Effect of heat treatment on the hydrogen delayed fracture of high strength spring steel

The purpose of this study is to investigate the effect of the changes in the tempering temperature on the hydrogen-delayed fracture resistance of high strength spring steel. The specimens with various microstructures are produced by differentiating the tempering temperature range, their mechanical characteristics are evaluated, and an impact test is carried out. The correlation with the change in mechanical properties is studied through the analysis of the microstructures with a scanning electron microscope. Hydrogen is forcefully injected inside the notched specimen electrochemically, and the hydrogen-delayed fracture resistance is evaluated using a constant loading test. The fractography of the fractured specimen is observed, and through a thermal desorption spectrometer analysis, the main trapping site of the diffusible hydrogen is investigated. The diffusible hydrogen in high strength spring steel is primarily trapped in dislocation, and the hydrogen-delayed fracture resistance increased depending on the increase in the tempering temperature.