Fracture Crack Relative to Weld Interface

in a Pressure Vesse1 Steel

A Thesis

Submitted to the Faculty of Graduate Stuàies and Research

In Partial Fu l lhent of the Requirements

for the Degree of Master of Applied Science

in Industrial S ysterns Engineering

University of Regina

by

Samit Sharma

Regina, Saskatchewan

April, 1998

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Abstract

Joining components of pressure vessels in industry uses welding. The high heat input of the

weld leads to a signincant variation in the mechanical and rnetallurgical properties of the

weldrnents, thus resulting in a crack prone wetdment.

In this project. 25.4mm ( 1") thick C-Mn steel plates of the type, ASTM A5 16-Grade70, were

welded using subrnerged arc welding (SAW). Three point bend (SENB) specimens were

made of the weldments which were then notched at different positions relative to the weld

interface. These specirnens were fatigue pre-cracked and frac tured according to plane strain

fracture toughness standard ASTM E399-90 on an MTS 8 10 servo-hydraulic system.

The study found that in weldment the base metal (BM) has a femte - pearlite structure,

foiiowed by a coarse grained structure consisthg of acicular femte, bainite and a network of

cementite in the heat affecteci zone (HAZ), while the weld metal (WM) had a large

proportion of coarse grain boundary femte dong with side plate femte, fine acicular femte

and pro bably bainite. Hardness across the weldments generaiiy sho wed slight variations,

except for hardness peaks close to the weld interface.

On fiacture testing of SENB specimens, it was found that inclusions, hardness, brittieness and

grain size played an important role in fracture crack propagation relative to interface.

In the weld plate with a low volume of inclusions, the crack path was influenced by its initial

orientation relative to the weld interface and w t by the point of initiation. Visuai observation

and scanning electron microscope fhctographs showed that the fracture crack was ductile

and had a marked preference for WM, aiigning itself dong îhe interface, provided the fatigue

crack, interface and the s t r a i n intensification were a i i in line.

However, in the weld plate with a high volume of inclusions, the crack propagated dong the

interface irrespective of its point of initiation and orientation. In this plate inclusions had a

tendency to aiign themselves dong the weld interface. Fractographs showed a combination

of ductile and brittie fiacture modes, with nucleation of voids around inclusions and

transgranular fracture in the coarse grained HAZ.

Acknowledgments

1 take great pleasure in acknowledging my supervisor, Dr. S. D. Bhole, for his uivaiuable

guidance, and advice during the various stages of research and thesis preparation. The

fiinding provided by the Natural Sciences and Engineering Research Council of Canada

(NSERC) is gratefuily acknowledged.

Thanks are due to James S. Quickfall and Lome Good of IPSCO (Research), for lending

their ftacture testing gnps and helping me out with their workshop facilities. The excellent

service of Inter-Library Loans of the University of Regina library deserves appreciation.

1 would Like to express my appreciation and thanks for the support and Company provided by

ail my friends, which made my stay in Regina a pleasant and mernorable one.

Finally, without the encouragement and blessings of my mother and sister, this stage wodd

have remained unattainable. I dedicate this thesis to my family.

iii

Nomenclature

P

Crm

YS

U

a, a

B

B M

CGHAZ

CMOD

CTOD

DCRP

E

EPFM

F

Fe$

FZ

Length of plastic zone

Micrometer

Yield strength (MPa)

Poisson's ratio

Crack length (mm), includes notch plus fatigue pre-crack

Specimen thickness (mm)

Base metal

Coane grained heat affected zone

Crack mouth opening displacement

Crack tip opening displacement

Direct curent reverse polarïty

Modulus of elasticity in plane stress

or E

, Modulus of elasticity in plane s t r a h (1 - u 2 )

Elastic plastic fiachue mechanics

Frequency (Hz)

Cementite

Fusion zone

G

GMAW

GTAW

HAZ

HSLA

Kt

Kc

L m

Ib

LEFM

P

ps

P m

PQ

R

r~

Rsb

S

SAW

SENB

UTS

W

WM

Energy release rate

Gas metal arc weld

Gas tungsten arc weld

Heat-affected zone

High strength low aiioy

Stress intensity factor ( M P ~ JkZ )

Plane strain fracture toughness ( m a 6 )

Maximum stress intensity ( M P ~ JE )

Criticai stress intensity factor (MP~& )

Linear elastic fracture mechanics

Load (N)

5% secant line to elastic loading slope (N)

Ultimate Ioad (N)

Criticai load (N)

Load ratio

Radius of the plastic zone

Specimen strength ratio

spm (mm)

Submerged arc welding

Single edge notched bend

Ultimate tende strength ( m a )

Specimen width (mm)

Weld metal

Table of Contents

A bstract

Acknowiedgment

Nomenclature

Table of Contents

List of Tables

List of Figures

1. INTRODUCTION 1

1.1 Scope of work .......................... .. ........................................................... 1

1.2 Organkation of thesis.. . ... ... . .... . ... . ... .. . .... ..-. . . . . . . . . . . . . . . . . . . . . . . . 3

2. LITERATURE REVIEW 4

2.1 Pressure vesse1 steel .-.................. .. .................................................................. 4

2.2 Submerged arc welding (SAW) .......... .. .. .. ....... .... .......... . . .... .. ... ...... ..... . ... ........ . .5

2.3 Fracture mechanics .. ...........,,,.. ... ...... ....*...-... . ... ......... . . .. .... ... .......... ............. 7

2.3.1 Linear elastic fracture mechanics (LEFM) . ...... . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . -8

2.3.1.1 Plane strain and plane stress.. ........ ........................ . . . . . . 10

2.3.2 Elastic-plastic fracture mechanics ...... .. .. .... . ... . . . .. . ...--.. . . . . . . . . . . . . . 1 1

....................................................... 2.3.3 Stress triaxiality and crack growth 12

.............................................. 2.3 -4 Shear Iip formation during crack growth 12

2.4 Weldment microstructure and properties ...................... .... .... .......-............ 13

. . .................................... 2 -5 Crack in bi-matenai mterface ............ ... 15

............................................ 2.6 Crack in dual phase microstructure ............. .... 17

................................................................................................... 2.7 Weld stresses 18

............................................... 2.8 Inclusion effects ., . . . 19

3 . MATERIAL AND TESTING 21

3.1 Material ................... .. .................................................................................. 21

...................................................................... 3.2 Submerged arc welding (S AW) -22

.......................................................................... 3.2.1 Welding parameten 2 3

........................................................................... 3.2.2 Electrodes and fluxes 23

.................................................................................. 3.2.3 Weld metal joint 2 4

3.2.4 Weld metal properties ................... .. ............................................ 2 5

3.3 Fracture rnechanics test method ....................................................................... 25

3 -4 Plane strain fracture toughness testing ...................... ...... ........................... 26

3.4.1 Specirnens .............. .. ............................... .... ........................... 27

3.4.2 The test ............................................................................................... 28

3.4.2.1 V-notch placement ......................................................... 2 9

3.4.2.2 Fatigue precracking .............................................................. 30

3.4.2.3 Fracture cracking (loading) .................. ...... ................. 3 1

vii

.............................................................. ............. 3 .4.2.4 Analysis .. 3 1

................................................................. 3.4.3 Conformation to standard 33

.......................................................... ..................... 3.5 Hardness measurement .,. 3 4

3.6 Photornicrographs ............................................ ........................................ 35

3.7 Fractographs ......................... ..... ........................................................... 3 5

3.8 Imagemalysis ..................... ... ................................................................... 36

4 . RESULTS

5 . DISCUSSION

Visual observation ...................................... . . ................................................. 37

Fracture toughness ......................... .... ........................................................... 39

Hardness ........................................................................................................ 4û

...................................................................... 4.3.1 Hardness grid position -40

...................................... .................*..........*............. 4.3 -2 Hardness graphs .. 41

Photomicrographs of weldment microstructures ..................... .... ............... 44

Fractographs ................................................................................................. -47

Image analysis .................................................................................................. 48

5 . 1 Weldment microstructure ................................ ,. ........................................... - 5 1

5.2 Fracture toughness values ............................................................................... -52

5.3 Fracture crack inclined to weld interface ...................................................... 53

5.3.1 Double vee welded plate ..................... .. ........................................... 53

.-......-........ ................*............................ 5.3.2 Single bevel welded plate .... 55

5.4 Frac- crack parallel to weld interface ... ... ..... ... .. . ..... . .. ... . ... . ... . -. . . -..- .. - ..-. .. -57

5.4.1 Double vee welded plate ..... . .. . ... . ..... .... ... ....-.-.. - -.-... . .-..- ..... ... . .--.. ....- 57

5.4.2 Single bevel welded plate .....................~..~...........~................ . . . . 5 9

6. CONCLUSIONS 61

6.1 Conclusions .................................................................................... ....- -.-.-..61

6.2 Suggestions for funue work .................................................................. . ...... 63

BIBLIOGRAPHY

List of Tables

Composition of A5 16-Grade70 pressure vesse1 steel ........................................... 2 2

Mechanical properties of A5 16-Grade70 pressure vesse1 steel ............................... 22

Values of welding (SAW) parameters ................ .. ............................................... 23

Composition of the welding electrode EM l2K ................................................... 2 3

Mec hanical properties of F7A6 flux ................. .... .......................................... 2 4

Weld metai composition ...................................................................................... 25

Mechanical properties of weld metal ...................................................................... 25

Fracture toughness data for Plate LI ( double vee weld design ) .............................. 39

Fracture toughness data for Plate 1 ( single bevel weld design ) .............................. 40

Proportion of inclusions in AS 16-Grade70 steel weldments ................................... 48

A . L ASTM specifications for pressure vesse1 quality carbon steel plate ......................... 71

