Development and research of a new technology for the production of hardened screw fittings with a gradient ultrafine structure

The paper presents the results of experimental studies of a new combined method of severe plastic deformation for the production of bolted joints, consisting of stages of radial shear rolling followed by twisting in a screw matrix. A 30 mm diameter billet of 25G2S structural steel was rolled to a diameter of 24 mm in two passes. In the third pass, the workpiece was rolled to a diameter of 19 mm and fed into a screw die. As a result, a gradient structure was obtained in the cross section of the bar - equiaxed grains with a size of 12–15 μm were revealed on the surface, deformed grains elongated in the rolling direction were revealed in the center. Analysis of the tensile test results showed that the initial tensile strength value of 625 MPa after deformation increased to 818 MPa in the surface layer and 763 MPa in the central layer. The initial elongation of 25.6 % decreased to 15.9 % in the surface layer and 17.1 % in the middle layer. Cross sectional microhardness analysis confirmed the gradient distribution of properties. • New SPD method for the production of screw fittings was investigated. • Method consists of radial-shear rolling and subsequent twisting in a screw matrix. • Gradient structure was obtained in the cross section of the bar.