ISIJ International, Vol. 41 (2001 ), Supplement, pp. S126-SI30

Degradation of CreepStrength in WeldedJoint ofgoloCr Steel

MasakazuMATSUI, Masaaki TABUCHl,Takashi WATANABE,Kiyoshi KUBO,Junichi KINUGAWAand Fujio ABEFrontier ResearchCenter for Structural Materiafs, National Research Institute for Metals,1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan

The degradation of creep strength in the heat affected zone (HAZ) of welded joint has beeninvestigated for a tungsten-strengthened 9Cr steel, 9Cr-0_5M0-1.8W-VNb.The creep test wascarried outfor the simulated HAZspecimens and the welded joint at 923 K. The creep rupture strength of thewelded joint is almost the sameas that of the base metal at high stresses but it decreases rapidly andthen it becomesafmost the sameas that of the Ac3 simulated HAZspecimen at low stresses. Thecreepfracture of the welded joint occurs at the fine-grained zone of HAZ, corresponding to the Ac3 heating, at

low stresses. The fine-grained zone of HAZcontains higher density of dislocations than the base metal.The recovery of higher density of dislocations and the sparse distribution of large M23C6carbidespromote the formation of coarse subgrains near prior austenite grain boundaries. This results in theconcentration of creep deformation in the coarse subgrains, which accelerates eventual creep fracture.

KEYWORDS:creep rupture strength, 9'/.Cr ferritic steel, welded joint, heat affected zone

l. Introduction

In consideration of reduction of C02and energy savings,

several types of advanced 9-12010Cr ferritic steels

strengthened by Whave been developed in order to

improve the thermal efficiency of power plants. 9Cr-

0.5M0-1.8W-VNb(NF616)and 11Cr-O.4M0-2W-CuVNb(HCM12A)steels are being nowperformed for application

to boiler componentsof ultra-supercritical (USC) powerplants operating at 898 K.1)

Since 1997, National Research Institute for Metals

(NRIM) has been conducting a research and developmentproject on advanced ferritic steels for USCboilers at 923K.2) The resistance to creep cracking in the heat affected

zone (HAZ) of welded joints is one of the serious problemsfor the 9 -

12010Cr ferritic steels for construction of thick

section boiler components.3~5) It is reported for 9Cr-

IMoVNbsteel that the creep rupture strength of weldedjoint is lower than that of base metal and that the difference

in creep rupture strength becomesmore significant withincreasing temperature, especially above873 K.6)

At present, the mechanismsresponsible for a drop of

creep rupture strength in the welded joint are not fully

understood. The purpose of the pTesent research is to

investigate the relationship between the microstructure of

HAZand creep deformation behavior and to clarify the

mechanisms responsible for a drop of creep rupturestrength in the welded joint for a tungsten-strengthened

9Cr steel, 9Cr-0.5M0-1.8W-VNb.The creep tests were carried out at 923 Kfor up to about

10000 h for simulated HAZspecimens and welded joint.

Although the HAZof real welded joint is limited to only

a small volume, uniform microstructure can be obtained

throughout creep specimens for the simulated HAZmaterials. The results on 9Cr-0.5M0-1.8W-VNb are

comparedwith those on a 12Cr steel, 11Cr-0.4M0-2W-CuVNb.7)

2. Experimental Procedure

The chemical composition of 9Cr-0.5M0-1.8W-VNbexamined was 0.09C, 0.16Si, 0.47Mn, 8.72Cr, 1.87W,0.45Mo, 0.21V, 0.06Nb, 0.002B and 0.050 (mass%)N. Aplate of 31 mmthickness wassubjected to normalizing at

1343Kfor 7.2ks and tempering at 1053Kfor 7.2ks. Figure

1shows the heat treatment conditions for the preparation

of the simulated HAZsamples. At a heating rate of 0.33

K/s, the Acl and Ac3 temperatures were found to be about1123 and 1193 K, respectively. Eachsample waskept for

