SSC Resistance of Low-Alloy Steels.pdf

CORROSION SCIENCE

Effect of Molybdenumon Sulfide Stress Cracking Resistanceof Low-Alloy Steels✫

C.-M. Liao and J.-L. Lee*

ABSTRACT

The effect of molybdenum (Mo) content on the sulfide stresscracking (SSC) resistance of low-carbon (C), low-alloy steelsin hydrogen sulfide (H2S)-saturated sodium chloride (NaCl)solution was investigated by conducting a constant-load teston four steels containing 0% to 0.45% Mo. The microstruc-tures consisted of second phases dispersed in a matrix offerrite grains. Mo additions altered the second phases frompearlite bands to dispersed martensite/austenite (M/A)islands through suppression of the transformationtemperatures of second phases. SSC was found toincorporate two components. One component was parallel tothe loading stress and frequently at interfaces of manganesesulfide (MnS)/ferrite and banded second phase/ferrite. Theother was perpendicular to the loading stress and initiatedpreferentially at M/A islands. The first component diminishedwith increasing Mo as a result of the breakup of the bandedstructures. The second component was enhanced byincreasing Mo because of the formation of more M/A islands.By optimizing the two effects, the highest SSC resistance wasobtained with 0.3% Mo.

KEY WORDS: hydrogen-induced cracking, martensite/austenite islands, molybdenum, pearlite band, sulfide stresscracking

CORROSION–Vol. 50, No. 9

✫ Submitted for publication September 1993; in revised form, March 1994.* Steel and Aluminum Research and Development Department, China

Steel Corp., Hsiao Kang, Kaohsiung, 81233, Taiwan, R.O.C.

0010-9312/94/00016© 1994, NACE I

INTRODUCTION

With increasing energy exploration in recent years,there has been an increasing demand for linepipes fornatural gas or crude oil transmission. Candidate steelsfor linepipes must meet stringent safety requirements,which include high strength to cope with hightransporting pressures, high toughness for service inlow-temperature regions, and good corrosionresistance for the transportation of sour gas.

In environments containing sour gas, resistance tosulfide stress cracking (SSC) is one of the mostimportant properties a steel must have to ensure safegas transmission. Several accidents associated withSSC have been reported in the past four decadessince the first occurred in Canada.1 Because suchaccidents may shut down a plant, damage equipment,pollute environments, and even cause casualties, it isvery important that the SSC behavior be understoodand that the SSC resistance of linepipe steels beimproved.

Molybdenum (Mo) is an important alloying elementin the manufacture of linepipe steel. The addition of Mohas been reported to be able to achieve an optimumstrength-toughness balance through the formation offine-grained acicular ferrite at a relatively low carbon(C) concentration.2 Another merit of Mo-bearing steel isits continuous yielding behavior,3 which can alleviatestrength loss due to the Baushinger effect4 during pipeforming.

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TABLE 1 Compositions of the Steels Investigated (A)

Sample C Si Mn P S Al Nb Mo N

A 0.082 0.31 1.53 0.011 107 0.022 0.028 0 47B 0.083 0.31 1.52 0.015 92 0.039 0.030 0.15 46C 0.083 0.32 1.46 0.016 103 0.049 0.030 0.30 46D 0.081 0.34 1.51 0.018 123 0.037 0.030 0.45 46

(A) C = carbon, Si = silicon, Mn = manganese, P = phosphorus, S = sulfur, Al = aluminum, Nb = niobium,Mo = molybdenum, N = nitrogen; S and N in ppm, other elements in wt%; balance: Fe

TABLE 2 Strength, SSC Threshold Stress,

and the Ratios of Strengths to Threshold Stress

A B C D

sys (kg/mm2) 43.2 42.6 39.7 45.2sts (kg/mm2) 51.8 55.8 62.0 69.6sth (kg/mm2) 12.9 25.6 31.7 29.4sth/sys 0.30 0.60 0.80 0.65sth/sts 0.25 0.46 0.51 0.42

Sample

Ravi, et al., demonstrated Mo had a beneficialeffect on the SSC resistance of steels containing highsulfur (S).5 Morikawa, et al., pointed out that theaddition of 0.5% Mo could improve SSC resistance byreducing the grain boundary phosphorus (P)segregation.6 Other researchers also have observed abeneficial effect of Mo on SSC resistance of steels.7-8

However, Fraser, et al., found Mo was detrimental toSSC resistance.9

The objective of the present work was to clarify theeffect of Mo on SSC resistance. Microstructural effectswere emphasized because microstructure hasreceived little attention in the past.

