Fatigue of ultra-high-strength steel improved by pre-torsion heat-assisted deep rolling and heat-assisted ultrasonic rolling composite strengthening method

In addressing the torsional fatigue failure and fracture issues of ultra-high-strength steel, as well as the urgent need to enhance its fatigue resistance, a surface strengthening method of pre-torsion heat-assisted deep rolling and heat-assisted ultrasonic rolling is proposed. In comparison with grinding, pre-torsion, and pre-torsion heat-assisted deep rolling process methods, it is demonstrated to achieve the lowest surface roughness, higher surface hardness, the smallest grain size, the largest plastic deformation, the highest proportion of recrystallization, and the largest residual compressive stresses in the axial and circumferential directions on the surface. The maximum fatigue life of materials is obtained under different cyclic loading conditions. The lamellar nano-gradient structures, which are formed by the accumulation of amorphous, nanocrystalline, recrystallized subgrains, and precipitated phases, are obtained via this method. In comparison with the untreated samples, the tensile strength following the final strengthening process exhibited an increase of 2.91 %, while the elongation after fracture demonstrated a substantial increase of 22.11 %. The initiation and propagation of surface cracks are effectively inhibited, and the fatigue life of ultra-high-strength steel materials is enhanced by the composite strengthening method.