte dessa skillnader att koppla till uppmätt anisotropi för dessa rör. Inte heller anisotropin

visar några stora skillnader för de tre designerna.

Slutsatser

Inverkan av sammansättning, kornstorlek och tillverkningsprocess på anisotropin har

undersökts. Förutom en viss inverkan på texturen för olika sammansättningar så kunde ingen

inverkan påvisas. En förändrad textur ger troligen också skillnad i anisotropi.

Examensarbete 30 hp på civilingenjörsprogrammet

Teknisk fysik med materialvetenskap

Uppsala universitet, maj 2010

Acknowledgements

First of all, I would like to thank my mentors at Sandvik Guocai Chai, Katarina Persson,

Anders Nyström, and Fredrik Meurling for their help and guidance during this thesis. Most of all I

want to thank Guocai for giving me this opportunity and for supporting me the whole way with ideas

and comments.

Secondly, I would like to thank Sorina Ciurea for her help regarding the EBSD measurements and for

valuable discussions of the results.

During my thesis I have received a lot of help from numerous people at Sandvik. I would like to thank

all that have helped me or contributed with specimen fabrication, measurements, sample

preparation, etching, and knowledge about the equipment. I specially appreciate that I could use the

EBSD equipment during the weekends.

I also want to thank Göran Engberg and Stefan Jonsson for their time. With their expertise they

contributed with valuable discussions about the texture formation.

Last of all, I want to thank my family and friends, especially Maryam, for their support.

Table of Content 1. Introduction ......................................................................................................................................... 1

1.1 Objective........................................................................................................................................ 1

2. Literature Study ................................................................................................................................... 3

2.1 Well Construction and OCTG Materials ........................................................................................ 3

2.1.1 Requirements for OCTG Materials ......................................................................................... 4

2.1.2 Austenitic and Duplex Stainless Steels ................................................................................... 4

2.1.3 Influence of Alloying Elements on the Properties .................................................................. 5

2.2 Anisotropy ..................................................................................................................................... 6

2.2.1 Deformation and Dislocations ................................................................................................ 6

2.2.2 Texture and Anisotropy .......................................................................................................... 9

2.2.3 Presentation of Textures ........................................................................................................ 9

2.2.4 Analysis Method for Texture and Anisotropy ...................................................................... 10

2.2.5 Parameters that Affect Texture and Anisotropy .................................................................. 12

2.3 Manufacture of OCTG ................................................................................................................. 14

2.3.1 Methods for Manufacture of OCTG ..................................................................................... 14

2.3.2 Plastic Deformation Mechanisms and Anisotropy during Cold Working ............................. 16

2.4 Previous Work ............................................................................................................................. 17

3. Test Material ..................................................................................................................................... 18

3.1 Steel Grades................................................................................................................................. 18

3.1.1 Sanicro 28 ............................................................................................................................. 18

3.1.2 Sanicro 29 ............................................................................................................................. 19

3.1.3 Sanicro 30 ............................................................................................................................. 19

3.1.4 Sanicro 41 ............................................................................................................................. 19

3.1.5 3R60 ...................................................................................................................................... 19

3.1.6 3R69 ...................................................................................................................................... 20

3.1.7 254 SMO ............................................................................................................................... 20

3.2 Heat Treatment and Grain Size ................................................................................................... 20

3.3 Test Samples ................................................................................................................................ 21

4 Experimental ...................................................................................................................................... 22

4.1 Sample Straining .......................................................................................................................... 22

4.2 EBSD............................................................................................................................................. 23

4.3 LOM ............................................................................................................................................. 24

4.4 Vickers Hardness ......................................................................................................................... 24

4.5 Rockwell C Hardness ................................................................................................................... 24

5 Results and Discussion........................................................................................................................ 25

5.1 Microstructure of Reference Samples (LOM).............................................................................. 25

5.2 Microstructure After Deformation for Sanicro 29 (LOM) ........................................................... 26

5.3 Microstructure of Reference Samples (EBSD) ............................................................................. 27

5.4 Texture Formation (EBSD) ........................................................................................................... 29

5.4.1 Inverse Pole Figures .............................................................................................................. 29

5.4.2 Specific Orientation Measurements ..................................................................................... 33

5.5 Substructure of Deformed Samples (EBSD) ................................................................................ 39

5.6 Strain Hardening Coefficient ....................................................................................................... 41

5.7 Influence of Composition and Grain Structure on Texture ......................................................... 41

5.8 TEM .............................................................................................................................................. 42

5.9 Misorientation ............................................................................................................................. 43

5.10 Tool Design Evaluation .............................................................................................................. 44

5.10.1 Hardness Variations (Rockwell C) ....................................................................................... 44

5.10.2 Texture (EBSD) .................................................................................................................... 46

5.10.3 Anisotropy Values ............................................................................................................... 48

6. Conclusions ........................................................................................................................................ 49

7. References ......................................................................................................................................... 50

List of Abbreviations and Symbols OCTG Oil country tubular gods

EBSD Electron backscatter diffraction

SEM Scanning electron microscopy

FCC Face centered cubic

HCP Hexagonal close packed

SFE Stacking fault energy

sfe Stacking fault energy per area unit

b Burgers vector

b1, b2 Burgers vectors of Shockley partials

Fr Repulsive force between two partial dislocations

G Shear modulus

pi = 3.14…

Fa Attractive force between two partial dislocations

Shear force component

ODF Orientation distribution function

x, y, z Coordination system of the crystal

RD, TD, ND Coordination system of the sample (rolling, transversal and normal direction)

21 ,, Euler angles

B Bragg angle

dhkl Distance between parallel planes

R Resolved shear stress

Angle between slip plane normal and stress direction

Angle between slip and stress direction

Stress

CNC Computerized numerical control

LOM Light optical microscopy

P Load in kg

d Average diameter of a increment

HV5 Hardness value from a Vickers test with load 5 kg

IPF Inverse pole figure

TEM Transmission electron microscopy

Rp0.2 Tension in the material at strain offset 0.002 (yield strength)

1

1. Introduction The requirements for oil country tubular gods (OCTG) materials are getting higher at the same rate as

the easily exploited wells are running out. The oil is often found deep in the seabed at depths of up

to 7 km with pressures up to 1050 bar, possibly even higher [1]. In order to reach the required

strength the material needs to be cold-worked. When performing the cold-working, the tube can get

anisotropic properties. Normally the tensile strength is higher in the lengthwise direction of the tube

than in the transversal direction. This effect is a problem due to the different types of pressure

subjected to the tube when used in a well, see figure 1.

Figure 1. Illustration of the forces subjected to the tube; a)tension b)compression c)collapse and d)burst

The tubes have high strength requirements for all the load cases; tension, compression, collapse, and

burst. It is not possible to compensate for the low transversal yield strength by increasing the total

strength of the tube because this will cause corrosion problems, exceeding the limits of hardness and

give a brittle material [2]. The lower transversal yield strength can be compensated by increasing the

wall thickness, resulting in a more expensive and heavier tube. Large savings can be made by

decreasing material usage and make the dimensioning easier if the anisotropy could be reduced.

One of the sources of anisotropy in the case of cold-working is texture formation. Texture is when

the orientation of the crystals is not random. Almost all metallic materials, even if the material has

not been cold worked, have texture to some extent. This type of texture forms when the alloy is

made, due to its shape, or the cooling direction in the material, but it is often weak and the material

can thus be regarded as uniform. If the material is cold-worked deformation texture is formed as a

consequence of deformation mechanisms, which are dependent on the composition and

temperature. The deformation causes movements in the crystals and rotations of the crystal

directions.

1.1 Objective The aim of this thesis is to give an understanding of how parameters like composition, grain size, and

manufacture process affect the anisotropy formation in austenitic stainless steels. The results can

perhaps help to get ideas of how to prevent texture formation in austenitic materials for OCTG

applications. In a long term perspective the aim is to decrease the anisotropy levels in pilgered

tubing.

The main investigation in this thesis will be to measure texture formed during tensile deformation at

room temperature for several austenitic stainless steels in order to see what influence the chemical

2

composition has on texture formation, if any at all. The texture measurements will be carried out

using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). The same type

of measurement will be used to investigate if the grain size can have an influence on the texture.

Three pilgered tubes from the same heat but pilgered with different tool designs will be compared to

see if one of them is better than the other with respect to texture or homogeneity.

3

2. Literature Study

The purpose of this section is to give a deeper understanding of anisotropy in tube materials and to

give adequate information to be able to follow the results and discussion. At the end of this section

there is a short review of published scientific articles concerning texture formation and deformation

mechanisms.

2.1 Well Construction and OCTG Materials The main application of OCTG is to transport oil and gas from the well to the sea bed. At the top of

each hole down to the well there is a wellhead. When the oil or gas reaches the wellhead, it enters

an oil transportation system made up like a network that transports the oil to main land or to a

platform at the surface, see figure 2. In the hole there are two layers of OCTG, the outermost tube

protects the inner tube that transports the oil to the sea bed. The tubing is put together using a

threaded coupling. Other names for OCTG are down-hole tubing or production tubing [1].

Figure 2. Well construction: illustration of a) how the oil is transported to the surface and b) the OCTG usage.

Normally ordinary carbon steels are used as OCTG. However, there are many oil wells where ordinary

carbon steels are not sufficient due to highly corrosive environments or other requirements. Some of

the aspects that need to be considered when choosing OCTG material are temperature, partial

pressure of H2S and CO2, total pressure, pH-value, and chloride content. Carbon steel or low alloyed

steel is the cheapest alternative. Austenitic or duplex stainless steels can be cold worked to reach

very high strength at the same time as they have good corrosion resistance. Austenitic stainless

steels are expensive due to their high nickel content but are not as sensitive to H2S as duplex

stainless steels. Austenitic stainless steels are chosen if the partial pressure of H2S exceeds 3 psi [1].

4

2.1.1 Requirements for OCTG Materials

OCTG are affected by different forces when used, such as tension, compression, internal pressure

and collapse. It is important to have the proper strength since a lower strength does not meet the

strength requirements and a higher strength gives a more brittle material and can cause corrosion

problems.

To reach the required strength OCTG tubes normally need to be cold-worked. This can be done either

by cold pilgering or cold-drawing. This treatment can however give an undesired anisotropy that can

affect the design of the OCTG tube. When designing OCTG tubes the internal pressure is normally

calculated using the (American Petroleum Institute) API 5C3 formula that is based on the tensile yield

strength in the transversal direction and the tube dimensions. The transversal yield strength is in the

case of anisotropy normally lower than the longitudinal yield strength, which makes it necessary to

increase the wall thickness of the tube compared to if the material was isotropic. The tube will thus

require more material especially expensive nickel, furthermore it will make the tubes heavier [2].

