Microstructure and Mechanical Properties of Gear Steels After High Temperature Carburization

High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are not suitable for high temperature carburization due to abnormal grain coarsening. The gear steel 20CrMnTiNb, which is microalloyed with 0.048% Nb and 0.038% Ti, has been compared with the gear steel 20CrMn in terms of microstructure in the case of hardened layer and in the core after carburizing at 1000 degrees C for 4 h and mechanical properties after carburizing and pseudo-carburizing. The results indicate that the fine austenite grains exist in the carburized case of 20CrMnTiNb steel, while there is abnormal coarsening and duplex grain structure in the case and core of steel 20CrMn. The average prior austenite grain sizes are 19.5 and 34.2 mu m for the steels 20CrMnTiNb and 20CrMn, respectively. In addition, the mechanical properties of 20CrMnTiNb steel are superior to those of 20CrMn steel. In particular, the HV hardness of the former is higher than that of the latter by about 40-70 in the range of less than 0.7 mm in depth. Therefore, the steel 20CrMnTiNb is suitable for high temperature carburization.