180 s at the various peak temperatures between 1123 and1273 Kand then cooled in air. Finally, the post weld heat

treatment (PWHT)was carried out for each sampleincluding the base metal at 1013Kfor 16.8ks. In this paper,the specimens subjected to heating to the peaktemperatures followed by PWHTare called as the

simulated HAZspecimens. Thewelded joint wasprepared

by meansof GasTungsten Arc (GTA) welding using the

filler metal s) with matching composition of 11Cr-0.4Mo-2W-CuVNbsteel. The procedure conditions of multi-

layers GTAwelding are given in Table 1. The weldedjoint was also subjected to the PWHTsimilar to the

simulated HAZspecimens.Creep test was carried out at 923 K for up to 10000 h,

using specimensof 6mmgaugediameter and 30 mm

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ISIJ International, Vol. 41 (20al ), Supplement

Peaktemperature;

~ 1073- 1473K

pWHT~ O~3~Ll s

~ 1013KX16.8ks~~,

~ Creep test~~, --~ at 923KH

l*~,:~~~~eo.

."~~ ~; e Test specimen~ 6mmc,30mmG.L.

TimeFig.1 Conditions of heat treatment for simulated HAZspecimens.

Tabel l. Procedure of welded joint.

groove single bevel 20pre-heating >373Kwelding current 200-250Aarc voltage 10- 10.5Vwelding speed I . 2- I .5mm/smulti-la er 33~37 ass

PWHT1013Kfor 15.6ks

~

10

~Fig.2 Creep test specimensof welded joint.

gauge length for the simulated HAZspecimens and of 18

X5mmcross section and 100 mmgauge length for the

welded joint as shownin Fig.2. The longitudinal direction

of the creep specimenswasparallel to the rolling direction

of the plate for the simulated HAZspecimens andperpendicular to the welding direction for the welded joint.

The microstructure of the longitudinal cross section of the

creep specimens was observed by scanning andtransmission electron microscopes. The Vickers hardness

was measured on the longitudinal cross section of the

welded joint specimens at a load of 4.9 N at roomtemperature.

3. Results and discussion

3.1 Creepstrength and microstructure of simulated

HAZspecimensFigure 3 shows the creep rupture time and the Vickers

hardness before creep test for the simulated HAZspecimens as a function of peak temperature in the

simulated HAZheat treatment, where the observed Acl(1123 K) and Ac3 (1193 K) temperatures are also

indicated. The creep rupture time has its minimumafter

heating the specimens to the temperature near Ac3, at

which the hardness before creep test also has its minimum.Hasegawaand co-workers 9) also reported the similar

result that the hardness deteriorated markedly at just abovethe A3 transformalion temperature in the hardnessdistribution at the welded joint of NF616. But, this is

different from the result on 11Cr-0.4M0-2W-CuVNbsteel

that the hardness has its minimumafter heating to the Acltemperature.7) In Fig.3, there is not large difference in

hardness betweenthe Acl and Ac3simulated HAZ

,::

:,

c~::

o

sF:

1cooo

1ooo

Fig.3

,:

~)cs

c:~

(J

1oo

10

i

[~1 91,*3K, 130MPa

O923K, 110MPa

AVickers hardnessbefoTe creep

iAcl

l

j

l

9Cr-0.5Mo-1,8W-VNb

Ac3

l

IIl

260

250

~;240 ee

C~

::

230 ~~~:,

220 c~$

~:

210

2001200 1300Base Imo

PeakTemperature(K)

Changesof creep rupture time and hardness in HAZas a function of peak temperature.

10-l

O. Ol o. 1 1 ro Iooo 10000moTime(h)

Fig.4 Creepcurves of the simulated HAZspecimens.

before creep after creep

O-

9Cr-O. 5Mo-I .

