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 t reatments affecting the fatigue behavior of smooth and notched quenche d and tempered (Q&T) specimens made of a B-Mn SS2131 (approximate to A ISI 15B21H) steel. In this part, the effects of plasma nitriding (nitr iding temperature = 480 degrees C, time = 24 h) on the fatigue strengt h and notch sensitivity were investigated. Constant stress amplitude p lane reversed bending fatigue tests (R = -1) at 47 Hz were conducted u sing cylindrical plasma nitriding (PN) and Q&T steel specimens with K- t= 1.05 and 1.7. The compound layer was found to consists of E-phase a nd gamma'-phase. S-N curves show that plasma nitriding improves the fa tigue limit by 53 and 115% of Q&T smooth and notched specimens, respec tively. The fatigue strength of smooth specimens is improved through t he whole fatigue life but only for long fatigue lives for notched spec imens. Plasma nitriding reverses the low notch sensitivity (al short l ives) and high notch sensitivity (at long lives) exhibited by Q&T spec imens. It is shown that compressive stresses introduced by plasma nitr iding play the principal role on improvement of fatigue strength, subs urface crack nucleation, fish-eye asymmetry and reduction of stress in tensity factors. Fracture toughness, evaluated as a function of depth from the surface, decreases rapidly when approaching the surface. A si mple model based on crack retardation and I eduction of stress intensi ty factors produced by the net stress distribution is used to explain fish-eye formation and subsurface fatigue failure. (C) 1998 Elsevier S cience S.A.