Materials Science and Engineering A 528 (2011) 22772281

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Materials Science and Engineering A

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Effect o ndtransfo

Do-Yeala Technical Res publicb Department o , Geum

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Article history:Received 31 MReceived in reAccepted 8 DeAvailable onlin

Keywords:Austenitic 304Ni contentTensile strengElongationStrain-inducedStrain hardeni

on th) use196ponepoinansfoedmNi ST

1. Introduction

The stor(LNG) areshould be ststorage tankto an abrupgas and duelevel and pr

Type 304a metastablaustenite pthe strains aformation t-martensformation.content, strformed pha

Many stducted forHowever, von the mecstudy inves

CorresponE-mail add

and phase transformation for 304 STS at RT and lower tempera-tures.

0921-5093/$ doi:10.1016/j.age tanks for large volumes of liqueed natural gasconstructed from 304 stainless steel (STS). The LNGored and transported at162 C. Some parts of the LNGs are subjected to bending fatigue or tensile fatigue due

t temporal change in the charging anddischarging of theto repetitive deformation induced by variations of LNGessure [1].STS (Fe18Cr8.4Ni) consists of -austenite, which is

e phase at room temperature (RT). The stability of thehase varies with respect to the Ni content. In addition,pplied at a temperature less than themartensite trans-emperature (Md) transform themetastable austenite toite, which is termed strain-induced martensite trans-The factors affecting the phase transformation are Niain-rate and temperature. The amount of the trans-se inuences the tensile and fatigue properties.udies on the mechanical properties have been con-austenitic STS at RT and lower temperatures [27].ery few studies have focused on the effect of Ni contenthanical properties for 304 STS [810]. Therefore, thistigated the effect of Ni content on the tensile properties

ding author. Fax: +82 51 514 4457.ress: kangcy@pusan.ac.kr (C.-Y. Kang).

2. Experimental procedure

Thespecimenwas typical304STSwithaNi contentvarying from8.3wt.% to 12wt.%. A 50kg ingot with the designed compositionwas produced in a high-frequency induction furnace and its thick-ness reduced to 2mm by hot rolling and subsequent cold rolling.Regardless of the Ni content, the grain size for the all specimensremained mostly constant as specied in ASTM No. 6.87.0.

The JIS 13B specimen was used for the tensile test at RT. Thelow temperature tensile test was performed by using the cryogenicsub-size specimen at a strain rate of 1mm/min. After completion ofthe tensile experiment, theamountof strain-induced-martensitewasmeasuredbyusing a ferritescope (Fischer Co.). The ferritescopeuses an eddy current probe to measure ferrite content since fer-rite is ferromagnetism. A vibration sample magnetometer VSMhas a same principle with the ferritescope. The ferritescope isuseful specically in the eld since it is portable and can suc-cess small specimens. For quantitative analysis of the -amount,standard samples were produced by changing the temperatureduring the cold rolling. Therefore, the amount of produced -martensite ranged from 4.6 vol.% to 98vol.%, and was measuredwith the (VSM). The amount of -martensite measured from theVSM was applied to the ferritescope for the correction. The para-magnetic -martensite was observed by using X-ray diffraction

see front matter 2010 Elsevier B.V. All rights reserved.msea.2010.12.022f Ni content on the tensile properties armation for 304 stainless steel

Ryooa, Namhyun Kangb, Chung-Yun Kangb,

earch Laboratories, POSCO, 1 Goedong-dong, Nam-gu, Pohang, Gyeongbuk 790-785, Ref Materials Science and Engineering, Pusan National University, San 30, Jangjeon-dong

e i n f o

arch 2010vised form 27 October 2010cember 2010e 15 December 2010

stainless steel

th

martensite transformationng

a b s t r a c t

The effect of Ni content (8.312wt.%)ied on type 304 stainless steels (STStemperature was varied from 25 C to(tensile strength, strain hardening exthe 8.39.0wt.% Ni STS, a lower yieldthe dynamic strain softening and/or trincrease in the amount of strain-inducnor TRIPwas observed for the 12wt.%at the low strains.m/locate /msea

strain-induced martensite

of Koreajeong-gu, Busan 609-735, Republic of Korea

e tensile properties and strain hardening behavior was stud-d for the membrane of LNG storage tanks. The tensile testC. At room temperature, the hardening and ductility indices

nt and elongation) increased with decreasing Ni content. Fort was observed at temperatures below 60 C. It was due tormation-induced plasticity (TRIP) that accompanied the rapidartensite () at low strains. Neither dynamic strain softeningS because only the -martensite transformationwas produced

2010 Elsevier B.V. All rights reserved.

2278 D.-Y. Ryoo et al. / Materials Science and Engineering A 528 (2011) 22772281

Fig. 1. Effect oand yield stren

(XRD). Thethe XRD.

3. Results

3.1. Tensile

Fig. 1 sset), total eNi contentelongationstrength remvious studyindependen[8].

