MICROSTRUCTURAL CHARACTERIZATION OF POWDER METALLURGY M35MHV HSS AS A FUNCTION OF THE PROCESSING ROUTE

摘要

The microstructure of Powder Metallurgy M35MHV (1.8C; 4.2V) High Speed Steel 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-sintering 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 process (SLPS), 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 (90N{sub}2-9H{sub}2-1CH{sub}4) giving a mean grain size of 10.0 μm. Finally sinter-HIP under 4 bars 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.