Experimental study on surface integrity and fatigue life of an ultra-high strength steel by the composite strengthening process of pre-torsion and ultrasonic rolling

A new surface modification method, i.e., the composite strengthening process of pre-torsion and ultrasonic rolling, is proposed for surface strengthening of an ultra-high strength steel (45CrNiMoVA). Based on the enhancement of dislocation density in the internal structure of the material by pre-torsion strengthening process and the grain refinement effect of ultrasonic rolling strengthening process, the evolution law of surface integrity and fatigue life of ultra-high strength steel after the composite strengthening process of pre-torsion and ultrasonic rolling were analyzed. The results show that the composite strengthening process has an obvious surface finishing effect, in which ultrasonic rolling plays a key role. The coupling effect of pre-torsion and ultrasonic rolling causes the residual compressive stress in the surface layer of ultra-high strength steel with large amplitude and deep influenced layer. Compared with the ultrasonic rolling process, composite strengthening can produce deeper plastic deformation on the surface of ultra-high strength steel. After composite strengthening, the high cycle fatigue life of the specimens increases greatly, and the torsional fatigue fracture mode changes to quasi-cleavage fracture. In conclusion, the composite strengthening process of pre-torsion and ultrasonic rolling is of great significance to improve the surface integrity and fatigue life of the ultra-high strength steel.