The effeexamined fcurve at RT0.15, the incing Ni conteexponents (value decre

Fig. 2. Effect o

Fig. 3. Effect of Ni content on the strain hardening exponent (n) at room tempera-ture.

for both strain regimeswith increasingNi content,which suggestedration in formability.e 30K [1sitiolightspeciion atensten0% tohe feat RT.%Ni)ain ogestssedd foris res].theed whe tf Ni content on the tensile properties at room temperature: (a) tensilegth and (b) total and uniform elongation.

CuK characteristic X-ray (50kV, 150mA) was used for

and discussion

properties at room temperature (RT)

hows the tensile strength, yield strength (0.2% off-longation and uniform elongation as a function of the. The tensile strength, total elongation and uniformdecreased with increasing Ni content, while the yieldainedmostly constant. Thiswas coincidentwith a pre-nding that the 0.2% offset yield strength at RT wast of the Ni content if it was larger than some limits

ct of Ni content on the strain hardening behavior was

deterioTyp

Ms 4compofered sstudyformat-mar

Thefrom 2from tphase(12wtsile strthe larsuppreThe MRT. Thet al. [9

Forincreasfore, tor the 304 STS. Fig. 2 shows the true stresstrue strainas a function of the Ni content. For the true strain aboverement slope of the true stress decreasedwith increas-nt. Fig. 3 indicates the behavior of the strain-hardeningn) for the strain regimes of 2030% and 3040%. The n-ased with increasing the strain. The n-value decreased

fNi content on the true stresstrue strain curves at roomtemperature.

increased dtransformanecking moincreased t

Fig. 4. Volumtent and tensi4 STS has transition temperatures of Md 293K and1]. The Md value is very similar to RT. The specimenns of the present study, specically 8.39.0wt.% Ni, dif-ly from that of the previous study [11]. Therefore, themen can undergo a strain-induced martensitic trans-t RT, i.e., the metastable austenite () transforms to theite under straining.sile test specimens were subjected to various strains50%, and the amount of -martensite was measured

rritescope. Fig. 4 shows the volume fraction of the -as a function of Ni content and tensile strain. No. 4 steeldidnotproduce the-phase, evenup toanapplied ten-f 50%. The nickel element is the austenite stabilizer, andNi content of 12wt.% stabilized the austenite phase andthe phase transformation to the -martensite at RT.the 12wt.% Ni steel should be signicantly lower thanult coincided with the previous report from Fujikura

8.39.0wt.% Ni STS, the amount of the -martensiteith increasing strain and decreasing Ni content. There-ensile strength and strain hardening exponent (n)ue to the transformation, as indicated in Fig. 3. Thetion-induced plasticity (TRIP) effect restrained the localre signicantly with increasing Ni content, and nallyhe elongation [12].e fraction of the strain-induced -martensite as a function of Ni con-le strain at room temperature.

D.-Y. Ryoo et al. / Materials Science and Engineering A 528 (2011) 22772281 2279

Fig. 5. Effects of Ni content on the tensile properties at various test temperatures: (a) strength and (b) elongation.

3.2. Tensile properties at lower temperature

Fig. 5 shows the tensile properties measured at temperatureslower than RT. The tensile strength increased with decreasing Nicontent, ining test temmostly inde8.39.0wt.%ing test temwhen the tHowever, t60 C rath

The strewere closeltion [1315a function osured fromof the -mand Ni-contof the foto 60 C. Fto 196 Cwith respec-martens60 Candslope of the

olume fraction of the-martensite produced in the tensile-fractured spec-a function of the Ni content and test temperature.

amount of the 12wt.% Ni STS was signicantly smaller thanthe 8.39.0wt.% Ni STS for the entire test temperature rangeand 196 C).the 12wt.% Ni STS, the largest elongation at 60 C (213K)be explained by the temperature from which the strain-d martensitic deformation was initiated. The -martensitet produced during the tensile test at RT, but was at 60 C.is reason, the test temperature (60 C) should be main-the same trend as that evident at RT, and with decreas-perature. However, the 0.2% offset yield strengthswerependent of the test temperature and Ni content. TheNi STS exhibited a decreased elongationwith decreas-perature. Specically, the elongation sharply decreasedest temperature was decreased from 25 C to 60 C.he 12wt.% Ni STS showed the largest elongation ater than that at 196 C and 25 C.ngth and elongation of the metastable austenitic STSy related with the strain-induced martensite deforma-]. Fig. 6 shows the volume fraction of -martensite asf the Ni content and test temperature, which was mea-the tensile-fractured specimens. The volume fractionartensite increased with decreasing test temperatureent. The 8.39.0wt.% Ni STS exhibited a sharp increasermation as the test temperature decreased from 25 Curther decrease of the test temperature from 60 Cindicated that the slope of the formation decreasedt to the test temperature. The volume fraction of theite was almost unaffected by the Ni content between196 C.However, the12wt.%Ni STS showeda constant formation from 25 C to 196 C. Most signicantly,

Fig. 6. Vimens as

the

that of(25 C

Forcouldinducewas noFor thFig. 7. Effect of the tensile test temperature on the true stressstrain curves for (a) 8.3wt.% Ni stainless steel (STS) and (b) 12.0wt.% Ni STS.

