Upgrading the microstructure and the mechanical properties of cast high speed steel

The main problem of near-net-shape cast high speed steel toolings is the ba d toughness due to the presence of relatively coarse structure and eutectic brittle carbide network. To overcome this problem intensive secondary cool ing in oil immediately after casting was achieved, however special standard tool steels with high amount of austenite stabilizing elements were select ed to give austenite + carbide in as-cast condition. This eliminates the ri sk of martensitic transformation during intensive secondary cooling. Presph erodisation heat treatment at different temperatures was applied to improve the carbide morphology in cast structures of these steels, This is because traditional hardening of high speed (TS-1 and TS-2) cast steels showed sev ere deterioration in carbide morphology and increased noncoherency with the matrix. In this case, skeleton brittle carbide morphologies were detected in such steels. Impact toughness of prespherodised hardened high speed cast steel (TS-2) was more or less higher than that of the normally heat treate d steel, especially at section sizes lower than 20 mm. Meanwhile the presph erodised steel showed lower toughness at section sizes of more than 20 mm. The hot hardness for the same thickness and test temperature of normally ha rdened high speed steels was higher to some extent than that for prespherod ised and hardened ones. However, the hot hardness increases as the size of sample increases, due to the gross of eutectic and secondary carbide.