EXPERIMENTAL PROCEDURES

Four steels with Mo content ranging from 0 wt% to0.45 wt% were investigated. Chemical compositionsare listed in Table 1. All specimens were prepared in a250-kg vacuum induction furnace and were cast intoingots 160 mm by 160 mm by 500 mm. The ingotswere heated to 1,150°C and soaked for 1.5 h, hot-rolled to 15 mm, and finally cooled in air.

After grinding, polishing, and etching in 2% nital,the specimens were examined with a light microscope(LM) and a scanning electron microscope (SEM).Inclusions in these specimens were analyzed byenergy dispersive x-ray spectrometry (EDX). The grain

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size of ferrite and the volume fraction of secondphases were determined by mean linear intercept andpoint counting methods, respectively, under 500xmagnification. Microstructural details of second phasesalso were observed using transmission electronmicroscopy (TEM).

The transformation temperature was determinedusing a dilatometer. Hollow cylindrical specimens of12-mm length, 5-mm outside diam, and 1-mm wallthickness were cut from the plate. The specimens wereaustenitized at 950°C for 10 min in a vacuum of ~ 10–2

Pa in a high-speed dilatometer and then cooled at arate of 1°C/s to determine their transformationtemperatures.

The yield strengths (sys) and tensile strengths (sts)of these steels were determined using a tension testingmachine. Results are listed in Table 2. The deadweight-type, constant-load test was performed toevaluate the SSC resistance of steels in 0.5% glacialacetic acid plus 5% sodium chloride (NaCl) solution(i.e., NACE solution).10 Specimens were mounted in acell filled with the NACE solution and then loaded tocertain percentages of sys. Pure hydrogen sulfide gas(H2S) was purged continuously into the solution tokeep a saturated concentration during the testing. Thetime to failure of the specimen was recorded.Threshold stress (sth) was defined as the maximumstress under which fracture did not occur after 720 h.sth was used as a criterion for the evaluation of SSCresistance.

RESULTS AND DISCUSSION

MicrostructureMicrostructures of all the steels studied are shown

in Figure 1. Typically, all steels consisted of a ferritematrix and second phases arranged in a bandingpattern. With increasing Mo content, the grain size offerrite decreased slightly, and the morphology of theferrite grain did not change significantly. Nevertheless,the banded structure became more dispersed withincreasing Mo content.

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(b)(a)

(d)(c)

FIGURE 1. Light micrographs of the steels with various Mo contents: (a) 0% Mo, (b) 0.15% Mo, (c) 0.3% Mo,and (d) 0.45% Mo.

To identify the second phases, SEM observationswere made (Figure 2). The second phase of Mo-freesteel appeared to be a typical pearlite structure incontinuous alignment. The amount of second phasesincreased with the Mo content (Figure 3). In 0.15% Mosteel, most second phases turned into nonlamellarferrite-cementite aggregates, which generally arereferred to as pseudopearlite (Figure 2[b]). Thebanding pattern was retained, but the length of theindividual pseudopearlite band was reduced. When theMo content was increased to 0.3%, the second-phasemicrostructure consisted of parallel ferrite laths withinterlath cementite and, therefore, was a typical bainitestructure (Figure 2[c]). However, some pseudopearliteand unetched islands also could be seen. In 0.45% Mosteel, the predominant second phases were unetchedislands (Figure 2[d]). TEM showed these unetchedislands to be mainly lath martensite (Figure 4) or twinmartensite (Figure 5). Very small amounts of retainedaustenite sometimes was observed to coexist withmartensite. This type of second phase generally is


referred to as martensite/austenite (M/A) constit-uents11-12 or M/A islands. The phase species in themicrostructures of these four steels are listed inTable 3.

Figure 6 shows the phase transformationtemperatures of matrix and second phases of thesefour steels. Transformation temperatures of the matrixphase decreased slightly, while transformationtemperatures of second phases were depressedmarkedly with increasing Mo content. As austenitetransformed into ferrite at lower temperatures, moresites were available for the nucleation of ferrite.Therefore, the grain size was refined slightly as Mocontent increased.

The transformation temperature of the secondphases of the Mo-free steel was ~ 600°C. At thistemperature, the diffusion rate of C was so high thatthe cooperative growth of ferrite and cementite couldtake place by transporting C atoms from ferrite intocementite, resulting in a lamellar pearlite structure. Thetransformation temperature of second phases

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(d)(c)

FIGURE 2. SEM showing the second phases of the steels with various Mo contents: (a) 0% Mo, (b) 0.15% Mo,(c) 0.3% Mo, and (d) 0.45% Mo.

