ISIJ International, Vol. 34 (1 994), No. 2, pp, 217-223

Sulfide Stress CQrrosion Cracking in Welded Joints of WeldedLine pi pes

Shigeru ENDO.Moriyasu NAGAE.YasuoKOBAYASHIand Kazuyoshi UME1)

FukuyamaResearch Department, Materials and Processing Research Center, NKKCorporation, Kokan-cho. Fukuyama,Hiroshima-ken, 721 Japan. 1)FukuyamaWorks, NKKCorporation. Kokan-cho. Fukuyama,Hiroshima-ken, 721 Japan.

(Received on ;lugust 18. l993; accepted in final form on October 22. l993)

The sulfide stress cracking (SSC) behavior of welded linepipes has been studied using someSSCtests,

such as tensile SSCtest (NACE-TM0177-90 MethodA), full thickness SSCtests, four points bendingSSCtests and full scale SSCtests. Detailed metallographic examinations have been performed in order toundeTstand the influences of steel chemistries and heat inputs in welding on the SSCresistance of the heataffected zone. The effects of the geometry of the SSCspecimen and hydrogen concentration on the SSChave also been investigated. The resu[ts have indicated that homogenizedbainitic ferrite microstructure,

which can be attained by reducing carbon content and by applying accelerated cooling after controlledrolling in plate rolling process, improves the resistance of steel plates to the SSC.Meanwhile, mostspecimensfrom submergedarc welding welded joints fai] at the heat affected zone, regardless of differences in

the rr'icrostructures of base materials, and they tend to shownearly the samelevel of the ratio of thresholdstress to yield strength in the tensile SSCtests. Specimengeometry and the quantity of hydrogen con-centration in stee[ affect on the threshold stress, and hence reducing the hydrogen concentration improvesthe SSCresistance of the welded joints especially in lower hardness heat affected zone. Relations betweenthe resu[ts of the laboratory tests and full scale tests are also discussed.

KEYWORDS:sulfide stress cracking; hydrogen induced cracking; welds; heat affected zone, Iinepipes;

bainitic ferrite; ferrite-pearlite.

1. Introduction

Sulfide stress cracking (SSC) resistance as well ashydrogen-induced cracking (HIC) resistance should beconsidered important to linepipes for sour oillgas service

because the pipes are highly stressed by internal pres-sure. In fact, a numberof reports have indicated that

somefailures in sour gas pipelines could be caused bydevelopment of SSCnear welds.1~3) The hardnessdependency of SSCsusceptibility could not explaincracking in such low or mediumstrength steels of weldedlinepipes. Since those failures were reported, metallo-graphic investigation of welds has indicated that mi-crostructural changes, including secondary precipita-

tions during welding as well as hardness and residual

stress distributions, have a significant influence of the

SSCresistance of the welds.4~8)

In this paper, SSCbehavior in welded linepipes is

metallographically investigated focussing on the relation-

ship between microstructures and SSCresistance of theweld. Full thickness specimens with reinforcements

as well as NACETMO177-90 MethodAspecimenswereused to determine the effects of geometry and hydrogenconcentration of the specimenson the SSC.Using results

obtained from a variety of laboratory tests and full scale

SSCtests, a correlation betweenthe small scale SSCtest

results and the SSCoccurring in sour gas pipelines is

also discussed.

2. SSCResistance Obtained by Tensile SSCTest (NACETM0177-90 MethodA)

2.1. Experimental Procedures

Tensile SSCtests set forth in NACETM0177-90MethodAwere carried out with the base material andwelded joints of welded linepipes. A11 test specimens for

the welds were sampled such that their axes wereparallel to the circumferential direction and perpendicu-lar to the straight seamwelds. The test specimens for

welded joints were cut from internal welds of thelinepipes. The test material used were API 5L X42-X70steel plates and welded linepipes with various micro-structures. These steel plates were produced by thecontrolled rolling (CR) and accelerated cooling (AcC)following the CR, The microstructure of the plate

manufactured by CRprocess consisted of ferrite andpearlite, and that of additional AcCresulted in fine grainbainitic ferrite. Thechemistry range of all materials testedis shownin Table 1.

