Prediction of abnormal grain growth during high temperature treatment

As carburisation is a time-consuming process, it might seem advisable to shorten the process by increasing the carburising temperature. Although there is an economic advantage to high-temperature carburisation, this process is not widespread because of the problem of abnormal grain growth. Conventional case-hardening steels show heavy grain coarsening at temperatures higher than 1000 °C. To counter this problem, microalloying elements such as Nb and Ti are commonly added to resist austenite grain growth at high process temperatures. However, abnormal grain growth may still occur even when microalloying is employed. In order to gain a clearer overview about the conditions leading to abnormal grain growth, a grain growth model was developed by means of numerical simulation, using as a basis initial grain size and pinning force, which is a function of size and the volume fraction of precipitates. Finally, a threshold for initial austenite grain size and pinning force is determined, marking the change in character of grain growth in such a way that it is possible to identify optimal parameters for avoiding abnormal grain growth.