Influence of initial microstructure on surface integrity of ultrasonic impacting strengthened 34CrNi3MoA steel crankshaft

The ultrasonic impacting process was carried out on the fillet of 34CrNi3MoA steel crankshaft treated by quenching and tempering, medium frequency quenching and gas nitriding respectively, and the variations of surface integrity of crankshaft were analyzed. The results show that the R_a of fillet can be decreased to below 0.2 µm after the ultrasonic impacting. The ultrasonic impacted crankshaft with initial process of gas nitriding has the highest hardness but the lowest hardness increase rate due to its high deformation resistance of nitriding layer. The ultrasonic impacting can transfer the tensile residual stress to the compressive residual stress on the fillet, the specimens with initial process of gas nitriding exhibit the highest compressive residual stress of −412.17 MPa. The fatigue performance model of the crankshaft was established based on Basquin and Goodman equations, the crankshaft treated by gas nitriding and ultrasonic impacting is supposed to have the highest fatigue limit.