8W-VNb

10-2923K,I lOMPa

~ ,10-3 ~oo ~

o ~~10-4 O A1't,

o o10-5 OBasemetal

l~~:O~OO~,OI

~ Acl simulated HAZ• Ac3simulated HAZ

10-61

Basemetal

Ac1

Ac3

stress

Fig.5 SEMmicrostructures of simulated HAZspecimensof 9Cr-

0.5M0-1.8W-VNbsteel before and after creep at 923K,

130MPa.

specimens.Figure 4showsthe creep curves, at 923Kand 110MPa,of

the simulated HAZspecimens of 9Cr-0.5M0-1.8W-VNbsteel. It should be noted in Fig.4 that the transient creepregion, where the creep rate decreases with time, continues

S127 o 2001 ISI J

ISIJ International, Vof. 41 (2001 ), Supplement

Fig.6 Transmission electron micrographs ofextraction replicas of simulated HAZspecimensof 9Cr-0.5Mo-I .8W-VNbbefore creep.(a)Base metal (b) Acl simulated HAZ(c)Ac3 simulated HAZ

20~~

for up to longer times in the Acl simulated HAZspecimenand the base metal than in the Ac3 simulated HAZspecimen. This results in the lower minimumcreep rate

and hence the longer rupture time in the Acl simulated

HAZspecimen and the base metal than in the Ac3simulated HAZspecimen. The longer duration of transient

creep region would result from the stabilization ofmicrostructure for up to longer times.

Figure 5shows the SEMmicrographs of the base metaland the Acl and Ac3 simulated HAZspecimensbefore andafter creep at 923Kand 130 MPa.The precipitates in the

three specimensbefore creep test were identified as M23C6carbides and MXcarbonitrides. The amountof extractedresidue was approximately the same among the three

specimens; 1.75, 1.55 and 1.65 mass%for the base metaland the Aa and Ac3 simulated HAZ specimens,respectively.

In Fig.5, the prior austenite grain size before creep test is

about 30 /L mfor both the base metal and the Acl simulated

HAZspecimen, while it is only about 7,Lm for the Ac3simulated HAZ specimen. The grain size becamedecreased with increasing peak temperature in the

temperature range between the Acl and Ac3 temperatures.The microstructure is observed to be lath martensitic for

both the base metal and the Acl simulated HAZspecimen,where the M23C6carbides are distributed mainly along lath

boundaries and prior austenite grain boundaries. In the

Ac3 simulated HAZspecimen, the SEMmicrograph gives

no strong evidence of the distribution of thin plates of lath

within grain and the M23C6carbides are distributed

mainly along grain boundaries,which was also typical for

11Cr-0.4M0-2W-CuVNbsteel.7) TheSEMmicrostructureof the three specimens scarcely changes during creep,because of short test duration. The time to rupture was541.4, 156.7 and 27.4 hfor the base metal and the Acl andAc3simulated HAZspecimens, respectively.

Figure 6 shows the TEMmicrographs of extractedreplicas of the three specimens before creep test. TheM23C6carbides are muchlarger in the Ac3simulated HAZspecimenthan that in the base metal and the Acl simulated

HAZspecimen, while the size~ of MXcarbonitrides are

very fine of 10 to several tens nmand are approximatelythe sameamongthe three specimens. The larger size of

M23C6carbides and the disappearance of thin plates of lath

within grain in the Ac3 simulated HAZspecimen areresponsible for the lower hardness and lower creepstrength (Fig.3) than the base metal and the Acl simulated

HAZspecimen.Thehardness of the Acl simulated HAZspecimenbefore

creep is lower than that of the base metal (Fig.3), althoughthe both specimens have similar lath microstructure(Fig.5). Because the Acl temperature (1123 K) is muchhigher than the tempering temperature (1053 K) of the basemetal, the Acl simulated HAZspecimenwas subjected to

tempering at higher temperature than the base metal. Thiswould result in a lower hardness in the Acl simulated HAZspecimen than that in the base metal. The stabilization oflath by the array of M23C6carbides along lath boundariesis considered to cause the longer duration of the transient

creep region in the base metal and the Acl simulated HAZspecimen than in the Ac3simulated HAZspecimen (Fig.4).

3.2 Creepstrength and microstructure of weldedjointsFigure 7 shows the creep rupture data for the welded

joint, the base metal (plate), the Acl and Ac3 simulated

HAZspecimens at 923 K, where the data for a different

heat of base metal (pipe) Io)are also shownfor comparison.

