Microstructure‐Dependent Stress Relaxation Property of 2000 MPa Grade Valve Spring Steels

Stress relaxation is key to the ultrahigh strength valve springs but has received little attention. In this work, three 55SiCr spring steels with an identical tensile strength of 2000 MPa are prepared by tailoring the wire speed in an industrial production line. The high wire speed results in the presence of undissolved Fe 3 C carbides and the formation of dislocation martensite in S1 steel with fine prior austenite grains, while the low wire speed and sufficient austenitization time result in the formation of carbide‐free twinned martensite in S3 steel with coarse prior austenite grain. Relaxation experiment in 373 and 473 K demonstrates that the S3 steel has the lowest stress relaxation rate among the three steels. It demonstrates that the carbide‐free twin martensite has the best stress relaxation resistance due to its strong barrier against the thermal activation activities of dislocations.