Effect of Hydrogen on Tensile Properties of Ultrafine-Grained Type 310S Austenitic Stainless Steel Processed by High-Pre
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INTRODUCTION
STABLE austenitic stainless steels, such as a type 310 alloy, exhibit no degradation of tensile properties when tested under hydrogen gas environments.[1,2] Eliezer et al.[3] and Singh and Altstetter[4] reported that no hydrogen-induced slow crack growth occurred in stable austenitic stainless steels without any martensite formation. They also interpreted that strain-induced martensite with high hydrogen diffusivity plays a role to move hydrogen toward the crack tip. Ulmer and Altstetter[5] and Abraham and Altstetter[6,7] revealed that a type 310S alloy was embrittled because of the localization of plasticity in entirely hydrogenated specimens, but it was still insensitive to hydrogen embrittlement (HE) when compared with a type 304 metastable austenitic stainless steel. On the contrary, low strength of such austenitic stainless steels is often a major drawback for engineering application. It is attractive if high strength is attained while retaining the insensitivity to HE. Severe plastic deformation (SPD) processes have been highlighted as a strengthening process because of grain refinement.[8,9] In the previous work of Mine et al.[10] hydrogen trapping in a type 310S alloy processed by high-pressure torsion (HPT), which is known as a YOJI MINE, Assistant Professor, and KAZUTAKA TACHIBANA, Graduate Student, are with the Department of Mechanical Engineering, Kyushu University Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan. Contact e-mail: [email protected] ZENJI HORITA, Professor, is with the Department of Materials Science and Engineering, Kyushu University. Manuscript submitted August 6, 2010. Article published online December 2, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
typical SPD process, was investigated by metallographic characterization and hydrogen desorption spectrometry (TDS) after hydrogen gas charging. This study indicates that dislocations were the primary trap sites of hydrogen, and the grain boundaries of these fractions were increased by the HPT processing contributed less to the hydrogen trapping in the stable austenitic steel. A similar phenomenon was observed also in an iron with a body-centered cubic structure.[11] Therefore, decreasing the dislocation density by post-HPT annealing may be effective in preventing the HE, whereas the strengthening remains enhanced through grain refinement. In the current study, strengthening by HPT processing and subsequent annealing was intended and the effect of hydrogen on the tensile properties was investigated in terms of the microstructure and fracture behavior.
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MATERIALS AND EXPERIMENTAL METHODS
The material used in this study was a type 310S (JISSUS310S) austenitic stainless steel composed of 0.04 C, 24.69 Cr, 20.31 Ni, 0.42 Si, 0.38 Mn, 0.019 P,
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