aws 03nov behavior of welded joints of creep-resistant steels at service temperature beres

7/27/2019 AWS 03nov Behavior of Welded Joints of Creep-Resistant Steels at Service Temperature BERES

1/7

WELDING RESEARCH

NOVEMBER 2003-S330

ABSTRACT. Power station pipelines andother structural fabrications operating athigh temperature are predominantlymade of creep-resistant steels. Thesesteels may become martensitic even if thecooling medium was air due to their rela-tively high Cr, Mo, and V content, whichcan retard the bainitic transformations.These steels must, therefore, be pre-heated, and filler materials must be care-

fully selected. In heterogeneous joints,substantial diffusion takes place duringhigh-temperature service conditions. Thispaper describes the layered formation inthe heterogeneous welded joint and re-ports on the investigation of the conse-quences of such diffusion. The principalgoal of this investigation was to reduce theprobability of cracking during service.

Introduction

A substantial number of creep-resistantsteels are used in the fabrication of nuclear

and fossil fuel power stations. These steelsare alloyed using high-content chromium and molybdenum withthe consequence that, after welding, in spiteof air cooling, their microstructure can be-come quenched to a bainitic-martensitic orfully martensitic structure. To prevent thisundesirable outcome, these steels are pre-heated to 200350C (based on empiricaldata) prior to welding. When a welding pro-cedure is being elaborated, the preheattemperature is selected in accordance with

wall thickness. This practice is not whollycorrect, because the optimum preheat tem-perature for the concentration range of

heats permitted by the relevant steel stan-dards may be different. If the compositionis left out of consideration, the crack sensi-tivity of the steels made of certain heats maybe increased.

In many power stations there may beseveral welded joints in which the base met-als and the filler materials are not of thesame type. These joints are referred to as

heterogeneous joints. It is well known thatin such a joint, diffusion occurs at increasedtemperatures, and diffusional layers areformed that may cause crack formation

within a short service time. Diffusional lay-ers are the zones formed due to the diffu-sion movement of the carbon on both sidesof the interface between the weld and theheat-affected zone (HAZ), typically car-bide layers and decarburized zones. The mi-

crostructure of the formed diffusional lay-ers significantly differs from theneighboring parts of the joints. The charac-teristic dimension of these layers is thethickness, i.e., the size perpendicular to the

weld interface between the weld and theHAZ. The rate of dimensional change ofthese layers is relative to the chemical com-position: the greater the composition (firstof all Cr) difference between weld and basemetals, the greater the dimensional change.Temperature also has the same effect onthis rate. In a heterogeneous joint of differ-ent base and filler materials, the acceptable

service temperature cannot be as high aswould be possible for homogeneous joints.The allowable service temperature for theseheterogeneous joints has not been preciselydetermined and it is necessary to rely onempirical observations for these values.

In order to prevent welding imperfec-tions, this paper suggests a method that isbased on metallurgical knowledge and is inharmony with modern quality assuranceprinciples. During the last decade, many ex-periments have been carried out on thistopic and a number of the results have beenpublished (see references) in addition to thepresent paper, which deals with the service

behavior of heterogeneous joints.

Preheating and Temperingof Homogeneous Joints

During the last century, characteristictypes of creep-resistant steels were devel-oped all over the world. In Table 1, the dataof the last column demonstrate that the per-mitted service temperature increases in theslightly oxidizing smoke gas atmosphere, ifthe quantity of alloying elements is higher

(Refs. 1, 2). Similarly, another effect of al-loying, namely the quenching susceptibility,is stronger in alloyed steel grades and,therefore, all steels except the ferrite-pearlitic (F+P) and austenitic (A) groupsshould be preheated.

According to experimental results(Ref. 3), the effect of chemical composi-tion on the preheating temperatures (Tp)seems as significant as the wall thicknessin the case of the bainite-martensitic(B+M) group. On the basis of investiga-tions, it became possible to reduce the

widely used temperature range of pre-

heating as follows:

(1)

Equation 1 is valid with the followingprovisos: If the sum in parentheses is less than

zero, preheat is not needed. The upper temperature limit is sug-

gested for castings and wrought prod-ucts of wall thickness greater than 40

mm. The lower temperature limit should be

applied when the thickness is less than15 mm.

Metallurgical characteristics of steelsof B+M and M (martensitic) groups dif-fer markedly from each other. These dif-ferences must be taken into account whencreep-resistant steels are classified orstandards are prepared (Ref. 4).

