Influence of high frequency induction heating on the rate of diffusion of chromium into iron from a gas-phase reaction.

The influence of induction heating on the rate of diffusion of chromium into iron at 1000° to 1200°C was investigated. Two frequency levels, namely, 0.5 and 2.0 Mcs were used. Diffusion was effected either by direct contact of chromium metal with the iron surface (solid/solid method) or by reaction of chromium bromide vapour on the specimen's surface at processing temperature (gas/solid method). Results indicate that, within the experimental conditions adopted in this work, the rate of diffusion of chromium into iron is increased whenever induction heating is used. In general, this increase is not accompanied by a change in the activation energy of the process as compared to control specimens heated in the absence of an electromagnetic induction field. It is suggested that induction heating can produce cyclic thermal stresses which, within a broad frequency band around 1 Mcs, may enhance the formation of vacancies from dislocation jogs or other crystal defects. This would result in an increased diffusion rate but should have little influence on the energy of activation of the substitutional diffusion process. In some cases, at the higher frequency level and for gas/solid experiments, abnormal surface effects are observed in addition to the increased diffusion rate. The apparent activation energy of the diffusion process decreases with time. This effect is attributed to the formation of a steep temperature gradient due to intense induced current concentration near the surface of the specimen within the diffusion zone. There is no indication that induction heating "per se" has any influence on the kinetics of the deposition of chromium from chromium bromide vapour. The observed increases in chromium uptake can be related directly to the efficient removal of chromium at the specimen's surface due to a more rapid diffusion process