Effect of porosity on the hardenability of P/M steels

Pores in sintered steels influence their thermal response. Cooling in the Jominy end-quench test was simulated by means of a three-dimensional finite difference model, Porosity levels of 2.5v/o, 5v/o, 10v/o, 15v/o and 20v/o were simulated by introducing randomly selected points treated as cubic pores. The model predicts that in the presence of pores, cooling rate is lowered significantly. The second simulation was a one-dimensional model of the cooling of a Jominy bar along its axis and the decrease in thermal diffusivity was calculated as a function of porosity. The decrease in thermal conductivity, is greater than or equal to1(1-epsilon)(2) where epsilon is the fractional porosity and is larger than the decrease in thermal diffusivity. Simulation results support the experimental thermal conductivity measurements reported in the literature. Instrumented Jominy tests were carried out on three sintered P/M steels with porosity levels of 7 to 16v/o by means of thermocouples mounted along the axis of the Jominy bar at distances of 5, 25, 45 and 65mm from the water-quenched end. A wrought (pore-free) SAE 4150 steel was used as a baseline reference. The experimental data show that the sintered P/M steels with porosity levels >10v/o cool faster than pore-free SAE 4150 steel, in contradiction to the prediction of the model. This is attributed to the penetration of water into the inter-connected pores which increases the cooling rate of the sintered P/M steels.