The suggested preheat temperature forthe martensitic welding of steels belongingto the martensitic group should be equal tothe interpass temperature (Ref. 5).

For steels of C=0.2%,

++ +

- 0 4 20. )Mo Ni Si

15C

Tp = + ++

+180 200 (CCr V

5

Mn

6

Behavior of Welded Joints of Creep-Resistant Steels at Service Temperature

Creep-resistant steels were examined in an attempt to reducethe probability of in-service cracking

BY L. BRES, A. BALOGH, W. IRMER, AND C. S. KIRK

L. BRES and A. BALOGH are Associate Pro-fessors, Universit y of Miskolc, Hungary. W.IRMER is a Professor at University of Magdeburg,Germany. C. S. KIRK is Senior Researcher, Uni-

versity of Bath, Great Britain.

KEYWORDS

Creep-Resistant SteelHeterogeneous JointsCrack SensitivityNi-Based Filler MaterialPreheating TemperaturePostwelding TemperingDiffusionFerrite LayersCarbide-Rich Layer

Service Temperature


2/7

WELDING RESEARCH

-S331WELDING JOURNAL

(2)

while for the C=0.1% steel in the samegroup,

(3)

The Ms temperature in Equations 2and 3 can be calculated from the compo-sition (Ref. 5):

(4)

In martensitic steel groups, the effectof wall thickness can be negligible com-pared to the effect of chemical composi-tion (Ref. 6); therefore, for wall thick-nesses more than 40 mm, it is sufficient toadd 10C to the suggested average.

On the basis of their experimental re-sults given in the literature, Steven,Haynes, and Kauhausen state that if the

welding was carried out at other than the

advised preheating temperature, 10C dif-

ference from the suggested average value

causes 8% difference in martensite con-tent forming during welding (Ref. 5).

These observations are visualized inFigs. 1 and 2. These figures show the

widely used preheating interval and thecomposition effect on the amount offormed martensite during cooling.

It can be seen in Fig. 1 that the mi-crostructure in the HAZ of the welded

joint of base metals whose compositionequals the lower limit of the relevant stan-dard contains 20% martensite and 80%austenite if the preheating temperature

was the maximum possible, i.e., 300C.

When the preheat is lower, e.g., 220C, the

+ + + + - (Cr Mo 1.5 Si W V) 21 Cu

Ms = - + - 454 210 C4.2

C

9.5

T Mp s0 1 90 10. ( )= - C

T Mp s0 2 60 10.

( )= - C

Fig. 1 Martensite content of 0.2C-12Cr-1Mo-type creep-resistant steel dur-ing welding. Msl is Ms temperature for heat of lower limit of the standard(Msl=324C [Ref. 5]); Msu is Ms temperature for heat of upper limit of the stan-dard (Msu=254C); PTI is preheat temperature interval in the practice.

Fig. 3 Thickness of carbide-rich layer after annealing at 550C. Base metal:0.15C-1.1Cr-0.6Mo. Filler material: 8 18Cr-8Ni; 20 22Cr-20Ni; 35 20Cr-35Ni; 70 70Ni-20Cr.

Fig. 4 Thickness of ferrite layer after a 100-h annealing at different temper-atures. Base metals: C 0.15C-0.8Mn; Mo 0.15C-0.8Mn-0.3Mo; MoV 0.15C-0.6Mo-0.3V; 1 0.15C-1.1Cr-0.6Mo; 2 0.15C-2.2Cr-1Mo; 5 0.1C-5Cr-0.6Mo; Filler material: 70Ni-20Cr.

Fig. 2 Martensite content of a 0.1C-9Cr-1Mo-type creep-resistant steel dur-ing welding. Msl is Ms temperature for heat of lower limit of the standard(Msl=393C [Ref. 5]); Msu is Ms temperature for heat of upper limit of the stan-dard (Msu=339C); PTI is preheat temperature interval in the practice.


3/7

WELDING RESEARCH

NOVEMBER 2003-S332

microstructure contains 82% martensiteand 18% austenite. If the steel heat be-longs to the upper limit of the standard,

and the preheating temperature is thesame (220C), then only 28% martensiteforms, and at the preheating temperatureover 254C, there is no martensite at all.

