Effect of Temperature on Microstructure, Corrosion Resistance, and Toughness of Salt Bath Nitrided Tool Steel

In this study, a type of hot work tool steel was modified through salt bath nitriding for 4 h at 540 and 560 degrees C, and post-oxidation was subsequently performed. Surface and cross-sectional hardness test results revealed that the surface hardness increased after the treatment because of the formation of compound layers and diffusion zones. Microstructures and phase analyses showed that more homogeneous compound layers and Fe3O4-phase could be generated after treatment at 560 than at 540 degrees C. As a result, the corrosion potential was elevated, and the corrosion current density was clearly reduced. The thickness and porosity of the compound layer were also increased with the elevated nitriding temperature. Because of the nitrogen atom solution, XRD diffraction peaks broadened, and the position of the peaks shifted to a lower angle in different degrees at different depths, thus showing the same tendency as the hardness curves. Salt bath nitriding significantly deteriorated the impact toughness from 32.3 to 5.2 J.