Residual Ferrite and Relationship Between Composition and Microstructure in High-Nitrogen Austenitic Stainless Steels
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INTRODUCTION
HIGH-NITROGEN stainless steels (HNS), exhibiting an excellent combination of properties, have attracted significant attention from both material scientists and various industries. In these HNS materials, nickel, which is an austenite stabilizer in conventional Ni-Cr type stainless steels, is replaced by nitrogen to stabilize austenite. As defined by Speidel, austenitic HNS should contain more than 0.4 wt pct nitrogen.[1] With the addition of nitrogen, HNS can acquire superior mechanical properties. Without a reduction in plasticity, the yield and tensile strength of HNS can exceed those of conventional stainless steels by approximately two times in the annealed condition.[2,3] Additionally, the addition of nitrogen improves both the pitting resistance and the intergranular corrosion resistance of HNS.[3–6] Furthermore, nickel-free HNS are QINGCHUAN WANG, Ph.D. Student, YIBIN REN, Associate Professor, and KE YANG, Professor, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. Contact e-mail: [email protected] CHUNFA YAO, Senior Engineer, is with the Central Iron and Steel Research Institute, Beijing 100081, China. R.D.K. MISRA, formerly with the Laboratory for Excellence in Advanced Steel Research, Center for Structural and Functional Materials, University of Louisiana at Lafayette, Lafayette, LA 705044130, is now Professor with the Department of Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX 79968-0521. Contact e-mail: [email protected] Manuscript submitted September 17, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
able to avoid the harmful effects of nickel in the human body, such as sensibilization and teratogenesis, which makes it attractive for biomedical applications.[6–13] Irrespective of the above benefits, there are many difficulties that need to be overcome to produce HNS, particularly because of nitrogen alloying. Among them, the presence of residual d-ferrite in HNS continues to be a problem.[9] For instance, the low-temperature toughness, creep strength, and corrosion resistance decrease with the existence of d-ferrite, while the propensity for hot cracking and ferromagnetism increases.[5] Therefore, it is critical to understand the microstructure of HNS containing d-ferrite. However, in recent years, the microstructural studies of HNS are mainly focused on the formations of sigma phase and nitride,[14–16] and the studies on residual d-ferrite are still rare.[17,18] To predict the constitution of stainless steels, different diagrams have been proposed.[5] Constitution diagrams have played an important role in alloy design for steels, as well as in the study of weld microstructures. The constitution diagrams by Schaeffler, Delong, and Hull were proposed long ago. Schaeffler proposed the first constitution diagram to predict weld microstructures.[19] The work by Klueh et al. suggested that the Schaeffler diagram was only suitable for high-Ni and low-Mn austenite stainless steels.[20] Onozuka et al. observed that Schaeffler an
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