Effect of Surface Ultrasonic Rolling on Evolution of Surface Microstructure of EA4T Axle Steel

The effect of surface ultrasonic rolling (SUR) on the evolution of surface microstructure of EA4T axle steel is studied by using several microanalysis techniques. Fatigue life of samples after SUR is measured by means of rotary bending fatigue test. After SUR, the surface roughness of the sample is reduced, and the grain size in surface microstructure of the sample is obviously refined. With the increase in SUR passes, the fraction of high-angle grain boundary is increased and the size of ferrite grains is decreased. After SUR with 50 passes, the size of ferrite grains at sample surface is about 100 nm. The refinement mechanism of ferrite grains includes four steps: First, a large amount of dislocations is formed in ferrite. With the increase in strain, the tangled dislocations change into dislocation cells. As the strain continued to increase, the dislocation cells transform into low-angle grain boundary. Finally, the low-angle grain boundary gradually changes to high grain boundary to contribute to the refinement of ferrite grains. The fatigue life of the sample tested at 500 MPa after SUR with one pass is about 23 times higher than that of the sample after turning. However, after SUR with 10, 30 and 50 passes, the excessive plastic deformation reaches ductility limit to cause the surface damage during SUR. The surface damage, as the fatigue cracks initiation sites, can reduce fatigue life.