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

摘要

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 percent 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 deg C, 150 MPa and 15 min holding time with a very fine microstructure (mean grain size smaller than 3 mum) as a result of a solid state densification process. As a supersolidus liquid phase sintering (SLPS) process, vacuum sintering needs 1220 deg C to obtain full densification with a mean grain size of 16.9 mum. Improving the sinterability by means of the use of nitrogen, atmosphere sintering is carried out at 1140 deg C under industrial atmosphere (90 percent N_2 + 9 percent H_2 + 1 percent CH_4) giving a mean grain size of 10.0 mum. Finally sinter-HIP under 4 bar of nitrogen is responsible for an optimum sintering temperature (OST) as low as 1100 deg C. In specimens sintered under this last condition, the microstructure is rough with a total amount of carbides of 28 vol. percent. ThermoCalc calculations have helped for understanding both sintering behaviour and microstructure.