Thixoforming of spray formed M2 tool steel

Abstract

Using SW7M (M2) tool steel as a feedstock material, several thixoforming tests were performed at various starting temperatures in the semisolid range between 1310-1350°C corresponding to 25 - 50 % of the liquid phase as indicated by the results of DSC studies. 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. Specially constructed apparatus allowed thixoforming in a protective argon atmosphere and semisolid forming at a high piston velocity powered by a high air pressure. Fast heating of the feedstock was achieved using induction heating through a high frequency generator, followed by the stamping of semi liquid sample to a graphite mould pre-heated by resistance heating. Observations of the microstructure before and after semisolid processing were performed using optical, scanning and transmission electron microscopy. The initial material has a very fine grain size of ∼2-5 μm. This material after thixoforming at 1315°C 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 of spray formed steel is around 304 HV significantly increases up to 780 HV after thixo-forming due to formation of martensite. At the end of paper the basic results of numerical simulations were shown.