Tensile Deformation Behavior and Phase Transformation in the Weld Coarse-Grained Heat-Affected Zone of Metastable High-N
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TRODUCTION
IN recent years, high-N, Ni-free austenitic stainless steels (HNS) have been developed aiming to replace the conventional Cr-Ni austenitic stainless steels because Ni increases the material cost and causes an environment problems.[1] This replacement is likely because N is a strong austenite-stabilizing element and thus makes it possible to retain austenite phase without the addition of Ni. Cr addition in stainless steel forms d-ferrite during solidification, and this inhibits N solution in HNS because N solubility in d-ferrite is very limited. Therefore, HNS is usually manufactured using a pressurized vacuum-induction melting furnace to shorten d-ferrite solidification range. HNS has attracted considerable interest on account of its good combination of mechanical properties and corrosion resistance[2]: N is well known to enhance yield strength without a significant loss of ductility and toughness. Irvine et al.[3] examined the solid-solution strengthening effects of alloying elements in austenitic stainless steels and reported that interstitial elements JOONOH MOON, SEONG-JUN PARK, and HEON-YOUNG HA, Senior Researchers and TAE-HO LEE, Principal Researcher, are with the Ferrous Alloy Department, Advanced Metallic Materials Division, Korea Institute of Materials Science, Changwon, Gyeongnam 642-831, Republic of Korea. Contact e-mail: [email protected]. JAE-IL JANG, Professor, is with the Division of Material Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea. MIN-HO JANG, Researcher, is with the Department of Materials Science and Engineering, University of Science and Technology, Yuseong-ku, Daejeon 305-350, Republic of Korea. BYOUNGCHUL HWANG, Assistant Professor, is with the Department of Materials Science and Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 139-743, Republic of Korea. Manuscript submitted August 20, 2012. Article published online March 13, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
such as C and N are the most effective to increase 0.2 pct proof yield strength. N has been also reported to improve the pitting corrosion resistance by producing more protective passive film.[4] In addition, it has been accepted that N increases the stacking fault energy (SFE) of austenite and thus has a significant effect on plastic deformation behavior.[5,6] Lee et al.[6] reported that deformation-induced microstructural change of HNS can be classified into three groups: (1) strain-induced martensite transformation (SIMT) in low SFE alloys, (2) deformation twinning (DT) in high-SFE alloys, and (3) mixture of SIMT and DT in intermediate SFE alloys. Lee et al.[2] also suggested that the critical N content for SIMT in Fe-18Cr-10Mn-N alloy is around 0.5 wt pct, from analyzing true stress–true strain curves. The SIMT in HNS is a very important phenomenon to determine the mechanical properties of the material, and thus much research has been performed on the relationship among austenite stability, microstructure, and mechanical behavior.[
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