2.2. Test Results

In Fig. l, the SSCtest results of Gr. X42to X70weldedlinepipes are shown. The SSCresistance of both weld-ed joints and base materials are shown in the figure.

21 7 C 1994 ISIJ

ISIJ International, Vol.

The vertical axis of the figure represents the ratioof threshold stress to yield strength (crth/(Ty). The ratiosfluctuated in the range from 0.6 to O.9 for base materialsand no strength dependencywas seen in the steels ofthese grade. Fromthe SSCresistance of the basematerialswith yield strength of about 500N/mm2,it is clear thatthe base material specimens with bainitic ferrite micro-structure (BF-) which can be obtained by the AcCproc-ess, tends to showbetter SSCresistance than that withferrite and pearlite microstructure (F-P-). The ratios

Table 1. Chemical composition of the tested steels. ('/.)

Grade t (mm) C Si Mn P S Others

34 (1 994).

X42

X70

16

25

0.02

0.17

0.19 0.89 0.004 0.001

0.30 1,9 1 O.021 0.004

Cu, Ni, Cr, Nb

V, Ti, B, Ca

No. 2

of cr,,,/ay of the base materials with bainitic ferrite

microstructure lay between 0.8 and 0.9 and those withferrite/pearlite microstructure are 0.5 to 0.7. On thecontrary, the a,hl(Ty ratios of the weldedjoints lay between0.5 and 0.9 regardless of strength and microstructures ofbase materials. TheSSCin the welded joints wasalwaysobserved in the intercritical HAZthat was reheated too( +y temperature by welding thermal cycle.

This indicates that fine and homogenized bainiticferrite microstructures free from pearlite lead to a higherresistance of the base material to the SSC,but the SSCresistance of the welds seemsto be influenced by othermetallurgical factors rather than the microstructure ofthe base materials.

J:a,

O,,,

11(D

>,n,,,

(D

,O

~O,:,,,

C,

=h

0~

~o

1.o

0.9

0.8

0.7

o.6

o.5

0.4

O

D

F-P•Bas

F*P*Weld

BF•Base

BF-Weld

oe oOpen;BasemetalSolid; Weldedjoint

D

EIIDalle

elo(l

DE]

l

Fig. 1.

300 4OO 700500 600

Yield strength,YSINlmm2

SSCproperties of base material and welded joints oflinepipe steels,

3. The Governing Factors of the SSCin WeldedJoints

3. 1. Experimental ProcedureIn order to makeclear metallurgical factors influencing

the SSCof the welded joints of linepipes, full thicknessspecimens of the 38mmwide and 50mmlon~ gaugelengths with welded joints of four linepipe steels werealso used for SSCtests. Their chemistries are shownin

Table 2. Steel A is' API Gr. X52and its microstructureconsists offerrite andpearlite. TheSteels B, CandDareAPI Gr. X65, X70andX80, respectively. Microstructureof the Steels B, Cand Dis composedof fine grain bain-itic ferrite, therefore, the ratios of athl(Ty of their basematerials are 0.8. Simulation straight seamSAWwelding

wascarried out on these materials in the laboratory. Theratios of athl(Ty of their welded joints in NACETM0177-90 MethodA are also shown in the Table. Fullthickness with weld reinforcement SSCtest as well as

Table 2. NACE-SSCtest results.

SteelMicro-

structureGrade C

(mm) ("/')

Si

("/.)Mn(o/')

P(ppm)

S(ppm)

Othersathla

yay* Base'Tthl(ry

(N/mm2)metal SAW

ABCD

F-PF-BFBFBF

X52X65X70x80

19

2519

19

0.09

o,040.020,05

O.29

0.28

0.25O. 19

0.85

l .55

l.91

l .90

7040210110

lO

lO

lO

lO

Nb, V, CaCu, Ni, Nb, V, Ca

Nb, B, CaMo, Nb, B, Ca

404

502566612

o.60 0.600.80 0.60o.80 0.55

Fig. 2.

