Microstructural evolution and the effect on hardness and plasticityof S31042 heat-resistant steel during creep

Yahong Yang a, Lihui Zhu a,n, Qijiang Wang b, Changchun Zhu c

a Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, Shanghai 200072, Chinab Baoshan Iron and Steel Co., Ltd, Shanghai 201900, Chinac Baosteel Special Metals Co., Ltd, Shanghai 200940, China

a r t i c l e i n f o

Article history:Received 6 February 2014Received in revised form15 April 2014Accepted 18 April 2014Available online 29 April 2014

Keywords:Heat-resistant steelCreep rupturePrecipitationPlasticity

a b s t r a c t

The microstructural evolution and mechanical properties of S31042 heat-resistant steel after crept at923 K were systematically investigated. Results show that secondary NbCrN and M23C6 are thepredominant precipitates during creep. Long-term creep results in the precipitation of a small amountof s phase and a few Cr3Ni2SiX. Secondary NbCrN is the most important strengthening precipitate ofS31042 steel. At least 75% of the precipitation hardening results from secondary NbCrN. Furthermore,secondary NbCrN is the main factor to affect the variation of hardness of S31042 steel during creep. Thedramatic degradation of plasticity is mainly caused by the precipitation of M23C6 at the grain boundaries.Chain-like M23C6, cuboids M23C6 and undissolved NbCrN are also harmful to the creep plasticity ofS31042 steel.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The demands of clean energy and protection of the globalenvironment have been accelerating the application of ultra-supercritical (USC) plants. The USC plants with improved steam para-meters of 923 K and 34.3 MPa have been successively constructed[1]. To ensure the safety during the service under USC conditions,the materials with high creep rupture strength, superior resistanceto oxidation and corrosion at high temperature are required.

Due to good comprehensive properties, S31042 (25Cr–20Ni–Nb–N)is one of the promising austenitic heat-resistant steels, and iswidely used as super-heaters and re-heaters in USC boilers. Thecreep rupture strength of S31042 is about 45% higher than that ofTP347 at 923 K [2]. It also exhibits higher tensile strength thanTP347HFG (18Cr–12Ni–Nb) and SUPER304H (18Cr–9Ni–3Cu–Nb–N)[3]. The resistance to hot corrosion and steam oxidation of S31042is superior to conventional 18% Cr heat-resistant steels [2–6].

During creep and aging of S31042 steel, the mechanical proper-ties tend to change owing to microstructural evolution. Previousinvestigations show that in addition to a small amount of MX,s-phase and Cr3Ni2SiX, the main precipitates are NbCrN andM23C6 during creep/aging or after service [2–6]. The excellentcreep rupture strength of S31042 is contributed by finely dispersedNbCrN and M23C6 [2–6]. However, the effect of precipitates on thevariation of hardness during creep/aging/service has not been

quantified, and which precipitate is dominant is not clear. It isworth noting that S31042 shows a significant decrease in impacttoughness after aging or service [2,3,7,8]. The reason of theembrittlement is still in dispute. Some scholars believed that theprecipitation of M23C6 at the grain boundaries is the main reasonfor the reduction of the impact toughness after aging [7,8]. It iswell-known that s phase at grain boundaries has the detrimentaleffect on creep properties [6,9]. Okada ascribed the embrittlementto blocky Cr3Ni2SiC at the grain boundaries [3]. Li pointed thataging embrittlement is caused by blocky Cr3Ni2SiC, s-phase andM23C6 when they precipitate at the grain boundaries [2].

In order to further improve the mechanical properties of S31042, itis necessary to find the main strengthening precipitate and dominantprecipitate in the degradation of toughness of S31042 steel. In thispaper, the mechanical properties and microstructure evolution ofS31042 during creep at 923 K were investigated, with the emphasison precipitation behavior. In order to determine which strengtheningprecipitate is the most important for S31042 steel, the precipitationstrengthening of NbCrN, M23C6 and s phase was calculated, and thentheir effects on hardness during creep were studied. At the same time,the reason of the degradation in the plasticity of S31042 steel duringcreep was discussed, aiming to explore which precipitate results inpoor plasticity.

