Influence of plasma nitriding on fatigue strength and fracture of a B-Mn steel

The first part of a systematic investigation is presented of surface treatments affecting the fatigue behavior of smooth and notched quenched and tempered (Q&T) specimens made of a B-Mn SS2131 (≈AISI 15B21H) steel. In this part, the effects of plasma nitriding (nitriding temperature=480°C, time=24 h) on the fatigue strength and notch sensitivity were investigated. Constant stress amplitude plane reversed bending fatigue tests (R=−1) at 47 Hz were conducted using cylindrical plasma nitriding (PN) and Q&T steel specimens with Kt=1.05 and 1.7. The compound layer was found to consists of ϵ-phase and γ′-phase. S-N curves show that plasma nitriding improves the fatigue limit by 53 and 115% of Q&T smooth and notched specimens, respectively. The fatigue strength of smooth specimens is improved through the whole fatigue life but only for long fatigue lives for notched specimens. Plasma nitriding reverses the low notch sensitivity (at short lives) and high notch sensitivity (at long lives) exhibited by Q&T specimens. It is shown that compressive stresses introduced by plasma nitriding play the principal role on improvement of fatigue strength, subsurface crack nucleation, fish-eye asymmetry and reduction of stress intensity factors. Fracture toughness, evaluated as a function of depth from the surface, decreases rapidly when approaching the surface. A simple model based on crack retardation and reduction of stress intensity factors produced by the net stress distribution is used to explain fish-eye formation and subsurface fatigue failure.