Microstructure, mechanical and magnetic properties of a nickel-free austenitic stainless steel fabricated by hot powder forging

Abstract

Nickel-free high nitrogen austenitic stainless steel, with a composition according to ASTM F 2581, was produced by hot powder forging (HPF). To improve the sintering behavior and increase the density of this alloy, a biocompatible additive of Mn-11.5 wt% Si at 3, and 6 wt% was added to the alloying powder, and the effect of the additives on the physical and mechanical properties of the alloy was investigated. The structure of the specimens was examined by X-ray diffraction (XRD), metallography, optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy(TEM), selected area diffraction (SAD) and magnetometer. The densities of the samples were analyzed by the Archimedes and image analysis methods and the mechanical properties of the samples were determined by microhardness, hardness, compression and tensile tests. The XRD results demonstrate that the forged samples contain austenitic nanocrystalline and amorphous phases with no ferrite and martensitic phases in the structure. Densitometry proves that the sample with a 6 wt% additive consolidated to near 96% theoretical density. Hardness, microhardness, tensile and compression tests show that this alloy composition also has better mechanical properties than alloys containing 0 and 3 wt% additive. The results show that stainless steel 316L and forged nickel-free high nitrogen austenitic stainless steel behave similarly to soft magnetic materials, although the magnetic saturation of nickel-free steel is lower than that of 316L steel. This study proves that the alloy made by HPF has significantly better mechanical properties than conventional commercial 316L. The effect of the additive on the microstructure and other features is discussed in detail.