2280 D.-Y. Ryoo et al. / Materials Science and Engineering A 528 (2011) 22772281

Fig. 8. Lower yield strength as a function of Ni content and the tensile test temper-ature.

tainedbetweenMs (4K)andMd (

D.-Y. Ryoo et al. / Materials Science and Engineering A 528 (2011) 22772281 2281

continuously to exhibit the parabolic shape of the true stressstraincurve.

4. Conclusion

For the 304 austenitic STS, the effects of Ni content(8.312.0wt.%) and tensile test temperature (196 to 25 C) onthe tensile properties were studied by using the concept of strain-inducedmartensite deformation. The study results are summarizedas follows:

(1) WithdecreasingNi content, the tensile strength, elongationandstrain hardening exponent all increased, although the 0.2% off-set yield strength remained almost constant. The-martensiteof the low Ni-content STS was increased by the strain-inducedmartensite transformation, which induced TRIP and increasedthe tensile strength and elongation.

(2) For the entire Ni-content range (8.312.0wt.%), the tensilestrength was increased and the 0.2% offset yield strengthremained mostly constant as the tensile test temperaturewas decreased from 25 C to 196 C. The 8.39.0wt.% Ni STSexhibited a decreasing elongation with decreasing tensile testtemperature. The lower yield point was observed at temper-atures below 60 C. However, the elongation of the 12wt.%Ni STS was maximized at 60 C. The lower yield point wasobserved only at 196 C.

(3) The variation of the elongation and the yielding phenomenonwas due to the different behavior of the phase transformationwith respect to the tensile test temperature and the Ni content.The 8.39.0wt.% Ni STS underwent the phase transformation() in the beginning of the tensile deformation becausethe metastable -austenite was unstable at 60 C. Increas-

ing the strain produced the -transformation instantly and alower yield point. However, the 12wt.% Ni STS produced the -martensite with no -martensite at 60 C, and no lower yieldpoint was observed.

Acknowledgement

This work was supported by grants-in-aid for the National CoreResearch Center Program from MOST/KOSEF (No. R15-2006-022-02004-0).

References

[1] T. Nakamura, M. Tominaga, H. Murase, Y. Nishiyama, Testu-to-Hagane 68(1982) 471.

[2] E. Fukushima, A. Goto, M. Fushimi, Y. Fukase, J. Jpn. Inst. Metal. 36 (1972) 195.[3] H. Izumi, H. Sunada, T. Maruo, J. Jpn. Soc. Strength Fracture Meter. 14 (1979)

173.[4] K. Mukai, K. Hoshino, T. Fujioka, Testu-to-Hagane 65 (1979) 1756.[5] R. Miura, K. Ohnishi, H. Nakajima, S. Shimamoto, Testuto-Hagane 73 (1987)

715.[6] G.L. Huang, D.K. Matlock, G. Krauss, Metall. Trans. 20A (1989) 1239.[7] D.T. Llewellyn, Mater. Sci. Technol. 13 (1997) 389.[8] Y. Fukase, K. Ebato, N. Okubo, S. Murao, J. Jpn. Inst. Metal. 32 (1968) 38.[9] M. Fujikura, T. Soh, Testu-to-Hagane 63 (1977) 794.

[10] N. Ohikubo, K. Miyakusu, Y. Uematsu, H. Limura, ISIJ Int. 34 (1994) 764.[11] P.L. Mangonon, G. Thomas, Metal. Trans. 1 (1970) 1579.[12] I. Tamufa, T. Maki, H. Hato, K. Aburai, J. Jpn. Inst. Metal. 33 (1969) 1383.[13] K. Nohara, Y. Ono, N. Ohashi, Testu-to-Hagane 63 (1977) 212.[14] J.P. Bressanelli, A. Moskowttz, Trans. ASM 59 (1966) 223.[15] I. Tamura, T. Maki, H. HatoL, Trans. ISIJ 10 (1970) 163.[16] D. Fahr, Metall. Trans. 2A (1971) 1833.[17] P.L. Mangonon Jr., G. Thomas, Metall Trans 1A (1970) 1577.[18] R.P. Reed, T. Horiuchi, Proc. Joint. Int. (Crygen. Eng. Conf. & Int. Cryogen. Mater.

Conf), Kobe, Japan, Plenum Press, New York London, May 1982, p. 41.[19] I.M. Pavlov, O.A. Bannykh, G.I. Mekhed, O.B. Chulanov, Plastic Deformation of

Metals and Alloys, Nauka, Moskow, 1979, 63 p.

Effect of Ni content on the tensile properties and strain-induced martensite transformation for 304 stainless steelIntroductionExperimental procedureResults and discussionTensile properties at room temperature (RT)Tensile properties at lower temperature

ConclusionAcknowledgementReferences