Consequently, if the chemical compo-sition is not taken into account and thepreheating and the interpass tempera-tures are kept as 220C, martensite con-tent can vary between 28 and 82%, de-pending on the actual chemicalcomposition of the steels to be welded. Itis understandable, therefore, that a person

who has welded creep-resistant steels with30% martensite content for a long time

will be surprised by a new steel heat inwhich 80% martensite can be formed dur-ing welding, because the weld and HAZshow increased crack susceptibility.

After some practice, an experienced,qualified welder can successfully weld thedifferent steel heats by applying the usualpreheating temperature interval (in Fig. 1it is denoted as PTI), but the crack risk canbe minimized if the welding is done at apreheating temperature to which the samemartensite amount belongs. This preheat-ing temperature is considered an optimalone. The optimal preheating temperature

can be calculated by Equation 2. It follows

from Fig. 1 that the martensite content ofany heat is approximately 50% if it waspreheated to (Ms 60)C. Applying the

advised preheated temperature, joints willbe uniform from the viewpoint of the qual-ity, and much time can be saved, mainly inrepair or on-site welding, because there isno need for testing.

The structure of Fig. 2, belonging tothe 0.1C-9Cr-1Mo steel, is similar to Fig.1, except the PTI is now 250350C. FromFig. 2, the same results can be drawn. Ex-periments demonstrated that as high as70% martensite content can be allowedfor this steel, because of the lower C con-centration and lower hardness. The opti-mal preheating temperature can be calcu-

lated by Equation 3.Following completion of the welding ofsteels with B+M and M microstructurethe HAZ becomes rather hard and brittle.In order to reduce hardness to the accept-able level, the joints should be tempered.During tempering, a certain upper limitgiven in Table 2 must not be exceeded, oth-erwise the austenitized part of the jointscould be transformed into martensite.Tempering at lower temperatures than theadvised values would reduce the effective-ness of tempering. In the literature, pa-pers unanimously claim (Refs. 6, 7) that

the hardness increases by approximately

15 HV when the tempering temperaturediffers up or down from the suggested

value by 10C. At temperatures 6080C

below the lower limit of the advised inter-val, tempering is practically ineffective. Tobe on the safe side, it is widely acceptedthat the tempering temperature alwaysshould be below theAc1 temperature by 20to 30C (Ref. 8).

Heterogeneous JointsContaining Austenitic Filler Metal

In homogeneous joints, the hardness ofthe weld and the HAZ are almost equal,i.e., considerably brittle before tempering.The frequently used austenitic filler mate-

rial has an advantage, because the weldmetal is tough and ductile during weldingand cooling.

Since joints containing creep-resistantsteels and austenitic filler material differin composition, diffusional processesoccur between the weld and HAZ of the

welded joints at high (service or temper-ing) temperatures.

The Cr content of austenitic filler ma-terials is approximately 20%. This con-centration is much more than that of steelsbelonging to the F+P, B+M, or M group,so they can be applied to joints operating

under 300C and need not be tempered

Table 1 Classification of Creep-Resistant Steels

Group of Steels Composition Type Chemical Composition, wt-% Service Temp.,C Mn Cr Mo Ni C

Ferrite-Pearlitic 0.15C-0.8Mn 0.12 0.40 480F+P 0.25 1.20

0.15C-0.8Mn-0.3Mo 0.12 0.40 0.20 5000.20 1.00 0.40

Bainite-Martensitic 0.15C-0.6Mo-0.3V 0.10 0.40 0.30 0.40 560B+M 0.20 0.70 0.60 0.70

0.15C-1.1Cr-0.6Mo 0.10 0.40 0.70 0.40 5700.20 0.70 1.30 0.70

0.15C-2.2Cr-1Mo 0.07 0.40 2.00 0.80 6000.18 0.70 2.50 1.20

Martensitic 0.10C-5.0Cr-0.6Mo 0.08 0.30 4.50 0.45 650M 0.14 0.60 6.50 0.65

0.20C-12Cr-1.0Mo 0.17 0.40 10.0 0.80 6500.25 0.70 12.5 1.20

0.10C-9.0Cr-1.0Mo 0.07 0.40 7.50 0.80 6500.14 0.70 9.50 1.10

Austenitic 0.06C-16Cr-13Ni 0.04 0.80 15.0 12.0 750A 0.10 1.50 17.0 13.5

0.06C-16Cr-16Ni 0.04 0.80 15.0 1.50 15.0 7500.10 1.50 17.0 2.00 16.5


4/7

WELDING RESEARCH


(Ref. 9). Above this temperature, thethickening rate of diffusional layers signif-icantly increases, which offers a good pos-sibility of determining the increase rate oflayer thickness. For these welded joints,the filler metal should be chosen so thatmartensite does not appear in the dilutedfirst layer (Ref. 10).

