| Nitriding is a thermochemical treatment which, by insertion of nitrogen in solid solution and nitride precipitation, generates on the surface of the treated part a profile of hardness and compressive residual stresses. It is used to improve the wear, corrosion and fatigue resistances of mechanical components such as gears, pistons and extrusion dies.; The purpose of this study is to understand the fatigue behaviour of a gas-nitrided 4140 steel by studying in particular the influence of the nitriding parameters and that of the geometry/loading configuration on the improvement of the base metal fatigue strength.; The analysis of literature shows that nitriding increases the fatigue strength in the high cycle fatigue regime. The improvement is associated to a transition from the usual external cracking mechanism to an internal cracking mechanism; a circular "fish-eye" crack initiates from a defect, e.g. a non metallic inclusion, inside the unnitrided core. Most of the time, results reported in the literature are based on rotary bending tests. Because of the bending stress gradient, cracks always initiate at the boundary between the nitrided case and the core. An increase in nitriding depth thus results in an increase in initiation depth and in an increase in the fatigue limit. However, this effect is artificial and fatigue results obtained in rotational bending are particularly sensitive to size effect.; For a better assessment of the fatigue behaviour evolution of nitrided steel with the nitriding parameters and, in particular, with the nitriding depth, axial fatigue tests were performed under fully reversed loading. Three nitriding depths (400, 700 and 1 000 mum) were achieved and characterized by hardness and residual stress measurements in depth. The fatigues curves obtained for these three case depths show that, in high cycle fatigue, nitriding brings an improvement of 20% as compared to the fatigue strength of the base metal. Fatigue of nitrided steel is a competition between an external crack, which initiates and propagates from the hard nitrided surface under compressive residual stresses, and an internal crack, which grows in the unnitrided core without the influence of the surrounding atmosphere. When nitriding is deep enough to initiate a surface crack faster than an internal crack, fatigue strength of the nitrided steel depends only on the intrinsic strength of the base metal, which is a function of metal hardness and defects size. (Abstract shortened by UMI.)... |
