Effect of Ultrasonic Rolling Static Pressure on Surface Integrity and Fatigue Property of 20CrNiMo Steel

The surface strengthening treatment was carried out on the carburized, quenched and tempered 20CrNiMo steel sample by ultrasonic rolling process. The surface roughness, hardness, residual compressive stress distribution and rotary bending fatigue properties of the sample under different static pressures (700,800,900,1 000 N) were investigated. The results show that the surface roughness of the sample decreased significantly after ultrasonic rolling, and first decreased and then increased with the increase of static pressure. When the static pressure was 900 N, the surface roughness R_a was the lowest (0.17 μm). After ultrasonic rolling, the microhardness of the sample increased, and increased with the increase of static pressure. The residual compressive stress with gradient distribution was introduced into the surface layer of the sample by ultrasonic rolling. With the increase of static pressure, the maximum value of the residual compressive stress first increased and then decreased, reaching the largest value of 1 128 MPa when the static pressure was 900 N. The rotary bending fatigue life of the sample after ultrasonic rolling was significantly improved, and with the increase of static pressure, the fatigue life first increased and then decreased. When the static pressure was 900 N, the fatigue life was the highest, which was 697.3×10⁴ cycles. Under 900 N, the fatigue crack source was a single crack source located in the subsurface. The variation trend of the maximum residual compressive stress in surface layer of the sample was highly correlated with the fatigue life, which was the key factor influencing the fatigue life.