In experiments, a 0.15C-1.1Cr-0.6Mo-type creep-resistant steel was used. Thedimensions of specimens were 100 mmlong, 50 mm wide, and 812 mm thick. An80-mm-long bead was welded onto the

surface of the specimen by the shieldedmetal arc welding (SMAW) process. Thespecimens were soaked at 550C for dif-ferent holding times. After the heat effect,a cross section was made for measuringlayer thickness. The specimens wereetched by aqua regia, and the appliedmagnification of the optical microscope

was 250.Discussion of the theoretical basis

(Refs. 11, 12) of the diffusional processesis not a chief aim of this paper, but it is wellknown that the diffusional rate of carbonis much higher than the same of other al-

loying elements (Ref. 13). Transport ofcarbon depends, among other factors, onthe concentration difference between

weld and base metals, the service temper-ature, and the microstructure of matrix(ferritic or austenitic). The carbon move-ment is directed from the lower Cr part tothe higher since the carbon demand of cre-ating chrome carbides dynamically de-creases the carbon concentrations, caus-ing diffusional fluxes. Consequently, a softferrite layer will form in the HAZ and acarbide-rich layer in the weld. Layer for-mation increases crack sensitivity in the

welded joint (Ref. 14).

The dimensional rate of change of thecarbide-rich layer is shown in Fig. 3 as anexample. It can be clearly seen that thelayer begins to form only after a definiteincubation time. The thickening rate (y)follows exponential law in time (t) and canbe written as follows:

(5)

wherea0 is the threshold of the layer for-

mation, anda

1 is the temperature-depen-dent thickening rate of the ferritic or car-bide-rich layer.

Equation 5 shows that the thickeningrate slows down as a function of time. Thisdigressive process is characteristic for theferrite layer as well, and can be explainedby increasing of diffusion distances, whilethe diffusional coefficient of carbon is notsignificantly changed.

The thickening rate decreases as the Niconcentration increases, but considerablereduction can be achieved by applying Ni-based (70% Ni + 20% Cr) filler materials.It follows from the position and shape of

curves that if the traces of layer formationcan be observed by optical microscopic ex-amination as late as 100 h, then it is prob-able that a dangerous-sized continuouslayer will not occur in a time longer bythree orders of magnitude.

As observed in our experiments, thediffusional layer formation began after100-h holding time at 520C when 70/20-type filler metal was used. The curve des-ignated as 70 in Fig. 3 belongs to the an-

nealing at 550C, would be much less steepat 520C due to the lower temperature,and the threshold of the layer formation

would be postponed to the longer holdingtime. The aforementioned consequencescan be read from Figs. 4 and 5.

On the basis of our theory, the highestsafe service temperature (the permittedservice temperature) can be defined as thetemperature belonging to the traces of thediffusional (ferritic or carbide-rich) layerformation after 100-h soaking time.

The design lifetime of fabrications thatuse creep-resistant steels is generally200,000 hours (approximately 20 to 25

y a a t= + 0 1 ln

Fig. 5 Thickness of the carbide-rich layer after a 100-h annealing. Base met-als: C 0.15C-0.8Mn; Mo 0.15C-0.8Mn-0.3Mo; MoV 0.15C-0.6Mo-0.3V; 1 0.15C-1.1Cr-0.6Mo; 2 0.15C-2.2Cr-1Mo; 5 0.1C-5Cr-0.6Mo,9 0.1C-9Cr-1Mo; 11 0.2C-12Cr-1Mo. Filler material: 70Ni-20Cr.

Fig. 6 Microstructure and hardness distribution of a heterogeneous joint,which had been operated for 35,000 hours in a main steam pipeline. Base met-als: 0.15C-0.6Mo-0.3V. Filler material: 70Ni-20Cr. Service conditions: 520Ctemperature and 120-bar pressure. Etching with aqua regia.

