The Effect of Stress Pulses on the Limited Endurance Under Cyclic Loading of Thermal-Hardened Carbon Steel

Abstract

Purpose. The assess effect of stress pulses on the cyclic endurance of thermal-hardened carbon steel. Methodology. A sheet with a thickness of 1 mm was selected for the study, made of steel with 0.42% С after thermal hardening. The treatment consisted of quenching and tempering at 300 °С. The samples were subjected to cyclic loading on the Saturn-10 machine, under a symmetrical cycle of alternating bending, with a frequency of 100 min^-1. Treatment with pulses of stress was carried out under conditions of «Iskra–23». To determine the effect pulses of stress on the cyclic endurance, samples after 50–60% by limit of endurance were subjected to doing pulses of stress. After completion treatment pulse of stress, the samples continued to be cyclically loaded until the moment of failure. The density of dislocations was measured by method of X-ray structural analysis on a DRON-3 diffractometer, by interferences (110), (211), (321). The complex of properties after thermal strengthening was determined under static tension, at a strain rate of 10^-3 s^-1. Findings. After processing with pulses of stress studied, thermally hardened steel with a hardness of 46–47 HRC, an increase in hardness by 11 % was obtained. According to the analysis of the cyclic loading curves of thermally hardened carbon steel, it was determined that due to the action pulses of stress, an increase in limit of endurance occurs in a wide range of cyclic overload. Structural studies have determined that, in proportion to decrease at magnitude of cyclic overload, an increase limit of endurance corresponds to higher number accumulated dislocations by different slip systems. Originality. The increase at density of dislocations from the action pulses of stress is due to the development processes of partial unlocking of dislocations after thermal strengthening and activation systems of sliding, which are not characteristic of these loading conditions of the steels. As a result of the action pulses of stress, the propagation deformation per cycle occurs at lower amplitudes of load, due to formation of an additional number of dislocations. According to analysis lines of French, it was determined that participation of an increased number of dislocations at propagation of deformation per cycle shifts a moment transformation of reversible damages into irreversible ones, towards an increase at number of cycles. Practical value. The obtained research results can be useful for assessing by influence of an external source of stress on the behavior of a carbon steel product under cyclic loading.