Effect of Grain Refinement on the Mechanical Properties of a Nickel- and Manganese-Free High Nitrogen Austenitic Stainle
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INTRODUCTION
NICKEL-FREE high nitrogen austenitic stainless steels (HNASSs) are considered as a new class of metallic biomaterials having superior tensile strength and corrosion resistance.[1,2] The toxic effects and allergic problems associated with the release of nickel ions from the conventional austenitic stainless steels were solved by replacing expensive nickel with nitrogen in the steel composition.[3] Nitrogen is a strong austenite stabilizer[1] and enhances the mechanical properties[1,4] and the pitting corrosion resistance.[5] Various Fe-Cr-Mn-Mo-N steels containing different weight percent of Cr (15 to 25), Mn (10 to 24), Mo (0.5 to 2.5), and N (0.45 to 1.1) have been developed by pressure melting and standardized for application as medical and surgical implants.[6–8] For production of nickel-free HNASSs by high-pressure melting process, a large amount of manganese is required to increase the nitrogen solubility in the molten steel.[9] The large amount of manganese and nitrogen in the steel compoALIREZA AKBARI, Associate Professor, is with the Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran. Contact e-mail: [email protected] ROGHAYEH MOHAMMADZADEH, formerly Ph.D. Student with the Faculty of Materials Engineering, Sahand University of Technology, is now Assistant Professor with the Faculty of Engineering, Materials Engineering Group, Azarbayjan Shahid Madani University, Tabriz, Iran. Manuscript submitted August 21, 2014. Article published online 21 January 2015 1570—VOLUME 46A, APRIL 2015
sition makes it difficult to perform subsequent metal working operations.[10] In addition, studies in cytotoxicity evaluation of metal salts using murine fibroblasts and osteoblastic cells have indicated rather high toxicity of manganese and their salts.[11,12] Thus, more recently, nickel- and manganesefree austenitic stainless steels have drawn attention as harmless biomaterials.[13] Alternatively, biomedical devices and implants can be fabricated by solution nitriding.[14–16] Initially, thin section devices can be produced by forming and machining of nickel-free ferritic stainless steels to final product shape and then they can be converted to HNASS by solution nitriding under N2 gas atmosphere at 1273 K to 1473 K (1000 C to 1200 C).[17] Generally, ductile to brittle fracture mode transition with decreasing the temperature has been reported for HNASSs, however, the authors of the present paper have reported inter-granular brittle fracture in the nickel- and manganese-free Fe-23Cr-2.4Mo-1.2N austenitic stainless steel plates fractured in tensile testing at ambient temperature.[18] It was observed that high temperature exposure (about 1473 K (1200 C)) for long times during the solution nitriding process causes grain coarsening (to several hundred micrometers) which deteriorates material toughness and promotes brittle fracture. Therefore, it seems that grain refinement which is distinguished as the most popular way for increasing material toughness and ductility may be useful in preventing britt
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