B . 1 Hardness values across grid in double vee butt weld ............................................. 73

............................................ B.2 Hardness values across grid in single bevel butt weld -75

List of Figures

2.1 Sketch of the SAW process ..................................................................................... 5

2.2 Invin plastic zone ......................... .... ....................................................*............. 9

2.3 Strip yield plastic zone ........................... ... ................................................... 10

2.4 Three dimensional stress field ................................................................................ 1 1

2.5 Three dimensional deformation at the tip of a crack ................. ... ..................... 12

2.6 Ductile growth of an edge crack ............................................................................ 13

2.7 Void nucleation. growth. and coalescence in ductile metals ................................... 20

3.1 Double vee butt weld joint .................................................................................... 24

3.2 Single bevel butt weld joint .................................................................................. 24

3.3 Fracture toughness versus thickness .................... .. ....................................... 27

3.3 Clip gage attachent in test specimen ............................................................ 28

3.4 Generd view of the testing equipment .................... .... .................................... 29

3.5 Three types of ioad-displacement behavior in a Kc test ....................................... 31

3.6 Load-displacement curve for an invalid Kic test .................... .. ........................ 3 3

4.1 Fracture path with fatigue crack tip originating in weldmnt. having parailel and inclined orientation to the weld interface ................................ ..-. ...................... 3 8

4.2 Hardness grid of double vee butt weld joint ........................................................... 40

........................... ...............*........*.. 4.3 Hardness grid of single bevel butt weld joint ... -41

4.4 Hardness graph for double vee butt weld joint at A l position ................................ 41

4.5 Hardness graph for double vee butt weld joint at B 1 position ................................ 42

................................ 4.6 Hardness graph for double vee butt weld joint at Cl position 42

............................... 4.7 Hardness graph for double vee butt weld joint at D 1 position -42

4.8 Hardness graph for double vee butt weld joint at El position .............. ... .......... 43

4.9 Hardness graph for single bevel butt weld joint at A2 position ............................... 43

4.10 Hardness graph for single bevel butt weld joint at B2 position ............................... 43

4.1 1 Hardness graph for single bevel butt weld joint at C2 position ............................... 44

4.12 BM microstructure with bands of proeutectoid ferrite (white) and peariite (dark) (Ma@cation 2ûûX) .......................................................................................... 45

4.1 3 HAZ rnicrostructure sho wing acicular ferrite surrounded by prior aus tenite grain boundary and cementite network (Magnification 2 O X ) ........................................ -46

4.14 WM microstructure showing grain boundary ferrite. side plate ferrite. Widrnanstatten femte and probably bainite (Magnification 200X) .......................... 46

4.15 Fractograph showing equiaxed dimples in WM fiactured surface near weld .................................................................. interface. in double vee welded plate 47

4.16 Fractograph showing mixed mode kacture with facet and dimples in the HAZ near weld interface. in double vee welded plate .................................................... 47

4.17 Fractogaph showing transition fkom a mixed mode large dimpled fiacture to a small dimpled ductile fracture at weld interface. in double vee welded plate ........... 48

4.18 Inclusion size distribution in BM of double vee welded plate ................................. 49

4.19 Inclusion size distribution in BM of single bevel welded plate ................................ 49

4.20 EDAX spectrograph of the inclusion in the BM showing manganese and silicon among other things ........................................................................................... 50

4.21 EDAX spectrograph of the inclusion in the BM showing calcium, silicon and zirconium among other things ........................................................................... 50

xii

5.1 Fractograph showing elongated chples in WM fktured surface away fkom weld ........................................ ....................... interface in double vee welded plate ... 54

5.2 Distribution of inclusions in the weldment of single bevel welded plate, with the inclusions aiigning themselves dong the WM interface (Magnincation 5X) ............ 55

5.3 Fractograph of ductile fiactured surface at the weld interface of single bevel welded plate, shows large proportion of inclusions and microvoids nucleation ....... 56

5.4 Fractograph showing interfacial fracture in case of single bevel welded plate. ....... ................. metal fails by a combination of ductile and brittle fhcture .. 57

5.5 Fractograph showing interfacial fracture in case of double vee welded plate, nietal fails by a combination of ductile and bat le kacture, also are seen microvoids

................................... nucleation due to spherical hclusions ........................... .. 58

5.6 Higher mgnification photomicrograph showing inclusion d i s t r i ion and alignment at the weld interface of double vee welded plate (Magrufication 200X) .. 59

xiii

Chapter One

INTRODUCTION

1.1 Scope of work

Crack propagation in the welded parts of steel is a very serious problem This becomes ail

the more serious when the crack occurs in pressure vessel steel, because pressure vessels are

widely used in oil and gas, power utilities, chernicd industries, etc.

When two or more parts of pressure vessel steel are welded together, the properties of the

met al in the weldment Vary signincantly : weldments here are he terogeneous structural

elements composed of at least three microstructural regions; base metal (BM), weld metal

(WM), and heat affected zone (HAZ). The variation in properùes may be due to high heat

input during welding, slag inclusions, chemicai reactions of the metal with atmosphere or flux

or filler material, disproportionate heating or cooling rates [3,5,3 11 etc.

As a result, the material properties change gradually fkom WM to HAS to BM. These

changes include changes in chernical properties, mechanical properties, microstructure, grain

size etc., with each of the regions having a dinerent yield strength, tende strrngth, and

fiacture toughness. The region near the weld has coarser grains which gradualiy becomes

b e r towards base metd This change in property is most pronounced at the weld interface,

which is the boundary separating the WM from the HAZ. Thus when a crack develops in the

weld metal or outside the weld metal in the k a t affected zone and starts propagating, it

encounters different set of properties a s it advances. In particular, when it encounters the

weld interface, its behavior changes accordmg to its orientation and point of initiation m

weldment relative to weld interface.

The present research is directed towards study and analysis of crack propagation or behavior

at the weld interface of ASTM A516-Grade70, plain carbon pressure vessel steel. For the

study, 25.4mm (1") thick plates of the above steel were welded by SAW (submerged arc

welding) using ANSYAWS A 5.17-89 & section M ASME boiier & pressure vessel code.

welding procedure 135. Three point bend (SENB) specimens were made out of these plates

using standard ASTM E399. These specimens afier grinding, polishing and etching were

tested on a WS-810 servo hydraulic system, using software based on ASTM E399-90.

During testing there is a fatigue pre-crack created parailel or inclined relative to the weld

interface. Thereafter the specimen is fractured. Fractographs. photomicrographs, image

analysis, and hardness tests, are used to study the propagation path of the resulting fracture

crack. The f?acture crack path is studied in te= of the fatigue pre-crack tip in the weldment

and its orientation to the weld interface. The reasons for this behavior are analyzed and

discussed based on mechanical and metallurgical knowledge.

In Chapter Two, a more detailed background of plam carbon pressure vessel steel,

submerged arc welding, hcture mechanics, fracture mechanics testing, weldment

microstructure, a literature review on studies in crack propagation, weld stresses, hclusion

effects etc., are detailed.

In Chapter Three, material and testing are discussed. This includes material ioformation on

AS 16-Grade70 plain carbon pressure vessel steel, submerged arc welding parameters, ASTM

standards and test procedures.

In Chapter Four, the results of the testing are presented. Crack propagation results.

hardness, fracture toughness. photomicrographs, fractographs. image analysis reports, etc.,

are reported.

in Chapter Five, the results are compared and discussed with references to the literature on

the similar topics.

In Chapter Six, concIusions and recomrnendations for further study are summarized.

Chapter Two

LITERATURE REVIl3W

2.1 Pressure vessel steel

The steel used in the fabrication of pressure vessels is usually of two h d s , viz. carbon steel

and dioy steel. The specifications of ASTM grades and types of carbon steel plates for

pressure vessels are listed in APPENDIX A - Table A. 1.

In pressure vessel steels, carbon is of prime importance because of its strengthening effect. It

ako raises the transition temperature, lowers the maximum energy values and widens the

temperature range between completely tough and completely brittle behavior. Manganese on

the other hand (up to 1.5%) improves low temperature properties [l].

Of all the different kinds of steel, those produced in greatest quantity fall within the low

carbon classification. These steels generally contain less than about 0.25 wt. % C and are

unresponsive to heat treatments intended to fom martemite; strengthening is accomplished

by cold work. Microstructures con& of ferrite and pearlite constituents. As a consequence,

these aiioys are relatively soft and weak, but have outstandmg ductility and toughness; in

addition they are machinable. weldable, and of all steek are the les t expensive to produce.

They typically have a yield strength of 275 MPa, tensile strengths between 415 and 550 MPa

and a ductility of 25% EL. A5 16Grade70 is one such kind of steel and has applications in

low-temperature pressure vessels [2].

2.2 Submerged arc welding ( SAW )

Submerged arc welding (SAW) is an arc welding process in which coalescence of metals is

produced by heating them with an arc between a bare consurnable elecuode and the

workpiece, with the arc king shielded by a blanket of granular, fusible material placed over

the welding area [3]. Figure 2.1 - [3], is a sketch of the SAW process.

G r a n u i > q k

- ; 1 1 1 f lu* Molkn slag Arc Solidifiecl slaq

Figure 2.1 : Sketch of the SAW process 131

This process is different from other arc welding processes in that the welding area, including

the arc, is covered by the granular, fusible materid.

Because of the protecting and r e f ~ g action of the slag, very clean welds c m be obtained

with the SAW process. Since the arc is covered by the molten slag and the granular flux,

high welding currents can be used without causing violent arcs, and the heat loss to the

surrounding is very smalL Both alloying elernents and rœtal powders can be added to the

granular flux. The former help control the weld metal composition, and the latter increase

the deposition rate. Directtument reverse polarity (DCRP) is m s t often used in SAW.