Table 2 Postweld Tempering Temperature and Hardness of Bainite-Martensitic andMartensitic Creep-Resistant Steels

Composition Type Temp., C Hardness in HV10of Steels Ac1 Tempering Before After

Tempering Tempering

0.15C-0.6Mo-0.3V 735 700720 260320 2002500.15C-1.1Cr-0.6Mo 745 660700 260320 1802300.15C-2.2Cr-1.0Mo 780 690750 270340 2002500.10C-5.0Cr-0.6Mo 785 720750 280320 2002500.20C-12.0Cr-1.0Mo 820 740760 350450 2002600.10C-9.0Cr-1.0Mo 810 740780 300370 200250


5/7

WELDING RESEARCH

NOVEMBER 2003-S334

years). In our opinion, the temperaturethat can be considered as the permittedmaximum service temperature is that at

which only traces of the layer formationcan be discovered after 100 h of soaking.

Heterogeneous Joints Made by70Ni/20Cr Filler Material

For operation at temperatures higherthan 300C, only nickel-based alloys canbe used. Typical of this group is the 70%

Ni and 20% Cr (70/20) alloy. Comparedwith creep-resistant steels, this alloy haslife strength greater than martensitic steel,and its elongation is approximately 40%regardless of the heat-treated condition.

One-hundred-hour experiments werecarried out at different soaking tempera-tures with specimens prepared as de-scribed above. The formed layer thick-nesses were measured using an opticalmicroscope.

After statistical analysis of the results,it was found (Ref. 15) that the best fitcould be achieved by a second order poly-nomial as follows.

(6)

whered is the thickness of the ferrite orcarbide-rich layer in mm, Tis temperaturein C, and ai are regression coefficientsgiven in Table 3.

Thicknesses as a function of tempera-ture are illustrated in Figs. 4 and 5. Curvesintersect the horizontal axis at the tem-perature at which the traces of layer for-mation appear. Formation of ferrite and

carbide-rich layers does not necessarilybegin at the same temperature, e.g., in thesteels No. 9 and No. 11, a ferrite-rich layerdid not appear at all. The suggested ser-

vice temperature is the lower of them. Wechecked the experimental results a fewtimes and stated them precisely. Thesetemperatures are collected and system-atized in Table 3.

It is clear from the data in Table 3 thatthe recommended service temperaturesincrease sharply with the Cr content of thebase metal.

For example, let the base metal be0.15C-0.6Mo-0.3V-type creep-resistant

steel and the filler metal 70Ni-20Cr-typeNi-based alloy. This case can occur whenferrite-pearlitic and austenitic creep-resis-tant steels are welded together. Traces offerrite layer can be observed after 100-hannealing at 506.8C, just as a carbide-richlayer appears in the joint. It follows fromthe given data that the allowed servicetemperature can be chosen as the lower ofthem, i.e., 495C.

On the basis of extensive investigation,it is probable that the first layer, diluted by

the base metal chemical composition, hasnot equalized (Ref. 16) and, consequently,the layer hardness is affected by the pre-heating temperature (Ref. 17).

In the last decade, new results werepublished about the construction of diffu-sional layers (Refs. 18, 19), which helpedto reveal the mechanism of the layer thick-ening.

It is clear from the previous works thatexperimental investigations into the ser-

vice life of the creep-resistant steels areexpensive and time consuming. For thisreason, a computer model to simulate thediffusional processes taking place at in-

d a a T a T= + + 0 1 22

Table 3 Regression Coefficients, Threshold Temperatures, and Rounded Values of Recommended (Maximum) Service

Composition Symbol of Base Ferrite Layer (Fig. 4) Carbide-Rich Layer (Fig. 5) RecommendedType of Base Metals in Coefficients Threshold Temp. Coefficients Threshold Temp. Service Temp.,Metals Figs. 4 and 5 a0 a0 C

a1 T(yF=0) a1 T(yC=0) (Lesser of a2 a2 Threshold Temps.)

