Influence of electropulsing on fatigue crack growth retardation in hot-work die steel

Thermal fatigue-induced crack propagation is a major damage mode in molds, and effective countermeasures are required to reduce manufacturing costs. Although crack healing using high-density pulsed electric current has been studied, its effectiveness in high-yield-strength mold steels remains unclear. In this study, the effect of pulsed electric current application time on fatigue crack propagation in SKD61 hot-work tool steel was investigated experimentally and analytically. Compact tension specimens with 0.2 mm fatigue pre-cracks were subjected to pulsed currents of 9 kA with durations of 1.0 ms, 1.5 ms, and 2.0 ms. Microstructural changes were analyzed by electron backscatter diffraction (EBSD), and thermal and stress fields were evaluated using finite element method (FEM) simulations. Although the crack opening displacement increased after current application, fatigue tests revealed that only the 9 kA-1.0 ms condition delayed crack length by approximately 10 % up to 4.0 mm. EBSD showed that this condition promoted retained austenite formation due to rapid heating and cooling. FEM results showed that the duration at the A3 temperature and above decreased as the current application duration time decreased. This result suggests that restricting carbon diffusion, increases retained austenite. This retained austenite is considered to retard crack propagation through strain-induced martensitic transformation and associated compressive stresses near the crack tip.