2.1.2 Austenitic and Duplex Stainless Steels

Stainless steels are normally considered to be iron-based alloys with high fractions of chromium (10.5

wt %). When the chromium exceeds 10.5-12 wt % it enables the forming of a thin self healing

chromium oxide film that prevents further oxidation of the steel [3]. Other frequently used alloying

elements in stainless steels are nickel, manganese and silicon [4]. The stainless steels can be divided

into five basic groups where austenitic and duplex stainless steels are the two commonly used for

OCTG applications.

The development of stainless steels started independently both in Germany by the company Krupp

and in Sheffield, Great Britain around 1910. Since then the use of nitrogen as an alloying element and

new process techniques have improved the steels. Due to the nitrogen’s stabilizing effect on the

austenitic phase, higher amounts of chromium and molybdenum can be used to further improve the

corrosion resistance [3].

The austenitic steels are as the name implies, consisting of one phase, the austenite phase. This is a

face centered cubic (FCC) structure which gives the material high ductility and good formability

compared to other groups of stainless steel. There are three ways to harden a stable austenitic

material, strain hardening, grain refinement through cold working followed by recrystallization, and

solid solution hardening [4].

The duplex steels are consisting of two phases in significant properties. Duplex stainless steels are

normally ferritic/austenitic steels with 30 to 70 % ferrite. The first commercial products were

marketed around 1930 and were then used for autoclaves, valves and coolers. Since then several

improvements have been made on the corrosion resistance. The advantages over austenitic stainless

steels are better resistance to chloride stress corrosion, higher strength and the low nickel content

[5].

5

2.1.3 Influence of Alloying Elements on the Properties

All steel grades in this thesis are austenitic stainless steels, this means that there are only supposed

to be one phase present, namely austenite. When alloying these steels the solute is added without

changing the phase of the material or causing other phases to form, this is referred to as solid

solution alloying. How much of a solute that can be solved in the solvent depends on several things;

size of the solute atom, crystal structure, electronegativity and valences. When a solute is added to a

solvent the atoms are placed either interstitially between the host atoms or substitutionally taking

the place of a host atom [6], se figure 3.

Figure 3. Illustration of how alloy atoms are placed in the solvent at solid solution.

The most important reason for further development of stainless steel has been to improve the

corrosion resistance. The most important element for this is chromium, but many other elements

also affect the corrosion properties. The alloying elements also have influence on the microstructure

and are divided into two groups either ferrite or austenite stabilizers. C, N, Mn, Co and Cu are typical

austenite stabilizers, while Si, V, Cr, Mo and W are ferrite stabilizers [3].

This thesis will discuss what influence N, Cu and Mo have on the structure of austenitic stainless

steels. It is therefore of interest to briefly describe their effect as alloying elements.

Nitrogen is a strong austenite stabilizing element that makes it possible to add more Cr and Mo to

the alloy. A negative aspect of this is that it increases the sensitivity of precipitation of undesired

intermetallic phases and nitrides. Nitrogen gives an increase in yield strength in austenitic steel due

to three reasons. First of all, more active glide planes are necessary for the same plastic strain,

secondly, more intersections of crossing active glide planes develop which act as dislocation barrier,

and last, dislocation barriers are more effective in nitrogen alloyed steels. Nitrogen also has a

positive effect on corrosion resistance, particularly on pitting corrosion. Nitrogen neutralizes the H+

ions forming NH4+ ions raising the pH-value, this enhances repasivation of the pit [3]. It is unclear if

nitrogen raises or decreases the stacking fault energy (SFE). Nitrogen has been shown to increase the

formation of planar dislocations in austenitic steels generally associated with low stacking fault

energy materials [7].

Copper may increase the corrosion resistance to general corrosion in sulphuric acid and phosphoric

acid environments. Copper may also enable considerable hardening, without excessive loss of

ductility in duplex steels. If copper diffuses to grain boundaries it may cause embrittlement [3]. Steels

are rarely alloyed with copper and when they are the amount is only exceeding one percent for

special alloys [8].

6

Molybdenum has a ferrite stabilizing and strengthening effect. With high Mo content the structural

stability of stainless steel may be lowered. This may cause precipitation during high temperature

exposure leading to embrittlement of the material. Molybdenum has the benefit to improve

resistance to pitting and crevice corrosion in chloride environments [3]. In OCTG applications Mo is

important in respect of stress corrosion cracking in H2S environments [1].

2.2 Anisotropy

A material is anisotropic when its properties depend on how the sample is orientated relative to the

direction of the measurement. In this thesis it is important to understand what anisotropy is and

what causes anisotropy.

2.2.1 Deformation and Dislocations

An FCC material has two basic methods of deformation; slip and twinning. It is mainly the materials

stacking fault energy ( sfe ) that determine if slip or twinning will be the dominating mechanism. If

the SFE is low it will be difficult for cross-slip to occur, and that will make it hard for the deformation

to form by slip alone and will lead to twinning [9]. Stacking fault energy will be described deeper later

in this report.

To be able to understand how the stacking fault energy influences the deformation it is necessary to

first know more about dislocations. In metals as in all materials the atoms are arranged in a way so

that they will reach as low energy state as possible, this gives the metal its structure. Which structure

that is most beneficial is different for different metals and alloys. This thesis centers around

austenitic stainless steels which have an FCC structure, see figure 4.

Figure 4. The face centered cubic structure where the densely packed planes are shown.

Atoms in a metal are strongly bound to each other and a perfect lattice has a very high strength,

much higher than what a normal metal is showing. The reason for this is dislocations, which are

linear crystal defects. When the metal is deformed it is due to movement of the dislocations causing

the material to change shape. It will be much easier to push one atom at the time to a different place

than to move all at once [10], see figure 5.

7

Figure 5. Dislocation movement, the extra half plane is moving to the right on the slip plane illustrating how

dislocations are moving in the metal, one step at the time.

Dislocations are normally propagating in the direction in the crystal that is the most favorable from

an energy perspective. The planes where the dislocations are moving are called slip planes. The slip

plane is often the densest plane. In an FCC crystal, the most dense plane is a {111} plane and the

most dense direction in that plane is a <110> direction. The slip system in an FCC crystal is therefore

<110>{111} [11].

In an FCC crystal the atomic layers are stacked in a sequence; ABCABCABC…, a deviation from this

stacking sequence is a stacking fault. The normal burger’s vector b in an FCC material is <110> type

and reaches between two identical positions regarding to the stacking sequence, see figure 6.

For some dislocations it is more energy efficient to divide into Shockley partials. This means that the

movement of the atom will take another way, first one slide to a position different from the first

regarding to stacking sequence, and then another slide to the final position. The dissociation will only

occur if Frank’s rule:

2

2

2

1

2 bbb Equation 1

is satisfied, meaning that the energy is lowered [10].

Figure 6. Stacking positions in FCC lattice and dissociation into Shockley partials of a burgers vector.

8

The intermediate position will result in a stacking fault, making a change in the stacking sequence.

The width of the stacking fault will be balanced through one repulsive force between the two partial

dislocations, and one attracting force due to the energy increase of the stacking fault area. The

repulsive force Fr for each length unit can be written as:

d

bGbFr

221 Equation 2

where G is the shear modulus and d is the distance between the two dislocations b1 and b2 according

to figure 7.

Figure 7. Illustration of how the distance of the partial dislocations is balanced by repulsive and attracting

forces.

The attracting force is equal to the stacking fault energy per unit area sfe and the distance between

the two dislocations is given by:

sfe

bbGd

2

21

Equation 3

When one of the partial dislocations gets locked up there will be a shear force component in the

direction of the burger’s vector reducing the distance between the Shockley partials, the attractive

force can be written:

2bFa

Equation 4

In order for the dislocation to move past the obstacle the dislocation can climb around it, but this is

only possible for a pure dislocation (not for partial dislocations). The force acquired to close the

dissociated partials and eliminate the stacking fault is increasing with decreasing stacking fault

energy according to equations 2 and 4 [11]. This is the reason that FCC materials with low stacking

fault energy have a tendency to form twins instead of deforming by slip.

9

2.2.2 Texture and Anisotropy

A single grain or a single crystal of a metallic material is anisotropic, that is its properties depend on

the direction. The typical property is the strength of the material. When the material is deformed

slips will occur in the crystal due to dislocation movements. This will happen in the crystals preferred

slip plane. Depending on how the slip planes are orientated relative to the deformation direction the

material will show different strength.

Texture is when the distribution of crystal orientations in a poly crystal is not random. This causes

anisotropy for the same reason as for the single crystal. There are many types of textures or

distributions of orientations and several reasons for them. The two main reasons for texture

formation are annealing and deformation, this thesis will focus on deformation texture. To make it

simple deformation texture can be divided in rolling texture and fiber texture. As the names imply

one is normally formed when rolling and one when deforming in one direction. How the texture is

formed will be explained later in this thesis report.

2.2.3 Presentation of Textures

When investigating structure and texture it can be very useful to be able to represent planes,

directions and angles in a two-dimensional diagram. The most frequently used way of doing this is

the stereographic projection. The crystal that should be represented is placed in the centre of a

reference sphere. The reference sphere is cut by an equatorial plane on which the two-dimensional

projection is made. To get the projection of a direction (e.g. a plane normal) a straight line is drawn

from the center of the crystal in the specific direction to a point p on the reference sphere. Then a

line is drawn from the south pole of the reference sphere to the point p and its intersection with the

equatorial plane is the stereographic projection, see figure 8. The projection can also be made for

planes which result in a great circle [12].

Figure 8. Illustration of the stereographic projection.

A pole figure is constructed by making the projection of a certain crystallographic direction (normally

111, 101 or 001 for FCC materials) for all measurement points and then calculating the concentration

of poles in every point on the projection.

An inverse pole figure is made by adding the crystallographic orientation parallel to the normal of the

sample surface in a 001 standard projection for every measurement point. The concentration of

orientations is then calculated in the same way as for the pole figure. A 001 standard projection is a

stereographic projection of the most common directions for a perfect single crystal with its 001

10

direction parallel to the plane normal. Due to crystal symmetry all orientations in an FCC material can

be represented within a small triangular area of the total projection, see figure 9.

Figure 9. 001 standard projection of an FCC material used for constructing an inverse pole figure. All

crystallographic orientations can be presented in the gray area or in any of the other triangles if symmetry is

used.

A more complete way of presenting the texture is by an orientation distribution function (ODF). The

coordinate system of the crystal is related to the coordinate system of the sample by three Euler

angles. To align the crystal coordinate system x, y and z with the sample coordinate system RD, TD

and ND it takes three rotations one for each Euler angle. First the coordinate system of the crystal is

rotated an angle 1 around ND so that z is parallel with the ND TD plane. Then a rotation around TD

is made with an angle so that z is parallel to ND. Last a rotation, 2 , is made around ND to align x

with RD [11]. In a cubic system all Euler angles are between zero and ninety degrees. The three Euler

angles put up a three-dimensional space named the Euler space. Texture is normally presented as

two-dimensional slices of the Euler space were one of the angles is held constant.