However, at very high welding currents (e.g., above 900A), aiternating curent is preferred in

order to minimize the arc biow.

The SAW process, owing to the relatively large vohunes of the molten slag and mtal pool, is

usuaily Iunited to flat position welding and circumferential welding (pipes). Also, because of

the relatively high heat input and the resuiting large weld pool. coane col~~nnar grains often

form in the fusion zone. This sometimes results in low toughness or even hot cracking of the

weld metal [3].

In SA welds, deposition rate can be increased by using two or more elecuodes in tandem As

a result of the high rate of deposition, the SAW process is suitable for welding heavier

sections that are encountered in pressure vesse1 steels as conipared to GTAW (gas tungsten

arc weld) or the GMAW (gas metal arc weld) process 131.

Furthemore, various doyuig elements and metal powdea can be added to the granular Qwc

to control the weld metai composition and improve the weld properties. One distinguishing

feature of SAW is its high eficiency, the highest among ail the processes [3].

SA welds have good ductility, unifonnity and density- Good impact strengths are obtainable

when specific procedures and techniques are evaiuated m combination with the electrode and

flux. A proper selection of the electrode and flux provides good corrosion mistance

depending upon the requirement, and ensures mechanical properties at least equal to that of

the BM. [4,5]

2.3 Fracture Mechanics

Fracture mechanics of a material is a masure of its resistance to crack growth. Different

methods have k e n developed to quantitatively masure fkacture toughness. This section wiii

review fracture mechanics st arthg wit h linear elas tic fkacture mechanics, stress intensity

factor (K), elastic-plastic fracture, and crack tip opening displacement (CTûD).

Axially loaded specimens fail when the ultimate tende strength is reached, but sometimes

Failure occurs at lower loads. These failures are due to stress concentration, due to poor

design, or due to flaws ( or cracks ) within the materiai.

Griffith [6] found a quantitative relationship between fracture stress and flaw size. His model

accurately predicted the relationship between fracture stress and fiaw size in glass. He found

that as the flaw size increases, fkacture stress decreases. Researchers were initially unable to

correlate his model to metals due to plasticity, so his model was only useful for brittle solids.

Griffith's fracture stress was caiculated based upon the surface energy of the material.

Subsequent research was split into linear eiastic fkacture mechanics (LEFM) and elastic-

plastic fracture mechanics (EPFM).

23.1 Linear elastic fracture mecbanics (LEFM)

LEFM applies when the materiai undergoes oniy a s m d amount of plastic deformation.

When characterizhg the kacture toughness of these materials they can be evaluated by

energy release rate (G), and stress intensity factor (KI), which are listed in the foliowing

fomulae:

L

K, = 06 (2-2)

where a is equal to the yield strength, 'a' is the half crack site, and 'E' is the modulus of

elasticity. The energy release rate cm be related to the stress intensity factor by the foIlowing

formula:

K, ' G = - E

(2.3)

As stated earlier LEFM only applies when very little plastic deformation occurs. If significant

plasticity occurs, the accuracy of Equations 2.1 and 2.2 decreases. To improve the accuracy

of these results, several researcbers including Irwin [7] ( who developed the energy release

rate and stress intensity factor), Dugdale [8], and Barenblatt [9] applied a correction. h i .

corrected for plasticity by assuming the existence of a circular plastic zone ahead of the crack

tip. He assumed that the half crack length increases by a factor, r, which represents the

radius of the plastic zone. The Invin plastic zone is s h o w in Figure 2.2 - [7] on the next

page.

Crack Plastic Zone

Fimire 2.2 : Irwin plastic zone

The equations for r, in plane stress and plane sixain conditions are given below.

An iterative technique is used to calculate &. Fist, is e s t k t e d and substituted into

Equation 2.4 - plane stress, or Equation 2.5 - plane strain, and then r, is caiculated. Second,

the crack length (a), and r, are added together and substituted back into Equation 2.2. The

new value of Ki is then substituted back into the appropriate equation above, these steps are

repeated until the results converge.

Dugdale and Barenblatt created the strip yield model. This model assumes a narrow suip of

yielding, while Invin assumed a circular plastic zone. The strip yield plastic zone has a length

of p. This is shown in Figure 2.3 - [7.8] on the next page.

Fimire 23 : Strip vieid plastic zone

To solve for the length, p, a few assumptions have to be made. Fist. the crack is assumed to

be in tension. Second, the plastic zone p has a closure ( compressive ) stress. equal to 4. To

solve it, the problem was split into two parts - a crack in tension, and a crack with closure

stress. Both of these problems c m be solved individudy and the fînd solution was then

found by adding the two elastic solutions. The boundary condition that must be met is that

the stress intensity factor fkom the closure stress must be equal to the stress intensity factor

for the tension stress. The length of the plastic zone is given in the equation below:

2.3.1.1 Plane s t r a i n and plane stress

The radius of the Irwin plastic zone is dinerent depending on whether plane strain or plane

stress conditions exist. For this reason the assuqtions of plane stress and strain wili be

explained within this section.

In real Me, stress and strain exists in all three planes, but three dimensional stress fields are

very difficult to solve. A three dimensional stress field is shown in Figure 2.4, ( this diagram

is in a simplified form and does not include shear stresses ). For this reason the asswllptions

of plane stress and plane strain were made. Plane strain is used for thick sheets and assumes

the stress in the z direction ( principal direction 3 ) is equal to u(ai+Q2). In other words. the

material in the z-direction resists the material's tendency to neck (E, = 0).

Figure 2.4 : Three dimensional stress field

Plane stress is used for thin sheets and it assumes that there is no stress in the z-direction. In

other words. there is not enough material in z-direction for stress variation to occur (4 =O).

The impact of the plate thickness will be investigated in section 3.4

2.3.2 Elastic-plastic fracture mechanics

When significant plastic deformation occurs LEFM ceases to be valid. even with a plastic

zone corrections. Wek[lO] proposed that the displacement of the crack faces be an alternate

fiacture toughness criteria In other words as a crack grows. the fixes of the crack are pulled

apart, and the further the crack fares c m be separated for a given crack length, the tougher

the material is. This idea became the basis of the crack tip opening displacement (CïOD)

method.

233 Stress triaxiality and crack growth

Figure 2.5 - [23], shows a cracked plate with thickness B subject to in-plane loading.

Assurning that the plastic zone is smd, the regions of the plate that are sufficiently far from

the crack tip must be ioaded in plane stress. Material near the crack tip is loaded to higher

stresses than the surroundhg material. The high nonnal stress at the crack tip causes

material near the surface to contract. but the material in the interior is constrained. resulting

in a viaxiai stress state 1231.

Figure 2.5 : Three dimensional deformation at the tip of a crack

For r cc B, plane straùi conditions exist in the interior of the plate. Materiai on the plate

surface is in a state of plane stress, however, because there are no stresses normal to the fiee

surface.

2.3.4 Shear lip formation during crack growth

When an edge crack in a plate grows by microvoid codescence, the crack exhibits a tunnelhg

effect, where it grows faster in the center of the plate, due to higher stress triaxiality. The

through-thickness variation of triaxiality ais0 produces shear Iips. where the crack growth

near the free surface occurs at a 4S0 angle fiom the maximum principal stress as illustrated in

Figure 2.6 - [23].

Figure 2.6 : Ductile growth of an edge crack

2.4 Weldment microstructure and properties

In C-Mn steels, the heat affected region closest to the weld interface transfomis to a complex

mixture of one or more of the following microstructures: (1) proeutectoid ferrite at prior

austenite grain boundaries; (2) transgranular Widmanstatten ferrite; ( 3 ) high-carbide-content

microstructures; such as pearlite; (4) upper bainite, and lower bainite; and (5) manensite [ I I l .

On the other hand submerged arc weld-metal microstructures generally are composed of a

complex mixture of microstructural constituents. These constituents. in case of HSLA steel.

are cornprised oE (1 ) proeutectoid ferrite. either in massive equiaxed fonn or as thin veins

delineating prior austenite grain boundaries; (2) sideplate Widmanstatten ferrite (pardel

ferrite laths emanating fkom prior austenite grain boundaries); (3) acicular ferrite (a tough

structure found within the body of prior austenite grain that is formed between 592°C to

667°C on cooling); (4) retained austenite and twinned or lath rnartensite (sometimes referred

to as martemite-austenite phases); (5) other products, including pearlite and bainite [I l , 121.

The most prominent microstructural features present in the low carbon steel weldments are

femte, acicular fen-ite. cementite, pearlite. and bainite.

Ferrite has a iimited solubility of carbon and is thus a relatively softer phase; it exists in other

forms as acicular femte, side plate femte, grain boundary ferrite, etc. The formation of even

a srnail volume fraction of grain boundary femte. ferrite side plates, or upper bainite is

considered detrimental to toughness, since these rnicrostmctures provide easy crack

propagation paths [L3,14,15.16].

Acicular femte on the other hand is responsible for high toughness. It is fotmed

intragranularly, resulting in randomly oriented short femte needles with a basket-weave lke

structure. This interlocking nature, together with its fine grain size, provides the maximum

resistance to crack propagation by cleavage and enhances the yield strength of the metai

[3,13,15,16,17,18].

An intermediate compound, iron carbide, is caIled cementite. It is formed when the solubility

ümit of carbon in a femte is exceeded below 727°C (for compositions w i t t a+Fe3C phase

region in the iron-iron carbide phase diagram) [2]. Mechanicaily, cementite is very hard and

brittle; strength of some steels is greatly enhanced by its presence.

