Influence of the Surface Structure and Properties on the Fatigue Strength of Electromechanically Strengthened Quenched Steel

Abstract—For the example of quenched carbon steel 45 and U8 steel, the influence of surface hardening (electromechanical treatment, surface plastic deformation, nonabrasive ultrasonic finishing, and various combinations) on the surface structure and microhardness, the cyclic durability of the hardened samples, and the mechanisms of fatigue failure is analyzed. Research by means of optical and scanning electron microscopy, microhardness measurement, and fatigue tests shows that, for the quenched carbon steels, high-speed pulsed thermal deformation in the course of electromechanical treatment increases the surface microhardness (by more than 50%) and decreases the fatigue limit (by 20–30%). That is associated with the formation of hard nonequilibrium ultradisperse phases of nonuniform chemical composition in the surface layer. The quenched structure close to the surface is tempered, with the formation of softening zones and the appearance of residual tensile stress. Accordingly, the microhardness in these zones declines and the fatigue limit falls. Such decrease in performance of the steel on surface hardening merits further study, along with potential technologies for improving its performance. Surface hardening of carbon steels by some combination of electromechanical treatment, surface plastic deformation, and nonabrasive ultrasonic finishing permits adjustment of the structure and phase composition and the stress–strain state of surface and subsurface layers of the steel by varying the temperature and deformation. By that means, balanced strength and fatigue characteristics of the samples may potentially be obtained by appropriate preliminary heat treatment. Intense surface plastic deformation and nonabrasive ultrasonic finishing after electromechanical hardening may be used to smooth the surface, mend subsurface defects, and correct the stress–strain state of the steel. That increases the microhardness in the tempering zone by 20–25% and the fatigue limit of the samples by 25–30%.