2.2.4 Analysis Method for Texture and Anisotropy

EBSD (electron back-scatter diffraction) is an add-on package to an SEM (scanning electron

microscopy) which has an ability to obtain microstructure-level information. Most other methods like

X-ray or neutron diffraction give diffraction intensities that are characteristic for a large contiguous

sample volume. The EBSD measurement gets the crystallographic information from a small sampled

volume, which usually is an individual crystallite. The orientation in such a small volume can be taken

to be uniform. The pattern gained from the EBSD analysis is named Kikuchi pattern. The

determination of the crystallographic orientation starts with indexing the Kikuchi pattern and then

the relative position of the poles with respect to an external reference frame is determined. This can

be made automatically by a computer, measuring and indexing several points per second [13].

The Kikuchi pattern is a gnomic projection. All poles are projected on a plane that is a tangent to the

north pole of the reference sphere, see figure 10. The computer identifies the desired direction and

adds intensity in the pole figure according to the stereographic projection.

11

Figure 10. Illustration of a gnomic projection which is a direct projection on to the projection plane.

The Kikuchi pattern forms due to electron diffraction when electrons interact with the atom planes

according to Bragg’s law. When electrons from an electron beam in a SEM enter a sample they are

diffusely scattered in all directions. This means that each plane in the crystal is hit by electrons in the

Bragg angle B . They can then undergo elastic scattering and form diffusion patterns when they

interact with elastically scattered electrons from planes in the same family. The diffraction pattern is

forming a cone known as the Kossel cone about the normal of the reflecting plane with the apex

angle B2180 . The reason for this is that the electrons are coming from all directions. One other

effect from this is that two cones are forming from each plane, one on each side, see figure 11.

Figure 11. The left picture shows the Kossel cone and the right shows how two cones form from each family of

planes.

The Bragg angle is about 0.5°, this means that the Kossel cones are rather flat and when they are

projected on a screen they will appear as two parallel lines, Kikuchi-lines. The width between the

lines is proportional to the distance between the planes dhkl. Each crystal structure has a specific

pattern consisting of pair of lines or bands where each band represents a specific plane and the

points where the bands cross represent certain directions in the crystal. By indexing the different

planes (the Kikuchi-lines on the screen) it is possible to identify the specific direction you are looking

along e.g. [100] or [111]. It is then possible to calculate its relative direction to a reference frame

depending on where the crossing is projected on the screen. All this is normally done by a computer

12

that can make one measurement in a fraction of a second. The SEM is then used to scan the sample

making measurements in a grid pattern. The result can be illustrated in several ways, either by a pole

figure using stereographic projection, an inverse pole figure or Euler space.

2.2.5 Parameters that Affect Texture and Anisotropy

The reason for deformation texture is crystal rotation. One way to explain the formation of texture is

to consider a single crystal which deforms in only one slip system. Slip will occur together with a

rotation since the crystal stays together and the points were the force is applied is not moved

sideways. When the material is stretched the angle between the pull direction and the slip direction

will decrease. When the material is compressed the opposite will occur [11], see figure 12.

Figure 12. Illustration of crystal rotation for a single crystal deformed in only one slip system.

An FCC material has 12 different slip systems consisting of a slip plane and a slip direction. The slip

plane is normally the most dense plane in the structure, in the case of FCC a {111} plane. There are

four different {111} planes, within each plane, three independent slip directions of type <110>. The

system activated first is the one with highest resolved shear stress R given by the equation:

coscosR Equation 5

where is the applied stress, is the angle between the slip and stress direction and is the angle

between the slip plane normal and stress direction, see figure 13 [6].

13

Figure 13. Geometrical relationships used for calculating the resolved shear stress.

The factor coscos is normally referred to as de Schmid factor. The largest value of the Schmid

factor, which gives the highest resolved shear stress, is 0.5. This happens when both angles are

45 degrees. All orientations have their own Schmid factor as can be seen in figure 14. The higher the

Schmid factor is the higher will the resolved shear stress be in the most favored slip system. In a

polycrystal it is likely that the deformation will start in a grain with a high Schmid factor. During the

deformation the crystal will turn and the Schmid factor will change.

Figure 14. Inverse pole figure showing the Schmid factor for different crystal orientations.

The crystal rotation and resolved shear stress are not a complete explanation of the texture

formation but a good start. Other factors have large impact on the texture, like what deformation

mechanism that is active during the deformation.

When an FCC material is tensile tested the slip directions are turned towards the stress direction

until two slip systems get equally beneficial. One direction where two slip systems have the same

Schmid factor is the <112> direction. What happens then is a deviation into bands were some turn to

a <111> direction and some to a <100> direction. The amount of the different orientations depends

on the materials stacking fault energy. Low SFE gives more <001> type texture. When the SFE

14

decreases the amount of twins rises, twinning shears the <111> oriented crystals to a <115> direction

from where they are turned to a <001> orientation [11].

The mechanical properties of austenitic stainless steels, particularly strain hardening is depending on

the SFE. When lowering the SFE the deformation mechanism will change from slip in perfect crystals

to slip in partials. With further decreasing SFE mechanical twinning will be activated, followed by

transformation to ε -martensite and/or α’-martensite. It is important to know that regardless of

deformation mechanism the greater part of plastic slip is accommodated through slip of dislocations

[14].

The composition can affect the deformation mechanism by changing the SFE. Attempts have been

made to create an equation to be able to calculate the SFE from the composition. Schramm and Reed

[15] constructed an equation for commercial-grade austenitic Fe-Cr-Ni, Fe-Mn-Ni, and Fe-Cr-Ni-Mn

steels. However it has later been concluded that no simple and universally valid composition

equations exist for the SFE [16]

Temperature can affect the deformation mechanism since it changes the SFE [21]. The influence of

temperature on texture formation will however not be included in this thesis.

2.3 Manufacture of OCTG

As mentioned earlier the OCTG have very high strength requirements and they are used in highly

corrosive environments. Only stainless steels with exceptionally good resistance are suitable for

OCTG applications. Due to the strength requirements cold working is necessary making austenitic

and duplex stainless steels the most suitable alternative due to their good properties when cold

worked.

2.3.1 Methods for Manufacture of OCTG

The cold pilgering process has many advantages over other tube processes such as cold draw. One

advantage is that it allows much larger deformation rates and it gives a good tolerance for

dimensions. The pilgering is performed by two rolls, or dies that roll over the tube and reshaping the

outside diameter. Each die has a groove were the diameter of the working section is shifting from

large to small to fit the tube before and after pilgering. Inside the tube there is a mandrel that

supports the tube and causes the wall reduction and shapes the inside diameter, see figure 15. The

shape of the mandrel is as important as for the dies since the space between them forms the tube

during the pilgering. The dies are supported by the saddle, during the pilgering the saddle is moving

back and forth making the dies roll over the tube. To reduce the material the tube is fed into the

tooling by controlled increments. The feeding is usually adjustable to allow for variations in material

and dimensions of the tube. For every fed increment the tooling rolls over the tube and reduces the

material, this step is defined a stroke. The feeding increment usually occurs when the dies are in their

entry position. The entry position is one of the end positions of the movement of the saddle where

the tube enters the tooling. There are machines that use double end feed where the feeding occurs

at both end positions of the saddle. In order to maintain roundness of the tube both the tube and the

mandrel are turned during the feeding [17].

15

Starting position

Position after one stroke

Mandrel

Billet, extruded Tube(

Finished Cold pilgered tube

Roll groove

Mandrel bar

Figure 15. Illustration of cold pilgering.

When the tube is pilgered the reduction in wall thickness is not necessary the same as reduction in

diameter. The relation between the reduction in wall thickness and diameter is known as the Q-

factor according to

in

outin

in

outin

d

dd

t

ttfactorQ

Equation 6

where int is the wall thickness of the extruded tube, outt is the wall thickness of the pilgered tube,

ind is the outside diameter of the extruded tube and outd is the outside diameter of the pilgered

tube. The Q-factor affects the surface finish and may be altered by changing the tool design [17]. It is

also possible to divide the pilgering in small intervals and then calculate the Q-factor for each small

interval to get more like a instant value for each point of the pilgering.

Sandvik AB uses the mean value of the inside and outside diameter when calculating the Q-factor

instead of the outside diameter of the tube.

The pilgering is a multi directional deformation process. When the dies roll over the tube a small

wave of material is pushed in front of the tool, this zone is called the squeeze zone. When the

material gets in between the die and mandrel it is in the forge zone, see figure 16. During the

pilgering the tube gets an elliptic shape, since it is pressed from two directions. This is good in one

aspect because it hinders the tube from getting stuck on the mandrel. The width of the contact area

between the tube and the die relative to the thickness of the wall determines how homogeneous the

deformation is. The most homogeneous deformation occurs when the contact length equals two

times the wall thickness (this corresponds to the criteria L/hm = 1 used for rolling processes where L is

the contact length and hm is the mean thickness). A small contact area relative to the wall thickness

causes inhomogeneous deformation were most deformation occurs close to the surface (L/hm < 1).

16

On the other hand if the contact area is much larger than the width of the wall, friction will occur

(L/hm > 1). The friction causes inhomogeneous deformation.

Figure 16. Illustration of the deformation of the tube during pilgering. The image is exaggerated.

The alternative to cold pilgering is cold drawing were the tube is pulled once or twice through a tool.

The drawing of OCTG is usually done by a draw bench that can draw the entire tube in one stretch.

For other applications the drawing is performed in steps were a new grip is made for every step.

There are several types of tools and ways of performing the drawing, some are illustrated in figure

17. Most of the methods are one step processes except rod drawing where the second step is

removing the rod. Cold drawing does not enable as large reductions as cold pilgering and is a much

faster process.

Figure 17. Principle of drawing processes, a) fixed plug draw, b) floating plug draw, c) rod drawing, ) sink

drawing and e) expanding

2.3.2 Plastic Deformation Mechanisms and Anisotropy during Cold Working

Cold drawn tubes reach about the same strength as a cold pilgered tubes even though the reduction

is lower. When performing the drawing the deformation in the tube is more or less in the same

direction throughout the entire process. The deformation can be compared with a one step rolling

operation. This assumption is backed by Park and Lee [18] that studied texture in an FCC material

during cold drawing. The drawing was made using plug drawing and the conclusion was that the

texture was very similar to rolling texture.

During pilgering the deformation is very complex and very few investigations have been made on the

texture on pilgered tubes, especially not on austenitic steels. When performing pilgering the

deformation is formed with small increments, one for each stroke. The deformation is shifting

direction since the rolls roll over the tube in both directions and the tube is turned during pilgering

17

changing the angle of deformation. Since the deformation shifts direction so does the slip plane

allowing large deformations. The Q-factor can have influence on the texture.

2.4 Previous Work

This section presents a small selection of scientific articles that gives a small hint of what can be

expected by this thesis. The results will be compared with these articles in order to confirm them.

