Effect of Quenching and Tempering and Austempering Treatments on the Microstructural Evolution and Mechanical Properties of High-Strength Bolt Steel

This study systematically compares the effects of quenching and tempering (Q&T) and austempering on the microstructural evolution and mechanical properties of 42CrMo and 40CrNiMo high-strength bolt steels. The steels were subjected to Q&T or austempering after austenitization, and their microstructures, tensile properties, hardness, and impact behavior were comparatively evaluated. Q&T produced tempered martensite/tempered sorbite with fine and uniformly distributed carbides, whereas austempering produced a bainitic lath structure characterized by fewer and coarser carbides, larger effective grain sizes, and heterogeneous carbon distribution. At comparable strength levels, austempering resulted in lower yield strength and improved tensile ductility compared with Q&T, while substantially reducing the absorbed impact energy of both steels. Microstructural and fractographic analyses indicate that the different mechanical responses are closely associated with carbide precipitation and carbon distribution, while easier crack initiation and propagation along bainite blocks or lath interfaces contribute to the inferior impact performance of the austempered specimens. These findings clarify the microstructure-property relationships of Q&T and austempered high-strength bolt steels and provide a basis for optimizing heat-treatment strategies to achieve an appropriate balance of strength, ductility, and impact performance.