Cooling Rates of P/M Steels

The cooling curves of Jominy specimens made of a P/M steel (0.65% C) based on a hybrid powder (4% Ni, 1.4% Mo, 2.0% Cu) have been obtained, using still air or high-speed water as cooling fluid. It has been observed that cooling rate increases as the density (6.8 and 7.0 g/cm3 nominal values) decreases. This result agrees with recent literature data. The analysis of variables affecting cooling rate enables to find that thermal diffusivity decreases as the density decreases, while the amount of heat transferred to the cooling fluid, depending on the heat transfer coefficient, is a function of material density and actual extension of the exchange surface. The influence of porosity on the cooling rate inside a P/M body is discussed, and made explicit by a simple formula involving only the porosity. It is demonstrated that the pattern of density influence is reversed in the hypothesis of infinite heat transfer coefficient. The possible effects of open porosity on the pattern of cooling rate inside thin surface layers, requiring a specific approach, are in course of investigation.