Plasma oxynitriding (PON) and plasma nitriding (PN) were carried out for AISI 1045 steel, and kinetics of the two cases were analysed and compared. Kinetics equations of d = 4.16 × 10 −3 exp ( − 1666.67/T)t √ and d = 1.87 e (−27333/T) t √ were obtained for PON and PN, respectively, which illustrates that much higher nitrogen diffusion coefficient and much lower activation energy for PON. Meanwhile, the reliability of obtained kinetics equations were verified by checking the agreement between the calculated compound layer thickness and those measured from the cross-sectional microstructures. The results confirmed the practicability of the kinetics equations.

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Axial loading fatigue tests of carburized 12Cr steel alloy in long‐life regime were performed under stress ratios of −1 and 0. Fatigue fracture can be divided into surface failure, interior failure without fine granular area (FGA), and interior failure with FGA. By considering the effects of the tensile limit, the evaluation of maximum crack sizes (inclusion sizes and FGA sizes), and residual stress distribution, the fatigue strength prediction model of carburized 12Cr steel alloy for inclusion‐FGA‐fisheye induced failure in low stress level region can be established. By comparing the predicted results evaluated by generalized extreme values (GEV) with generalized Pareto (GP), the GP distribution is more suitable to predict the fatigue limit of the carburized 12Cr steel alloy. Furthermore, by using the relationship between inclusion sizes and FGA sizes, the fatigue limit prediction model for the design of components can be established, and the result is extremely accurate for the carburized 12Cr steel alloy.