Influence of the Nitrided Layers Microstructure on the Fatigue Life Improvements of AISI 4140 Steel

摘要

Nitriding is an important industrial process to improve the mechanical properties of components, especially by producing compressive residual stresses. Gas and ion nitriding has become a popular thermo-chemical surface treatment, which is being used to develop thermal/mechanical fatigue and wear characteristics of steels. In this study, the gas and ion nitriding of AISI 4140 steel was carried. The micro-structure, the micro-hardness, the residual stresses distribution and the crack resistance of the hardened steel are determined. These analysis and characterization are carried out using optical microscopy, scanning electronic microscopy, X-ray diffraction and mechanical measurements (micro hardness and residual stresses) of treated material. The results are intended to contribute in defining and optimizing the adequate choice of treatments for this type of steel in industrial conditions. The gains, expressed in term of endurance limit, brought by these treatments are established by three-points bending fatigue tests and discussed in relation to the residual stresses evolution under the cyclic loading conditions. The fatigue fracture resistance is analyzed by methods of fracture mechanisms. This reveals that the gain provided by the gas nitriding (50%) is about 8% against 32% for the ion nitriding. This is primarily allotted to a high level of compressive residual stresses for ionic nitrided state compared to the gas nitrided state.