Typical SSCin the full thickness SSCtest.

C 1994 ISIJ 218

ISIJ International, Vol.

NACEstandard tensile SSCtests were performed toinvestigate geometrical effects of the specimen on the

SSCoccurrence.Different microstructures in the HAZof Steel Awere

simulated by using the Gleeble Tester and examinedfor

the SSCresistance by the NACEstandard tensile SSCtest. In this test 70 "/, of the yield strength of the original

basematerial and 80 '/* of each simulated HAZspecimen

were employedas applied stresses for the SSCtest.

3.2. Observation of SSCin Full Thickness SSCTest

Typical SSCobserved in the full thickness SSCtests

are indicated in Fig. 2. SSCmorphologyof Steels AandC in their welded joints are shown in the figure. Thehardness of weld metal and HAZare also shownin thefigure. SSCwasobserved to propagate in the intercritical

HAZ(reheated to c( +y temperature) of Steel A, but nocrack developed near the fusion line that indicated the

maximumhardness and stress concentration in the HAZ.As shown in the Fig. 3, a numberof planner-orientedfine cracks were also found in the intercritical HAZ.Supercritical tensile stress is considered responsible for

the generation of these cracks because the weld showedno cracks in the sameregion during a test without stress.

Similar SSCmorphology to Steel A was observed in

the full thickness SSCtest of Steel B. In the SSCtest ofwelded joints of Steel C, SSCpropagated along withfusion line and intergranular fracture surface wasobserved by SEM.Similar cracking were observed in

HIC test and occurrence of these cracks were caused bydecreasing in the strength of the grain boundary ascompared to that of the matrix of the fusion line

associated with reduced carbon content in Nbbearingsteels.8) The weld of Steel D, in which the maximumhardness wasmore than Hv260, developed SSCacross

34 (1 994), No. 2

the weld metal and the HAZshowing the maximumhardness.

Therefore, the configurations of the cracks observedin HAZwere classified into three different morphologiesdepending on steel chemistry and HAZhardness.

3.3. SSCTest Results of Simulated HAZIn Fig. 4a, 70 o/o of the yield strength of the original

base material, was loaded to each specimen, and SSCresistance wascomparedin terms of time-to-failure. InFig. 4b, the yield strengths of the specimens heatedin different thermal cycles were determined, and eachspecimenwas loaded to 800/0 of its yield strength in the

SSCtest. Both results showed that heating at aboutl 100K, which wasequivalent to the peak temperature ofwelding thermal cycle for the intercritical HAZ,resultedin the highest susceptibility to SSC. It is also seen in

these figures that the highest SSCsusceptibility ofintercritical HAZis not only because of the loweststrength (hardness) but also because of other factors,

such as its microstructural characteristics. The thermalcycles of welding decreased the yield strength of theintercritical HAZby about 100/0 of the original yield

strength and this is significant because the (Tthl(Ty ratios

for the welds are usuall smaller by about 100/0 of theyield strength than those of the base materials withferrite/pearlite microstructures. The softened regionundergoesmacroscopically no localized plastic deforma-tion in a tensile test because of mechanical restraint bythe surrounding portion. Microscopically, however, thesofter region can be plastically deformed to the larger

107

/~i':~l

a 0.7oy of base material

o,

F,S

E 106

o:,

o,5LL

105

50Ohrs

1OOhrs

co

ha'

Eo5o,cLL

107

Asrolled

b

8OO 100O 1200 1400 1600 1800

Reheattemperature, TrlK

0.8oy (Actual YS)

106

105

lod

e

}~~){

!nS~:Gr!i~~Lsemp~~~~~~:

YS

500

400

Fig. 3. AnembryoSSCoccurring in HAZof Steel Aweldedjoint.

21 9

300

E~~_

U)>ia,*d,

~5

1'~;

>As 800 100O 1200 1400 1600 1800rolled

Reheattemperature, TrlK

Fig. 4. The effect of thermal cycles in welding on SSCresistance.

