Effect of hydrogen on mechanical properties of nitrided austenitic steels

Abstract

Hydrogen (H) embrittlement in austenitic stainless steels is restricted to the surface due to the low diffusion coefficient of H in face-centred cubic (fcc) structures. Depth-sensing indentation is a very important tool to investigate the mechanical properties of hydrogenated steels at near-surface regions. In the present work, the effect of glow discharge nitriding on the mechanical properties of steels submitted to cathodic hydrogenation was analyzed by instrumented indentation. Cracks are present at the surface after outgassing in the untreated sample. Hardness and elastic modulus are related to the nitriding process and charging time. Hardness of nitrided samples is higher at the near surface region. However, after hydrogenation, the hardness still increases, mainly due to the presence of martensitic phases. In nitrided samples, hydrogen and methane bubbles can be formed after hydrogenation, probably due to high hydrogen concentration under the nitrided layer. Load versus displacement curves in the nitrided and hydrogenated sample varies according to the indentation made on or outside a bubble region. Nitriding temperature affects the kinetics of bubble formation.

Acknowledgement

We thank the Brazilian Research Fund CNPq for financial support.