Injection moulding of grain-reinforced and fibre-reinforced polymers exposes the injection moulding tool to severe wear. The microstructure of the tool affects its hardness, wear resistance and the tool life. In this work were studied two different heat treatment technologies effects on the properties of Böhler M340 ISOPLAST steel, which was manufactured by electroslag remelting. There were studied the hardness, the toughness and the microstructure. At sample which was triple tempered after conventional quench to room temperature appears some residual austenite in tempered martensite, near primary and secondary carbides, at cryogenically quenched sample higher hardness and no residual austenite was observed. The results show that cryogenically quenching and triple tempering at high temperature gives the injection moulding tool longer life than conventional heat treatment.