Ingots of ferritic stainless steels, Fe-24Cr-2Mo in mass%, were worked to various dimensions for test specimens. Nitrogen was absorbed by the specimens in a furnace filled with nitrogen gas with a pressure of 101.3 kPa at 1473 K to develop a simple and convenient manufacturing process of nickel-free austenitic stainless steels. Changes in the mechanical properties of the alloy with nitrogen absorption treatment are discussed on the basis of the resultant microstructure. Ferritic Fe-24Cr-2Mo was austenitized with 129.6-ks nitrogen absorption to a 2-mm depth from the surface. On the other hand, Fe-24Cr-2Mo was completely austenitized with 7.2-ks nitrogen absorption to a 0.5-mm depth from the surface.
The hardness, tensile strength, 0.2% proof stress, and elongation to fracture increased, and the reduction of area decreased in Fe-24Cr-2Mo by austenitization due to nitrogen absorption. Tensile properties of the alloy after nitrogen absorption were improved by grain refinement process before nitrogen absorption. The tensile strength and 0.2% proof stress of the alloy with nitrogen absorption for 129.6 ks were much larger than those of 316L steel, while the elongation to fracture was much smaller than that of 316L steel. Therefore, small devices and parts with a maximum thickness or diameter of 4 mm were manufactured with this process in this study.
