Search

The contribution of the kinematics in local surface plastic deformation to intensification of the stress state at cylindrical surfaces is considered. On the basis of the finite-element method, a computer model of hardening is obtained so as to determine the stress state in the surface layer in different conditions.

Cold-drawn metal has a number of undeniable advantages over the hot-rolled one. Increased hardness, high surface quality, stability of the diametrical dimension along the length of the workpiece are the basis for choosing calibrated metal as effective workpieces for the manufacturing of long parts such as shafts, axles, and rods. In some cases, such workpieces require a small amount of machining, for example, threading or making necks at the ends of a bar. The wider use of the calibrated metal is hindered by residual stresses that are formed during its manufacturing. In the first part of this article, it was proposed to use small plastic deformations to control residual stresses. By the example of a new process of surface plastic deformation, which is called orbital burnishing, the working and residual stresses in cylindrical workpieces are determined. In the second part of the article, the process of enveloping surface plastic deformation is considered, which, at high productivity, makes it possible to reduce the residual tensile stresses in the calibrated metal or form the surface layers of the workpiece compressive stress. A technique for the experimental determination of residual stresses in the volume of a body is based on layer-by-layer removal of the inner and outer layers of cylindrical samples. Influence of the main parameters of the enveloping deformation process on the components of the residual stress tensor is established. A range of relative compressions (0.1–1.0%) is revealed, at which residual compressive stresses are formed in the surface layers of the workpiece. It was found that at a relative compression of 0.5%, the maximum residual compression stresses are created. Enveloping surface deformation has a positive effect on the residual stress state and on cold-drawn metal: the residual tensile stresses can be reduced, removed, or converted into compressive ones.

The effect of the deforming tool parameters on the roughness of hardened surfaces during orbital burnishing is considered. As result of experimental studies, it is revealed that after orbital burnishing, new surface is formed with significantly lower height of microscopes compared to the original microprofile, which has positive effect on improving of the shape accuracy and quality of hardened parts. It is found that the most significant effect on the roughness parameters during orbital burnishing is exerted by the orbital rotation radius, less significant — by the radius and working tool tilt angle. Increase in the working tool tilt angle the orbital rotation radius and decrease in the working tool radius leads to increase in roughness of surface.

Cold-drawn calibrated steel is an effective workpiece for the manufacturing of low-stiff cylindrical parts such as shafts and axles. A high accuracy of the diametric size along the workpiece length, low surface roughness, and increased hardness and strength of the surface layer compared to hot rolled products allow the production of a variety of parts with high metal utilization and high machining performance. The main disadvantage of calibrated metal is the residual stresses that occur during pressure processing. To reduce or change the nature of the distribution over the cross section, we propose to use small plastic deformations in the product’s surface layer. Surface plastic deformation (SPD) methods known in practice usually lead to the curvature of non-rigid workpieces. To intensify the stress-strain state in the deformation zone, a method of orbital surface deformation is proposed. The orbital deformation scheme is a rod indenter (tool), one end of which is equipped with a ball tip that is in contact with the surface to be treated. The second end of the rod indenter rotates about the vertical axis, forming a conical surface in space with a certain angle at the apex. A deforming force acts along the rod axis. Based on the finite element modeling, we consider the effect of the main parameters of orbital surface deformation on the stress state in the deformation zone and residual stresses in the finished products. Compared with the traditional SPD process, the stress intensity during orbital surface deformation will increase by 10–15%. The residual compressive stresses formed in the surface layers reach 70–85% of the material tensile strength. The relative radius of the orbital rotation and the radius of the operating tool, at which the formation of maximal temporary and residual stresses is ensured, were established. At an increase in the frequency of the orbital rotation of the operating tool, the temporary and residual compressive stresses increase. In the second part of the paper, we provide information on a more effective method of surface deformation and on the change in initial residual stresses that are formed during the calibration of cylindrical rods.