C 1994 ISIJ

ISIJ Internationa], Vol. 34 (1994), No, 2

extent whenthe welds is stressed to near yielding, which stressed to either 80 or 85 o/. of spccific minimumyield

maylead to a high possibility of developing SSCin the strength (SMYS)by pressurizing with a test solution.intercritical HAZ. The test solution wasprepared in a tank and circulated

Figure 5 shows cracks and their surrounding mi- to keep the solution chemistry constant in the pipe. 5olo

crostructure in the intercritical HAZof Steel A. The NaCl+0.50/* acetic acid saturated with I atm H2Sgas,intercritical HAZcan be characterized by ahigh tendency which is set forth in the NACETMOl 77-90 Methodtowards formation of Martensite-Austenite constituent A, wasemployed.(MA). In the case ofSteel A, Gr. X52ferrite and pearlite

4.2. The Effect of Hydrogen Concentration on the athsteel, the MAconstituent wasrarely found, andcrackingappeared at retained pearlite rather than at the MA. of SSCTests

Increasing in the alloy content increases the tendency Figure 7showsstress vs. time-to-failure curves obtainedtowards formation of the MAin the intercritical HAZ.For this reason, cracking at the MAis more likely to

appear in the intercritical HAZof Gr. X65 or higherstrength steefs. The HAZof the Steel B showed aconsiderable amountof the MA.However, cracks here

were developing along pearlite rather than the MA.These observations suggest that the presence of the

MAis not always a primary factor for the SSCsuscep-tibility of the intercritical HAZ.Adecrease in the yieldstrength combined with the metallographical featuresdescribed above can be responsible for the rupture ofthe intercritical HAZin the SSCtests, with slightly

smaller a,hl(Ty ratio than those of base materials.

4. The Effects of Testing Condition on the SSC4.1. Experimental Procedure

In order to examine the effect of hydrogen concen-tration in steels on the a,,, of the SSCtests, four typesof SSCtest specimenswere employed. Twoor three sur-face coated full thickness SSCspecimens as well asbare full thickness SSCspecimenwere used for the SSCtest to vary entered hydrogen concentration. NACEstandard tensile SSCtest specimenswere also tested forthe sake of comparison. Gr. X52 Iinepipe wasused forthis test.

Furthermore, 800/*C02-200/.H2S gas as well as 100~/o Flg. 5. EarlystageofcrackinginintercriticaIHAZ(steelA).H2Sgas were used for the NACEstandard tensile SSCtest and four point bending SSCtest to examinethe effect

of hydrogen concentration on the SSCsusceptibility.

Schematic drawing of the four point bending SSCtest

specimen and apparatus are shown in Fig. 6. Threemillimeter thick rectangular specimenswere stressed by 'oo*~

30 40bending to either 50, 100 or 130 o/o of the yield strength L1L2

of the base material and immersed in the NACETMOl 77-90 MethodAtest solution for 720hr. Gr. X46and X65 Iine pipe steels whose chemistries andmechanical properties are summarizedin Table 3wereused for these tests. In order to vary the hardness, SAWwelding with 5kJ/mmheat input and MAGweldingwith 0.3 to I.5kJ/mm heat inputs were performed on st**ss l*~'1 =

s=12.D.E.t/(3L1'-4L2')

the steels. t=3~~In the full scale SSCtest, two meter long pipes were Fig• 6. 4point bending SSCtest.

Table 3. Chemical composition and mechanical properties of the tested steels.

Grade C Si Mn P S Cv Ni Nb Ca YS TS a./af El 50"/. FATT("/.) ("/.) (*/.) (ppm) (ppm) ('/.) ('/.) ('/.) (ppm) (N/mm') (N/mm') ('/.) ('/.) (K)