2. Experimental

S31042 heat-resistant steel tubes were produced by BaoshanIron and Steel Co., Ltd. The chemical composition is given in

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Materials Science & Engineering A

http://dx.doi.org/10.1016/j.msea.2014.04.0730921-5093/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ86 21 56331462; fax: þ86 21 56331466.E-mail addresses: yhyang@shu.edu.cn (Y. Yang), lhzhu@mail.shu.edu.cn (L. Zhu).

Materials Science & Engineering A 608 (2014) 164–173

Table 1. Creep rupture test was carried out at 923 K under differentstresses in the range of 140–235 MPa. In addition to as-suppliedspecimen, the specimens crept for 1608, 4203, 8014, 10,526, 17,115and 27,943 h were chosen to study the microstructure evolu-tion and its effect on the hardness and plasticity. The hardnessof all the specimens was measured by MH-3 Vickers hardnesstester at a load of 200 g at room temperature. Microstructure ofas-supplied and as-crept specimens was examined by means ofNikon LV-150 optical microscope (OM), Hitachi Su-1510 scanningelectron microscope (SEM) and JEM-2010F transmission electronmicroscope (TEM).

3. Results

3.1. The hardness and plasticity of S31042 heat-resistant steel creptat 923 K

Fig. 1(a) shows the variation of hardness of S31042 steel duringcreep at 923 K for up to 27,943 h. It can be divided into threestages. In the first stage, the hardness increases rapidly andreaches the maximum at 4203 h. The hardness decreases graduallywhen the creep time prolongs from 4203 h to 10,526 h (the secondstage). In the third stage, the hardness tends to be stable aftercrept for 10,526 h. Even after crept for 27,943 h, the hardness ofS31042 still keeps high. The creep plasticity drops dramatically inthe early stage. The reduction of area is only 6% after crept for17,115 h. Subsequently it keeps stable, as shown in Fig. 1(b).

3.2. OM observation

Fig. 2 shows the optical micrographs of S31042 steel before andafter crept at 923 K. The microstructure of as-supplied S31042steel is typical austenite. A lot of twins can be observed, and theaverage grain size is 95 μm, see Fig. 2(a). During creep of S31042steel, the grain size increases and the twin number decreasesgradually. After 8014 h of creeping, the average grain size increasesto 125 μm rapidly, as shown in Fig. 2(b). When the creeping timeprolongs to 27,943 h, the grains grow to 135 μm and there is asmall amount of twins (Fig. 2(c)).

3.3. SEM observation

SEM micrographs of S31042 steel before and after crept at 923 Kare shown in Fig. 3. There are a few particles in the as-suppliedspecimen, see Fig. 3(a). Many particles precipitate as soon as creepbegins, and they tend to coarsen during the creep of S31042. Aftercrept for 1608 h, the particles precipitate preferentially at the grainboundaries and twin boundaries, see Fig. 3(b). After 4203 h ofcreeping, more particles precipitate at the grain boundaries and twinboundaries, and they grow up gradually (Fig. 3(c)). In the followingcreeping process, the precipitation inside the grains becomes sig-nificant. After crept for 8014 h, a large number of particles aredispersed uniformly in the matrix. In the specimen crept for17,115 h, some particles around the ends of twin boundaries are alsoobserved, and they are arrowed in Fig. 3(d). There is no obviousdifference in SEM micrographs with further creeping. It is worthnoting that some chain-like particles are observed in the grains inas-crept specimens, their amount and size increase with the increaseof creeping time.

3.4. TEM micrographs of as-supplied S31042 heat-resistant steel

The analysis of electron diffraction pattern and energy disper-sive X-ray spectrometry (EDS) indicates that the particles inas-supplied specimen consist of NbCrN and Nb(C, N). As shownin Fig. 4, the globular particles about 1000 nm in diameter areundissolved NbCrN, whereas granular particles about 200 nm indiameter are Nb(C, N).

