Influence of radical nitriding on fatigue strength of maraging steel

Rotating bending fatigue tests were carried out to investigate the influence of radical nitriding on surface integrity and fatigue strength for a maraging steel. Fatigue strength was increased by nitriding. The initiation site of fracture was die surface at high stress levels and the interior at low stress levels in a nitrided steel, though it was the surface at all the stress levels in an aged steel. Consequently, S-N curve of the nitrided steel showed a double staged one, though that of the aged steel was an ordinary S-N one. In the nitrided steel, the main reason for the suface fracture was surface embrittlement while the one for the interior fracture was surface hardening, due to nitriding.