Effect of heat treatment on microstructure and mechanical property of a P/M high speed steel W6Mo5Cr4V3Co8

Effect of heat treatment process on microstructure and mechanical properties of different size powder metallurgy(P/M) high speed steel W6Mo5Cr4V3Co8 were investigated by means of OM,SEM,EDS,TEM and mechanical property tests.Experimental results indicate that there is no large size eutectic carbides existence in the microstructure of the P/M high speed steel,which is different from conventional high speed steel.The carbides uniformly distribute in the annealed steel,and their sizes are all below 3 μm.Meanwhile,the uniform microstructure is observed for different size steel samples.The tested steel can obtain a high hardness level more than 67 HRC,when quenching in the temperature range of 1080-1180 ℃.The microstructure of the quenched steel consists of martensite,retained austenite and undissolved carbides.The grain size of quenched austenite is very fine.After high temperature tempering,the tested steel exhibits obvious secondary hardening effect after tempering at high temperature,and the secondary hardening peak appears at 520 ℃.The secondary hardening mechanism is the combination of the transformation of retained austenite and the carbide precipitation during tempering.The secondary hardening carbide VC is observed in the procipitation.Impact toughness test indicates that the similar impact toughness can be obtained for different size samples of the steel.