The chemical composition of the pipe heat of 9Cr-0.5Mo-1.8W-VNb, ASME-P92,was O.11C, 0.10Si, 0.45Mn,8.82Cr, 1.87W, 0.42Mo, 0.19V, 0.06Nb, 0.002B and 0.047(mass%)N. The normalizing and tempering conditions

were the samebetween the pipe and plate heats. The creeprupture strength of the welded joint is almost the sameasthat of the base metal at high stress and shorttimeconditions but it decreases rapidly and then it becomesalmost the same as that of the Ac3 simulated HAZspecimenat low stress and long time conditions.

C~$

(~~:

(1)

1O IOO IOOO IOOOOTime to rupture (h)

Fig.7 Creep rupture time of base metal,welded joints and simu-lated HAZspecimensof 9Cr-O.5M0-1.8W-VNb.

1509Cr-O.5 Mo-I.8W-VNb

923K

100

90

80 O Basemetal (Pipe. P92)

70 A Basemetal (Plate)

A Weldedjoint60 [] Acl Simulated HAZ

l Ac3Simulated HAZ50

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ISIJ International, Vol 41 (2001 ), Supplement

(a)

(b)

Weldmctal

~~::_:tBase metalBasemeta[

20mm

HAZ

Fig.8 Theappearanceof creep ruptured specimensof weldedjoints of 9Cr-O.5M0-1.8W-VNbsteel.

(a) ruptured in base metal; 110MPa,923K, tr =1099,5h, El.=7.49a/o

(b) ruptured in HAZ; 60MPa,923K, tr =9194.0h, El.=0.67a/o

280

260ZCh~: 240:!:

~ 220q,F:

1:,

* 200cs

~:

180

160

before creep test

\ Fine grained zone~-lA*3

Ac]

lafter creep test;

923K, 60MPa.tr=9 194h

weld metal ~J~_1 Basemetal

-3 -2 O-1 321Distance from fusion line(mm)

4

Fine-grained HAZ BasemetalTemperedzone

Fig.9 Distribution of Vickers hardness and scanning electronmicrographs on creep ruptured specimenof 9Cr-0.5Mo-1.8W-VNbwelded joint.

The similar behavior was also observed for 11Cr-0.4Mo-2W-CuVNbsteel.7) The creep rupture strength of the

Aclsimulated HAZspecimen seemsto coincide with that

of the sameheat of base metal (plate) at long times. Thewelded joint is fractured in the portion of base metal at ahigh stress of 110 MPa,while the fracture portion is in the

HAZwith a bevel angle of 20 degree but near the interface

between the base metal and HAZat low stresses below 90MPaas shownin Fig.8, indicating the type IV fracture at

low stresses. The total elongation during creep was aslarge as about 7%in the base metal but it was lower than

1%in the HAZat 60 MPa.The distribution of hardness across the HAZand base

metal in the welded joint and the microstructure after creep

were examined using a part, containing the HAZwith abevel angle of Odegree, of the pair of creep rupturedspecimen. Becausethe creep fracture did not occur in the

HAZof this part even at low stresses, the change in

hardness and microstructure across the HAZcan beevaluated for this part. The results are shownin Fig.9 for

the specimenafter creep ruptured at 923 Kand 60 MPafor

Fig.10 Transmission electron micrographs of thin foils at basemetal and fine-grained HAZof 9Cr-0.5M0-1.8W-VNbweldedjoint.

(a) Basemetal before creep(b) Basemetal crept, 923K60MPa,9194h(c) Fine-grained HAZbefore creep(d) Fine-grained HAZcrept, 923K60MPa,9194h

9194 h. The softening occurs throughout the welded joint

during creep but the distribution of hardness is similar

between the specimens before and after creep. Thehardness minimumis located at the outside of the Aclheating, corresponding to a low temperature below the

Acl, but not at the portion of the Ac3 heating. This is

different from the result on the simulated HAZspecimensshownin Fig.3 where the hardness has its minimumnearthe Ac3 heating. The shift of the hardness minimumto aportion of lower heating temperature maybe caused by aresidual strain in the welded joint during cooling after

welding, although detailed mechanismsare not clear at

present. The hardness of 11Cr-0.4M0-2W-CuVNbsteel

showednearly the samevalue for both the simulated HAZspecimens and the welded joint, suggesting a negligiblysmall effect of residual strain in the welded joint.7) In Fig.9,

the portion heated to the Ac3 temperature in the weldedjoint consists of fine grains, which is the sameas the Ac3simulated HAZspecimen. A Iarge numberof creep voids

are observed to have formed mainly at prior austenite grainboundaries in the fine-grained zone, corresponding to the

Ac3 heating. In the base metal, no creep void is observedand the lath martensitic microstructure is stable for up to

long times.

