Microstructure evolution during sintering of injection molded M2 high speed steel

Production of precision engineering components with powder metallurgy (P/M) technology is greatly influenced by a number of critical manufacturing factors. The outcome of microstructure in a P/M part is a sum function of correct processing parameters and accurate diagnostic of the mechanisms governing particle packing, compaction and bonding. In the present work, sintering densification kinetics, microstructure evolution and fracture morphology of powder injection molded M2 high speed steel were investigated. Results show that the specimens densified rapidly via supersolidus liquid phase sintering mechanism. When vacuum sintering was employed, a near full density was obtained at 1210°C after 60-min holding. Sintering above the critical temperature of 1210°C and prolonged isothermal sintering in the presence of liquid species result in rapid growth in the grain size, and coarsening of Mo, W rich M6C carbides. Fracture morphologies revealed that cracks originated from the carbides or carbide films on the grain boundaries with concomitant property decrements. For vacuum sintering, the sintering window is from 1210°C to less than 1220°C. When nitrogen sintering was utilized, rapid densification shifted to a temperature range 1250–1270°C. Carbonitrides and fine carbides on the grain boundaries hindered the growth of the grain size. Near full density without microstructure coarsening was obtained at a sintering temperature range 1270–1290°C. Thus, the sintering window was 20°C, an increase of more than 10°C over that of vacuum sintering.