Pearlite consists of altemating layers of lamellae of the two phases, femte and cementite

(Fe3C), that forrn simultaneously during the transformation fkom austenite. Mechanically,

pearlite has properties intermdiate between the soft, ductile femte and the hard, brittle

cementite.

Bainite is the other microconstituent produced during austenite transformation. It consists of

femte and cementite phases, and is in the form of needles or plates. Bainite and pearlite

formation are cornpetitive processes during transformation fiom austenite. Since bainite is a

finer structure (i.e.. smaller cernentite particles in the ferrite matrix), it is generaily stronger

and harder than pearlite; yet bainite exhibits a desirable combination of strength and ductility

Pl .

2.5 Crack in bi-material interface

A study was done by Shih [26]. to study the fatigue crack propagation nomial to a fenite-

austenite interface. It was observed that coniinued advance or arrest of a crack is dictated by

whether the crack is approaching the interface fiom the ferritic steel (sofier phase) or the

austenitic steel (harder phase). When the crack approached the interface nomaüy fiom the

stronger (harder phase) austenitic steel, it penetrated the interface unjmpeded into the weaker

(softer phase) femtic steel. By contrast when the crack approaches the interface normally

fiom the weaker ferritic phase and the plastic zone at the crack tip spreads into the stronger

material across the interface, the crack is arrested. Finite elenient analyses of the interactions

of plastic zones ahead of stationary fatigue cracks with normaily oriented bimateriat interface

have been conducted [40]. These snidies show that when the plastic zone spreads from the

weaker side of the interface to the stronger side (as fatigue crack approaches the interface

from the weaker material), the cyclic opening stresses at the crack tip are signincantly

reduced conipared to those in the homogeneous stronger phase. This pomts to the possibility

of crack arrest. However, when the fatigue crack approaches the interface from the stronger

materiai, the spread of the plastic zone across the interface produces cyclic opening stresses

which are considerably larger than those in the homogeneous weaker phase. This implies an

acceleration of crack growth.

A study s d a r to Stiih's was undertaken by Tschegg [27], where fatigue cracks were

generated in ferritic steel close and parallel to the femte-austenite steel interface. These

cracks were generated at different distances varying fiom 0.05 to 1.10 mm from the interface.

In ail cases the cracks showed a slight tendency to veer away from the interface into the

softer femtic steel phase.

An elastic-plastic analysis of cracks in a bi-material interface (331 showed that when an

initidy sharp crack blunts, and thus loses the strong cowtraint at its tip, the maximum

stresses are shifted somewhat ahead of its tip. When this occurs at an interface crack,

especidy at the interface of elastic-plastic and elastic materiais, it was found that the viaxial

stresses are somewhat larger than those that develop at the crack tip in a homogeneous

material. Thus, the plastic zones, plastic suaius, and the crack tip openings that evolve near

bi-material interface are considerably larger than those in a homogeneous medium The

stresses within the nnite zones are also higher. In addition, a localized zone of high

hydrostatic stresses develops near the crack tip but then expands rapidly within the weaker

material as the plasticity spreads across the ligament. As a result, weaker material is

subjected to large stresses as weli as strain-states, which promote ductile fracture processes.

At the same Ume accompanying high interfaciai stresses can promote interfacial fracture.

2.6 Crack in dual phase microstructure

Ln a paper by Suh [14], structural steel welds were made on high strength low alloy (HSLA)

steel and the resulting femte - martensite structure was loaded. These were then examined

for microstructurai factors controlling local microcrack initiation and propagation. Since the

rnartensite with a higher hardness level is very brittle compared to the femte, it is natural to

expec t that as a ferrite - martensite microstructure is Ioaded, microcracks nucleate

preferentiaiiy by intemal fracture of hard martensite or by separation at the femte - martensite

interface, and then propagate dong these locaiiy damaged areas. However, microfracture

occurs in the suain-intensified region of the femte rnatrix rather than the rnartensite. This is

m d y due to the fact that the yield strength of femte is much lower than that of martensite

so that the deformation is initiaily localized in the soft ferrite.

Austempered ductile iron (ADT) composed of femte and retained austenite was fracture

tested by Rao [IS] according to the ASTM E399 standard. It was found that the width of

the femte plate plays an important role in fracture crack propagation across the austenite

region. As plastic deformation takes place in femte, dislocation pileups wiii form within

femte at the interface. There wiii be a high stress concentration at the head of the pileup

wbich, if sufficiently large, can initiate a crack within the austenite. When the femte plate is

large. the dislocation piieup will be large, and crack initiation will be easy. It was found that

AD1 with lower bainitic structure with fine acicular ferrite imparts better îkacture toughness

than an upper bainitic structure with coarse feathery bainitic femte.

The above studies on cracks across a bi-material interface and dual phase microstructure

show that the presence of ferrite is more conducive to fracture initiation and propagation.

relative to austenite or martemite. Furthemore, a weaker material (softer phase) in an

interface is more likely to experience higher strains and stresses and, because of large plastic

zones. it would be prone to ductile fractures.

2.7 Weldstresses

After two steel plates are welded together, various kinds of transverse and longitudinal

stresses arise in the weldment due to disproportionate heating and cooiing rates. The residual

stresses Vary fkom tension on the surface of the weld to compression in the center [29]. Post

weld heat treatment heQs reduce these non uniform stresses m the weldmnts, but their

existence cannot be completely eliminated.

Accordmg to Fang [30, 311, the residuai stress in a weldment is usually highest dong the

weld centerliw, diminishes in the transverse direction, and is balanced by the compressive

stresses in the rest of the weldment. Also the normal columnar growth mode of fusion welds

leads to the formation of a plane of weahess at the center of the bead, where the two

solidification fÏonts fkom opposite side of the weld impinge [32].

2.8 Inclusion effects

Materials that contain hclusions / impurities fail at much lower strains. Microvoids nucleate

at inclusions; the voids grow together to form a rnacroscopic flaw, which leads to failure. The

various stages in ductiie fracture are [35-391:

I ) Formation of free surface at an inclusion by either interface de-cohesion or particle

cracking.

2) Growth of the void around the particle, by means of plastic strain and hydrostatic stress.

3) Coalescence of the growing void with adjacent voids.

As seen in Figure 2.7 [23], void nucleation is often the cntical step; thereafter the fracture

crack properties are controued by the growth and coalescence of voids; the growing voids

reach a cntical size. relative to their spacing, and a local plastic instabiiity develops between

voids, resulting in ductile failure.

t t t

(a) Inclusions in a ductile matrix.

(CI Void growth-

(b) Void nucleation,

(dl Strain localization between voids.

t t t

(el Necking between voids. (fl Void coalescence and fracture,

Figure 2.7 : Void nucleation, erowth, and coalescence in ductile metals

Chapter Three

MATERIALS AND TESTING

The test material used was a pressure vesse1 steel of the type ASTM A5 ldGrade70. which is

a norrnalized C-Mn steel. The 28.575 mm (1-1/8") thick plates were manufactured by

Algoma steel inc., of Sault Ste. Marie, Ontario and obtained fkom Edmonton Exchanger of

Edmonton, Alberta.

Nonnalized steel plates are given a heat treatment to refine the grains (i.e. to decrease the

average grain size). It requires heating to approximately 55" to 85OC (100° to 150°F) above

the upper critical temperature. depending upon the composition. The steel in this study was

thus heated kom 87S0 to 9ûû°C and then air cooled. The resulting microstructure is that of

tough fine grained pearlitic steel.

The A516-Grade70, plain carbon steel contains two main elements aod some other minor

ones. The two main constituent elements are carbon (C) and manganese (Mn). Minor

doying elements include sulfur (S). phosphorus (P), silicon (Si), chromium (Cr), nickel (Ni),

copper (Cu), molybdenum (Mo), and vanadium (V). The relative amounts of each

constituent ailoying elernent can be seen in Table 3.1.

Table 3.1 : Composition of A516 - Grade70 Pressure vesse1 steel

F STM std.wt.% 0.28 1.25 (max*) (max.)

ctual wt. % 0.19 1.10

The mechanical properties of A5 16-Grade70 plane carbon steel are shown in Table 3.2.

Table 3.2 : Mechanical properties of A516-Grade70 Pressure vesse1 steel

ASTM std. (min.) Actual

- - - - - - - -

T e d e Strength

MPa ksi

Yidd Strength 1 Impact Energy Elongation in 5(knm I

3.2 Submerged arc welding (SAW)

AU the submerged arc welds were performed by Edmonton Exchanger of Edmonton, Alberta.

Two welds with different weld designs of double vee butt and smgle bevel butt were made

using ANSUAWS AS.17-89 and section M ASME boiler and pressure vessei, welding

procedure 1 35.

3.2.1 Welding parameters

The preheat temperature, interpass temperature. voltage. arnperage and speed for the

submerged arc weld can be seen in the Table 3.3 below:

Table 3 3 : VaIues of weldine (SAW Parameters

3.2.2 Electrode and Fluxes

Welding was done using medium rnanganese electrode AWS No. EMlZK (trade name king

Lincoln L61 wire). The composition of this electrode is given in the following Table 3.4 -

[201

Table 3.4 : Composition of the weldine electrode EM12K

h ~ e r a ~ e -ps

Voltage volts

Preheat OC

The flux used for welding with the electrode EMl2K is FIA6 (trade name king Lincoln 882

flux). The mechanical properties of this neutrai flux are given in the Table 3.5 - [20].

Travel Speed mmimin.

Interpass OC

Table 3 5 : Mechanical properties of F7A6 flux

1 T e d e Strength 1 Yield Strength (min.) 1 Elongation in 50 mm (min.) 1 MPa 1 MPa 1 1

3.23 Weld metal joint

Weld metal joint designs for the two plates welded are double vee butt and single bevel butt

as shown in Figure 3.1 and Figure 3.2. Both welds were made using multipass mns; the

double vee had 14 passes and the single bevel had 9 passes.