X46 0.09 0.3 0.8 70X65 0.04 O.3 1.3 70

1O 0.23 O, IOlO 0.26 O, 16

0.01

O.052325

415522

500587

83

884857

178168

C 1994 ISIJ 220

ISIJ International, Vol. 34 (1 994), No. 2

,,,

>

o

~s

sa

EE

1.21OOhrs 720hrs

1.o

0.8

0.6

o.4

e.o

~~~3~

104

ORoundbar

OFull thlckness,

sidesurface coated

OFull thlckness,slde surface

and outslde weld coated

e Full thickness, noncoated

Fig. 7.

Table

105 1071OG

Timeto failure, Tfls

Theeffect of H2Sconcentration on the SSCproperties

of welded joints.

4. The effect of H2S concentrationproperties :of welded joints.

on the SSC

(1,

>,5:;

OO'O

~~~,l~OO~

1.o

SAW1OOhrs 720hrs

0.9

0.8

0.7

0.6

oe

1no'/.*H 2S

80e/.CO2-20e/QH2S

0.5104

Max.Hv (98N)

crth

o t~

Q~

Steel

(1)

>

~C9

O

~~aOO~

1.o

MAG

105 1Oe

Timeto failure, Tls

100hrs 720hrs

107

lOOo/o H2S800/0C02-200/~H2S

~OO"/. H2S800/*C0220"/oH2S

0.9

X46(F-P)

Basemetal

SAW(5 kJ/mm)

l57

l92(21 l)

0.6ay0.8aJ,

0.6ay

O.8ay

X65(BF)

Basemetal

SAW(5 kJ/mm)

MAG(0.5 kJ/mm)

i93

200(226)

262(397)

0.8(Ty

0.9a),

0.6a.

O.8ay

0.6~y

0.7ay

1000/0 H2S80"/oC02200/0H2S

lOOo/o H2S800/0C02-20"/oH2S

lOO"/o H2S800/*C02-200/0H2S

0.8

0.7

0.6

0.5

oo

1OO'Ie-H2S

80'1eC02-2OeloH2S

IPec~

104

Fig. 8.

( ): Maxmaumhardness of weld metal

for the bare specimen, the coated specimensand NACEstandard tensile SSCspecimens. The ratios of a,h to cry

were similar in the NACETMO1 77-90 MethodAandthe full thickness SSCtest with the bare specimens. Larger(Tthl(Ty ratios were observed in the full thickness coatedspecimens as comparedwith the former two tests, andratio was between 0.8 and 0.9. The ratio was approxi-

mately 40 o/o larger than those of the bare specimens,

whenonly one surface wasexposed to the environment.The ratios of ath to ay of X46and X65welded joints

under 100o/o H2S and 800/0C02-200/0H2S saturated

conditions are shownin Table 4. Themaximumhardnessofthe HAZare also indicated in the table. Themaximumhardness of SAWwelded joints and MAGwelded joint

is approximately Hv200and Hv260, respectively. Therelationship between the applied stress and the time tofailure of the SAWand that of MAGwelded joints of

X65 are shown in Fig. 8. The SSCtest results underO. I MPaof 100o/o H2Sand 800/0C02-20"/oH2Ssaturated

conditions are shown in the figure. Increasing ratio ofcr,h/ay With reducing H2Sconcentration wasobserved in

all the tested welded joints. The incremental increase in

the ratio is larger in the low hardness HAZthan in the

high hardness HAZ, for the ratio of the SAWweldedjoint of both the X46and the X65increases from 0.6 to

0.8 or more, while the ratio became0.7 in the MAGwelded joint of the X65by reducing H2Sconcentration.

Thesesituations coincide well with the test results of full

thickness SSCtest with and without coated specimens.

TheSSCmorphologyof the SAWwelded joint of X46

105 107106

Time to failure, Tls

The SSCproperty of welded joints.

Fig. 9. The SSCmorphology of the SAWand MAGweldedjoint of X65.

and that of the MAGwelded joint of X65are shownin

Fig. 9. TheSSCpropagation behavior changesdepending

on hardness of the HAZ. SSCappeared at the fusionline, that showedapproximately Hv260, the HAZof

MAGwelded joint. Onthe contrary, SSCappeared andpropagated along the intercritical HAZin SAWweldedjoints that showedlower hardness.

Table 5 shows diffusible hydrogen content in the

NACEstandard tensile SSCtest specimens after 8.6 x

221 C 1994 ISIJ

Table 5.

