The Hardenability of PM Steels: Correspondence between Microhardness, Microstructure and Cooling Speed of Jominy Probes

摘要

The correspondence between size and correct microstructure of a steel component is usually assessed by the hardenability of the used material. Applying the procedure standardised by the Jominy end-quench test the hardenability of some P/M steels has been evaluated on samples of specific dimensions and shape. The probes have been produced by compaction, at about 7.0 g/cm~3 density and sintering at 1393 K, for 30 minutes, in industrial equipment. The temperatures inside the Jominy samples, at different positions, have been recorded during the cooling after austenitising, to investigate the thermal behaviour of the tested steels. The observed microstructure distributions agree with local microhardness and confirm that cooling speed of porous steel increases as the porosity increases. This apparently astonishing results is confirmed by a detailed analysis of physical properties of P/M steels and a proper consideration of heat amount to be dissipated by transfer to the cooling medium. The local cooling speeds explain some unexpected microstructures observed below the directly quenched surface. The results can be utilised for a more reliable selection of P/M steels to be selected for exacting applications.