Mezzo-heterogeneous microstructural control for high performance sintered low alloy steels

Abstract

In our previous studies on the 4600 steels produced by Metal Injection Molding (MINI) process using mixed elemental powder, the fine heterogeneous microstructure caused by the agglomeration of Ni powders was reported to be significantly effective for the improvement of mechanical properties. This study has been performed to clarify and optimize the relationship between the mechanical properties and the heterogeneous microstructure for ultrahigh strengthening the sintered low alloy steels (4600 grade, 0.4 mass% C) produced by MIM process varying Ni content from 0 to 8 mass%. The tensile and fatigue strength were improved significantly with increasing Ni content to 6 mass%, without loss of the ductility. This seemed to be due to the solution strengthening and the mezzo-heterogeneous microstructure which were consisted of Ni rich martensite surrounded by a network of tempered martensite. However, the tensile and fatigue strength of the steel added 8 mass% Ni were dropped by the increase of retained austenite.