ISIJ International, Vol.

Diffusible hydrogen content in NACEstandardtensile SSCspecimens.

Hydrogenconcentration. (ppm)8.6x 104* 1.8x lOs* 3.6x 105*

34 (1 994), No. 2

Solution Saturating gas

50/0NaCI~).50/0 1000/0 H2Sacetic acid 800/0C02-200/0H2S

* Immersion time, (s).

100'10 H2S

1.4

0.8

1.3

0.5

1.4

a:Do O:l e I1.2

1.o D:DO U:9cDD Io.8 D=

oF,c,rrtte, ~ljbea,! ti,itj'e, i+=

Bal hl'ti,c,:F,idFr,,ie : Sd,k,lif

::::,N,o,=jC,= 'raek+:::

==::C;rae,1'kj+ l, =:::

o.6

D:DO u!a~ o DD I0.4

1.O

0.4

C')

>

~O~~~CL~OO~a:

Maximumhardness : Hv285 (MAG)

Fig, 11. Typical SSCobserved in 4point bending SSCtest.

lco 2OO 25D 300

Hardness*Hvl98N

80'/• C02-20•/• H2S

350

1.4

a:Do O:! oao I I1.2

1.O D:OO U:l a:~D o I0.8

::Q;+:=Ferrl ft, e:/peer,1~te*~=:+:O.

.

