Cyclic strength of parts with overlapping zones formed by roller burnishing

Abstract

and V. V. SubbotinaThe authors study residual stresses in and cyclic strength of shaft with overlapping zones formed during roller burnishing. The most significant decrease in fatigue strength is shown to occur in the presence of an overlapping zone with the areas arising due to a lowered roller burnishing pressure in the second pass.The problem of improving strength, reliability, and service life of critical heavy-duty engineering components of high-strength steels takes on great importance, especially considering the action of repeated oscillating loads that entail fatigue phenomena.Prominent among advanced methods aimed at a better reliability and longer service life of such products is the hardening by surface plastic deformation (SPD) [1]. Owing to their high efficiency and practicability, SPD methods contribute to a greater quality margin of engineering products due to cold work hardening of a workpiece surface layer and inducing compressive residual stresses in it [2].It was found [3] that generally the single-pass roller burnishing without any feed interruptions and non-burnished zones ensured the greatest increase in cyclic strength [4]. However, for long-length or complex-shaped workpieces which are impossible or inadvisable to roller-burnish in a single pass, this operation is performed in several stages. For example, the roller burnishing of workpieces with transitional fillets (such as torsion shafts) is most commonly done in two stages: the cylindrical part and the fillets are roller-burnished separately. In this case, the workpiece inevitably has some overlapping zones (OZ), i.e., the areas that have been hardened twice.The overlapping zones always contain sections formed under transient burnishing conditions: they are the start and end sections of the single-and double-pass roller burnishing. It is these sections which were found [5] to have nonuniformly distributed residual stresses, with an eventual effect on the cyclic strength of parts.Based on the fact that residual stresses in SPD-machined parts have a significant effect on their cyclic strength, the objective of the present work has been to study extensively the residual stress state in the overlapping zones formed in various roller burning modes and to clarify the OZ influence on the cyclic strength. The residual stress state was assessed by the axial component of residual stresses, which is the crucial one for the parts subject to bending. We investigated the distribution of axial residual stresses on the surface of a shaft along its generatrix.The study was carried out on 30-mm-dia. 30KhGSN2A steel shafts and 48-mm-dia. 45KhNMFA steel (in a high-strength state with HRC 50-55) shafts. The roller burnishing was performed by means of a triple-roller hydraulic tool, with a pressure P =10 kN, feed rate S = 0 35. mm/rev, machining speed of 350 rpm, roller diameter of 60 mm, and profile radius of 5 mm.