Failure Analysis of Cryogenically Treated and Gas Nitrided Die Steel in Rotating Bending Fatigue

AISI H13 hot work tool steel is commonly used for applications such as hot forging and hot extrusion in mechanical working operations that face thermal and mechanical stress fluctuations, leading to premature failures. Cryogenic treatment was applied for AISI H13 steel to improve the surface hardness and thereby fatigue resistance. This work involves failure analysis of H13 steel specimens subjected to cryogenic treatment and gas nitriding. The specimens were heated to 1020°C, oil quenched followed by double tempering at 550°C for 2 h, and subsequently, deep cryogenically treated at −185°C in the cryochamber. Gas nitriding was carried out for 24 h at 500°C for 200 μm case depth in NH₃ surroundings. The specimens were subjected to rotating bending fatigue at constant amplitude loading at room temperature. Measurement of surface roughness, hardness, and microstructural analysis indicated improved fatigue life for cryogenically treated specimens as compared to gas nitride, which could be attributed to fine carbide precipitation accountable for the delayed crack initiation and propagation.