Abstract
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. DOI: http://dx.doi.org/10.5755/j01.mech.21.5.11399
Highlights
It is known that initial acts of plastic flow are connected to the surface layer of the material
The thickness of compound layer and microhardness measurements of the samples nitrided at different temperatures are given in Figs. 2 and 3, respectively
The results of this study show that growth of fatigue limit of nitrided samples is not directly related to the thickness of compound layer and case depth
Summary
It is known that initial acts of plastic flow are connected to the surface layer of the material. The fatigue behaviour in comparison with static loading has unusually high sensitivity to conditions of surface layer. The large influence of condition of the metal surface on fatigue strength is caused by earlier damage of the surface layer (in comparison with other volume of metal), i. At strains, which are much below yield strength of material, the plastic deformation develops in thin surface layer (the size 1 – 2 grains). Using surface strengthening (hardening) treatments it is possible to improve considerably the resistance to the fatigue crack initiation and increase fatigue life of metals and alloys. The material surface condition has a predominant effect on high-cycle fatigue and less important for lowcycle fatigue
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