1411.0 260.90

0.15C-0.8Mn-0.3Mo Mo 5.98270 479.6 1.13312 492.2 4800.0063400 0.0012252

1503.0 239.650.15C-0.8Mn C 6.41525 471.9 1.07533 483.2 470

0.0068452 0.001199

1764.8 322.80.15C-0.6Mo-0.3V MoV 7.30860 506.8 1.36065 495.3 495

0.0075500 0.0014313

3856.7 605.90.15C-1.1Cr-0.6Mo 1 14.43680 544.0 2.34608 530.3 530

0.0135060 0.0022695

3668.6 800.6

0.15C-2.2Cr-1Mo 2 13.34390 561.7 2.94639 560.3 5600.0121286 0.0027084

3979.5 1053.50.10C-5.0Cr-0.6Mo 5 13.77860 597.2 3.70239 589.5 590

0.0119140 0.0032490

1752.00.10C-9.0Cr-1.0Mo 9 5.82170 619.0 620

0.0048325

914.00.20C-12Cr-1.0Mo 12 3.02564 629.2 630

0.0025000


6/7

WELDING RESEARCH


creased temperature in the equipment ofpower stations was constructed using thefinite volume method (Ref. 20).

To illustrate the reliability of data, sev-eral cases are reported below. In an in-dustrial case, a welded joint operated inthe main steam pipe of a power station ata temperature 25C higher than recom-mended is considered. In the microphototaken from the section of the pipe (Fig. 6),indentations of a micro-Vickers diamondpyramid loaded by 50-g weight can beseen. The diffusion process directed into

the weld metal penetrated much deeper atthe grain boundaries than into the grainsthemselves.

The hardness of the weld was notgreater than 200 MHV. Proceeding to-

ward the fusion zone, the hardness in-creased because more and more carbidegrains formed during service life. Thehardness of the darkly colored carbide-rich layer is 339 MHV. From the neigh-boring heat-affected zones, the carbon

went off and a ferrite layer (183 MHV)began to form. The next indentation is lo-cated at the border of the ferrite layer, and

thereafter the hardness of base metalranges between 185 and 191 MHV.This photograph shows the fine grains

in the ferrite layer. This is due to the Vcontent of the steel. In the vanadium-freesteels, the ferrite layer has a coarse grainstructure.

The investigated joint was available forinspection because of the reconstructionof the pipeline system. This joint had beenin service for 35,000 hours. According toour conclusion, a crack should haveformed in the next three to four years inthe ferrite layer. This crack-sensitivestructure could not have formed if the ser-

vice temperature had not exceeded thepermitted 495C.

When two steels with different Cr con-tent are welded, service temperature is de-termined by the side with the lower Cr.Tempering temperature should also belower thanAc1 temperature by 20 to 30C.

Nowadays, a few constructional planscan still be found for which 0.15C-1.1Cr-0.6Mo- and 0.1C-9Cr-1Mo-type steelsmust be welded. This is now seen to be in-

correct since there is no suitable temper-ing temperature for both sides: 700C istoo low for 9% Cr steel, and 750C is veryhigh for 1.1% Cr steel, because a thick fer-rite layer may form even during tempering Fig. 7. Welded joints of these steelsmust not be used operationally. To preventthe problem of incompatibility, a mini-mum 2-m-long pipe made from a 0.15C-2.2Cr-1Mo-type creep-resistant steelshould be inserted. The microstructure ofFig. 7 points out that the duration of thetempering cannot not be too long since thehardness of the joint decreases signifi-cantly in the first hour, and later the soft-

ening slows down, but the diffusional lay-ers thicken considerably from the secondhour.

In cases when a steel of B+M or Mgroups is to be welded to austenitic or0.15C-0.8Mn unalloyed steel grade, an 8-to 10-mm-thick buttering layer should bemade using 70/20 type Ni-based alloy.

After tempering, the joint welding be-tween the buttering layer and theaustenitic or ferrite-pearlitic steel should

be carried out with the same filler mater-ial, without preheating (Ref. 12). With thismethod, we can prevent the dilution that

would be formed between the Ni-basedalloy and any other type of filler material,as well as the hot crack sensitivity causedby the dilution.

Heterogeneous Joint withQuenchable Filler Material

In such a joint, the composition differ-ence between parts of the joint is lowerthan the same in joints made withaustenitic filler material. However, the

Table 4 Recommended Service Temperature of Welded Joints Made of Quenchable Filler Ma-terial

Composition Type Cr Content in Cr Content in Recommendedof Base Metals Base Metal, wt-% Weld, wt-% Service Temp., C

0.15C-2.2Cr-1Mo 2.2 9 4900.15C-0.8Mn-0.3Mo 2.2 11 4900.15C-2.2Cr-1Mo 2.2 5 5600.10C-9.0Cr-1.0Mo 9.0 5 6000.20C-12Cr-1.0Mo 12.0 5 5900.10C-9.0Cr-1.0Mo 9.0 2.2 5600.20C-12Cr-1.0Mo 12.0 2.2 540

Fig. 7 Microstructure of the welded joint after tempering of 750C/3 h. Basemetals: 0.15C-1.1Cr-0.6Mo. Filler material: 70Ni-20Cr. Etching with aquaregia.

