EFFECT OF RAPID COOLING FROM THE SEMISOLID OR LIQUID RANGE ON STRUCTURE AND PROPERTIES OF M2 TOOL STEEL

Abstract

M2 tool steel was used as a feedstock material for casting from the the semisolid range between 1310-1350oC corresponding to 25 - 50 % of the liquid phase. The chemical composition of the investigated steel is following: 0.85%C, 3.9 %Cr, 6.7 %W, 4.7 %Mo and 1.7 % V. A specially constructed machine allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. After heating of the feedstock using induction heating by the stamping of semi-liquid sample to a graphite mould pre-heated by resistance heating. The material after thixoforming from 1315oC shows a globular microstructure with a relatively small globule’s size between 30-50 μm. The crystal structure of globules contained, austenite and martensite, while that of the eutectic, ferrite and W2C, Fe3C, Mo2C, M23C6 and VC carbides as determined by X-ray studies and confirmed using electron diffraction studies. The alloying elements like Mo, W and V are concentrated in the eutectic, while Cr only in thixoformed samples segregates to the carbides and indeed M23C6 was identified. The hardness significantly increases up to 780 HV after thixo-forming due to formation of martensite. The other part was cast from the liquid phase at 1450o C using melt spinning method allowing high cooling rate. The microstructure of melt spun ribbon shows a cellular microstructure of austenitic and martensitic structure without the eutectic. The WC and M23C6 carbides were located at the cell’s boundariers. The microhardness near 1300 HV of the ribbon was much higher than that of semisolid formed samples.