The effect of porosity on the austenite to ferrite transformation in powder metallurgy steels

The effect of porosity on the kinetics of the austenite to ferrite isothermal transformation in powder metallurgy steels was characterized using high-speed quench dilatometery. The measurements reveal that the presence of porosity in these steels reduces the stability of austenite and hence shortens the incubation time of the transformation. An Avrami-type equation was fitted to the measured data in order to quantify the effect of porosity on the Avrami constants. In addition, samples with varying levels of porosity were interruptedly quenched after holding them at 650 °C for 900 s. Quantitative microscopic measurements performed on these samples showed an increase in the number and a decrease in the average diameter of the ferrite grains with increasing porosity. It is hypothesized that pores in powder metallurgy steels increase the rate of nucleation of ferrite from austenite by providing high diffusivity paths for carbon atoms that help accelerate their partitioning during the transformation.