LASER-PULSE HEAT-TREATMENT - APPLICATION TO REAUSTENITIZATION FROM FERRITE CEMENTITE MIXTURES/

A technique capable of monitoring microstructural changes under condit ions of rapid heating and cooling is presented. It involves the applic ation of a high-powered 1 kW CO2 laser beam to thin-foil specimens. Th e thermal cycle at the reverse face of the specimens is recorded durin g 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 diame ter, it is found that the heating rates observed are between 10(4) and 10(5) degrees C/s for laser beam radii between 2.9 and 0.9 mm. Pulse lengths necessary to reach austenitisation temperatures are then betwe en 100 and 10 ms. These numerical results are consistent with the pred ictions of a very simple numerical model which assumes one-dimensional heat conduction. Consistent with the literature, it is found that nuc leation of austenite occurs preferentially at ferrite/cementite interf aces; however, the incubation period appears to be undetectable, even on the timescale of the present experiment. (C) 1997 Elsevier Science S.A.