Figure 3.1 : Double vee butt weId joint

Figure 3.2 : Single bevel butt weld joint

24

3.2.4 Weld metal properties

The composition and properties of the resuiting submrged arc weld are given below in the

Table 3.6 and Table 3.7 respectively .

Table 3.6 : Weld metal comdtion

Table 3.7 : Mechanical properties of weld metai

Element

Weight %

3.3 Fracture mechanics test method

There are several methods by which the welds can be tested. The weldments cm be tested

for fracture toughness, crack-tip opening displacement, impact strength, tensile strength, and

hardness. I n h g [2 11 investigated destmctive testing of weldments and found that tensile and

hardness tests are not sufficient. Better aadysis would be to pexform fiachire toughness

tests. These tests are better because they can mdividuaily evaluate the fusion and HAZ, while

a tensile test cannot. Selection of a kacture toughness test involve evaluatmg its

C

0.070

Uitirnate Load kN

M n

1.140

Tende Strength MPa

impact Energy (J) EAZ WeId Zone

P

0.014

S

0.006

Si

0.360

Ni

0.013

Cr

0.020

M o

0.010

Cu

0.060

Nb

0.005

effectiveaess, given the material constrâints (thickness of plate, yield strength etc.). The three

maui fracture mechanics tests are plane s t ra in fkacture toughness, crack tip opening

displacement, and charpy impact tests. The test method widely used to evaluate the hcture

toughness of the material is plane saain fracture test.

3.4 Plane strain fracture toughness testing

These tests are performed in accordance with ASTM E399-90 [22]. This test is very

stringent and a valid test has to satisfy several criteria regarding specimens thickness, crack

length, and crack length to width ratio.

The specimens thickness (B) and crack length (a) must be greater than or equal to 2.5 times

the square of stress intensity divided by yield stress. This is s h o w in Equation 3.1.

The crack length must be between 45 and 55 percent of the width (W). This is shown in

Equation 3 -2.

It is very important to have the plastic zone small compared to the specîmens thickness in

order to achieve plane strain conditions at the crack tip. If the thickness is too small (or,

equivalently, if the plastic zone is too large) the constraint at the crack tip relaxes. A lower

degree of stress triaxiality usualIy results in higher toughness [23].

If al l the above test requirements are met, the test falls w i t b the plane strain region and KQ

is equal to the plane strain bcture toughness of the materiaL The plane strain region cm be

seen in Figure 3.3. If the test results fall wi& the plane stress region, an infiated Krc wiU

result and the toughness will be overestimated

Plane stress 1 Pbne strain behavior 1 behavior

I I

Thickness B

Figure 3.3 : Fracture touehness versus thickness

3.4.1 Specimens

The vast majority of fracture toughness tests are performed on either compact or SENB

(single edge notched bend) specirnens. The SENB specimen is more flexible with respect to

size, and its span cm be adjusted continuously to any value that is within its capacity. Thus

SENB speckns with a wide range of thickness can be tested with a single fixture. Besicies

this, SENB configurations are preferable for weldment testing, because of ease of fabrication

and less weld metd being consumed in some orientations.

The SENB specmiew used for the plane suain fiacture toughness testing had dimensions

according to ASTM std. E399-90, as foilows; k 125 mm, W=25.4mm. B=12.7nmi. Ail the

welded specimns had the surface porosity and irregulanty removed by fine grinding. The

above dimensions were the resulting dimensions of the surface finish after grinding. These

specimens were etched with 296 nitai and 5% picral to see the resulting weldment

microstructure boundaries. Notches were then placed at dinerent positions relative to weld

interface, both inside the WM and m the HAZ. Figure 3.1 and Figure 3.2 show the diagram

of wo such kind of specirnens, with different weld design.

3.4.2 The test

Afier the V-notch placement, there are three parts of Kc (plane strain fiacrure toughness)

testing. These include fatigue pre-cracking, fracture cracking, and anaiysis. During testing,

the displacernent of the crack mouth and the load appiied are recorded, these are then used in

the calculation of the fracture crack. Figure 3.4 shows the ciip gage king heid by sharp knife

edges attached to the notched three point bend specimen.

m e 3.4 : C l i ~ eape attachment in test swcimen

The clip gage. which attaches ta the mouth of the crack, consists of four resistance strain

gages bonded to a pair of cantilever bearns. Deflection of the bearns results in a change in

voltage across the strain gages, which varies lin6arly with displacement. Load measurement

on the other hand is done using linear variable differential transformer (LVDT). AU these

outputs are taken by the software (Teststar II) mnning the fracture test. Thereafier

computations based on ASTM standard E399 are done by the software to give the plane

strain fracture toughness values. AU the fracture tests were c d e d out on 810 Material

Test h g System (MTS) servo-hydraulic system, using the teststar software. The general view

of the testing equipment is given in Figure 3.5 - [24].

Figure 3.5 : General view of the testing esuipment

3.4.2.1 V-notch placement

A V-notch is machined in the three point bend specimens, both in the WM and HAZ.

Notches 10 mm in length and notch tip less than 90" are placed in the microstructure.

ensuring their orientation both parallel and perpendicular to the weld metal interface.

3.4.2.2 Fatigue precracking

The e s t part of the test is to generate a fatigue crack within the specimens. A fatigue crack

is generated by cyclically loading the specimens until a crack of the desked length is made.

The crack must end near the weld interface in the region to be evaluated, that is, HAZ or

WM. A fatigue crack is introduced in such a way, so as not to adversely infiuence the

toughness values to be measured.

Based on prelimmary calculation, values of EC- as 12.9 M P ~ & , load ratio (R) of O. I and

fiequency (f) of 10 Hz were assigned to the software to generate the crack, but after hours of

cyclic loading and no fatigue crack these values were changed to 15.0.1 and 20 respectively.

Again there was no success in initiating a crack after hours of loading, so these values of

K-. R and f were changed to 17.0.07 and 30 respectively, but without success. Reducing

the load ratio to 0.001 led to high vibraiions and shifung of the load. Thereafter. based on

experience gained so far. more precise calculations were done according to ASTM Std.

E399-90, and thus IG, was reduced to 16 M P ~ & , load ratio was raised to 0.01 and

frequency was increased to 35 Hz. The cyclic loading with these values, did provide fatigue

crack propagation, to give the nnal fatigue crack tip close to the interface of interest. The

ASTM standard 1221 was foilowed so that the fatigue crack length was more than the

minimum of 1.3 mm or 0.025W and the crack length fell between 45 and 55 percent of the

width (W).

3.4.2.3 Fracture cracking (ioading)

Afier fatigue cracking, the test specimens were slowly loaded until they fded. The loadulg

range was between 0.55 to 2.75 MPa & per second. The loading rate was set at 900 N ~ s ,

within the loading range of 0.3 kN/s to 1.4 kN/s as allowed by the standard 1221. During the

test the MTS-810 servo-hydraulic testing machine recorded the load, crack length and the

dis placement.

3.4.2.4 Analysis

When a pre-cracked test sample is loaded to failure, load and displacernent are monitored.

Three types of load-displacement curves are shown in Figure 3.6.

Pmax

LOAD

- DISPLACEMENT

Figure 3.6 : Three types of load-displacement behavior in a Krc test.

The critical load, PQ, is defmed in one of several ways, dependhg on the type of curve. A

5% secant iine (Le. a line from origin with a dope equal to 95% of the initiai elastic loading

dope) is constmcted to dete- Ps. In the type 1 nonlinear curve caused by plasticity

and/or subcritical crack growth, PpP5. With the type II curve, an unstable crack growth

(ie. a popin) occurs; in this case PQ is deked at the pop-m. In the type III curve, the

specimen fails completely before achieving 5% noniinearïty; in such cases, PQ = P-.

Since the crack length has a tendency to Vary through the thickness, it is dehed as the

average of three evenly spaced measurements and m s t be measured from the fracture

surface. The detennination of PQ and crack length are used in the computation of provisional

fracture toughness, &, fiom the following relationship 1231:

where f(a/W) is a dimensionless function of alW [23] and is expressed as:

The Kp value computed fYom Equation 3.3 is a valid Kc result, only if alI the validity

requirements in the standard are met, including Equations 3.1,3.2 and 3.5 below:

P-5 1.10 PQ (3.5)

The reason for using Equation 3.5 as a third validity requirement for fracture toughness test

is explained now. Consider a fracture toughness test that displays considerable plastic

deformation pnor to faiIure as shown in Figure 3.7 - [23]. Since this is a type 1 curve,

(Figure 3.6) Pp = P5. A Kq value computed fkom PQ however, would have little relevaoce to

the fracture toughness of the material. since the specimen fails weii beyond Pp; the KQ vaiue

in this case would grossly underestimate the true toughness of the material. Consequently,

the third validity requirernent, Equation 3.5, is necessary to ensure that a value is

indicative of the true toughness of the materiai.

LOAD

t

DISPLACEMENT

Fieure 3.7 : Load-displacement cuwe for an invalid Krc test

3.4.3 Conformation to standard

The dimensions of the SENB specimen are k125nm-1, W=25.4mm, B=12.7mm, and

a=13mm (notch plus fatigue length). These dimensions were chosen such that alW=0.51.

which satisfies the ASTM std. E399-90 section 7.3.2.1 - [22], according io which:

0.45 I a/W 5 0.55

The maximum stress intensity during the fatigue pre-cracking was taken as, &=16

~ ~ a f i . This satisfies the ASTM std. E399-90, section A2.1.2 - [22], according to which:

& 1 80% of the calculated Kit value during initiation and,

& 5 60% of the calculated Krc value during final crack size.

