Strong effect of carbides and alloy elements in gradient nanostructured 32Cr2Mo1VE steel on nitrogen atom diffusion

Highlights: • After surface ultrasonic rolling process (SURP), the surface grains were refined. • After SURP, the carbides were refined and precipitated on the grain boundaries. • The Cr₂N precipitated on the grain boundaries of the SURP sample after gas nitriding. • The carbides and Cr₂N formed on the grain boundaries hinder the diffusion of N atoms. Metal surface nanocrystallization can improve the surface activity, reduce the diffusion activation energy, and then reduce the diffusion temperature or time. However, for gradient nanostructured surface layer on 32Cr2Mo1VE prepared by surface ultrasonic rolling process (SURP), the surface nanocrystallization seriously hindered the diffusion of N atoms during gas nitriding (430 °C, 480 °C and 530 °C). The effect of the distribution of alloying elements on the diffusion of active N atoms was studied in detail. For multi-element medium‑carbon alloy steels, it should be that carbides or nitrides formed by alloying elements usually precipitate easily at grain boundaries. So, precipitates along grain boundaries will block the rapid diffusion channel of nanocrystalline grain boundaries and also reduces the concentration gradient drive.