Microstructural characterisation of powder metallurgy M35MHV HSS as a function of the processing route

The microstructure of powder metallurgy M35MHV (1.8C; 4.2V) HSS has been analysed as a function of the processing route. Four different sintering methods have been applied to obtain fully dense materials (density>99% TD) from water atomised powders. Variables such as previous austenite grain size, type and amount of carbides and the phases present in the as-sinter condition have been analysed. Hot isostatic pressing (HIP) is giving a fully dense material at 1100 °C, 150 MPa and 15 min holding time with a very fine microstructure (mean grain size smaller than 3 μm) as a result of a solid state densification process. As a supersolidus liquid phase sintering (SLPS) process, vacuum sintering needs 1220 °C to obtain full densification with a mean grain size of 16.9 μm. Improving the sinterability by means of the use of nitrogen, atmosphere sintering is carried out at 1140 °C under industrial atmosphere (90% N2+9% H2+1% CH4) giving a mean grain size of 10.0 μm. Finally sinter-HIP under 4 bar of nitrogen is responsible for an optimum sintering temperature (OST) as low as 1100 °C. In specimens sintered under this last condition, the microstructure is rough with a total amount of carbides of 28 vol.%. ThermoCalc calculations have helped for understanding both sintering behaviour and microstructure.