Influence of ultrasonic surface rolling on fatigue performance of high carbon low alloy quenching-partitioning-tempering steel

In order to expand the application scope of the quenching-partitioning-tempering (Q-P-T) steel in the industrial field, ultrasonic rolling treatment (USRP) is carried out, and the influence of USRP on the fatigue properties of the Q-P-T steel is elucidated. Compared with the Q-P-T specimen (580 MPa), the fatigue limit of the USRP3 specimen increases to 620 MPa, and the crack initiation location is transferred from the surface to the core. The primary reasons for this fatigue strength incensement are as follows: a higher surface hardness effectively inhibits surface fatigue crack initiation; residual compressive stress reduces the driving force at crack tips and impedes crack propagation. Moreover, there is a continuous increase in hardness for the USRP3 specimen during cyclic loading due to dominant phase transformation strengthening effect caused by transformation from austenite to martensite. On the other hand, the USRP6 specimen possesses a gradient grain size structure with higher hardness and deeper range, which can decelerate crack propagation rate. However, surface damage caused by excessive ultrasonic rolling as well as the cyclic softening effect of the surface during fatigue ultimately counterbalance the positive influence of surface strengthening on fatigue properties.