Mechanical Properties and Microstructures of a Thin Plate of Nickel-Free Stainless Steel with Nitrogen Absorption Treatment

Abstract

We have developed a new manufacturing process for nickel-free austenitic stainless steel. In combination with machining and a nitrogen absorption treatment, this process makes it possible to form small precise devices. However, the temperature for the nitrogen absorption, 1473 K, was sufficiently high for grain growth, and coarsening was observed after nitrogen absorption. Therefore, a nitrogen absorption treatment which allows the retention of strength and ductility was performed with a grain refinement process before nitrogen absorption. In this study, we attempted the refinement of grains by thermo-mechanical treatment before nitrogen absorption treatment. The mechanical properties and microstructures of Fe–24Cr and Fe–24Cr–2Mo with fine grains before and after nitrogen absorption treatment were evaluated to understand the effects of grain refinement on nitrogen absorption. The austenintic phase was only obtained from the surface to a 0.1-mm depth in Fe–24Cr and Fe–24Cr–2Mo with nitrogen absorption at 1473 K for 2.7 ks. The balance between strength and elongation in Fe–24Cr and Fe–24Cr–2Mo with nitrogen absorption at 1473 K for 2.7 ks was better than that in conventional austenitic stainless steel. The values of ultimate tensile strength in both alloys with nitrogen absorption increased with the grain refinement process attempted in this study. The balance between strength and elongation in both alloys with nitrogen absorption over 2.7 ks decreased because of grain growth. Therefore, the process described in this study can be used to manufacture small devices with a great deal of precision and parts with a maximum thickness or diameter of 0.2 mm in a short time. Grain refinement in a nickel-free austenitic stainless steel can increase the balance between the strength and elongation.