Fig. 8 Microstructure of the welded joint after tempering of 750C/2 h. Basemetals: 0.1C-9Cr-1Mo. Filler material: 0.15C-1.1Cr-0.6Mo. Etching with aquaregia.


7/7

WELDING RESEARCH

NOVEMBER 2003-S336

weld metal does not contain nickel, ham-pering diffusion movement; therefore,danger of layer formation is increased.

In the course of the experiments it wasfound that the thickening rate of the dif-fusional layers can be estimated by thesame type of regression equation as wasshown for the joints of austenitic filler ma-terial. For the quenchable joints, morefiller material can be used, so the number

of variations is larger. Table 4 shows likelycombinations that can be used.

It was concluded from the experimen-tal results that if the Cr difference be-tween weld metal and base metal is lessthan 2.5%, diffusional layers harmful forstrength parameters of the joint are notformed either during tempering or ser-

vice. This is true for circumstances wherethe service temperature does not exceedthe permitted temperature valid for theless-Cr-containing steel part of the joint,according to Table 1.

Using a combination in which the Cr

difference between base and filler metalsis larger than given in Table 4, thick layerscan appear in the joint even during tem-pering, which can crack after a relativelyshort service time.

This phenomenon is illustrated by thephoto in Fig. 8. A few years ago, there wasa design between a 0.1C-9Cr-1Mo and a0.15C-1.1Cr-0.6Mo steel pipe for which a0.15C-1.1Cr-0.6Mo-type filler material

was planned. If this joint had been made,a very thick diffusion layer would havebeen formed at one side of the joint dur-ing tempering, and this layer would have

cracked in the first few years.In this example, the ferrite layer devel-oped in the weld. If the Cr content of thebase metal is lower, the ferrite layer willform in it.

A conclusion can be drawn from the ex-amined cases relating to the joints withdifferent Cr steels as follows: joints shouldbe welded by a filler material having Crcontent near the average Cr percentage ofthe base metals.

These joints should be tempered attemperature lower by 2030C than theAc1 temperature of the steel of lower Crcontent part (Table 2). In cases whenhighly different Cr-containing steels haveto be welded, a 1-m-long pipe made from0.15C-2.2Cr-1Mo steel should be inserted,similarly to the example above.

Our experimental results are in har-mony with the theory published in Refs. 21and 22.

Conclusions

It follows from the principles of qualityassurance that welded joints should be re-producible and of the best quality at anytime. Accordingly, homogeneous joints

should be welded at a broad preheatingtemperature interval determined on thebasis of chemical composition. It is impor-tant to minimize crack sensitivity during

welding, because postweld tempering isnot able to equalize the difference in hard-ness in parts of the joint.

It is suggested that there is no such tem-pering temperature optimal for the ele-ments of a given heterogeneous joint. Due

to the concentration differences, strongdiffusional movement takes place betweenthe weld and heat-affected zone. This dif-fusional process accelerates when the ser-

vice temperature of new power stationshas been increased. Both ferrite and car-bide-rich zones are undesirable, becauseof their high-level crack sensitivity. On thebasis of detailed metallurgical investiga-tions, attention is directed to requirementsthat can be prescribed for the welds andheat-affected zones of these joints andmethods to prevent the in-service crackingof heterogeneous welded joints.

References

1. Boese, V., Werner, D., and Wirtz, F. 1984.

Das Verhalten der Sthle beim Schweissen (Be-havior of Steels during Welding). Dsseldorf:DVS Verlag, 265.

2. Mannesmann Rhrenwerke. 1989.High-Temperature Steels, Austenitic High-TemperatureSteels and Ni-Fe-Cr Alloys. Data Sheets, Dssel-dorf.

3. Bres, L., Irmer, W., and Balogh, A. 1997.

Inconsistency of classification of creep resistant

steels in European standard EN 288-3. Science

and Technology of Welding and Joining. 2(5):236238.4. Irving, B. 1999. International Standard-

ization: A wake-up call for American industry.

Welding Journal 78(9): 3537.5. Bres, L., Balogh, A., and Irmer, W. 2001.