Karaman et al. [19] studied the influence of nitrogen on the stress-strain behavior of low stacking

fault energy AISI 316L austenitic stainless steel. They used single crystals and performed tensile tests

at room temperature for different crystallographic directions. Transmission electron microscopy was

used to study the twinning. The test showed twin formation at strains as low as 3 % for some

directions and that twinning is a strong barrier to dislocation motion by confining the mean free path

of the dislocations. When adding 0.4 % nitrogen to the alloy the critical resolved shear stress increase

due to higher lattice friction and short range order formation. This leads to a change in deformation

mechanisms and deviations from Schmid Law. Twinning is suppressed in all directions which

decrease the strain hardening coefficient. The change in deformations mechanisms can be explained

by the increase in stacking fault energy caused by the addition of nitrogen.

Lee et al. [20] studied the deformation twinning in high-nitrogen austenitic stainless steels. They

used a steel with composition: 17.94Cr-18.60Mn-2.09Mo-0.89N-0.04C-balance Fe (wt %). With in-situ

EBSD they showed that grains oriented in the <001> or <111> orientation kept their original

orientation or rotated along the <001>-<111> boundary line towards the <112> orientation. Grains

with other orientations (in particular near <110> orientation) rotated towards the primary slip

direction until reaching the <001>-<111> boundary line. They confirmed that the orientation change

showed the same trend as for FCC single crystals. They also used TEM to study the twin formation

showing stacking faults and deformation twinning in the high-strain regime while the microstructure

in the low strain region consisted of planar dislocation arrays. The twinning showed strong

orientation dependence. Grains oriented with <100>, <110> and <111> direction parallel to the

tensile direction showed no deformation twinning, one activated twinning system and primary and

conjugate systems cooperated, respectively.

Dumay et al. [21] used thermochemical modeling for calculation of how the stacking fault energy is

affected by addition elements in austenitic Fe-Mn-C stainless steel. Their material (Fe22Mn0.6C) show

high formation of microtwins for SFE around 20 mJ/m3. When the SFE drops below 18 mJ/m3

microtwin formation are replaced by HCP ε-martnsite and when it drops below 12 mJ/m3

α´-martensite is formed. When the SFE is increased the amount of twinning decreases and other

deformation mechanisms take place. Twinning forms due to stacking-faults in the FCC structure

extending in parallel adjacent planes. When the twinning extends to every two planes it leads to the

formation of ε -martensite. They also showed that copper increases the stacking fault energy.

Simmons [7] wrote a review of high-nitrogen alloying of stainless steels. It is stated that austenitic

stainless steels typically have low stacking fault energy. Their deformation structure is characterized

by planar dislocation arrays while high SFE materials generally form cell-type dislocation structure.

The planar dislocations reduce the cross-slip leading to slip-band formation and enhanced strain

hardening.

18

3. Test Material The thesis is divided in two parts, the first and most extensive concerns the texture formation during

plastic deformation. The purpose of the examination is to learn what influence the alloying elements

have on the crystal rotation for austenitic stainless steels that are suitable for OCTG applications. The

reason for this is that crystal rotation is one source of anisotropy, which is a problem for cold

pilgered tubes. The focus lies on the elements molybdenum, nitrogen and copper. Austenitic steels

have been selected in pairs where there is a difference in each of these elements according to table

1. The examination is expanded with examinations of synergy effects and the influence of grain size.

Table 1. Steel grades used for texture evolution study (3R69, Sanicro 30 and 254 SMO are not

used for OCTG applications by Sandvik).

Influence Steel grade Heat Shape

Molybdenum { Sanicro 28 high N 522031 7” tube

Sanicro 29 522137 7” tube

Nitrogen { 3R69 713413 Bar, diameter 15 mm

3R60 498916 Tube

Copper { Sanicro 30 772082 Bar, diameter 22 and 32 mm

Sanicro 41 463664 Bar, diameter 22 mm

Nitrogen { Sanicro 28 520553 7” tube

Sanicro 28 high N 522031 7” tube

Synergy 254 SMO 474816 Bar, 15 mm

Grain size Sanicro 30 772082 Bar, diameter 22 and 32 mm

The other part of the thesis concerns evaluation of the tool design used for the cold pilgering. For this

evaluation three different 7” tubes are examined all from the same heat but pilgered with different

tool designs. The steel grade used is Sanicro 29 heat 522148.

3.1 Steel Grades

All steels used in this thesis are austenitic stainless steels with face centered cubic crystal structure.

The eight different steel grades have been chosen for their chemical composition more than for their

suitable applications. The chemical composition is very important to enable the possibility to

examine the influence of specific alloying elements, however some of the steel grades are very well

suited for OCTG applications such as Sanicro 28 and 29.

3.1.1 Sanicro 28

Sanicro 28 is a multi-purpose austenitic stainless steel well suited for OCTG applications and is

attended for service in highly corrosive conditions. For chemical composition see table 2. In this

investigation two versions of Sanicro 28 is used one with slightly higher nitrogen content. The

nitrogen contents measured are 0.047 and 0.091 respectively.

19

Table 2. Chemical composition (nominal %) of Sanicro 28 according to datasheet at Sandvik AB

webpage [22].

C Si Mn P S Cr Ni Mo Cu

< 0.020 < 0.6 < 2.0 < 0.025 < 0.015 27 31 3.5 1.0

3.1.2 Sanicro 29 Sanicro 29 is suited for OCTG applications in particularly corrosive conditions. The alloy is developed

for improved localized corrosion resistance in environments with high chloride concentrations, CO2

and H2S at high temperatures. It is highly alloyed, see table 3, to be able to have very good corrosion

resistance.

Table 3. Chemical composition (nominal %) of Sanicro 29 according to datasheet at Sandvik AB

webpage [22].

C Si Mn P S Cr Ni Mo Cu

< 0.020 < 1.0 < 2.5 < 0.025 < 0.015 27 31.5 4.4 1.0

3.1.3 Sanicro 30

Sanicro 30 is used for tubing in heat exchangers and steam generators in nuclear stations for

temperatures up to about 550 °C were it is important with good resistance to stress corrosion

cracking and intergranular corrosion. Its chemical composition can be seen in table 4.

Table 4. Chemical composition (nominal %) of Sanicro 30 according to datasheet at Sandvik AB

webpage [22].

C Si Mn P S Cr Ni Cu Ti Al

< 0.030 0.5 0.6 < 0.020 < 0.015 20 32 < 0.10 0.5 0.3

3.1.4 Sanicro 41

Sanicro 41 is an austenitic, corrosion resistant Ni-Fe-Cr alloy. It is used for heat exchanger tubing and

for tubing and casing in oil and gas production, especially suited for use in environments with high

concentrations of hydrogen sulphide and chloride. See table 5 for chemical composition.

Table 5. Chemical composition (nominal %) of Sanicro 41 according to datasheet at Sandvik AB

webpage [22].

C Si Mn P S Cr Ni Cu Ti Mo

< 0.030 < 0.5 0.8 < 0.025 < 0.015 20 38.2 1.7 0.7 2.6

3.1.5 3R60

3R60 is a multi-purpose austenitic steel where there is need of good corrosion resistance like heat

exchangers, condensers, pipelines, cooling and heating coils in the chemical, petrochemical, pulp and

paper and food industries. Its chemical composition can be seen in table 6.

20

Table 6. Chemical composition (nominal %) of 3R60 according to datasheet at Sandvik AB webpage

[22].

C Si Mn P S Cr Ni Mo

< 0.030 0.4 1.7 < 0.040 < 0.015 17.5 13 2.6

3.1.6 3R69 3R69 is similar to 3R60 but has a higher content of nitrogen, see table 7.

Table 7. Chemical composition (nominal %) of 3R69 according to datasheet at Sandvik AB webpage

[22].

C Si Mn P S Cr Ni Mo N

< 0.030 0.4 1.7 < 0.030 < 0.015 17.5 13 2.6 0.18

3.1.7 254 SMO 254 SMO is a austenitic steel approved for seamless pipe in boiler and pressure vessel applications

and for sulfide stress cracking resistant material for oil field equipment. Its chemical composition can

be seen in table 8.

Table 8. Chemical composition (nominal %) of 254 SMO according to datasheet at Sandvik AB

webpage [22].

C Si Mn P S Cr Ni Cu Mo N

< 0.020 < 0.80 < 1.00 < 0.030 < 0.010 20 18 0.7 6.1 0.20

3.2 Heat Treatment and Grain Size In order to get comparable results it is important that the materials have the same starting situation

with respect to residual stress and grain size. The ideal starting material would be a completely

relaxed material with uniform grain size and no other phases than austenite. The best way to get

close to this situation is to heat treat the material to get it recrystalized. Most of the materials were

heat treated in order to get a desired structure with similar grain size and undeformed grains, see

table 9.

Table 9. Heat treatments of the steel grades, all

materials were quenched in water.

Steel grade Temperature (°C) Hold time (min)

Sanicro 28 1125 3

Sanicro 29 1180 3

Sanicro 30 1050 25

3R60 1100 20

254 SMO 1180 5

To get larger grains in Sanicro 30 it was held at 1050 °C for 90 minutes and then quenched in water.

To further increase the grain size it was held at 1100 °C for 60 minutes and then quenched in water.

The measured grain sizes after the heat treatments will be presented in the result section.

21

3.3 Test Samples

The test samples were in shape of tensile testing specimen with diameter 7.0 ± 0.4 mm and active

length 70 mm, denotation DR7C70, see appendix I for geometries and drawing. All test samples were

taken in the lengthwise direction of the rod or tube, see figure 18. The specimens were fabricated by

turning in a CNC operated lathe at Sandvik AB in Sandviken.

Figure 18. Illustration of how the tensile specimens were taken out from tubes and solid bars and which parts

of the specimen that were used for evaluation.

After straining, samples for LOM, EBSD and hardness testing were taken out from each tensile

specimen.

For the evaluation of tool designs Rockwell hardness measurements were made. The test samples for

hardness measurements were in shape of cut out rings, all were cut out close to the middle of the

tubes. The rings were cut out using a lathe and were then ground to get smooth and parallel sides.

The samples for EBSD texture measurements were also cut out close to the middle of the tube

according to figure 19.

Figure 19. Illustration of where the samples were cut out for evaluation of tool designs. The tube length is not

scaled to its diameter.

22

4 Experimental This section will describe how the measurements were performed.

4.1 Sample Straining

In ordered to be able to investigate how the chemical composition affects the texture formation the

specimen need to be deformed in a controlled way. The easiest way to do this is to use tensile testing

equipment. A test pull was performed for all materials in order to see at which strain the necking

begins. All deformation up to the point of maximum stress were the necking begins is uniform

throughout the narrow region of the tensile specimen [6]. This is of course valid in a macroscopic

perspective due to that strain hardening is larger than area decrease and requires that the material is

homogenous. It is important for the investigation since several samples were taken out from each

specimen. Each steel grade was deformed to five different strains that were evenly distributed in the

interval up to necking, see table 10. All tensile specimens were measured before and after

deformation and the area reduction of the cross section were calculated. The diameter

measurements were performed with a digital micrometer from Mitutoyo and the area was marked

so that the EBSD sample was taken from the same place if there would be any deviations along the

active part of the specimen. All straining was performed with a Roell & Korthaus tensile testing

machine controlled by the program Cyclic by Inersjö Systems AB. The stain rate was 1 mm/min.