3.5. TEM micrographs of S31042 heat-resistant steel crept at 923 K

In the as-crept specimens, some particles with string-like shapeare observed to be dispersed in the matrix, as shown in Fig. 5. Theanalysis of electron diffraction pattern and EDS indicates that thenewly formed precipitates are NbCrN. It can be seen from the darkfield image that the string-like NbCrN is formed by the chain of smallparticles along dislocation lines. The secondary NbCrN is fine, and theaverage diameter is 8 nm after crept for 1608 h (Fig. 5(a)). With theincrease of creeping time, more and more NbCrN precipitate, andthey are very stable during creep. Although crept for 27,943 h, theaverage diameter of secondary NbCrN is only 20 nm, see Fig. 5(b).

Granular particles are observed in the specimen crept for 1608 h,and most of them precipitate at the grain boundaries and twinboundaries. Electron diffraction pattern and EDS analysis prove thatthey belong to M23C6. SEM observation has shown the occurrence ofchain-like particles during creep. TEM analysis indicates that the

Table 1Chemical composition of S31042 steel (mass%).

C Mn Si S P Cr Ni Nb N Fe

0.06 1.18 0.35 0.002 0.020 24.84 20.54 0.41 0.230 Bal.

Fig. 1. Mechanical properties of S31042 steel during creep at 923 K: (a) hardness and (b) reduction of area.

Y. Yang et al. / Materials Science & Engineering A 608 (2014) 164–173 165

chain-like precipitates are granular M23C6 arrayed in the form of thechains. Long-term creep leads to the further development in themorphology of granular M23C6 inside the grains. They grow intocuboids after crept for 17,115 h. Fang et al. also observed cuboidsM23C6 in S31042 steel aged at 1023 K [5]. TEM micrographs of M23C6particles with typical morphology are shown in Fig. 6. TEM observa-tion also shows M23C6 particles coarsen obviously in the course ofcreep process. The average diameter of M23C6 increases quickly from120 to 280 nm, when the creeping time prolongs from 1608 h to17,115 h. However, with further creeping M23C6 seldom changesexcept that the average diameter increases slightly to 290 nm aftercrept for 27,943 h.

After crept for 17,115 h, some parallel plate particles begin toprecipitate around the grain boundaries and the ends of twinboundaries. The average diameter is about 260 nm. The particlesare identified to be s phase by TEM, as shown in Fig. 7. In thespecimen crept for 17,115 h a few Cr3Ni2SiC, which are about1300 nm in length and 100 nm in width, are also found inside thegrains, see Fig. 8. There is no obvious change in s phase andCr3Ni2SiC with further creeping.

3.6. Microstructure of near-fracture of S31042 heat-resistant steelcrept at 923 K

Fig. 9 shows SEM micrographs of near-fracture of S31042 steelcrept at 923 K. There are many cavities and cracks at the grainboundaries, indicating that the inter-granular fracture dominates.From the above-mentioned experimental results, the major precipi-tate at the grain boundaries belongs to M23C6. Therefore, most of thecavities and cracks are related to M23C6 at the grain boundaries, see

Fig. 9(a and b). Some cavities and cracks inside the grains are alsoobserved. The shape of cavities and cracks indicates that theynucleate at chain-like M23C6 or cuboids M23C6, see Fig. 9(c and d).Besides, EDS analysis shows that some cavities also form aroundundissolved NbCrN particles, regardless of NbCrN at the grainboundaries or inside the grains, as shown in Fig. 9(c–e).