(b)(a)

decreased sharply to ~ 500°C by adding 0.15% Mo. Atthis medium temperature, C diffusion was not fastenough to sustain the coupled growth of ferrite andcementite. An intermediate transformation product,pseudopearlite, was obtained. Further increases of Mosuppressed C diffusion by lowering the transformationtemperature, thus promoting the formation of bainiteand M/A islands.

In addition to the ferrite and second phases, manymanganese sulfide (MnS) inclusions were present in allfour steels (Figure 7). Most MnS inclusions wereelongated, regardless of Mo content. The direction ofelongated MnS was parallel to the rolling direction.

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These four steels had nearly identical numbers andshapes of MnS inclusions because they had almostidentical compositions (except for Mo) and were notinclusion shape-treated by calcium (Ca) or other rareearth elements.

Resistance to SSCThe relationship between time to failure and

loading stress of all the specimens is shown in Figure8. Loading stress obviously was an important factorinfluencing SSC resistance. Time to failure for eachspecimen decreased with the increase in loadingstress.


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FIGURE 3. Volume fraction of second phase in the steelscontaining various Mo contents.

(a)

(b)

FIGURE 4. Lath martensite in the Steel D examined by TEM:(a) bright field and (b) dark field.

Table 2 shows the strengths, SSC sth, and theratio of sth to sys and sts of the four steels under investi-gation. Figure 9 illustrates the relationship between Mocontent and sts, sys, and sth. sys decreased withincreasing Mo content up to 0.3%, but then increased.

A gradual transition from discontinuous tocontinuous yielding behavior in the steels containing0% to 0.3% Mo was found to be the reason for thedecrease in sys. This transition in yielding behavior is acharacteristic of Mo-bearing steels.3 By contrast, sts

increased with increasing Mo. sth increased withincreasing Mo content from 0% to 0.3% and slightlydecreased as Mo content was increased to 0.45%.

Figure 10 shows the effects of Mo content on theratios of sth to sys and sts. As Mo content increasedfrom 0% to 0.3%, sth greatly increased from 0.3 to0.8 sys. Nevertheless, sth decreased to 0.65 sys at Mocontent of 0.45%. The change in the sth/sts ratioexhibited a similar trend. Although SSC resistancedecreased as Mo content increased to 0.45%, sth ofthis steel was still higher than that of Mo-free steel. Moeffectively improved the SSC resistance of steels.Many researchers have pointed out that Mo enhancesSSC resistance of steels.5-8,13 Asahi, et al., also showedthat the SSC sth of steels increased with increasing Mocontent.14-16 Nevertheless, in the present work, it wasfound that too much Mo diminished the beneficialeffect. The best SSC resistance was obtained at~ 0.3 wt% Mo content. This phenomenon wasconsistent with other studies.7,17-18

Crack Morphology of SSCCross sections of the specimens after SSC testing

were examined by SEM. Figure 11 shows the typical


crack morphology of Steel A. Morphologies of othersteels were similar to that of Steel A. The direction ofloading stress was parallel to the rolling plane. Twocomponents of cracking, one perpendicular and theother parallel to the direction of loading stress, wereobserved. The occurrence of cracks along the rollingplane is a general characteristic of hydrogen-induced

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TABLE 3 Microstructures of the Second Phases

in Steels with Various Mo Content

Steels P PP B 2nd M/A

A uuuB uu u uC u uu uuD u u uuu

(A) P = pearlite, PP = pseudopearlite, B2nd = bainite second phase, M/A = M/A constituents; uuu, uu, and u: volume fraction of secondphase in the sequence of high, medium, and low.

Second Phase Species (A)

FIGURE 5. Twin martensite in Steel D examined by TEM: (a) bright field and (b) dark field.

(b)(a)

cracking (HIC).19 On the other hand, stress corrosioncracking (SCC) generally proceeds perpendicular tothe applied stress.20 Therefore, SSC could beexplained as a combination of HIC and SCC.

Cracks parallel to the loading stress frequentlywere found to be associated with the elongated MnSinclusions and banded structures. Figure 12(a) showscracks accompanied by the elongated MnS inclusions.The interfaces between MnS inclusions and matrixusually acted as sinks of hydrogen (H) atoms. H atomswere apt to diffuse to these interfaces, where theyrecombined into H molecules and led to HIC.

