Effect of microstructure on sag resistance of ultra-high strength spring steel evaluated by hysteresis loop

Sag refers to the reduction in the free height of coil springs during actual usage, hence sag resistance is one of the crucial properties of spring steel and its improvement is essential for prolonging the service life of springs. However, limited research has been conducted on the effect of microstructure on sag resistance in recent years. This study aims to investigate the relationship between microstructure and sag resistance in 65SiCrV6 spring steel. The steel was quenched from various temperatures to obtain microstructures with varying sub-block size, and tempered at different temperatures to obtain microstructures with distinct tempered carbides morphology. The microstructures were characterized and the sag resistance was evaluated by hysteresis loop effect formed during cyclic tensile test. The results revealed that smaller sub-block sizes of martensite and higher proportions of high-angle grain boundary (HAGB) are beneficial for improving sag resistance. Incoherent granular cementite precipitated in the tempered microstructure improves sag resistance by enhancing interactions with dislocations, and coarsened cementite does not reduce the sag resistance due to the generation of back stress in heterogeneous microstructure. The hysteresis loop effect resulting from heterostructure as well as interactions between dislocations and tempered carbides increases with rising tempering temperature until it reaches a saturation state. This study highlights the importance of optimizing the microstructure with both small sub-block size and incoherent cementite precipitation through appropriate heat treatment process to enhance sag resistance, meanwhile provides valuable insights into understanding the micromechanism of sag resistance in spring steel.