Influence of nitriding temperature on surface structural characteristics and fatigue strength of steel

In this research the influence of nitriding temperature on surface structural characteristics and fatigue strength of plain carbon steel (C45) was investigated. The samples were gas nitrided at different temperatures at constant nitriding time. The influence of such surface structural characteristics as case depth, hardness of the compound layer and case hardness on fatigue behaviour of nitrided steel was investigated. Also, the fatigue fracture surfaces nitrided and nonnitrided samples were observed by a scanning electron microscope. It was determined that the hard compound layer and high compressive residual stresses in the nitrided layer are the two major factors in improving the fatigue strength of plain carbon steel. However, the deep case depth is not necessary to improve fatigue strength of examined steel. Investigation of fatigue fracture surfaces showed that the samples have higher fatigue resistance when the fatigue crack begins to develop under the nitrided (hardened) sample surface at the central part of fish eye" from structural stress concentrator - nonmetallic inclusion.