Evaluation of Delayed Fracture Properties of Hydrogenated Surface Hardened JIS SCM420H

Abstract

The delayed fracture properties were evaluated by conducting constant load test using notched round bar specimens with different stress concentration factor (K_t) of gas carburized JIS SCM420H. The amount of hydrogen added to specimens after gas carburizing were almost same regardless of K_t value. The testing method employed was a weight-lever method with constant load application, and the displacement of the specimen during the test was measured with a laser displacement meter. Both the deformation rate and the fracture time did not depend on K_t, but the delayed fracture limit decreased as K_t increased. Based on the morphology of fracture surface analysis and the distribution of hydrostatic stress (σ_m), this study was conducted in two cases for the carburizing region near the notch where hydrogen accumulated and for the internal non-carburizing region obtaining the averaged σ_m in each area, σ_mC and σ_mNC. The presented stresses could evaluate the delayed fracture limit and the fracture time.