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Therefore, elongated MnS inclusions were detrimentalto SSC resistance.21-24 Since the amounts and shapesof MnS inclusions were similar in the steels studied,the contribution of elongated MnS to horizontal cracksalso was similar.

Banded structures are preferential sites for HIC toinitiate and propagate, and cracks were observed to beassociated with the banded structures. As discussedpreviously, the banded structures included pearlite,pseudopearlite, and bainite in steels containingdifferent amounts of Mo. These three structures wereobserved to be preferential locations for cracks tooccur. Figure 12(b) shows a typical example for acrack occurring on a pearlite band, and Figure 12(c)shows an example of a crack on a bainite band.

The banded structures became more dispersedand less continuous as a result of Mo addition. Thus,the total length of cracks on the band structuresdecreased with Mo addition. Therefore, the total HICcrack length, including cracks along with MnS andbanded structures, decreased with increasing Mocontent. That is, the HIC component of the SSC wasdiminished by Mo addition.

Cracks frequently were found perpendicular to theloading stress in two main locations. One was in thebanded structures, the other was in the M/A islands.Figure 13(a) shows two cracks (indicated by arrow) inthe pearlite band of the Mo-free steel. The cracks


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FIGURE 6. Effect of Mo content on the transformationtemperature of the matrices and second phases.

FIGURE 7. Morphology of MnS inclusions in the steelsinvestigated.

FIGURE 8. Relationship between the time to failure andloading stress for the steels with various Mo contents.

propagated into ferrite as illustrated in Figure 13(b).Even when the banded structures changed frompearlite into pseudopearlite or bainite with Mo addition,cracks also could initiate in the banded structures.When the second phases were composed ofpseudopearlite, bainite, and M/A islands, the crackspreferentially initiated in M/A islands and propagatedinto ferrite (Figure 13[c]). Because the number of M/Aislands increased with Mo content, the cracks on M/Aislands and the total length of cracks perpendicular tothe loading stress increased with Mo content. The SCCcomponent to SSC was increased by Mo addition.

Factors Influencing SSC ResistanceThe difference in the harmful effect on SSC

resistance caused by MnS was negligible in these foursteels as discussed previously.

SSC resistance can be improved by reducing theferrite grain size,16 but in this study, grain size effectwas not believed to be the dominant factor becausethe difference in ferrite grain size of these four steelswas not significant and cracks appeared not to initiatepredominantly in ferrite grains or at ferrite grainboundaries.

The strength of a steel is also a factor affectingSSC resistance. It generally is believed that the higherthe strength of a steel, the lower its SSC resistancewill be.15,25-27 However, SSC resistance, judged eitherfrom sth (Figures 9 and 10) or from the sth/sts ratio


(Figure 10), was not found to decrease with increasingsts. Therefore, sts was not the significant factor in SSCresistance. Table 2 shows steel D did not exhibit thelowest sth even though it possessed the highest sys.The change in sth/sys ratio with Mo content had areverse trend to the change in sys (Figure 9). However.the relatively small difference in sys among the steelsseemed unlikely to give rise to such a large variation insth/sys ratio (Figure 10). Therefore, sys also was not

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FIGURE 9. Effect of Mo content on sts, sys, and SCC sth.FIGURE 10. Effect of Mo content on the ratios of sth to sys andsts.

FIGURE 11. Crack morphology of SSC in Steel A.

believed to be the dominant factor affecting SSCresistance.

It is well known that banded structures decreaseSSC resistance of steels.28-30 Therefore, scattering thebanded structures should have improved SSCresistance. The fact that 0.15% Mo steel had higherSSC sth than Mo-free steel could be attributed to itsmore dispersed banded structures. Although thebanded structures were dispersed by Mo addition,

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SSC resistance did not continue to increase withfurther Mo addition. For instance, the steel containing0.45% Mo had fewer continuous banded structuresthan the steel containing 0.3% Mo, but sth of the0.45% Mo steel was lower. Therefore, another factormust have influenced SSC resistance.

Snape31-33 and Payer, et al.,34 indicated thatuntempered martensite greatly damages SSC resis-tance. Pressouyre also pointed out that untemperedmartensite is the most susceptible structure in theferrite steels to H embrittlement.35 From microstructuralobservations, the number of M/A islands increasedwith Mo content as a result of the decrease intransformation temperature of second phases. In thesteel containing 0.45% Mo, most of the second phaseswere M/A islands. Therefore, the detrimental effect toSSC resistance caused by M/A islands increased withMo content.

