Effects of alloying elements and heat treatment on hydrogen diffusion in SCRAM steels

Highlights: • The effective diffusion coefficient decreased with increasing Ti and N content. • Optimized heat treatment could refine both martensitic lath and precipitates. • The effects of MX and M23C6 phase on hydrogen permeation were studied. • Arrhenius relationships of two SCRAM steels were determined. - Abstract: In this paper, the effects of alloying elements Ti and N, different heat treatments and permeation temperature on hydrogen diffusion in SCRAM steels were investigated. Devanathan-Stachurshi electrochemical hydrogen permeation technique was used to determine the hydrogen diffusivity. The results show that increasing the content of Ti and N can decrease the hydrogen diffusion coefficient of SCRAM steels. Compared with the traditional quenching and tempering heat treatment process, the process including an intermediate hold can refine the M23C6 phase, increase the volume fraction of MX phase but coarsen the martensitic lath. The twice quenching and tempering heat treatment process can not only refine the martensitic lath and M23C6 phase, but also increase the volume fraction of MX phase, resulting in a decrease of hydrogen diffusivity. The hydrogen permeation steady-state current density and the hydrogen diffusivity increase with the rise of permeation temperature. The temperature dependence of hydrogen diffusion coefficient conforms with the Arrhenius relationship. And the activation energy for hydrogen diffusion of two SCRAM steels with different heat treatments was determined.