Factors influencing fretting fatigue properties of plasma-nitrided low alloy steel

709M40 steel samples were plasma-nitrided under various processing conditions. Fretting fatigue tests were carried out with a Wöhler rotating bending fatigue machine, using S–N curves to evaluate the fretting fatigue properties of the untreated and plasma-nitrided steel. The influences of such structural factors as compound layer, case depth, case hardness and compressive residual stress on the fretting fatigue behaviour of plasma-nitrided steel were investigated. It has been shown that the hard compound layer on the nitrided surface increases the resistance to fretting fatigue crack initiation, and the high compressive residual stress in the nitrided case increases the resistance to fretting fatigue crack propagation. However, a deep case depth and a high hardness in the diffusion zone may not make a significant contribution to the fretting fatigue strength enhancement. A higher nitrogen potential results in a thicker compound layer, thereby increasing the fretting fatigue strength further. A longer nitriding time reduces the surface hardness and compressive residual stress level in the near surface region, thus having no benefit regarding the further improvement of the fretting fatigue properties of the steel.