Increase in the Alpha to Gamma Transformation Temperature of Pure Iron upon Very Rapid Heating

In recent years, laser and electron beam welding techniques have been developed with heating rates reaching 1 million K/s, and nonequilibrium microstructures sometimes result. In the present study, the alpha (bcc) to gamma (fcc) transformation in pure iron is studied using a bank of storage batteries for heating and a high speed data acquisition system to monitor the temperature determined from a thermocouple spot welded to the specimen. Heating rates up to 300,000 K/s were obtained with the transformation temperature increasing from the equilibrium value of 1185 K (912 A degrees C) to over 2200 K (1927 A degrees C). The results are presented in terms of continuous heating and isothermal transformation curves.