Evaluation of Delayed Fracture Properties of Hydrogenated Surface Hardened JIS SCM420H

The delayed fracture properties were evaluated by conducting constant load test using notched round bar specimens with different stress concentration factor (Kt) of gas carburized JIS SCM420H. The amount of hydrogen added to specimens after gas carburizing were almost same regardless of Kt 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 Kt, but the delayed fracture limit decreased as Kt increased. Based on the morphology of fracture surface analysis and the distribution of hydrostatic stress (σm), 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 σm in each area, σmC and σmNC. The presented stresses could evaluate the delayed fracture limit and the fracture time.