The load ratio during the fatigue crack was, R d . 0 1 and fkequency was 63SHz. They bath

satisfied the ASTM std. E399-90, section 2.4 - [22], according to the section:

-1 I R S + O . l and,

f < 100 Hz

The ramp or the loading rate was set at 900N/s, which satisfied the ASTM std. E399-90.

section A3.4.2.1 - 1221. According to this section. the rate of Ioading for the standard

(B=0.5W) 25.4mm thick specimen should be between 0.3 kN/s to 1.5 kN/s

The pre-cycle load was selected as 90ûûN. which satisfies ASTM std. E399-90, section 9.1.1

- [22]. According to this section the pre-cycle load should be less than the stress intensity

level in the final stages of fatigue cracking.

3.5 Hardness measurement

Hardness measurements were performed on the BM, HAZ and the WM zone of the

specirnen. The hardness readings were perfomed in a grid pattern. A Vickers microhardness

tester with a lkgf load maintaineci for 15 seconds was used. The distance between the

readings was 0.8mm (I/32"), which was at lest five diameters of indentation. The main

purpose of the testing was to determine hardness values near the weldment interface and

crack tip locations. The results of this cm be seen m chapter four in the f o m of hardness

graphs, while the corresponding vdues are given in APPEM>D( B.

Photomicrograph

The foiiowing sequence of steps was u&ed to prepare the specimens for rnicroscopic

examination:

(a) Grinding on 120 grit silicon carbide paper.

(b) Grinding on 240 grit silicon carbide paper.

(c) Grinding on 400 grit siücon carbide paper.

(d) Grinding on 600 grit silicon carbide paper.

( e ) Polishing with 15 micron diamond paste on nylon cloth.

(0 Polishing with 1 micron diamond paste on nylon cloth.

(g) Etching with 2% nital and 5% picral.

The resuliing microstructure of the weldment was opticaiiy examined using an inverted Nikon

Epiphot Microscope with attached video graphic terminal and camera for 35 mm and 4" X 5"

films. Photomicrographs of various magnitications at dinerent positions of the weldment

were taken. The photomicrographs are shown in chapter four.

3.7 Fractograph

The fiactured pressure vessel steel surfaces were anaiyzed under a scannùig electron

microscope (SEM) to observe the bnttie and ductile fractured surfaces. The SEM was a

Philips SEM with an EDAX spectroanaiysis system attachmeat. The spectroanalysis of the

inclusions was done to determine their composition. The analyses are presented and

discussed in detail in chapters four and five.

3.8 Image analysis

One of the pressure vessel steel plates used in the fracture toughness test was found to have a

significant proportion of inclusions fiom its photomicrograph. On visual inspection and

analysis of the fiactured surface under SEM, it was further found that these inclusions were

affecthg the fracture crack path in a major way. The density of these idusions differed both

in double vee welded plate and single bevel welded plate. Therefore, to study the effect of

inclusions on fkacture crack propagation, an image analyis of the plates was done usmg

Omnimet and IRS software, to note the relative size distn'bution and number of inclusions

present. For the purpose, the specmien was fine polished with alumina (suspension), and an

area of 26.40 mm' was sampled at magrilfication of lOOX for image analysis, using an

instrument calibration of 1.59 pm / pixel. The onmimet IRS reports and observations are

discussed in detail in chapters four and five.

Chapter Four

RESULTS

In this chapter the test resuits will be reported. Detailed analysis of these resdts WU be done

in chapter five.

4.1 Visual observation

SENB specimens with the weld joint designs of double vee butt and single bevel butt were

fiacture cracked. For the purpose, surface notch was put at different positions relative to the

weld interface, both in the WM and in the HAZ The specimens were fatigue pre-cracked,

and the fatigue crack tip was Iocated close to the weld interface. On visual inspection of the

fracture crack path, the following configurations as seen in Figure 4.1 were noted. It was

seen that in Plate II with single bevel weld design, all the fracture cracks irrespective of their

orientation and point of initiation in the microstructure propagated dong the weld interface.

In Plate 1 with double vee weld design, the crack path was iduenced by its initial orientation

relative to the weld interface and not by the point of initiation. The fracture crack in Plate 1

showed a rnarked preference for the WM, aligning itseif dong the mterface, when the fatigue

crack, interface and the strain intensification were dl in line.

Low Inclusion Plate4 (double vee] - - - - -- - - - - - - - - - -

Eigh Inclusion Plate41 (single bevel)

Weld Metal

Heat Mec- ted Zone

1 results out of 1 3 results out of 3 6 results out of 7 5 results out of 5

1 result out of 7

8 results out of 9 2 results out of 2 7 results out of 7 2 results out of 2

1 result out of 9

Figure 4.1 : Fracture paths with fatigue cracks having iigi.allel and inched orientation to the weld interface (Dashed line indicates notch DIUS fatigue ~re-crack, fracture starts at the tip of the soiid arrow head and the s u b ~ u e n t solid line indieates fraehve crack)

38

4.2 Fracture toughness

The fkacture toughness tests were performed on a servo-hydraulic Material Testing System

machine, using Teststar II, a software based on ASTM standard E399-90. The total number

of tests performed were thirty six; for some of the tests it was not possible to get any tangible

data. The data for the other tests is given below in Table 4.2 and Table 4.3.

Table 4.1 : Fracture toughess data for Plate II ( double vee weld design 1

Interface Orientation

Inclined Inclined Inclined Inclined Inclined Inclined hclined ParaIlel Inclined Inclineci Inclined Inclined

& in the table above is specimen strength ratio. This strength ratio is defined in ASTM

standard E399-90 and is calculated as shown below in Equation 4.1. It is a fûnction of the

maximum load the specimen can sustain, its initial dimensions, and the yield strength of the

materiai.

Table 4.2 : Fracture toudmess data for Plate I ( single bevei weld design )

Microstructure of on*

KAZ HAZ HAZ HAZ HAZ HAZ HAZ WM WM WM WM WM

4.3 Hardness

43.1 Hardness grid position

Hardness was performed in a grid form. The grid positions for the two weld joints of double

vee butt and single bevel butt are s h o w in Figure 4.2 and Figure 4.3 below. For the double

vee welded plate, the grid lines were 4 mm from each other and the plate edge, while for

single bevel welded plate. the grid iines were 6 mm from each other and the plate edge.

Interface Orientation

Parallel Paralie1 Parallel Parailel Parallel Parailel Parallei Parailel Parailel Parailel Parailel Paralle1

Fimire 4.2 : Hardness grid of double vee bu# weld joint

40

PQ N

8305.9 7263 -4 8830.0 7000.0 8800.0 8740.0 7750.0 8724.3 8022.9 7053 .O 8069.3 7000.0

Figure 4.3: Hardness grid of sinde bevel butt weld joint

43.2 Hardness graphs

The hardness values for double vee butt weld joint are given m Figures 4.4,4.5,4.6. 4.7, and

4.8. and those for single bevel butt weld joint are given in Figures 4.9, 4.10 and 4.1 1. These

hardness graphs are drawn fiom the hardness values listed in APPENDM B.

Graph of Vickers Microhardness, A l position

M icrostructure Position ( Dis tance be tw een points is 0.8m m )

Figure 4.4 : Hardness graph for double vee butt weld loint at A l position

Graph of Vickem Micro hardnese, 81 position

Microsttudure Postüon ( Dlrtmœ ûetween point. 4 0 . h m )

Fimve 4.5 : Hardness manh for double vee butt weld ioint at B1 mition

Graph of Vickers Microhardness, Ci position

Mitrostrudura Position ( 01stnce beniveen points b 0.&nm )

Fipre 4.6 : Aardness mgph for double vee butt weld joint at Cl position

Graph of Vickers Microhardness, Dl poaiüon

Miuostruchire Position ( Dlrtance betwean points k OBmm )

Figure 4.7 : Hardnesspa~h for double vee butt weld joint at D l Dosition

Graph of Vidaers Mictohardnear, El podtron

Microstructure Poeition ( astance between points b 0.8mm )

Figure 4.8 : Hardness -ph for double vee butt weld joint at El wsition

Graph of Vickers Microhardness, A2 position

Microstructure Position ( Wstance between points is OBmm )

Figure 4.9 : Hardness -ph for sinele bevel butt weld joint et A2 -tien

Graph of Vicûers Microhardneas, 82 position

Microstructure Position ( #.tance betwean points b 0.ûmm )

Figure 4.10 : Hardness g r a ~ h for sinde bevel butt weld joint at B2 msition

Graph of Vidaers Microhardness, C2 pomb'on

Miffostrudure Position ( Ustmœ betwsen points b U m m )

Figure 4.11 : Hardness graph for single bevel butt weld ioint at C2 position

In Figures 4.4 to 4.1 1, the values of hardness varied not ody across the weldment. but across

a particular microstructure as well. Given below are the variation of hardness for particular

microstructures. The hardness showed a dramatic increase and decrease at the interface.

1 Vickers hardness for BM varied from 155 HV to 184 HV.

2 Vickers hardness for WM varied from 163 HV to 193 HV.

3 Vickers hardness for HAZ varied from 173 HV to 2 10 )TV.

4.4 Photomicrographs of weldment microstructures

The ground specimen was fine polished and etched with 2% nital and 5% picral. The

resulting microstructure was analyzed and photographed usmg the inverted Nikon Epiphot

Microscope. Photomicrographs of varying magnifications were taken at various positions of

BM, WM and HAZ. The resulting microstructure photograpbs were found to be the same

for both the plates, that is, double vee welded plate (Plate II) and single bevel welded plate

(Plate 1), except that the single bevel plate had more inciusions Some of these

photomicrographs are represented nom Figures 4.12 to 4.14.