The banded structure and M/A islands areimportant to SSC resistance of steels. A schematic ofthe beneficial effect from reducing banded structureand the harmful effect of increasing the number of M/Aislands is shown in Figure 14. The contribution to SSCresistance as a result of the scattering of bandedsecond phases increased remarkably with increasingMo content up to 0.3% because of the transition fromcontinuous to discontinuous banded structure. Thisbeneficial effect saturated after 0.3% Mo since all thecontinuous bands were broken into small segments.However, the deterioration of SSC resistance causedby the initiation of cracks on M/A islands becamesevere at 0.3% Mo because of the formation of largenumbers of M/A islands (Table 3). Combining thesetwo effects, the best SSC resistance was obtained at0.3% Mo.


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(a) (b) (c)

(a) (c)

FIGURE 13. Cracks perpendicular to the direction of loading: (a) initiating in the pearlite, indicated by arrow,(b) propagating through the pearlite and ferrite, and (c) initiating in M/A islands.

FIGURE 12. Cracks parallel to the direction of loading: (a) along an elongated MnS, (b) along a pearlite band,and (c) along a bainite band.

(b)

ACKNOWLEDGMENTS

The authors acknowledge the assistance of theChina Steel Corp., C.F. Wu, J.M. Hsu, G.Y. Lee, andR.I. Hsieh.

CONCLUSIONS

Four steels with Mo content from 0% to 0.45%were prepared to study the microstructural effectassociated with Mo additions on SSC.❖ The addition of Mo decreased the transformation


temperatures of the ferrite matrices and secondphases, causing a change of second phases frompearlite bands to M/A islands.❖ SSC was observed to be composed of twocomponents: cracks parallel to and perpendicular tothe loading stress. The former was found frequently toinitiate at the interfaces of MnS/ferrite and bandedsecond phases/ferrite. The latter was found to initiatepredominantly at M/A islands.❖ The contribution of second phases to cracks parallelto the loading stress was diminished with increasingMo content because of the breakup of the banded

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FIGURE 14. Schematic of the effect of banded structure andM/A islands on SSC resistance.

structure. By contrast, cracking perpendicular to theloading stress was enhanced by Mo additions as aresult of the formation of a larger number of M/Aislands.❖ The best SSC resistance was obtained at 0.3% Moas a result of the optimization of the beneficial effectfrom dispersed banded structure and the disadvantagefrom increasing volume fraction of M/A islands.

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Stress Cracking at Ambient Temperature” (Houston, TX: NACE, 1977).11. J.L. Lee, M.H. Hon, G.H. Cheng, Scrip. Metal. 21 (1987): p. 293.12. J.L. Lee, M.H. Hon, G.H. Cheng, Mat. Sci. 22 (1987): p. 2,767.13. Y. Yoshino, Corrosion 38 (1982): p. 156.14. H. Asahi, Y. Sogo, M. Ueno, H. Higashiyama, Metall. T-A, 19A (1988):

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21. Y. Nakai, H. Kurahashi, T. Emi, O. Haida, Trans. ISIJ 19 (1979): p. 401.22. E.M. Moore, J.J. Warga, MP 15 (1976): p. 17.23. B.J. Berkowitz, F.H. Heubaum, Corrosion 40 (1984): p. 240.24. T. Taira, Y. Kobayashi, H. Ichinose, Nippon Kokan Technical Report,

Overseas 34 (1982): p. 66.25. T. Taira. K. Tsukaka, Y. Kobayashi, M. Tanimura, H. Inagaki, N. Seki,

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Sumitomo Search 37 (1988): p. 83.30. M. Kimura, N. Totsuka, T. Kurisu, K. Amano, J. Matsuyama, Y. Nakai,

“Sulfide Stress Corrosion Cracking of Linepipe,” CORROSION/86, paperno. 160 (Houston, TX: NACE, 1986).

31. E. Snape, Corrosion 23 (1967): p. 154.32. E. Snape, Corrosion 24 (1968): p. 261.33. E. Snape, F.W. Schaller, R.M.F. Jones, Corrosion 25 (1969): p. 380.34. J.H. Payer, S.P. Pednekar, W.K. Body, Metall. T-A 17A (1986): p. 1,601.35. G.M. Pressouyre, in Current Solutions to Hydrogen Problems in Steels,

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SSC Resistance of Low-Alloy Steels.pdf