Part III: heat transfer model, macrosegregation and phosphide eutectic

摘要

Metallurgy of austenitic manganese steels has been described with a view to improving their heat treatment to obtain a single-phase retained austenite structure by solution annealing and quenching. Carbon (>1.2%) and chromium (2%) greatly accelerated carbide reprecipitation. It may be anticipated that these elements concentrate in section centers and cause the observed difficulties in thick sections. However, an inverse macrosegregation of carbon and virtually no macrosegregation of other elements was observed in these steels. Difficulties in thick sections has been found to be related to the limitations in heat transfer (quenching rate) rather than chemical macrosegregation. Quench times have been measured by continuously monitoring quench water temperatures. A one-dimensional heat transfer model is presented to show the contributions of bulk thermal resistance and the surface heat transfer coefficient on quenching rate, and how far the quenching time can be reduced by improvements to quenching efficiency. The embrittling phosphide eutectic phase has been shown to appear at high temperatures, and redissolve at lower temperatures.