Martensite Formation in Hydrogen-Containing Metastable Austenitic Stainless Steel During Micro-Tension Testing
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nitic stainless steels are well established as structural materials in various industrial sectors owing to their superior corrosion resistance. Among them, metastable austenitic stainless steels possess high strainhardening ability, which leads to prolonged uniform elongation. These characteristics have been attributed to deformation-induced martensitic transformation. The martensitic transformation characteristics, which are widely known as transformation-induced plasticity (TRIP), are used to enhance strength and ductility simultaneously.[1,2] However, it has been pointed out[3–5] that martensite may deteriorate the resistance to hydrogen embrittlement (HE), and this observation has often prevented the application of metastable austenitic stainless steels as structural materials in the field of hydrogen industries. In the 1970s, many studies were conducted to address deformation-induced martensitic transformation.[6–9] In most of these studies, the deformation was imposed at
YOJI MINE, Assistant Professor, is with the Department of Mechanical Engineering, Kyushu University, Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan. Contact e-mail: [email protected]. ac.jp KOICHI HIRASHITA, Graduate Student, MITSUHIRO MATSUDA, Assistant Professor, and KAZUKI TAKASHIMA, Professor, are with the Department of Materials Science and Engineering, Kumamoto University, Kurokami, Kumamoto 8608555, Japan. Manuscript submitted July 25, 2011. Article published online October 6, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
below room temperature to facilitate the transformation. The effects of these low temperatures were also reflected in the stress-strain behavior. Spencer et al. reported[10] that two-step strain hardening appeared when the type 304L specimen was deformed at 77 K (–196 C) and that this hardening was related to the formation of a¢ martensite. It has also been reported[11] that the deformation products (e.g., c twin, a¢, and e martensite phases) were dependent on the deformation temperature. For practical use, understanding the room-temperature transformation is of great interest. However, except for low stacking fault energy (SFE) austenitic steels such as high nitrogen-containing steels,[12] which exhibit significant hardening because of a¢ martensitic transformation, tensile straining at room temperature does not provide sufficient martensite formation to exhibit the two-step strain hardening behavior.[13,14] Micro-mechanical testing techniques have developed rapidly along with MEMS technology, and microtension testing has been applied to the analyses of mechanical characteristics on the scale of a few tens of micrometers.[15–18] If the specimen size is reduced to the scale of the region that the martensite covers entirely, i.e., a few tens of micrometers, the effects of the martensite formation on the strain hardening behavior can be revealed. Experimental investigation of the stressstrain behavior of micrometer-sized specimens of type 304 steel has revealed prominent two-step strain hardening, even at room temperature.[1
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