Figure 4.12 : BM microstructure with bands of proeutectoid ferrite (white) and marute (dark) -mation 200Q

Fimire 4.13 : HAZ microstructure showhg acicular ferrite surrounded bv ri or austenite main boundarv and cementite network (MagnXcation 2WX)

Figure 4.14 : WM microstructure showing grajn boundarv femte, side plate femte, Widmanstatten ferrite and ~robably bainite (Magnification 2WX)

46

4.5 Fractographs

Some of the fiactured SENB spechens were observed in the SEM. The fkactographs for

some of the weldment are shown below in Figures 4.15 to 4.17.

Figure 4.15 : Fractograeh showine equiaxed dimoles in WM fmctured surface near weld interface, in double vee welded plate

F M 4.16 : Fractomph showinn mixed mode fmcture with facet and dimples in the H A 2 near weld interface, in double vee welded plate

Figure 4.17 : Fractomph showinn transition from a mixed mode lame dim~led fracture to a small dimpled ductile fracture at weld interface. in double vee welded

plate

4.6 Image analysis

On image analysis, the proportion, size and distribution of the inclusions present in the

weldment was noted. Their proportion in the weldment is given in Table 4.3.

Table 4 3 : Prowrtion of inclusions in AS16 Grade70 steel weldment

Double vee welded plate Single bevel welded plate I (Plate 1) % I Inclusions in BM bclusions in WM

Since the proportion of inclusions in double vee and single bevel welded plates is dinerent in

BM and almost the same in WM, the inclusion size and distribution in BM 1 W of the two

plates is presented in the histograms of Figures 4.18 and 4.19.

lndudon Size MsMbution in Ba= Metal of Double V Welded Sbeel Plate

3.5- 4.0- 4.5 5.0- 50- 60, 6.5- 7.0- 7 ao- a5 9.0- 9.5- >IO 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

Cirwlar Ofmeter (micrometet)

Figure 4.18 : Inclusion size distribution in BM of doubIe vee welded lat te

Inclusion Size Distribution in Single Bevel Welded Sbe l Plate

Clrculu üiamater (m krometer)

Figure 4.19 : Inclusion size distribution in BM of single bevel welded plate

Because of the significant proportion of inclusions in the single bevel welded steel plate,

analytical spectrographs (SEM - EDAX ) of the inclusions was done. Two of the

spectrographs are given in Figurr 4.20 and Figure 4.2 1. The spectrographs show the

inclusions consisting of the elements calcium, silicon, zirconium and manganese.

The results of the experiments and tests are discussed in the next chapter.

Si Ka

' IO Ka ' Ailla

Figure 4.20 : EDAX spectrograph of the inclusion in the B M showing manganese and silicon among other things

Figure 4.21 : EDAX spectr~~raph of the inclusion in the BM showing calcium, siücon and zirconium amona other thinas

Chapter Five

DISCUSSION

The main purpose of this research was to study the fracture crack propagation relative to

weld interface in the weldments of A516-Grade70 C-Mn pressure vesse1 steel The

properties of the microstructure near the weld interface, that is in the HAZ and WM, were

evaluated. In this chapter the fkacture crack behavior near the weld interface is discussed

based on mechanical and me taIIurgical kno wledge gat hered kom hardness. microstruc tue.

weld properties. Iiterature, etc.

5.1 Weldment microstructure

Various types of microstnictures are formed in the weldments due to submrged arc welding.

The prominent microstructures formed, as seen in photomicrographs of F i p s 4.12, 4.13

and 4.14, are femte - pearlitic structure in the BM, foilowed by coarse grained structure

consisting of, interlocking laths of acicular femte, network of prior austenite grain boundary

and cementite in the HAZ, whiie the WM has a large volume kaction of coarse grain

boundary femte dong with side plate femte, fine acicular femte, Widmanstatten ferrite and

probably bainite. Weldments of double vee welded and single bevel welded plates consist of

the sarne type of microstructure.

The image analysis reports in Table 4.3, Figures 4.18 and 4.19, show that there is general

distribution of inclusions in the microstructure. But the proportion of inclusions in the single

bevel welded plate is significantiy more than in double vee welded plate.

5.2 Fracture toughness values

As cm be seen from the data m Tables 4.1 and 4.2, the P-/PQ ratio in almost ail the cases û

more than 1.10. Thus they do not sa&@ the plane strain condition, as specined by Equation

3.5 (page 32), which States that critical stress intensity factor (KQ) is a valid plane strain

toughness value (&), only when P-/PQ S 1.10. These values when plotted, essentiaily

represent Figure 3.7, where the spechens fails weii beyond Pp; and so, the KQ value thus

grossiy underestimates the uue toughness of the material. This suggests that AS 16 Grade70

steel used in the present study is quite ductile, as its toughness and thickness precludes a valid

test. Therefore the data in the Tables 4.1 and 4.2, cannot be used to evaiuate the bcture

toughness of the material and couid only be used for cornparison purposes between double

vee welded plate and single bevel plate, as it SU gives qualitative idea regarding cntical stress

intensity factor.

Two scenarios discussed are crack propagation hclined to the we1d interface and crack

propagation parailel to the interface. These are Wher subdivided into cracks in double vee

welded plate and cracks in single bevel welded plate.

5.3 Fracture crack inclined to weld interface

5.3.1 Double vee welded plate

The double vee welded plate has low inclusions. Here the WM predorninantly has large

volume fkaction of grain boundary femte as compared to fine interlockhg laths of acicular

ferrite in the HAZ. From the hardwss graphs of Figures 4.4 to 4.8 it is quite apparent that

WM zone is relatively sofi, as compared to the hard HAZ Furthemore, the impact strength

of the WM is high compared to HAZ, as seen in Table 3.4 and Table 3.7, suggesting a

relatively non brinle zone in WM with respect to HAZ.

Irrespective of its crack tip in WM or HAZ, when a fatigue pre-crack lying inclined to weld

interface is strain intensined as seen in Figure 4.1, a localized zone of high hydrostatic

stresses develops near the crack tip. These stresses then expand rapidly within the weaker

(softer) material as the plasticity spreads across the ligament. Thus, the constraint at the

crack tip relaxes. the crack tip blunts and the stresses are shined somwhat ahead of the

crack tip. This region ahead of the crack tip is the weld metal. Here, a lower degree of stress

triaxiality due to large plastic zone, usuaNy results in higher toughness [23]. As a result.

weaker material is subjected to large stresses as weil as scrain-states, which promotes ductile

fracture. This result is sixniIar to the observation by Shih [26,33] in bi-material interfaces.

Visual inspection of the specimens showed shear Lips. which also indicate ductile fracture.

The shear iips are fomed due to microvoid coalescence. whereby the crack exhi is a

tunnelhg effect and grows faster in the center of the plate, due to higher stress aiaxiality.

The microvoids can be formed due to soft femtic structure and high inclusion proportion in

the WM compared to HAZ /BM, as can be seen kom the Hnage analysis report in Table 4.3.

The observation agrees very weil with the fkactograph in Figure 5.1. which shows the

elongated h p l e s in the WM a little faher fiom the interface. suggesting a zone of high

plasticity and ductile fracture. This is quite unüke and distinct from the fractograph of Figure

4.15, where equiaxed diropies in the WM near the interface, suggest a zone of rehtively

lower plasticity or hard matenal, which is consistent with the hardness graphs.

Fipure 5.1 : Fractomph showhg eloneated dim~les in fractureci surface away from weld interface in double vee welded plate

Aiso. the fine interlocking laths of acicular femte in the HAZ, present a tough structure for

the crack, thereby stopping it fkom propagating into HAZ along the weld interface. The role

of weld stresses is not clear, but it is quite possible that residual stresses [29,30,31], which

are usudy highest along the weld centerliw, may also provide for additional suain

intensification in the weld metai zone, providing in turn a path of least fhcture resistance.

Only in two cases out of 16 did the crack propagate along the interface, before veering mto

the weld metal. This could be attrïbuted to drastic change of interfacial hardness, some

interfacial discontinuity or the presence of inclusions dong the interface.

53.2 Single bevel welded plate

The single bevel welded plate consists of signiucant proportion of Inclusions, h s t 0.066%

compared to 0.0 176% in the double vee welded plate, as seen in Table 4.3, Figures 4.18 and

4.19. These inclusions as seen in the photornicrograph in Figure 5.2 below, are distniuted

throughout the base metal, with a tendency to align themselves dong the weld interface.

m r e 5.2 : Distribution of inclusions in the weldment of single bevel welded date, with the inclusions aligning themselves alone the WM interface (Madcation 5Q

As seen in Figure 4.1, a fatigue pre-crack lying inched to the weld interface and irrespective

of its crack tip in WM or HAZ is strain intensified due to load application. The strain

intensification in the interfacial zone having hclusions leads to the formation of fke surtace

or void nucleation around inclusions, by either interface de-cohesion or particle cracking as

Uustrated in Figure 2.7. This fixther results in the growth of voids around the particle, by

meam of plastic strain and hydrostatic stress. The growmg voids reach a cntical size, relative

to their spacing, and a local plastic instability develops between voids - whereby coalescence

of the growing voids takes place to form a macroscopic Baw [35-391, leading to a ductile

fracture. In Figure 5.3 is shown a SEM fhctograph, showing large proportion of inclusions

and microvoid nucleation present at the interface.

