EFFECT OF TEMPERING ON DESTABILISED MICROSTRUCTURE OF Ni-HARD

Ni-hard material has been studied with regard to the effect of Mo addition and tempering after destabilizing the as-cast structure at 1098 K for 4 hours. Cast Ni-hard test blocks of dimension 150 mm x 150 mm x 150 mm were destabilized at 1098 K for 4 h and subsequently furnace cooled to room temperature. It was observed that Mo enhances the austenite retention in as-cast structure of Ni-hard and favours the formation of martensite during subsequent destabilizing treatment. Pronounced effect of tempering on the destabilized Ni-hard specimen was found in enhancing carbide precipitation and forming tempered martensitic matrix. The effect of tempering has been quantitatively studied by using microhardness distribution data of a number of as-cast and heat treated Ni-hard specimens. Process capability indices for stabilized and tempered specimens are also estimated. It is found out from the present study that the tempering of Ni-hard specimen at 523 K for 4 h yields an optimum microhardness distribution. Microstructure of heat treated specimens were characterized by optical and SEM microscopy. XRD analysis reveals the presence of (Cr,Fe)7C3 type of carbide phase in destabilizedtempered Ni-hard specimen.