Table 10. Strain and area reduction for the specimens.

Sanicro 28 Strain (mm) 7 14 21 28 35

Area reduction (%) 6.7 13.2 19.0 24.4 30.3

Sanicro 28

high N

Strain (mm) 7 14 21 28 35

Area reduction (%) 7.6 14.0 19.7 25.3 30.9

Sanicro 29 Strain (mm) 7 14 21 28 35

Area reduction (%) 7.3 13.1 18.7 23.9 28.6

Sanicro 30 Strain (mm) 6 12 18 24 30

Area reduction (%) 5.9 11.4 16.2 21.3 25.7

Sanicro 41 Strain (mm) 7 14 21 28 35

Area reduction (%) 7.1 13.3 18.5 23.7 28.5

3R60 Strain (mm) 7 14 21 28 35

Area reduction (%) 6.7 12.8 18.9 24.3 29.8

3R69 Strain (mm) 3 7 11 15 19

Area reduction (%) 2.8 10.6 11.6 16.0 24.8

254 SMO Strain (mm) 7 14 21 28 35

Area reduction (%) 6.7 13.0 18.8 23.2 28.5

Sanicro 30

large grains

Strain (mm) 6 12 30

Area reduction (%) 5.8 10.9 25.4

23

4.2 EBSD

Electron back scatter diffraction was used to measure the texture. The technique is described in the

theory section. The instrument used was a Zeiss 1540 EsB Cross beam FIB-SEM equipped with EBSD

apparatus by Oxford Instruments. The middle part of each rod was carefully cut out not to damage

the microstructure or texture using equipment by Struers. Measurements were performed at the

transversal cross section of each rod. The samples were prepared for measurements by the following

steps:

1. In-molded in 25 mm pellets of Struers Polyfast 2. Surface-grinding, removing at least 1 mm to be sure to eliminate any plastic deformation

from the cutting 3. 6μ grinding 4. 3μ grinding 5. OPS-polishing with hydrogen peroxide (100:15 OPS-solution and hydrogen peroxide) 6. The surface was carefully cleaned using ethanol soaked cotton pads

All sample preparation was performed using equipment by Struers.

The measurements were made at half radius of the sample with acceleration 20 kV, step size 1.8 µm

in a raster of 1173x244 points, se figure 20. The parameters were chosen with respect of the texture

measurement only. During the measurements the sample was tilted 70 degrees and the indexing was

made with the HKL Channel 5 program Flamenco by Oxford Instruments.

Figure 20. Illustration of were on the sample the measurements were made.

EBSD measurements were also carried out on the Sanicro 29 tubes pilgered with different tool

designs. Here the measurement parameters were acceleration 20 kV, step size 1.8 µm and the raster

were 500x500 points. All measurements were made in the transversal direction but on three

different locations, close to the outside surface, in the middle of the wall and close to the inside

surface for the different tool designs.

24

4.3 LOM

The microstructure was examined with light optical microscopy (LOM). Optical lenses are used to

give a magnified image of the specimen. Visible light is used which makes it easy to use because it

gives a direct picture that can be captured by a normal camera. The technique has limitations in a

low focus depth and resolution of only about 2 μm due to the wave length of visible light.

The samples were prepared by grinding and polishing down to 3 μm and etching. The etching was

performed in V2A or electrolytically in oxalic acid (voltage 3V). The V2A etch is consisting of 50 ml

hydrochloric acid, 50 ml water, 5 ml nitric acid and 2 drops of Rodine. The V2A etch was heated to

about 60°C before etching.

4.4 Vickers Hardness

When measuring Vickers hardness a small square-based diamond pyramid is pressed on the surface

with loads normally between 1 and 10 kg, sometimes lower but is then referred to as microhardness.

The pyramid has an angle of 136° and Vickers hardness is then calculated with the formula:

2/854.1 dPHV Equation 7

where P is the load in kg and d is the average diameter of the increment [6].

In this thesis HV5 were used (load 5 kg) to study deformation hardening during deformation. The

samples where baked in Bakelite and ground down to 3 µm. Each sample was measured with three

indents on both transversal and lengthwise cuts.

4.5 Rockwell C Hardness

When measuring Rockwell C a diamond cone is pressed towards the surface, the hardness number is

determined by the difference in penetration depth between an initial minor load and a following

major load. The cone angle is 120°, the minor load is 10 kg and the major is 150 kg [6].

Measurements were performed at Sandvik AB in Sandviken. All measurements were made on a

transversal cut of the tube. Increments were made at three positions, 2.54 mm from the inner and

outer edge and in the middle of the wall. The measurements were made every ten degrees around

the tube.

25

5 Results and Discussion All results will be presented in this section together with a discussion.

5.1 Microstructure of Reference Samples (LOM)

The steel grades 3R69 and Sanicro 41 were not heat treated due to that their microstructure

appeared undeformed and grain growth wanted to be avoided. The microstructure is shown in

figure 21. 3R69 had large amounts of twinning and a large variation in grain size while Sanicro 41 had

more uniform grain size and a small amount of twinning.

Figure 21. Microstructure of a) 3R69 and b) Sanicro 41.

The source materials for Sanicro 28 and 29 were cold pilgered tubes with large deformations. The

deformed and annealed structure of Sanicro 29 can be seen in figure 22. The reference sample of

Sanicro 29 had a microstructure with twins and no other obvious phases than austenite. The

reference samples of the two Sanicro 28 grades had very similar structure to Sanicro 29 after the

heat treatment. The only difference was that Sanicro 29 had a slightly larger grain size.

Figure 22. Microstructure of Sanicro 29, a) as received and b) after heat treatment.

Sanicro 30 had much smaller grains than the other materials before the heat treatment. After heat

treatment the structure showed an amount of twinning similar to Sanicro 29 but the grain size was

still smaller than both Sanicro 28 and Sanicro 29. Four specimen of Sanicro 30 were heat treated to

get larger grains. This heat treatment resulted in inhomogeneous grain growth with some areas with

very large grains and some with grain sizes similar to Sanicro 28, see figure 23.

26

Figure 23. Sanicro 30 heat treated to get large grains showing inhomogeneous grain growth, a) area with large

grains and b) area with smaller grains.

The microstructure was also evaluated using EBSD which will be presented later in this report

including measured grain size and amount of twinning.

5.2 Microstructure After Deformation for Sanicro 29 (LOM) The deformed structure is shown for one steel grade. Sanicro 29 has similar microstructure as several

other steel grades in this investigation and its deformation can be regarded as typical for these

steels. Figure 24 shows the undeformed material, where it is possible to see the distribution of twins.

There is no difference in the structure obtained from the transversal compared to the lengthwise cut.

Figure 24. Reference sample of Sanicro 29, a) lengthwise and b) transversal.

Figure 25 shows the structure at maximum deformation, just before necking. The lengthwise

structure is clearly deformed and stretched in the strain direction. Both in the lengthwise and

transversal structures slip bands are clearly formed. The transversal structure seems to have smaller

grain size in the deformed structure compared to the reference sample. This is due to the stretching

of the grains, the section area of the grains will decrease as the section area of the tensile sample

decrease during deformation.

27

Figure 25. Deformed 28.6 %, just before necking. Images a) lengthwise structure showing grain deformation

and slip bands (the pull direction is horizontally) and b) transversal structure showing slip bands.

Figure 26 shows the slip bands at higher magnification. The slip bands start to show at deformations

around 20% area reduction.

Figure 26. The formation of slip bands at high magnification, a) transversal at 23.9 % deformation and b)

transversal at 28.6 % deformation.

5.3 Microstructure of Reference Samples (EBSD)

When performing an EBSD measurement the electron beam from the SEM is used to scan the

sample. The beam stops at each measurement point and the crystal orientation and the quality of

the signal is detected (see the theory section). The results from the EBSD measurements can be

presented in different ways to highlight different properties of the samples. One of these possibilities

is to present maps of the sample like the one in figure 27 that show the microstructure of the

Sanicro 28 reference sample. Each measurement point’s orientation is compared with its neighbor’s

orientation, when there is a difference between the orientations it is registered as a boundary. How

large this difference has to be to be registered as a boundary can be specified. In figure 27 each

boundary larger than 2 degrees is registered and differences of 60 degrees are registered as a twin

boundary. The evaluation was made with the HKL Channel 5 program Tango.

28

Figure 27. Sanicro 28, grain boundaries are shown with black and twins with red lines. Band contrast is also

used; the brightness corresponds to the signal quality which is related to the orientation.

The software was used to count the grains within the measured area. All grains with area less than 10 μm2 were discarded due to the possibility that the detected grain might be noise instead of an actual grain. To get a value of how much annealing twins each material has, all boundaries are measured, see figure 28. The value measured is the accumulated value of number of boundaries up to the specific angle. It is possible to calculate the fraction of twin boundaries compared to grain boundaries. Boundaries of small angles (less than 2 degrees) are not considered to be a grain boundary but more likely a small misorientation. The most frequent annealing twin in austenitic steels is a Σ3 type twin. The Σ3 boundary can be described through a 60° rotation around a <111> axis [23]. When calculating the twin fraction all boundaries between 59 and 61 degrees are regarded to be a twin boundary, see figure 28.

Figure 28. The diagram shows the number of boundaries up to a specific angle and the points were the values

are taken for calculating the twin fraction for Sanicro 28.

All materials were investigated in the same way so that they can be compared with each other, the

result can be seen in table 11. The ASTM grain size number G was calculated using standard

conversion formula according to 2009 ASTM standards [24].

29

When performing the EBSD measurement it became evident that the 3R69 reference sample was not

free from deformation as assumed by the LOM observation. Because of the deformations it is

impossible to use the same parameters as for the other steels to establish grain size and amount of

twinning. In fact it is very hard to get a good value at all. However the main objective is to study

texture formation and this is still possible, but it is important to keep in mind that the material does

not have the same undeformed structure as the other steels as this might affect the results.

5.4 Texture Formation (EBSD) EBSD were used to measure the texture in the different austenitic steel grades. The results are

presented as inverse pole figures and as relative values of how large fractions of specific orientations

that are oriented within a specified angle from the strain direction. The texture formed during the

straining is fiber texture. It is what can be expected and is confirmed by an ODF comparison for

Sanicro 28 at 30.3 % deformation, see appendix II.

