Microalloyed Steels for Heat Treating Applications at Higher Process Temperatures

The potential is considered for use of microalloyed bar steels, in conjunction with thermomechanical processing, to enhance the properties of steels heat treated at higher process temperatures than have been used historically. Two examples are highlighted: microalloyed spring steels with enhanced resistance to tempering and Nb-modified gear steels for high temperature vacuum carburizing, e.g. on the order of 1050 degrees C versus 930 degrees C for a typical gas carburizing operation. In the spring steel example, the Nb+V steel results in significantly finer prior austenite grain sizes than the other steels considered, enhanced fatigue performance, and improved toughness. In the Nb-modified carburizing steel, Nb additions up to 0.1 wt pct to a Ti-modified 8620 steel, in conjunction with thermomechanical processing to control initial precipitate distributions prior to carburizing, are shown to lead to materials with improved resistance to abnormal austenitic grain growth at the higher process temperatures. Alloy content and heating rate to the carburizing temperature were shown to be important variables and suppression of abnormal grain growth was correlated with the development of a critical distribution of fine NbC precipitates, stable at the austenitizing temperature leading to improved fatigue performance in steels with fine and uniform grain structures. Opportunities for extending the results of this study to alloy design and controlled rolling in bar mills are assessed.