4. Discussion

4.1. Microstructural evolution of S31042 heat-resistant steel crept at923 K

Creep of S31042 at 923 K results in the increase of grain size andthe decrease of twin number. A large amount of secondary NbCrNand M23C6 form and tend to coarsen during creep. Long-term creepsuch as 17,115 h leads to the precipitation of a small amount of sphase and a few Cr3Ni2SiC. Image ProPlus software was used toanalyze the volume fraction of the precipitates. Considering thatthere is a few Cr3Ni2SiC in as-crept specimens, the volume fraction ofCr3Ni2SiC was not analyzed for simplification. fM23C6 and fs werecalculated from SEM micrographs, while fNbCrN was calculated fromthe dark field images of TEM micrographs, since secondary NbCrN istoo fine to be observed by SEM. The variation in the size and volumefraction of NbCrN, M23C6 and s phase during creep at 923 K isshown in Fig. 10. It can be seen from Fig. 10(a) that M23C6 coarsensmore seriously than secondary NbCrN. Especially before 17,115 h thediameter of M23C6 particles increases quickly from 120 to 280 nm. Onthe contrary, secondary NbCrN is stable and coarsens slightly from8 to 20 nm in spite of creeping for 27,943 h.

Fig. 2. OM micrographs of S31042 steel before and after crept at 923 K: (a) as supplied, (b) crept for 8014 h and (c) crept for 27,943 h.

Y. Yang et al. / Materials Science & Engineering A 608 (2014) 164–173166

4.2. Effect of microstructural evolution on hardness

The increase of grain size and the decrease of twin numberreduce the hardness of S31042 during creep. Most important of all,the change of hardness during creep results from the evolution ofprecipitates, for example, the precipitation and coarsening of sec-ondary NbCrN, M23C6, s phase and Cr3Ni2SiC. Orowan bypassmechanism governs, once the size of particles exceeds a criticalvalue. Generally speaking, the critical value ranges from 1.5 nm to6 nm [10]. Therefore, in S31042 steel the dislocations tend to bypass

the precipitates by the Orowan process. The pinning force Δτ iscalculated by Eq. (1) [10]

Δτ¼ Gb2πK

f 1=2

ð0:854�1:2f 1=2ÞDln

D2b

� �ð1Þ

where G stands for shear modulus (about 57.7 GPa for S31042 steel at923 K). b represents Burgers vector of dislocation (about 0.2 nm). K is aconstant depending on the kind of dislocation, and K is 1 when

Fig. 3. SEM micrographs of S31042 steel before and after crept at 923 K: (a) as supplied, (b) crept for 1608 h, (c) crept for 4203 h and (d) crept for 17,115 h.

Fig. 4. TEM micrographs of as-supplied S31042 steel: (a) undissolved NbCrN and (b) Nb(C, N).

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Fig. 5. TEM micrographs of secondary NbCrN in S31042 steel crept at 923 K: (a) crept for 1608 h, (b) crept for 27,943 h and (c) electron diffraction pattern of NbCrN.

Y. Yang et al. / Materials Science & Engineering A 608 (2014) 164–173168

Fig. 6. TEM micrographs of M23C6 in S31042 steel crept at 923 K: (a) crept for 4203 h, (b) crept for 8014 h, (c) crept for 17,115 h, (d) crept for 27,943 h, and (e) electrondiffraction pattern and chemical composition of M23C6.

Y. Yang et al. / Materials Science & Engineering A 608 (2014) 164–173 169

assuming that all dislocations are screw dislocations. D is the averagediameter of precipitates. f is the volume fraction.

Cr3Ni2SiC with large size does not form until crept for 17,115 h.In order to simplify calculation, the effect of Cr3Ni2SiC on hardnessduring creep of S31042 steel was neglected. The pinning force ofsecondary NbCrN, M23C6 and s phase in S31042 steel wascalculated and shown in Fig. 11(a). The total value of calculatedpinning force as a function of creeping time is given in Fig. 11(b). Inorder to compare, the hardness is also shown in Fig. 11(b). Thetendency of calculated pinning force with creeping time is inagreement with measured hardness.