5.4.1 Inverse Pole Figures

Inverse pole figures have been chosen to present the texture formation. This is because they are easy

to understand and in a good way show what is important in this investigation. The color scale is the

same for all materials making it easier to compare how strong the texture is in the different grades,

the scale ranges from close to zero to mud value 8. Each height curve represents one mud. Mud is a

relative scale used for presentation of texture densities, the mean value over the entire pole figure is

one mud. All inverse pole figures show the texture components in the transversal direction since it is

fiber texture that is studied. Every point in the figure represents a specific crystal orientation, the

corners represent <001>, <101> and <111> directions for example. The color of each point indicates

the density or concentration of that specific crystallographic orientation in the transversal direction

(tensile direction). For example if the 001 corner is red it means that a large fraction of the measured

points have their <001> direction parallel or close to parallel with the tensile direction. The inverse

pole figures were created with the HKL Channel5 program Mambo.

Table 11. Grain size and fraction of twinning in undeformed samples (No. is the number of grains within the

measured area).

Steel grade No. No./mm2 d average/µm

Grain size

ASTM no. G

Fraction of

twinning %

3R60 349 376 55 5.6 35.1

254 SMO 306 330 60 5.4 21.0

Sanicro 28 335 361 56 5.5 55.0

Sanicro 28 high N 291 314 61 5.3 51.7

Sanicro 29 172 185 80 4.6 51.6

Sanicro 41 370 399 53 5.7 20.2

Sanicro 30 855 922 32 6.9 58.0

Sanicro 30 (larger grains) 282 304 55 5.3 55.2

30

Sanicro 28

The reference sample show annealing texture, mainly <001> but also <111> texture none of which is

strong, see figure 29. The deformed material shows more and more <111> texture with larger

deformation, at the same time the intensity increases along the line between the <001> and <111>

direction and especially at the <112> direction.

Figure 29. Texture in Sanicro 28 at different rates of deformation.

The other Sanicro 28 grade, with slightly higher nitrogen content, shows a very similar texture

formation during deformation, see figure 30. The reference sample has an annealing texture of

mainly <111> texture but as for the other Sanicro 28 grade it is a weak texture. The texture at

maximum deformation is however slightly smaller than for the lower nitrogen alloy.

Figure 30. Texture in Sanicro 28 with high nitrogen content at different rates of deformation.

Sanicro 29

The reference sample of Sanicro 29 has a small annealing texture just like for Sanicro 28. During

deformation the texture formation indicates an increase of <001> texture at about 20 % deformation

that decreases again at higher deformations. At maximum deformation Sanicro 29 show a strong

<111> texture, stronger than Sanicro 28, see figure 31.

Figure 31. Texture in Sanicro 29 at different rates of deformation.

31

Sanicro 30

The reference sample of Sanicro 30 has a weak annealing texture. When deformed the <001> texture

goes up and down indicating that the <001> direction is not stable or that there are variations in

different samples. However the <111> texture is steadily increasing with increasing deformation. It

shows the same type of texture formation as both Sanicro 28 and 29 with an increase along the line

between <001> and <111> directions, especially at the <112> direction. The texture at maximum

deformation is lower than for Sanicro 28 and 29 however the deformation is not as high. Inverse pole

figures are displayed in figure 32.

Figure 32. Texture in Sanicro 30 at different rates of deformation.

The samples with larger grain size show almost the same texture, see figures 32 and 33. A small

difference can be seen at about 11 % deformation, were the sample with larger grains has a slightly

stronger texture.

Figure 33. Texture in Sanicro 30 with larger grain size at different rates of deformation.

Sanicro 41

Sanicro 41 shows a completely different texture formation than Sanicro 28, 29 and 30, see figure 34.

The texture formation is much weaker and the annealing texture is kept at a much higher extent. The

crystal rotation is still directed so that the orientation is turned towards the line between the <001>

and <111> direction. Then the texture starts to divide between <001> and <111> texture.

Figure 34. Texture in Sanicro 41 at different rates of deformation.

32

3R60

Unfortunately the annealing texture is very strong in the reference sample of 3R60, with a <001>

texture. When deformed the <001> texture is initially weakened and the <111> texture is formed, at

high deformations both <001> and <111> texture is increasing with larger deformations, see figure

35. Unlike some of the other steel grades the intensity along the line between <001> and <111>

directions is low.

Figure 35. Texture in 3R60 at different rates of deformation.

3R69

The texture formation in 3R69 resembles the texture formation in Sanicro 30. However there is no

smooth formation. Remember that the material was deformed already before straining causing a

more inhomogeneous deformation then for the other grades, see figure 36.

Figure 36. Texture in 3R69 at different rates of deformation.

254 SMO

254 SMO shows a very slow texture formation that much resembles the formation in Sanicro 41 but

even slower with no evident <001> or <111> texture, see figure 37. The density is increasing on the

line between the <011> and <111> directions. The annealing texture seems to rotate toward that line

were it slowly is scattered along the line.

Figure 37. Inverse pole figures of 254 SMO at different rates of deformation.

All samples show some kind of texture at zero deformation. The texture depends on how the

material has been treated. It is not possible to totally erase the texture with a heat treatment. But it

33

is possible to decrease and change the texture. Texture gained for instance during cold pilgering will

leave traces after the heat treatment.

Texture during compression

Most of the austenitic stainless steels tested show clear <111> and <001> fiber texture after

straining. 254 SMO and Sanicro 41 show initial texture rotation according to what can be expected

from the crystal rotation theory. According to the theory a compressed sample should show <101>

fiber texture. No compression test was made during this thesis. However David Hedstöm [25]

performed EBSD measurements on unidirectional compressed Sanicro 28 during his thesis performed

at Sandvik 2008/2009, see figure 38. The sample show clear <101> fiber texture as was expected.

Figure 38. Inverse pole figure for Sanicro 28 deformed in one direction 20 %.

5.4.2 Specific Orientation Measurements

With the feature “texture component” in the software Tango used for EBSD evaluation it is possible

to get information from only a specific crystallographic direction. It is possible to get the accumulated

value of points up to any desired deviation angle from the tensile direction. When knowing the total

amount of measurement points the fraction can easily be calculated. When the fraction were

calculated only measurement points with a value were used, that is compensation were made for

zero solutions.

Figure 39. Illustration of where the measurements were made for the reliability test, performed at Sanicro 28

with high nitrogen, deformed 30.3 %. Three measurements were made at half radius, one in the centre and one

close to the edge.

34

To get an idea of how reliable these measurements are, five different measurements were made on

the same sample, see figure 39. Sanicro 28 with high nitrogen was chosen due to that it gave the

lowest count of zero solutions among the maximum strained samples. Then empirical testing showed

that the best results were attained at a maximum deviation angle of ten degrees, see figure 40. A

smaller angle gives higher scatter, while a larger angle does not show the differences between the

crystallographic directions as well. The measurements were made in a raster of 400x400 points with

step length 1.8 µm. The amount of measured points is about half as many as in the real

measurement applying that the real measurement should have smaller scatter than this one.

Figure 40. The diagram show scattering between five different measurement points on the same sample. The

maximum deviation angle from the tensile direction is ten degrees.

The scatter for the 111 direction is as high as 20 % but when adding several directions together to

form a value of the total texture the scatter is much lower. No evident difference can be seen

regarding how far from the center the measurement has been made.

The same calculations were made to show the texture formation during deformation. The diagram

for Sanicro 28 can be seen in figure 41. Even if the material was totally free from texture the texture

component in this case would not show zero at zero deformation and the different directions would

not show the same value since some directions are more common than others. It is however possible

to see how the texture is changed, what orientations that are favored and what fiber texture that is

formed. In the beginning all the chosen directions is favored by the deformation but the 101

direction soon decreases and almost completely disappears at large deformations. The 112 direction

seems to be the dominant direction at the beginning, remember however that the high starting point

is due to the many equivalent directions. At higher deformations it decreases like the 112 position

35

only is a temporary point in the crystal rotation. The 111 and 100 directions seem to be competing

against each other finding some sort of equilibrium towards the end of the deformation, where 111

clearly dominate over 100 textures. The sum of 001, 010 and 111 texture component is included in

the diagram to represent the total texture and as can be expected it goes up with increasing

deformation.

Figure 41. The diagram shows the fiber texture evolution during deformation for Sanicro 28. The maximum

deviation angle is set to ten degrees.

The Sanicro 28 grade with higher nitrogen, Sanicro 29, Sanicro 30 (both grain sizes) and 3R69 show

the same type of texture evolution.

254 SMO show strong formation of <112> texture, that is continuously growing during the

deformation. The <001> texture grows initially to a fraction of 9 % at about 20 % deformation but is

then decreasing at higher deformations. <111> texture is growing during the whole deformation but

only to about 6 % at maximum deformation, see figure 42.

36

Figure 42. The diagram shows the fiber texture evolution during deformation for 245 SMO. The maximum

deviation angle is set to ten degrees.

Sanicro 41 has a high fraction of <001> texture from the beginning. Initially the <001> texture is

decreased up to 13 % area reduction and is then growing again. The <001> texture is higher than the

<111> texture unlike many of the other steel grades. <112> texture is growing during increasing

deformation. Overall the texture evolution of Sanicro 41 is similar to that of 254 SMO.

3R60 is very similar to Sanicro 41 with the only difference that <112> texture is decreasing towards

the end.

To be able to compare the different steel grades and to see any differences due to their composition

and grain structure several steel grades are presented in the same diagram. In figure 43 the texture

components <001> and <111> are presented for the steels Sanicro 28 and 29. The overall impression

is that the formation of <111> texture accelerates as the deformation increases while the <001> goes

up and down but stays within a certain level as if it is not a stable texture. Sanicro 29 shows a large

<111> texture increase towards maximum deformation while Sanicro 28 with high nitrogen shows a

low increase. This indicates that molybdenum has an increasing effect on texture while nitrogen has

a decreasing effect, this will be discussed further later in the report.

37

Figure 43. <001> and <111> texture components in Sanicro 28 and 29 at different rates of deformation. The

maximum deviation angle is ten degrees.

Figure 44 shows the <001> and <111> texture components for the steels Sanicro 30 and Sanicro 41.

Sanicro 30 shows a smooth increase of both <001> and <111> texture. When comparing the texture

evolution for the two different grain sizes there is no evident difference between them, they both

show the same trend and at maximum deformation the textures is almost identical. If the increase in

texture is considered the small grained steel has improved its texture more. As observed earlier in

the inverse pole figures, Sanicro 41 shows a different texture evolution with much weaker texture.

Sanicro 41 is supposed to be compared against Sanicro 30 with regard of difference in copper

composition. This implies that copper has a decreasing effect on texture formation. Observe that

Sanicro 41 not only has higher copper composition but also more molybdenum making it much more

complex, this will be discussed further later in this report.

38

Figure 44. <011> and <111> texture component in the steels Sanicro 30 and 41 (LG = large grains). Maximum

deviation angle from the strain direction is ten degrees.

