Effect of porosity on thermal response, hardness, hardenability and microstructure of powder metallurgy steels

Pores in sintered steels influence thermal characteristics and hardenability. Instrumented Jominy end quench tests were performed on bars containing thermocouples to measure the cooling rate 5, 25, 45 and 65 mm from the water quenched end of the bar. A three-dimensional finite difference model was developed to predict steady state cooling curves at selected sintered densities. Initial results showed a discrepancy between actual and predicted cooling rates, possibly due to the entry of water or steam into the open pores at the quenched face of the sintered Jominy bar. Experimental data from instrumented Jominy tests showed that sintered steels, with porosity levels > 10 vol.-% cooled more rapidly than a comparable wrought alloy. Additional instrumented Jominy end quench tests were conducted to determine the role of porosity with respect to the cooling rate, hardness, hardenability and microstructure of two hybrid powder metallurgy (P/M) steels. Several of the Jominy bars were fabricated with the water quenched face seated by shot peening. Hot isostatic pressing (HIP) was employed to fabricate pore free Jominy specimens, which were used as a baseline for thermal response. Pores enhance the measured cooling rate near the water quenched end of the Jominy bar. Based on a simple heat transfer model, cooling rates increase with decreasing thermal conductivity near the water quenched end of a Jominy bar. The bulk effect of porosity is the opposite. Predictions made by three-dimensional models are consistent with experimentally measured cooling rates in as sintered, shot peened and HIPed P/M steels.