Abstract
In this work the 42CrMo4 steel ground surface integrity was evaluated at different grinding conditions before and after nitriding. An experimental study was carried out to investigate the influence of depth of cut on work piece surface integrity by grinding. Surface integrity was characterized by roughness, micro-hardness, residual stress distribution and ground surface morphology. These properties were compared to nitriding state. These improvements in grindability and in surface integrity are particularly favoured by low depth of cut values. It is also shown that these improvements are essentially due to the reduction of the grinding temperature, which decrease the tensile residual stresses and favours the material removal by shearing and limits the ground surface damage. The roughness of the ground work piece surfaces increases with the depth of cut, which was attributed to plastically deformed coatings on the work piece surface occurring with elevated temperature. The results presented in this study shows that the material history before nitriding does not have any influence on the residual stresses distribution after nitriding. This is due to that during the nitriding treatment, especially in the heating phase; the tensile stresses generated by grinding are cancelled under the temperature effect.
Highlights
It has been shown that the fatigue life improvement of steels by the nitriding treatments depend on the characteristics of the nitrided layers, in particular of the residual stresses distributions and the work hardening [1,2,3]
The surface work hardening was evaluated by micro-hardness measurements using a Shimadzu micro-hardness tester HMV2000 and the near surface residual stress were evaluated using the tilt X-ray method by means of a SET-X type diffractometer under the conditions listed in table 4
Effects on the Surface Defects scanning electron microscopy (SEM) observations of the ground surface generated under fine grinding conditions show that these surfaces are exempt from thermal defects (Fig. 2)
Summary
It has been shown that the fatigue life improvement of steels by the nitriding treatments depend on the characteristics of the nitrided layers, in particular of the residual stresses distributions and the work hardening [1,2,3]. It is reported that these characteristics depend on the treatment conditions (lasted, atmosphere and temperature), on steel nature and in particular its contents of alloy elements contributing to the nitride formation and controlling their residual stresses level and in consequence their resistance to the fatigue crack initiation. The surfaces preparation mode before nitriding, the surface layers preliminary characteristics and their eventual effect on the resistance to fatigue were not approached in the literature. One target the grinding preparation mode explores three cutting conditions by this process, supposed to confer on the surface layers of the very different initial characteristics. The effects of the process parameters on these surface characteristics can be summarized as follows:
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