Figure 45 show a comparison between the steels 3R60, 3R69 and 254 SMO. 3R60 has a very high

annealing texture making it very hard to compare it with the other steels. It shows an increase mainly

in the <111> direction as many of the other steel grades but the texture at maximum strain is

dominated by <001> texture. 3R69 behaves similar to Sanicro 30 but with a smaller increase in <001>

texture. 254 SMO show very little increase in both <001> and <111> texture as expected from the

inverse pole figures. The <112> component has been added to show that 254 SMO still has some

texture formation.

39

Figure 45. Measurements of the texture components <001> and <111> for the steels 254 SMO, 3R69 and 3R60

and the <112> component for 254 SMO. Maximum deviation angle is ten degrees.

5.5 Substructure of Deformed Samples (EBSD)

An attempt was made to see if any differences in substructure could be seen between Sanicro 28 and

254 SMO. If the differences in texture is due to activated deformation mechanisms there is likely that

they will not have the same type of substructure. The measurements were performed with step

length 0.1 µm in a 500x500 raster. The area analyzed is, as for the other measurements, at half radius

of the sample. The area was chosen so that information should be attained from several grains.

Figure 46 show maps of the measured areas. The deformations can be seen clearly for both materials

as parallel lines through the grains. These lines could be piles of planar dislocations, since planar

dislocations are normally expected in austenitic stainless steels. There are no obvious differences in

the substructure between the two steel grades. Some pixels in the pictures show completely

different orientation relative to their surroundings. It is unclear if these are a result of dislocations or

just measurement errors.

40

Figure 46. Inverse pole figure colored maps according to the legend. The left image show 254 SMO deformed

18.8 % and the right Sanicro 28 deformed 19.0 %.

The rotation of the crystallographic orientation also gives a change in the Schmid factor. In figure 47

the Schmid factor is presented for the same samples as in figure 44 and for a Sanicro 28 sample

deformed 13.2 %. The parallel bands can be seen clearly in all the maps. In one of the grains in the

Sanicro 28 sample deformed 13.2 % the lines are visible in two directions. Remember from figure 14

that this grain has a <111> direction due to its low Schmid factor.

Figure 47. Schmid factor variations in the grains for 254 SMO (deformed 18.8 %) and Sanicro 28 (deformed 19.0

and 13.2 %) respectively.

41

5.6 Strain Hardening Coefficient

Vickers hardness measurements have been made on the samples because the hardening coefficient

can give information of which deformation mechanism that is active. A high hardening rate can be a

result from more twin formation [21]. The HV5 value used for calculating the hardening coefficient is

a mean value attained from both transversal and lengthwise measurements, three on each side, see

table 12.

Table 12. Curve approximation on the form HV5=kxn

were made for the measurements. n is the hardening coefficient.

Steel grade k n HV5, 0 %

254 SMO 117 0.30 164

3R60 105 0.27 132

3R69 252 0.09 266

Sanicro 28 106 0.29 150

Sanicro 28 High N 98 0.31 153

Sanicro 29 97 0.32 147

Sanicro 30 107 0.25 130

Sanicro 30 larger grains 94 0.28 125

Sanicro 41 88 0.33 128

There are no large differences in the hardening coefficient and most grades have a hardening

coefficient close to 0.3. When comparing 254 SMO and Sanicro 41 with the other steels like Sanicro

28 and 29, that showed a completely different texture formation, no differences can be seen. 3R69

has very deviating values due to its already deformed state before straining.

5.7 Influence of Composition and Grain Structure on Texture

One of the purposes of this investigation was to establish what influences N, Cu and Mo have on

texture formation during cold working of austenitic steels.

The influence of nitrogen should be attained by comparing 3R69 and 3R60. It turned out that 3R69

was deformed before straining making the evaluation much harder. 3R60 had a very strong texture

even after heat treatment adding on the difficulty of the comparison. However both steels show

increasing <111> fiber texture with deformation. It is hard to say anything about the fraction of

<100> texture compared to <111> texture for both steels.

Regarding Sanicro 28, it seems that the grade with more nitrogen has lower texture. The dominating

texture in both cases is <111> texture. According to Engberg [11] lower SFE gives less <111> texture

but should also increase <001> texture. It is unclear if nitrogen raises or decreases the SFE but it is

affecting the material as if it decreased it, see the theory section. The differences in measured <100>

texture is small compared to the jumps during deformation and the assumed accuracy of the

measurement. In total the low accuracy of the measurement makes it impossible to say in what way

nitrogen affects the texture formation. However the results rather support the assumed behavior

than contradicts it.

The influence of molybdenum was studied by comparing the texture formation of Sanicro 28 (high N)

with Sanicro 29. Sanicro 29 has a higher concentration of molybdenum and gives a stronger <111>

texture. This is in agreement with Engbergs [11] explanation if molybdenum is raising the SFE.

42

Schramm et al. [15] claims that molybdenum increases the SFE in commercial –grade austenitic Fe-

Cr-Ni, Fe-Mn-Ni and Fe-Cr-Ni-Mn steels. However it is important to remember the large uncertainty

in the measurements, but as for the influence of nitrogen in Sanicro 28 the results is in line with the

theory.

The influence of copper was studied in Sanicro 41 compared to Sanicro 30. Sanicro 41 has very high

copper content combined with both higher Mo and Ni compared to Sanicro 30. The texture

formation of Sanicro 41 is completely different and appears to form at a much lower rate (slower).

Since not only the copper content is higher this cannot be concluded to be caused by copper alone.

The result of this is a much lower texture which could be good from an anisotropy perspective. Since

there are high levels of Mo and Cu in Sanicro 41 it is likely that it has a high SFE. A very high SFE could

cause changes in deformation mechanism.

254 SMO were used to see synergy effects by combination of high Mo, N and Cu content. As for

Sanicro 41 it showed less texture formation were the crystal rotation appears slower. This

strengthens the reason to believe that the anisotropy problem caused by texture formation during

cold pilgering can be reduced by changing composition. As for Sanicro 41 the high levels of Mo and

Cu can increase the SFE so that the deformation mechanism changes. This is of course just

assumptions. Measurements of the SFE could give an answer if this is the reason.

The effect of grain size on texture formation was studied on Sanicro 30. The heat treatment

supposed to enlarge the grains gave inhomogeneous grain growth. The actual area for the

measurement had only slightly larger grain size. The results show no strong effect on the texture

since the textures at necking is almost the same. The increase of texture during the total deformation

was however stronger for the samples with smaller grains. It cannot be concluded that the grain size

has any effect on the texture. It is likely that if grain size has an impact on texture the effect is small.

5.8 TEM

No transmission electron microscopy (TEM) measurements were performed in this thesis. However

David Hedström [25] performed TEM measurements on Sanicro 28 during his thesis at Sandvik

2008/2009. TEM pictures of Sanicro 28 compressed 20 % are presented in figure 48.

The TEM analysis of the dislocation structure shows dislocation bands that support the substructure

measurements performed by EBSD. They also show tangled dislocations very similar to sub-cell

formation. This type of formations could possibly give the type of orientation changes in small areas

shown by the EBSD substructure measurements. If it is so this is a difference in deformation

mechanisms. Sanicro 28 shows a larger fraction of sub-cell like entanglements relative to planar

dislocations than 254 SMO. This contradicts the assumption that 254 SMO should have higher SFE

than for instance Sanicro 28. A tendency to form planar dislocations instead of sub-cells is related to

decreasing the SFE [7].

43

Figure 48. TEM pictures of Sanicro 28. The sample has been compressed 20 % in one direction. The pictures are

showing: a) dislocations in specific crystal planes, b) thick dislocation bands and c) heavily tangled dislocations.

5.9 Misorientation The EBSD maps can be used to show how the material is deformed. In the same way as the maps can

be used to detect grain boundaries and twins it can be used to detect small rotations inside the

grains. The small rotations are referred to as misorientations. These rotations are an effect of

dislocation movement inside the material. The measurements in this thesis is not optimized to show

this behavior but to show texture evolution. When performing a texture measurement it is important

to get information from many grains and because of this the step length is rather long, 1.8 µm.

Misorientations are smaller than this with the effect that several misorientations can be within a

step. Figure 49 show the undeformed sample of Sanicro 28, where there is almost no visible

misorientations and the ones present are most likely low angle grain boundaries mistaken for

misorientations. This is typical for an undeformed material.

Figure 49. Undeformed sample of Sanicro 28. The black lines are grain boundaries, the brown are twins and the

colored ones are misorientations with rotation axis according to the legend at the right.

Figure 50 show the same material deformed to 13.2 % area reduction. Here the formation of

misorientations has started. The amount of deformation is different in different grains due to the

grains orientation relative to the tensile direction. Notice that the grains seem to be dominated by a

certain color meaning that the rotations occur about the same axis, this is also due to the orientation

of the grain.

44

Figure 50. Sanicro 28 deformed to 13.2 % area reduction. Black lines are detected grain boundaries, brown are

twin boundaries and misorientations are colored according to the legend at the right.

At even higher deformation the deformation spreads to cover entire grains, see figure 51. If the

measurement was performed at higher resolution it probably would be possible to see conceivable

dislocation cell formation were dislocations form cell walls. Every cell contains a small volume of

material with few faults like small grains within the grain.

Figure 51. Sanicro 28 deformed to area reduction 24.4%. The black lines are grain boundaries, the brown are

twins and the colored ones are misorientations with rotation axis according to the legend at the right.

5.10 Tool Design Evaluation The second part of this thesis concerns the evaluation of three different tool designs used for cold

pilgering of OCTG. The three designs are referred to as design A, B and C. All designs have the same

total deformation. The Q-value is however different at intermediate positions during the pilgering.

Design B has a more homogeneous deformation, especially towards the end of the pilgering.

Design C has a larger Q-value than the others in the beginning of the pilgering and a smaller towards

the end. Therefore the reduction of the wall is large compared with the reduction of the diameter in

the beginning of the pilgering and the opposite towards the end for design C.

5.10.1 Hardness Variations (Rockwell C)

Rockwell C hardness measurements were made every ten degrees around the tube at three different

locations 2.54 mm from respective edge and in the centre of the wall. There were no large

differences between the tubes pilgered with the different designs. All designs makes the tube harder

at the outermost position, see table 13. Design B shows the smallest difference between the

outermost and innermost hardness values, while design A has the lowest standard deviation of the

values around the tube.

Table 13. Rockwell C hardness measurement on tubes pilgered with the different tool designs. Every position was measured at 36 locations around the tube.

Design A Design B Design C

Outermost Centre Inner Outermost Centre Inner Outermost Centre Inner

Average 32.94 30.08 29.60 32.70 29.51 29.48 32.75 29.80 29.44 St.dev. 0.42 0.36 0.39 0.42 0.56 0.49 0.33 0.50 0.41

45

In order to see if there were any relation between the thickness of the wall and the hardness value at

that position the thickness of the wall were measured. When the tube is extruded there is always a

risk that the thickness can vary around the tube. The pilgering smoothens these differences out

causing larger deformations where the tube is thickest, this might result in a higher hardness at these

locations. Figure 52 shows the variations for design A. There is no visible correlation between the

hardness variation and the wall thickness. There is no visible relation between the hardness

measurements at the different positions either.

