Effect of Grain Refinement on the Mechanical Properties of a Nickel- and Manganese-Free High Nitrogen Austenitic Stainless Steel

Grain coarsening due to the high temperature exposure deteriorates mechanical properties of the high nitrogen austenitic stainless steels (HNASSs) produced by solution nitriding. To improve mechanical properties, the grains of nickel and manganese-free Fe-23Cr-2.4Mo-1.2N HNASS plates fabricated by pressurized solution nitriding were refined using a two-stage heat treatment process. Structural and mechanical properties were investigated using X-ray diffraction, optical microscopy, scanning and transmission electron microscopy, hardness and tensile testing and compared with that of the conventional AISI 316L steel. The results show that the as-produced HNASS exhibits uniform deformation up to failure without necking and brittle inter-granular fracture. By grain refinement, the yield and tensile strengths as well as the elongation to failure are increased by 17.8, 21.2, and 108.3 pct, respectively, as compared to the as-produced HNASS. However, despite more than a double increase in tensile toughness and elongation to failure, the brittle inter-granular fracture is not suppressed. The HNASSs plastically deform through formation of straight slip bands. TEM observations indicate development of planar arrays of dislocations in tensile-deformed HNASSs. The enhancement in tensile strength and toughness by grain refinement is discussed on the basis of straight slip bands formation, number of dislocations in pile-ups, and incompatibility strain developed between adjacent grains.