Although NbCrN and M23C6 grow up slightly in the first stage, thepinning force caused by NbCrN and M23C6 continually increases,since the volume fraction of NbCrN and M23C6 keeps on increasing.Relatively speaking, secondary NbCrN is much smaller than M23C6.So the pinning force contributed by NbCrN is much stronger thanM23C6 according to Eq. (1). For example, after crept for 4203 h, theaverage diameter of NbCrN and M23C6 is separately 9 nm and140 nm. The pinning force of NbCrN is 10 times as strong as thatof M23C6. The increase in hardness in the first stage is attributed tothe precipitation of secondary NbCrN and M23C6. Compared withM23C6, secondary NbCrN is much more effective to increase thehardness.

In the second stage, M23C6 particles coarsen quickly, but thevolume fraction of M23C6 increases gradually. As a result, the pinningforce of M23C6 hardly changes according to Eq. (1). However, the slightgrowth of fine precipitates will lead to a significant decrease in thepinning force [11]. Although NbCrN grows from 9 nm to 18 nm, thepinning force decreases quickly when the creeping time prolongsfrom 4203 h to 10,526 h, as shown in Fig. 11(a). In a word, thehardness of S31042 steel decreases in the second stage due to thecoarsening of secondary NbCrN.

The reduction in the pinning force of NbCrN is not obvious in thethird stage, since the growth velocity of NbCrN slows down. On theother hand, despite the coarsening of M23C6, the volume fraction ofM23C6 and s increases gradually with the increasing creep time.Hence the pinning force contributed by M23C6 and s becomes strong,which compensates the slight reduction in the pinning force of NbCrN.Finally, the hardness of S31042 steel keeps stable in the third stage.

In summary, the change of hardness of S31042 steel during creep at923 K mainly results from the precipitation and coarsening of second-ary NbCrN. At least 75% of the precipitation hardening results fromsecondary NbCrN. It is secondary NbCrN particles, rather than M23C6 ors phase, play the most important role in the excellent strength ofS31042 steel. In order to optimize the strength, it is important tocontrol the precipitation and coarsening of secondary NbCrN.

Fig. 7. TEM micrograph, chemical composition and electron diffraction pattern of s phase in the specimen crept for 17,115 h.

Fig. 8. TEM micrograph, chemical composition and electron diffraction pattern of Cr3Ni2SiC in the specimen crept for 17,115 h.

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Fig. 9. SEM micrographs of near-fracture of S31042 steel crept at 923 K: (a) crept for 1608 h, (b) crept for 4203 h, (c) crept for 17,115 h, (d) crept for 27,943 h, and (e) EDS ofundissolved NbCrN in (d).

Fig. 10. The variation in the size and volume fraction of precipitates crept at 923 K: (a) average diameter and (b) f1/2.

Y. Yang et al. / Materials Science & Engineering A 608 (2014) 164–173 171

4.3. Effect of microstructural evolution on plasticity

The increase of grain size and decrease of twin number declinethe creep plasticity of S31042. Undissolved NbCrN is also harmfulto the creep plasticity of S31042 steel, since some cavities formaround undissolved NbCrN (see Fig. 9). It is consistent with Wang'sopinion that NbCrN particles with large size reduce the elevatedtemperature ductility [12]. Besides, the plasticity is stronglydependent on the size, distribution and morphology of precipi-tates (especially at the grain boundaries) [10,13,14]. Thus, theprecipitation behavior during creep at 923 K plays an importantrole in the degradation of plasticity.

From Fig. 1(b), the dramatic decrease in the reduction of areatakes place in the early stage of creep. After crept for 17,115 h, thecreep plasticity tends to be stable. Some researchers believed thats phase and Cr3Ni2SiC are responsible for the embrittlement ofS31042 steel [3,6,9]. But in the present study, a small amount of sphase and a few Cr3Ni2SiC are found to precipitate only after creptfor 17,115 h. Moreover, no cavity nucleates at s phase andCr3Ni2SiC. Hence, s phase and Cr3Ni2SiC are not the main reasonfor poor plasticity of S31042 steel during creep.

