Study on the fatigue properties of plasma nitriding S45C with a pre-ultrasonic nanocrystal surface modification process

► Nano-structured surface layers with different grain sizes and depths were nitrided. ► Thicker compound layer was produced for sample with nano-structured surface layers. ► Nitrogen enrichment was found for nitriding sample with smaller surface grain size. ► Nano-structured surface has little effect to nitrogen diffusion in sample interior. ► Surface voids were found in nitriding nano-structured surface sample for 48 h.