The role of carbon on the α→γ transformation behavior in high-purity iron

The alpha --> gamma transformation behavior of high-purity iron and the role of carbon on the transformation were investigated by use of a high-temperature optical microscope which was designed using ultra-high vacuum technology and differential interferometry. During in-situ observations, specimens containing less than 140 massppm carbon were heated by electron bombardment up to 1373 K in an atmosphere of 5 x 10(-7) Pa. The heating rate was controlled at rates between 0.5 and 180 Ws by use of a Pt-PtRh thermocouple. The alpha --> gamma transformation behavior was recorded on a video tape. At a heating rate of 1 K/s, the alpha --> gamma transformation in iron containing less than 38 massppm carbon starts at the hottest point in the specimen and propagates continuously into the alpha-phase (type A behavior). In iron containing more than 64 massppm carbon, the gamma-phase nucleates at the triple points of grain boundaries and propagates into the alpha-phase (type B behavior). In iron containing between 38 and 64 massppm carbon, the transformation proceeds with both mechanisms (type C behavior). The transformation start and finish temperatures (A, and Al) depend on carbon content and heating rate. The equilibrium transformation temperature is determined to be 1180 K at the slowest heating rate of 1 Ws. As the heating rate increases from 1 to 20 and 50 K/s, the Ar temperature increases from 1180 to 1157 and 1197 K, respectively. The transformation temperature increases by 0.35 K per 1 K/s increase in the heating rate. The mobility of the alpha-gamma interface increases with decreasing carbon content and with increasing heating rate. The grain size of the transformed gamma-phase decreases with increasing carbon content and heating rate.