The enhancement effect of salt bath preoxidation on salt bath nitriding for AISI 1045 steel

A rapid salt bath nitriding technology was primarily developed by conducting salt bath pre-oxidation (SPO) instead of conventional air pre-oxidation (APO). Characterization of the modified surface layer was made by means of optical microscopy, scanning electron microscope (SEM), micro-hardness tester and x-ray diffraction (XRD). The results showed that salt bath pre-oxidation could significantly enhance the nitriding efficiency than conventional air pre-oxidation. Under the same salt bath nitriding parameters, the thickness of compound layer is only 13.3 μm treated by air pre-oxidation, and it is increased to 20.8 μm by salt bath pre-oxidation, more than 60% increased. While nitrided at 560 °C for 120 min, higher cross-section hardness and thicker effective hardening layer are obtained by salt bath pre-oxidation. In addition, kinetics analysis shows that the nitrogen diffusion coefficients were increased significantly by salt bath pre-oxidation and the activation energy of nitrogen atom diffusion (Q) was decreased from 216 kJ·mol⁻¹ and 158 kJ·mol⁻¹, and the enhancement mechanism of salt bath pre-oxidation for salt bath nitriding was discussed.