Figure 5.3 : Fractoera~h of ductile fractured d a c e at the weld interface of singie bevel weided la te, shows large ~roporîion of inclusions and rnicrovoids nucleation

A material consisting of inclusions or impurities, f a at much lower strains, suggesting a

lower fracture toughness [35-391. This observation agrees quite weli with the fkacture

toughness testing results in Tables 4.1 and 4.2, where the critical stress htensity factor Kp in

case of single bevel welded plate having interfiid crack is comparativeiy lower ( 1200.18

M P ~ J ~ to lS9I -21 MP~,/=) than in the case of double vee welded plate, having no

interfacial crack (1677.00 MP~,/= to 1957.07 MPaJG). Since these inclusions are

lined in the HAZ side of the weld interface. the resulting fracture is a combination of ductile

and brittie fkacture, due to coalescence of rnicrovoids with adjacent voids and facet cleavage

of CGHAZ, as is seen in the fractogarph of Figure 5.4. Aiso, unlike ductile fracture in the

case of double vee welded plate, fracture in this case has no shear lip formation.

Figure 5.4 : Fractozra~h showine interfacial faetture in case of single bevel welded plate, metal fails bv a combination of ductile and briffle fmcture

5.4 Fracture crack parallel to weld interface

5.4.1 Double vee welded plate

When the fatigue pre-crack with its crack tip m the WM or the HAZ iies close and parailel to

the weld interface (Figure 4.11, the strain intensincation ahead of the crack due to the applied

load, also lies almost in iine and parallel to the interface. As a result, the strain intensification,

interface, and the crack are d pardel and nearly in iîne leading to high triaxiai stresses at the

interface. The tough, though bnttle nature of the HAZ as seen from impact data in Tables

3.2 and 3.7. contributes to the toughness of the interface. Thus, during crack propagation.

the plastic zone ahead of the crack is srnaLi and plane strain conditions exût. Because of

srnaII plastic zone, the uiaxial stress state is over a wider area and the region is capable of

canying higher stresses. Thus, less stress redistribution is necessary and leads to a sharp

crack and rapid crack propagation. This results in a mùced mode tkacture without shear lips,

consisting of ductile and brittie cleavage dong the interface, only and as long as the strain

intensification, crack and the interface are all neariy m line. In the absence of this condition,

the crack propagates into the WM. The fractograph of one such interfacial fracture is given

in Figure 5.5. As can be seen, it is quite similar to the fiactograph of Figure 5.4, thereby

suggesting a similar interfacial hcture mode in the double vee and single bevel welded steel.

F i m e 5 5 : Fractomph showine interfacial fracture in case of double vee welded plate, metal fails bv a combination of ductile and britüe fracture, also are seen

microvoids nucleation due to spherical inclusions

The double vee welded plate has low number of inclusions, and inclusions do not appear to

contribute signifïcantly to plastic instability. due to void nucleation, growth and coaiescence

at the interface. Nonetheless. the hctograph of Figure 5.5 does show some void nucleation

due to inclusions. Aiso. the photomicrograph of the polished and etched specimen did show

inclusion alignment near weld interface at few places, thereby suggesting a possible role of

void nucleation in interfaciai fiac tures. One such pho tomicrograph at higher mgnification is

shown in Figure 5.6 below.

Figure 5.6 : Higher magnification photomicroeraph showhg inclusion distribution and -- --

alignment at the weld interface of double vee welded plate (Mamiification 200x1

5.4.2 Single bevei welded plate

In thk case, when the fatigue pre-crack irrespective of its crack tip in WM or HAZ as seen in

Figure 4.1 lies close and pardei to the interface. the strain intensification ahead of the crack

due to load application would also lie close and pardel to the interface. As a result of this,

the stresses dong the interface would be high and an mterfacial fkacture would occur. Since

this plate has high aurnber of mclusions and inclusion alignment to the weld interface as seen

in section 5.3.2, is signincant, the void nucleation, growth and coalescence takes place. This

leads to an interfacial fracture consisting of a combination of ductile and bnttle fractures,

mainly because of void coaiescence at inclusions and facet cleavage of coarse grained HAZ.

In su-, inclusions were found to play an important role in influencing fracture crack

path in the case of pressure plates having high proportion of inclusions. The inclusions

tended to align themselves dong the weld interface and as such fiacture path too tended to

align themselves dong the weld interface, irrespective of its placement in the weldment and

its orientation to the intexface. While m case of plates with low inclusions, the fiacture path

was more influenced by its initial orientation relative to the weld interface and not by its

placement in the weldment. The fracture m such cases showed marked preference for the

softer femtic WM, aligning itself dong the interface, provided the fatigue crack, interface and

the strain intensification were all in line. The interfacial k t u r e in case of pressure plates

with low inclusions can be attnbuted in some part to inclusion alignment, high triaxial stresses

due to idine s h intensification and general interfacial elastic discontuiuity.

Chapter Six

CONCLUSIONS

The present study was canied out to determiw the propagation behavior of a fracture crack

relative to a weld interface. in C-Mn pressure vesse1 steel of the type A5 16 - Grade70. This

steel was welded by SAW, and SENB specimens were machined and notched at merent

positions relative to weld interface. Thereafier the SENB specimens were fatigue pre-cracked

and fracture cracked according to ASTM standard E399-90.

6.1 Conclusions

The main conclusions of this work are sumarized below:

The inclusions were found to play a major role in influencing fracture crack path In case of

welded plate with high volume of inclusions, the inclusions tended to align themselves dong

the interface. As such, the fiacture crack in these plates irrespective of its orientation to weld

interface and point of ongin in WM or HAZ was interfaciai in nature. The 6ractwe consisted

of a combination of ductile and brittle cleavage. The crack was ductile because of plastic

înstability caused due to void nucleation, growth and coalescence, while it was bnttie due to

transgranular fracture of the coarse grained HAZ.

However, in case of welded plate having low volume of mciusions, the fifracture crack path

was iduenced by its initial orientation relative to the weld interfice and not by the point of

initiation in the weldment. The fkacture crack was ductile with shear Lips and had rnarked

preference for the sofi femtic WM, this was mainiy due to large plastic zone in the sofi

femtic weld =ta1 leading to higher stresses and suain-states. The fracture crack aligned

itself dong the interface, provided the fatigue crack, interface and the strain intensification

were al1 in h e . The alignrnent of the crack aiong the interface can be attributed to somr

inclusion alignment aiong the interface leading to microvoid nucleation, or high triaxial

stresses due to idine strain intensification leading to sharper fkacture crack, or geneml elastic

discontinuity at the interface as seen from high hardness peaks near the weld interface.

6.2 Suggestions for future work

Some of the suggestions for future work are;

1) Shce the present material A5 16 Grade70 exhibits significant ductility, and nonhear

material behavior becomes significant, fracture toughness testing of the specirnens

with the same dimensions as used in this study should be done, using Crack tip

opening displacement (CTOD) or the J i n t e g d These tests discard stress intensity

and adopt crack tip parameter that takes material behavior into account. It wouid be

interesting to explain the fracture crack behavior in tenns of fracture toughness

values.

2 ) This study hcorporated kacture tests. Simiiar tests using fatigue tests should be

designed, to see the crack behavior, when a cyclic fatigue crack approaches the

interface at different orientations, from different points in weldment.

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pp. 61-91.

[36] Knott, J.F., 'Effects of Microstructure and Stress-State on Ductile Fracture in

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Brown University Report, Providence, RI, USA, 1993.

APPENDIX A

SPECIFICATIONS FOR PRESSURE VESSEL QUALITY

CARBON STEEL

In this Appendix the S pecifications for some of the ASTM grades of carbon steel (of pressure

vesse1 quality). are listed, they are taken from reference [ 11.

Table A.l : ASTM Specifications for ~ressure vesse1 qualitv carbon steel date

Steel tvae and Condition

Carbon steel plates of low or intermediate tensile strength Carbon-mangrnese-silif on steel plates Carbon steel plates for applications requiring low transition temperature Carbon-manganese steel plates of high tensile sû-ength Carbon-silicon steel plates for intermediate and higher temperature Carbon steel plates for moderate and lower temperature Heat treated carbon-manganese-silicon steel plates Titaaium-bearing carbon steel plates for glass or diffused rnetailic coatings Carbon steel plates of high tensile strength for moderate and lower temperature Carbon-manganese steel plates for moderate and lower temperature Quenched and ternpered carbon steel plates for layered pressure vessels not subject to p s t weld heat treatment Heat treated carbon-manganese-silicon steel plates for moderate and lower temperature

APPENDIX B

HARDNESS DATA ACROSS THE GRID FOR THE

PRESSURE STEEL WELDS

In this AppendUt the Vickers Microhardness value across the grid for the two steel welds, viz.

Double Vee butt weld and Single bevel butt weld as shown in Figures 4.2 and 4.3 (page 41)

are given in Tables B.1 and 8.2 respectively. The corresponding graphs are shown in Figures

4.4 - 4.11.

Hardness was taken by Vicken at a load of 1 kgf maintained for 15 seconds. The distance

between two consecutive points is 0.8mrn (lB2"), which is more than at least 5 diameters of

indentation.

Table B.1 : Haniness values across prrid in Double vee butt weld

Position A 1 -- -

Position C 1

B M 168.2 165.0 168.2 174.8 165.0 171.4 178.2 178.2 168.2 171.4 178.2 178.2 171.4 189.2

HAZ 193.1 193.1 205.5 2 14.4

WM 178.2 181.8 178.2 181.8 178.2 162.0 171.4

W 171.4 KAZ 168.2

168.2 165.0 168.2 171.4 171.4 174.8 165.0 168.2 171.4 168.2 171.4

Position El

Table B.l : Hardwss values across grid in double vee butt weld (Contd.:-1

Position A 1 Position B 1 Position C 1 Position D 1 Position E 1

Table B.2 : Hardness values across grid in Sinpile bevel wdd

Position B2

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