'= =n=: :,:I

*=+e ::=B~i~Inii'~ic_'i~d=r,rjit,' ei==:1;,:::S,;Si,id,' :;:,;::1

o.6

o:Bo u: a:lD o I:! U:l

o.4

~=~

*a

E'5a

U,

r

1

co>

~

?;

~~~aE

150 2OO 25O eoO 350Hardness, Hvl98N

The e~ects of H.S concentration and applied stress

on the SSCof 4point bending SSCtest.

0.1

0.01

0,001

175

Fig. 10.

Fig. 12.

NoFallureFailure

OO O ee

O(D oo eOo

SAW,MAG

SAW,MAGMAG

l04 and 1.8 x 105 sec immersion in NACETM0177-90Method A solution. The hydrogen content of thespecimen immersedin 100o/o H2Ssaturated condition is

about double of that of 800/0C02-200/0H2Ssaturatedcondition. Hydrogencontent of middle thickness of pipewall is one half of exposed surface of actual pipes andfull thickness SSCspecimenwith three surface coated.9)This hydrogen concentration pattern seemsto be a rea-son of the coincidence of the test results of lower H2Sconcentration condition atnd those of full thickness SSCtest with the coated specimens.

Figure 10 shows the four point bending SSCtestsresults under 0.1 MPaof 1000/0 H2Sand 8001.C02-200/0H2Sgas saturated conditions. Typical SSCobservedin four point bending SSCtest is shownin Fig. 11. SSCappeared at the fusion line where the maximumhard-ness wasobserved and propagated into the base materi-al. NoSSCwas observed in the intercritical HAZ.Themaximumhardness and applied stress rather than steel

chemistry affect occurrence of the SSCand critical

hardness, above which SSCcan be observed, decreasedwith decreasing applied stress. Figure 12 showsthe effect

C 1994 ISIJ

ti;a'

O'O,,,

~,cl

~LE~E.~Z~~~~,pt)

~va'

aa.,

2OO 225 25O 275 30O 325Maxlmumhardness in HAZHvl98N

Theeffect of H2Sconcentration on the SSCof 4pointbending SSCtest.

500

4oO

30o

200

1.36cth I .27~th

.,~•6

•l

OOO D

D

1:1

iDj C.R:j

SSC:NbsSC

222

Fig.

200 3OO 400 500Thresho'd stress for NAc2Estandard tensile ssc

ath/N•mm13. Therelationship betweenNACE-SSCtest results and

full scale SSCtest results.

of the hardness of the HAZand H2Sconcentration onthe SSC. Applied stress at the bending surface wasequivalent to 1300/, of the yield strength of eachspecimen. SSCtest results4) in 900/,C02-lO"/,H2S sat-

urated condition are also plotted in the figure for thesakeof comparison. Thecritical hardness becomesharderwith reducing H2Sconcentration. The critical hardnessis Hv260and Hv280 for 1000/0 H2Sand 80010C02-

ISIJ International, Vol. 34 (1994), No. 2

oo

Tested Pipe: x 65 71 1OD.x 19,1 WT.(mm)Full Scale SSCTest Condition: NACEsol./Stress, 80*/.SMYS/Duration, 2months

1mm

Fig. 14. Examplesof SSCat welded jomt observed mfull scale SSCtest

200/0H2S respectively. Howeverthe difference betweenthe critical hardness obtained in 100 "/* H2S and80"/*C02-20"/oH2S is approximately Hv20and this val-

ue is not so large practically.

Fromthe results, the SSCis not likely to occur in the

10werhardness HAZin actual pipelines becauseof lowerhydrogen content. Possibility of occurrence of SSCin

high hardness HAZseems to be high even in actual

pipelines, becauseoccurrence of the SSCin high hardness

HAZis insensible to H2Sconcentrations. Therefore it is

important to control HAZhardness less than critical

value to prevent the SSCand to reduce the possibility

of the SSC.Furthermore, SSCtest under lower H2Sconcentration

condition seemsto be morerealistic to evaluate the SSCresistance of actual linepipes.

4.3. Correlation of Laboratory SSCTests with SmallSpecimeris and Full Scale SSCTests

Comparisonof the SSCthreshold stress ((Tth) in the

NACEstandard tensile SSCtest and SSCbehavior in

the full scale test for a wide range of welded linepipes is

shownin Fig. 13. All pipes were stressed to 80 or 85 "/o

of SMYSin the full scale tests. The test were continuedfor two months without any leakage due to cracking.

The solid marks meanSSCdeveloping in the full scale

tests, while the openmarksmeanno SSC.Onepipe for

ordinary use, which meanssusceptible to HIC, showedSSCin the HAZin association with HIC(Fig. 14), whenthe stress is I .27 times the cr*h of the NACEstandardtensile SSCtest but others for ordinary use showeddifferent degree of HICdevelopment, dependingon their

HIC resistance, but they showedno SSC.Another SSCwas found in a pipe for sour service whenthe pipe wastested at 1.36 times the (T*h.

Theseobservations suggest that the occurrence of SSCin the pipe differs from that in the NACESSCtest becauseof the difference in the hydrogen concentration pattern,

and the threshold stress for SSCdevelopment in the

pipeline is higher by 30'/o than that in the NACETM0177-90 methodASSCtest. This relationship coincides

well with that observed in the SSCtests under lower H2Sconcentration condition

.

Further systematic studies are needed for under-standing of environmental effects on SSCin the welded

pi pes.

5. Conclusions

(1) Fine and homogenizedmicrostructures free frompearlite lead to a high resistance of the base material to

SSC.(2) Theconfigurationsofthe cracks observed i.n HAZ

were classified into the three different morphologiesdepending on steel chemistry and HAZhardness.

(3) A decrease in the yield strength combinedwiththe metallographical features such as existence of pearlite,

can be responsible for the rupture of the intercritical

HAZin the SSCtests, with slightly smaller athlcry ratios

of the weld than those of base materials.

(4) It seemsto be difficult to generate the SSCin the

10werhardness HAZin actual pipelines becauseof lowerhydrogen content in actual linepipes. Possibility of

occurrence of SSCin high hardness HAZseemsto bestill high in the actual pipelines. Therefore it is importantto control HAZhardness less than critical value to

prevent the SSCand reduce the possibility of the SSC.(5) The occurrence of SSCin the pipe differs from

that in the NACESSCtest because of the difference in

the hydrogen concentration pattern, and the threshold

stress for SSCdevelopment in the pipeline is higher by300/0 than that in the NACETM0177-90 MethodASSCtest.

REFERENCESl) A. A. Omar, R. D. Kaneand W. K. Boyd: CORROSION/81,

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