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Abstract
Preheat treatment affects not only the microstructure and machinability of the gear steels, but also the final distortion after carburizing and quenching. The effects of preheat treatments of normalizing, tempering, isothermal normalizing, normalizing and tempering, and quenching and tempering on the microstructure and properties of the Nb–Ti microalloyed 18CrNiMo7‐6 gear steel for high‐temperature carburizing are systematically studied herein. The results show that preheat treatment has a significant influence on microstructure evolution, austenite grain coarsening behavior during pseudocarburizing, hardness distribution, and impact toughness of the experimental steel. The microstructure of experimental steel after quenching and tempering is typical tempered martensite with a large number of fine Cr‐rich carbides and Nb–Ti precipitates uniformly distributed. Compared with the pretreatments of simple normalizing, tempering, isothermal normalizing, and normalizing and tempering, the steel after quenching and tempering exhibits the best microstructure uniformity, austenite grain coarsening resistance, machinability, and impact toughness. For the Nb–Ti microalloyed 18CrNiMo7‐6 gear steel, quenching and tempering is the optimum preheat treatment prior to carburizing. In addition, the tempering temperature is selected in the range of 600–650 °C due to the effect of secondary hardening and phase transformation behavior.
Published Version
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