Figure 10 shows the TEMmicrographs of the fine-

grained zone and base metal before and after creep at 923K and 60MPafor 9194 h. In the base metal, the recoveryof martensitic microstructure such as the recovery of

excess dislocations, the agglomeration of carbides andcoarsening of lath and the precipitation of Fe2WLavesphaseoccur during creep, but the lath microstructure

S129 C 2001 ISIJ

ISIJ International, Vol 41 (2001 ), Supplement

where the precipitates are distributed along lath

boundaries, is still maintained. The fine-grained zoneconsists of fine subgrains having a size of several hundreds

nmand contains a higher density of dislocations than the

base metal befoTe creep. After creep, the coarse subgrainsof several ~m, where the recovery of microstructure hasalready occurred, are produced in the region of fine

subgrains. The recovery of higher density of dislocations

to grain boundaries in the fine-grained zone promotes the

production of coarse subgrains in the vicinity of grain

boundaries. The sparse distribution of large M23C6carbides in the fine-grained zone can also promote the

recovery of microstructure. The inhomogeneousrecoveryof microstructure producing the coarse subgrainsaccelerates the production of weakregion. This results in

the concentration of creep deformation in the fine-grained

zone and hence the formation of a large numberof creepvoids, accelerating eventual creep fracture. Thedegradation mechanismsof creep strength in the HAZof

welded joint are considered to be similar for both 9Cr-O.5M0-1.8W-VNband 11Cr-0.4M0-2W-CuVNb.

In the simulated HAZspecimens shown in Fig.4, the

shorter duration of the transient creep region and the

shorter time to rupture for the Ac3HAZspecimen than the

Acl HAZspecimen and base metal also results from the

preferential recovery of higher density of dislocations andthe sparse distribution of large M23C6carbides, similar asthe welded joint described above. On the other hand, the

microstructure observations gave no strong evidence of the

inhomogeneous recovery of microstructure producingpreferentially recovered region in the Acl simulated HAZspecimen and the base metal where the fine M23C6carbides are distributed along lath boundaries and stabilize

the lath microstructure for up to long times.

4.Conclusion

The creep test was carried out for the simulated HAZspecimens and the welded joint of 9Cr-0.5M0-1.8W-VNbat 923 Kfor up to about 10000h.

(1) The creep rupture time of the simulated HAZspecimenshas its minimumafter Ac3 heating at which the

specimens exhibit the minimum hardness and the

minimumprior austenite grain size. The transient creepregion continues for up to longer times in the Aclsimulated HAZspecimen and the base metal than in the

Ac3 simulated HAZspecimen. This results in the lowerminimumcreep rate and hence the longer rupture time in

the Acl simulated HAZspecimenand the base metal thanin the Ac3simulated HAZspecimen.(2) Thecreep rupture strength of the welded joint is almostthe sameas that of the base metal at high stress and short

time conditions but it decreases rapidly and then it

becomesalmost the same as that of the Ac3 simulated

HAZspecimenat low stress and long time conditions. Thewelded joint is fractured in the HAZbut neaT the interface

between the base metal and HAZat low stresses below 90MPa,indicating the type IV fracture at low stresses.

(3) In the fine-grained zone, corresponding to the Ac3

heating, in the HAZof the welded joint, the recovery of

microstructure during creep is inhomogeneousas shownbythe formation of coarse subgrains in the region of fine

subgrains. The recovery of higher density of dislocations

and the sparse distribution of large M23C6carbides

promote the formation of coarse subgrains. Theinhomogeneousrecovery of microstructure forming the

coarse subgrains results in the concentration of creepdeformation in the fine-grained zone and hence accelerates

eventual creep fracture. In the portion of HAZheated to

Acl and the base metal of the welded joint, the fine M23C6carbides are distributed along lath boundaries and stabilize

the lath microstructure for up to long times.

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