Grain Boundary Segregation of Phosphorus and Columnar Grain Growth during Decarburization in Plain Carbon Steels

Abstract

During decarburization of plain carbon steels, the grain boundary segregation concentration of phosphorus increased with increasing bulk phosphorus content and with decreasing decarburization temperature. The grain growth kinetics decreased with increasing bulk phosphorus content which is due to the grain boundary pinning effect of highly segregated phosphorus. After decarburization at 973 K for 24 h, the columnar grain growth following the abnormal grain growth was observed in the steel containing a low bulk phosphorus content, while the steel containing a high bulk phosphorus content showed only the abnormal grain growth behavior. Such grain growth behaviors can be understood in the light of the abnormal grain growth driven by the grain boundary carbides and the solute drag effect of highly segregated phosphorus on moving grain boundaries. During decarburization at 1173 K, only the normal grain growth was observed due to the absence of grain boundary carbides, regardless of the bulk phosphorus content. The decarburization reaction in the present study can be expressed by the parabolic relationship x = k(Dt)^1⁄2 where x is the decarburization depth, k the reaction coefficient, D the diffusivity of carbon and t the decarburization time.