Improvement of operating characteristics of quenched steels by combined surface plastic deformation

Abstract

In recent years, new progressive technologies of hardening based on surface treatment of materials with high-energy fluxes (plasma, electron-beam, laser, electromechanical, etc.) have been applied more and more extensively. In many cases, this type of treatment leads to the formation of a specific high-strength state of the surface layer. However, it may also cause a decrease in both fatigue resistance and sensitivity to stress concentration. Therefore, surface plastic deformation (SPD) may be a quite useful final hardening operation. In the present paper, we study the prospects of the combination of electromechanical hardening (EMH) and SPD as applied to 45 steel with an initial hardness HV of 5 GPa subjected to quenching with low-temperature tempering at 473{degrees}K. The specimens underwent preliminary electromechanical treatment by local heating of the area of contact of a hard-alloy tool with the specimen surface at a current strength of up to 600 A. A high-strength 0.2-mm-thick layer with a hardness HV up to 9 GPa appeared as a result of thermal quenching of the surface layer because of extensive heat dissipation into the body of the specimen. Then cylindrical specimens with a diameter of 10 mm were subjected to SPD by two-stage rolling in a three-rollmore » device. The diameter of the rolls was 36 mm, the radius of curvature was 4 mm, and the force applied to the rolls amounted to 1 kN. Fatigue tests were carried out according to a pure-bending scheme as the specimen was rotated at a frequency of 50 Hz. As a result of preliminary electromechanical hardening, the endurance limit of the specimen increases by a factor of 5-7. At the same time, the fatigue resistance exhibits a substantial drop both in the region of limited endurance and at the endurance limit. This effect of electromechanical treatment can be explained by the inhomogeneity of the properties of the surface layer and the appearance of unfavorable residual stresses.« less