The Cause of Premature Tensile Fracture of Gas-Nitrocarburized Steel

The objective of the present study was to investigate the mechanism of premature tensile fracture of the gas-nitrocarburized steel. For this purpose, tempered martensitic steel specimens were nitrocarburized at 853 K (580 °C) for maximum 3.5 hours, and tensile-strained at room temperature. The prolongation of nitrocarburizing time caused the high strength, the short elongation, and the heavy serrations at the yield point elongation range of the tensile flow curve. The serrations were generated by vertical cracking at the specimen surface, which started at the bottom of the gauge portion, and were propagated to the top of the gauge portion with increasing tensile strain up to the yield point elongation. The vertical cracks were triggered at the porous compound layer when the local deformation occurred by the propagation of the Lüders band from the bottom to the top of the gauge portion. The cracks readily passed through the diffusion layer with an assistance of ε-carbonitride particles, which formed at the boundaries of tempered martensite during nitrocarburizing, resulting in brittle fracture at the diffusion layer and finally in premature failure of the gas-nitrocarburized specimens.