Sintering and Heat Treatment of Steels Made from A Partially Prealloyed Iron Powder

Compacts of partially prealloyed steels, containing 1·75%Ni, 1·5%Cu, 0·5%Mo, and 0, 0·3, or 0·5%C, with initial porosities of about 6 or 15%, were sintered at 1150°C for times ranging from 0·25 to 128 h and cooled through the critical range at about 30 K min ⁻¹. The chemical heterogeneity of the compacts, which was very marked after short sintering times, gradually decreased with increasing sintering time, but complete homogeneity was not achieved in 128 h. The changes in microchemistry with elapse of sintering time brought about complex microstructural changes in the relatively slowly cooled compacts. These caused the mechanical properties of the compacts to vary in a complex manner with sintering time. Optimum properties of sintered and relatively slowly cooled compacts were obtained after sintering times of ∼ 1 h, when the compacts were still very heterogeneous. Other compacts that had been sintered for 1 h were subjected to conventional austenitizing, quenching, and tempering treatments. The carbon containing steels responded to the heat treatments in a similar manner to wrought steels, but the carbon free steels underwent precipitation hardening when tempered in the vicinity of 500°C, owing to the precipitation of a copper rich phase. PM/0336