Welding of martensitic creep-resistant steels.

Welding Journal 80(8): 191-s to 195-s.6. Bres, L., and Balogh, A. 2001. A 912%

Cr tartalm melegszilrd aclok hegesztsi

repedsrzkenysge s elhrtsnak

lehetsge (Crack sensitivity of a welded joint

of 9 to 12% Cr creep-resistant steel and its pre-

vention). Hegesztstechnika (Budapest) 12(2):1725 (in Hungarian).

7. Kalwa, G., and Schnabel, E. 1999. Heat

treatment and microstructure transformation of

the martensitic steels with chromium content of

9 to 12%.Proceeding of Materials and Their Weld-ing. Dsseldorf, No. 1089, pp. 18. Duisburg,Germany: Mannesmann Forschungsinstitut.

8. Haarmann, K., and Vaillant, J. C. 1999.

The T91-P91 Book. Houston, Tex.: Valloures-Mannesmann Tubes Corp.

9. Pohle, C. 1994.Eigenschaften Geschweis-ster Mischverbindungen Zwischen Sthlen undChrom-Nickel Sthlen (Properties of Heteroge-neous Welded Joint between Steels and CrNiSteels). Dsseldorf, Germany: DVS Verlag.

10. Bres, L. 1998. Proposed modification to

Schaeffler diagram for chrome equivalent and

carbon for more accurate prediction of martensite

content. Welding Journal 77(7): 273-s to 276-s.11. Parikov, L. N., and Isaicsev, B. I. 1987.

Diffuzija v Metallah i Szplavah (Diffusion in Met-als and Alloys). Kiev: Nauka Dumka.

12. Bres, L. 1991. Ausbildung der kar-

bidreichen Schicht in Schwarz-Weiss-Ver-

bindungen. (Developing of carbide-rich layer in

black-white joints). Schweisstechnikf(Wien)45(4): 6264.

13. Folkhard, E. 1988. Welding Metallurgy ofStainless Steels. New York, N.Y.: Springer-Ver-lag, Wien.

14. Pohle, C. 1999. Schweissen der Werkstof-fkombinationen (Welding of Material Combina-tions). Dsseldorf, Germany: DVS Verlag.

15. Bres, L., and Irmer, W. 1995. Bestim-

mung der zulssigen Betriebstemperatur von

Schwarz-Weiss-Verbindungen (Determination

of the allowable service temperature of ferrite-

austenitic welded joints). Schweissen undSchneiden 47(10): 807812.

16. Missori, S., and Koerber, C. 1998. Pro-cedure development for improved quality sin-

gle and dual LBW of dissimilar metals. WeldingJournal 77(6): 232-s to 238-s.

17. Omar, A. A. 1998. Effects of welding pa-

rameters on hard zone formation at dissimilar

metal welds. Welding Journal 77(2): 86-s to 93-s.18. Nelson, T. W., Lippold, J. D., and Mills,

M. J. 1999. Nature and evolution of the fusion

boundary in ferrite-austenitic dissimilar weld

metals. Part 1: Nucleation and growth. WeldingJournal 78(10): 329-s to 337-s.

19. Nelson, T. W., Lippold, J. D., and Mills,

M. J. 2000. Nature and evolution of the fusion

boundary in ferrite-austenitic dissimilar weldmetals. Part 2: On-cooling transformations.

Welding Journal 79(10): 267-s to 277-s.20. Kirk, C. S., and Mileham, A. R. 2001.

EPSRC Report, Modelling and Simulation of Dif-fusion in Heterogeneous Welded Joints Using theFinite Volume Method.Archives, Department ofMechanical Engineering, University of Bath,

Bath, and EPSRC, Swindon, U.K.

21. Huang, M. L., and Wang, L. 1998. Car-

bon migration in 5Cr-0.5Mo/21Cr-12Ni dissim-

ilar metal welds. Metallurgical and MaterialsTransactions A 29A. 12: 30373046.

22. Race, J. M., and Bhadeshia, H. K. D. H.

1992. Precipitation sequences during carburiza-

tion of Cr-Mo steel.Materials Science and Tech-nology 8(10): 875882.

Change of Address?

Moving?

Make sure delivery of your WeldingJournal is not interrupted. Contact theMembership Department with yournew address information (800) 443-9353, ext. 480;[email protected].

aws 03nov behavior of welded joints of creep-resistant steels at service temperature beres

Documents