Figure 52. Variations around the tube. From the left: outermost, centre, inner and wall thickness. Hardness is

measured in HRC and thickness in mm.

Figure 53 shows the same thing for design B. There is a correlation between the thickness of the wall

and the hardness at the outermost position. The hardness seems to be higher at the same location as

the wall is thinnest, opposite of what could be expected. However it is not possible to say that this

not only is a coincidence since no other tool design show this behavior.

Figure 53. Variations around the tube. From the left: outermost, centre, inner and wall thickness. Hardness is

measured in HRC and thickness in mm.

Figure 54 show the variations around the tube pilgered with design C. It is not possible to see any

relations regarding hardness and wall thickness.

Figure 54. Variations around the tube. From the left: outermost, centre, inner and wall thickness. Hardness is

measured in HRC and thickness in mm.

46

5.10.2 Texture (EBSD)

The measurements were made with a 1.8 µm step in a 500x500 raster. The inner and outermost

measurement points were made 1 mm from each edge. The total width of the wall was about

12 mm. Since the tube has been cold pilgered the structure has been largely deformed with a large

fraction of zero solutions as a result during the measurements. The zero solutions is however evenly

scattered and the measured texture is assumed to be representative to the measured area. There

are large variations of the texture in the middle of the wall and close to the surface, see figure 55.

Design A

Design B

Design C

Inner Middle Outermost

Figure 55. ODF maps were 2 is held constant at 45 degrees, for the different tool designs. 1 horizontally

and vertically.

Since the pilgering process is very complex the textures attained are not a pure fiber or rolling

textures. The theoretical fiber and rolling textures can be seen in figure 56. The textures in the inner

and outermost measurements are similar. The texture seem to be close to a <111> fiber texture

parallel to the rolling direction. The texture in the middle of the wall is not similar to any of the pure

textures in figure 56. When comparing the different designs no large deviations exist but it seems

that design B is the least similar. This might be explained by the fact that it is deviating from the

others at the end of the pilgering were the Q-value is about 2.3 for design B and goes to zero for the

others.

47

Figure 56. Theoretical fiber and rolling textures attained with HKL Channel 5 software by Oxford Instruments.

1 horizontally, vertically and 2 is held constant at 45 degrees.

The textures are presented as IPFs in figure 57. Since the textures are complex and not only fiber

textures parallel with the rolling direction all three directions RD, TD and ND are shown. It appears

that the texture is strongest close to the inner surface with <111> directions parallel with RD and

<101> directions parallel with ND. The middle section show weak texture with no specific crystal

direction parallel with any of the samples coordinate axis. The texture close to the outermost surface

is similar to that close to the inner surface. The definitions of the directions are shown in figure 58.

Inner Middle Outermost

Figure 57. IPF representation of the textures after pilgering. The rows represent design A, B and C respectively.

Figure 58. Definition of how the directions ND, TD and RD are oriented relative to the tube/sample.

48

To get a value of how strong the texture is the fraction of the directions <111>, <101> and <001>

within ten degrees of RD, TD and ND are calculated. The strongest measured texture is found in the

RD close to the inner surface of the tube pilgered with design A. Here 29 % of the measured points

have their <111> direction within ten degrees of RD. The result for the other directions is presented

in appendix III. Overall the tube pilgered with design A shows the strongest texture, the tube pilgered

with design C shows the lowest.

5.10.3 Anisotropy Values

No anisotropy measurements were made during this investigation. However anisotropy values

measured on the same tubes are available from another investigation, see table 14.

Table 14. Anisotropy values of cold pilgered tubes from heat 522148 pilgered with different tool

designs. The values used for the calculations are Rp0.2 values.

Design

tensile transversal/tensile lengthwise compression lengthwise/tensile lengthwise

average min max average min max

Design A 1.02 1.00 1.06 0.92 0.88 0.98

Design B 1.01 0.98 1.04 0.84 0.82 0.86

Design C 0.94 0.91 0.97 0.91 0.87 0.94

The anisotropy values are quite good for the pilgered tubes and only small differences can be seen

between the different tool designs. When comparing the tensile Rp0.2 values for lengthwise and

transversal specimens taken from tubes pilgered with the designs A and B they show almost no

anisotropy. For tubes pilgered with design C there is possible to see weak anisotropy. This anisotropy

cannot be coupled to the measured texture. It is important to remember that the texture was very

similar for all three designs.

The tubes pilgered with design B shows the most anisotropy when comparing the Rp0.2 values

attained from compression and straining. Once again it is not possible to couple the anisotropy

values to the measured texture.

It seems that other anisotropy sources than texture is the most important for these tubes. Other

possible sources are dislocations and residual stress created during the pilgering. Keep in mind that

the textures were very similar for all designs. It cannot be concluded what influence the texture has

on the anisotropy for pilgered tubes.

49

6. Conclusions

When straining austenitic stainless steels crystal rotation occurs towards <111> and <001>

fiber texture.

Sanicro 41 and 254 SMO have a much slower texture formation than the other steel grades

in this investigation, and give a weaker texture. This is probably caused by synergy between

several alloying elements.

It is likely that the anisotropy is influenced by the composition since the texture formation is.

If this can be used to reduce the problem with anisotropy in OCTG applications has not yet

been confirmed.

Grain structure has no strong influence on texture formation.

The texture measurements on pilgered tube show no large difference between tubes

pilgered with the designs A, B and C. However tubes pilgered with design C has the weakest

measured texture.

There is a different type of texture in the middle of the wall compared with close to the

surface of pilgered OCTG.

The texture is stronger close to the surface of pilgered OCTG.

50

7. References [1] Katarina Persson, Power Point presentation, OCTG

[2] Persson, K (2008). Influence of cold-working process on the degree of anisortopy – A

litterature review. AB Sandvik Materials Technology – Research & Development.

[3] Heino, S (2000). The Preciptation and Deformation Behaviour of Superaustenitic

Stainless Steel. Gothenburg: Department of Engineering Metals, Chalmers university of

Technology.

[4] Lindstedt, U (1998). Deformation and Fatigue Behaviour of Porous and Dense

Austenitic Stainless Steel. Gothenburg: Department of Enginering Metals, Chalmers

university of Technology.

[5] Robert N Gunn, Duplex Stainless Steels – Microstructure, properties and applications,

Abington publishing, Cambridge England 1997.

[6] William D. Callister, Jr, Materials science and engineering an introduction, sixth

edition, 2003 John Wiley & Sons, Inc.

[7] J.W. Simmons, Materials Science and Engineering A207 (1996) 159-169

[8] Rune Lagneborg and Eva Waltersson, Guide för legeringsmetaller och spårelement i

stål, second edition, Jernkontoret 2004.

[9] Humphreys, F.J. and Hatherly, M (2004). Recrystallization and Related Anneling

Phenomena, second edition. Oxford: Elsevier.

[10] Jonsson, S (2008). Mechanical Properties of Metals and Dislocation Theory from an

Engineer’s Perspective. Stockholm: Royal Institute of Technology.

[11] Engberg, G (1998-2003). Materialbeteende vid plastisk bearbetning av metalliska

material. Unpublished manuscript. Borlänge: Högskolan Dalarna.

[12] Smallman, R.E (1985). Modern Physical Metallurgy. Vol. 4. Oxford: Butterworth-

Heinmann Ltd.

[13] Randle, V and Engler, O (2000). Introduction to Texture Analysis: Macrotexture,

Microtexture and Orientation Mapping. Boca Raton: CRC Press.

[14] L. Bracke, K. Verbeken, L. Kestens, J. Penning, Acta Materialia 57 (2009) 1512-1524

[15] R. E. Schramm and R. P. Reed, Metallurgical Transactions A, volume 6A 1975, 1345-

1351

[16] L. Vitos, J.-O. Nilsson, B. Johansson, Acta Materialia 54 (2006) 3821–3826

[17] Stapleton, G (1996). Cold Pilger Technology, first edition. Sheridan, Indiana.

51

[18] Hyun Park and Dong Nyung Lee, Journal of Materials Processing Technology 113

(2001) 551-555.

[19] I. Karaman, H. Sehitoglu, H. J. Maier and Y. I. Chumlyakov, Acta mater. 49 (2001) 3919-

3933.

[20] T.-H. Lee, C.-S. Oh, S.-J. Kim, S. Takaki, Acta Materialia 55 (2007) 3649-3662.

[21] A.Dumay, J.-P. Chateau, S. Allain, S. Migot and O. Bouaziz, Materials Science and

Engeneering A 483-484 (2008) 184-187.

[22] http://www.smt.sandvik.com/tube

[23] D. Jorge-Badiola, A. Iza-Mendia, I. Guti´errez, Materials Science and Engineering A 394

(2005) 445–454

[24] Annual book of ASTM standards 2009 section three, volume 03.01, E112

[25] David Hedström, Anisotropic behavior in worked austenitic and duplex stainless steels

52

Appendix I

Denotation n Tap m d (nominal) d (tolerance) Lc Lt R

DR7C70 15 mm M12 7 mm 0.04 mm 85 mm 125 mm 5 mm

53

Appendix II

ODF of Sanicro 28 deformed to 30.3 % area reduction. The theoretical positions for <111> and <100>

fiber textures are marked. 2 is held constant.

54

Appendix III

Fraction within ten degrees of rolling direction. All values are in % of total measured points.

Inner Middle Outermost

001 101 111 001 101 111 001 101 111

Design A 8.2 0.9 28.7 6.0 12.4 10.2 6.6 2.8 13.3 Design B 6.9 0.6 25.1 3.5 3.9 5.1 5.8 1.2 18.6 Design C 8.0 0.9 18.7 3.8 7.7 4.5 7.6 1.0 15.7

Fraction within ten degrees of transversal direction. All values are in % of total measured points.

Inner Middle Outermost

001 101 111 001 101 111 001 101 111

Design A 2.5 15.4 7.2 2.8 18.7 8.8 2.0 9.4 6.8 Design B 1.1 14.2 9.7 1.6 8.3 3.7 0.9 10.7 7.7 Design C 3.6 12.6 6.6 2.3 11.8 5.9 1.9 10.6 7.8

Fraction within ten degrees of normal direction. All values are in % of total measured points.

Inner Middle Outermost

001 101 111 001 101 111 001 101 111

Design A 0.9 24.8 0.5 10.1 13.0 6.7 3.2 17.0 3.0 Design B 0.7 25.6 0.4 5.6 5.9 3.2 1.3 20.2 2.3 Design C 1.3 21.3 0.5 7.0 8.7 5.0 2.7 21.1 2.2