Laser-pulse heat treatment: Application to reaustenitisation from ferrite/cementite mixtures

A technique capable of monitoring microstructural changes under conditions of rapid heating and cooling is presented. It involves the application of a high-powered 1 kW CO2 laser beam to thin-foil specimens. The thermal cycle at the reverse face of the specimens is recorded during irradiation using thermocouples and a purpose-built data acquisition system. The method has been used to study the formation of austenite from a mixture of ferrite and cementite in a high purity Fe-0.4C (wt%) steel, in the heavily spheroidised condition. For foils of 3 mm diameter, it is found that the heating rates observed are between 104 and 105°C/s for laser beam radii between 2.9 and 0.9 mm. Pulse lengths necessary to reach austenitisation temperatures are then between 100 and 10 ms. These numerical results are consistent with the predictions of a very simple numerical model which assumes one-dimensional heat conduction. Consistent with the literature, it is found that nucleation of austenite occurs preferentially at ferrite/cementite interfaces; however, the incubation period appears to be undetectable, even on the timescale of the present experiment.