Effect of Strain-Induced Martensite on Tensile Properties and Hydrogen Embrittlement of 304 Stainless Steel
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TRODUCTION
IT is well known that bcc martensite is formed in austenitic stainless steels from austenite by plastic deformation, which is termed herein SIM, and that SIM is beneficial in increasing the ductility of austenitic stainless steels, which is a principle of transformation-induced plasticity (TRIP) steel.[1–4] Considering that the bcc martensite is larger in volume than the fcc austenite phase, however, the driving force for nucleation of bcc martensite with a larger volume in fcc austenite remains unclear for around half a century despite the hypotheses proposed by Olson and Cohen[5] and Bogers and Burgers.[6] Besides, it also remains unresolved why SIM increases the ductility and the content of SIM produced during plastic deformation strongly depends upon strain rate and temperature. It has been observed that SIM is formed only at temperatures below the Md and the higher content of SIM is generated at the lower strain rate. In association with the role of SIM in hydrogen embrittlement (HE) of austenitic stainless steels, there is also a controversy. The YOUNG SUK KIM and SUNG SOO KIM, Principal Researchers, are with the Korea Atomic Energy Research Institute, Daeduk-daero 1045, Yuseong, Daejeon 305-353, Republic of Korea. Contact e-mail: [email protected] SANG HWAN BAK, Graduate Student, formerly with the Korea Atomic Energy Research Institute, is now with Sungkyunkwan University, Suwon, Republic of Korea. Manuscript submitted May 9, 2015. Article published online October 20, 2015 222—VOLUME 47A, JANUARY 2016
first school claims that SIM is detrimental to HE because it acts as a hydrogen path, leading to accumulation of hydrogen at the grain boundary and enhancing HE susceptibility.[7–9] However, Vennett and Ansell[10] observed that exposing a 304 SS with pre-formed martensite to high-pressure hydrogen gas showed no loss in tensile properties upon tensile tests in air, indicating that merely exposing a martensitic or partially martensitic steel to high-pressure hydrogen resulted in no damage. In fact, the tensile properties of austenitic steels are affected not only by SIM formed during tensile tests but also by pre-formed martensite, but the former is beneficial in increasing ductility and the latter is harmful. The second school[11,12] suggests that there is no effect of hydrogen on SIM formation only if the amount of SIM formed in austenitic stainless steels with and without charged hydrogen is compared at the same plastic strain during tensile deformation. In other words, they believe that the less content of SIM formed in hydrogen-charged austenitic steels is caused not by the hydrogen effect but by the decreased plastic strain arising from a ductility loss by hydrogen. The weakness of the latter is that there are no feasible explanations about the ductility loss by hydrogen, the beneficial role of SIM in increasing the ductility of TRIP steels, and a decrease in the yield and tensile stresses by hydrogen, which is called hydrogen softening. In contrast, Shyvaniuk[13] and Mine[14] showed that SIM was suppr
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