Statistical Assessment of Hardness Scatter Reduction in Shot-Peened Steel Springs via Low-Temperature Stabilization Annealing

Shot peening is widely used to improve the fatigue performance of steel springs by inducing compressive residual stresses and work hardening. However, the inherent non-uniformity of the process leads to significant spatial heterogeneity in hardness across the spring cross-section, contributing to fatigue life scatter. While previous studies have focused primarily on mean hardness values, the statistical dispersion of hardness has received limited attention. This study investigates the effect of low-temperature stabilization annealing (۲۳۰ °C for ۱۵ min) on both the mean and standard deviation of hardness at three radial positions (surface, intermediate, and core) in cold-coiled EN ۱۰۲۷۰-۱ spring steel. Results show that although the overall mean hardness remains virtually unchanged (۵۸۴.۰۹ HV vs. ۵۸۲.۱۹ HV), the standard deviation decreases markedly from ۳۳.۳۶ HV to ۲۸.۲۱ HV—a ۱۵.۴% reduction in scatter. The most significant homogenization occurs in the intermediate region, with a ۲۹% reduction in standard deviation. These findings demonstrate that post-peening annealing stabilizes the deformed microstructure and reduces local strain gradients without compromising the beneficial work-hardening effect of shot peening. The reduced hardness scatter, particularly in surface and subsurface regions, is expected to improve fatigue reliability by minimizing uncertainty in crack initiation sites. This study underscores the value of statistical dispersion measures as complementary criteria for optimizing post-peening heat treatments.

کلیدواژه ها:

Low-temperature annealing ، Hardness scatter ، Spring steel ، Cross-sectional hardness uniformity ، Post-peening heat treatment