Above-mentioned experimental results show that M23C6 andsecondary NbCrN are the main precipitates in the early stage ofcreep. Compared with secondary NbCrN, the size of M23C6 parti-cles is much larger. Hence, it is easier for the cavities to nucleate atM23C6 particles. Especially the precipitation of M23C6 at the grainboundaries reduces the bonding interface strength [7]. Corre-spondingly both the nucleation of cavities at the grain boundariesand the development of cracks along the grain boundaries becomeeasy during creep (see Fig. 9). Therefore, the precipitation of M23C6

at the grain boundaries is the main reason for the dramaticdegradation of the plasticity. Besides, some chain-like M23C6 andcuboids M23C6 are observed inside the grains. The particles withacute morphology are susceptible to the stress concentration andcavity nucleation [10,15]. It seems that the chain-like M23C6 andcuboids M23C6 have a detrimental influence on plasticity.

In conclusion, the precipitation of M23C6 at grain boundaries inthe early stage during creep is the main reason to impair the creepplasticity of S31042 steel. The decrease of plasticity is partlyascribed to chain-like M23C6 and cuboids M23C6 owing to acutemorphology. Besides, undissolved NbCrN is harmful to the plasti-city. From the viewpoint of plasticity, it is necessary to control theprecipitation and development of M23C6, especially at grainboundaries.

In order to suppress the precipitation of carbides at the grainboundaries and facilitate the precipitation of fine carbides inside

the grains, it is necessary to add the strong carbide formingelements such as Nb [7]. If there is no enough Nb to tie up C,M23C6 might precipitate at the grain boundaries. N element can actlike C by precipitating in the form of nitrides, but N remaining insolid solution has a much greater strengthening effect than that ofC. Especially it has a role in retarding the formation and coarseningof M23C6 [9]. The ratio of Nb/CþN is important and should beoptimized in S31042 steel to prevent the precipitation of M23C6 atthe grain boundaries as well as maximize the strengthening effect.Besides, increasing solution treatment temperature is also usefulto decrease the undissolved NbCrN and improve the creep plasti-city of S31042 steel. Further work will be focused on optimizingthe ratio of Nb/CþN and finding an appropriate solution treatmenttemperature.

5. Conclusions

(1) During creep of S31042 at 923 K, the predominant precipitatesare secondary NbCrN and M23C6. The typical morphology ofM23C6 particles are granular, chain-like and cuboids. Long-term creep results in the precipitation of a small amount ofs-phase and a few Cr3Ni2SiC.

(2) Secondary NbCrN is the most important strengthening pre-cipitate of S31042 steel. At least 75% of the precipitationhardening results from secondary NbCrN. The change in hard-ness of S31042 steel during creep at 923 K mainly results fromthe precipitation and coarsening of secondary NbCrN.

(3) The reduction in creep plasticity of S31042 steel is related toprecipitation behavior during creep. The poor plasticity ismainly caused by the precipitation of M23C6 at the grainboundaries in the early stage of creep. Chain-like M23C6 andcuboids M23C6 inside the grains also decline plasticity owing toacute morphology. Besides, undissolved NbCrN is harmful tothe plasticity.

Acknowledgments

The work was supported by the National Natural ScienceFoundation of China under the Grant 51171097 and National KeyTechnology R&D Program for the 12th Five-Year Plan under theGrant 2011BAK06B04. The authors are grateful for the helpprovided by the Instrumental Analysis & Research Center inShanghai University. We also would like to thank Baoshan Iron

Fig. 11. The effect of precipitation hardening of S31042 steel crept at 923 K: (a) the calculated pinning force of secondary NbCrN, M23C6, and s phase, (b) comparison of thecalculated pinning force and measured hardness.

Y. Yang et al. / Materials Science & Engineering A 608 (2014) 164–173172

and Steel Co., Ltd. and Baosteel Special Metals Co., Ltd. forproviding the specimens and properties data.

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