The present paper reports on a comparative study of the microstructure, hardness and cavitation erosion behaviour of Ti-6Al-4V laser nitrided under different gas atmospheres with nitrogen concentrations of 0-25%. By varying the nitrogen content of the gas atmosphere, the structure and hence the hardness and cavitation erosion behaviour of the laser nitrided layers can be controlled in a wide range. With increasing nitrogen content of the gas atmosphere, the structure within the laser nitrided layers characteristically changed from martensitic a9-Ti to a fine mixture of a-and b-Ti grains, finally leading to the formation of different titanium nitrides. The study revealed that the increase of hardness and cavitation erosion resistance can be mainly attributed to solid solution hardening of the nitrogen. It is shown that optimum cavitation erosion resistance is only achieved if crack free layers containing no brittle titanium nitrides are produced.
Starting from the known local fatigue strength concept problems of its application to martensitic surface layers, especially laser hardened layers have been discussed.A method for calculation of the fatigue limit of surface hardened specimens or components from quench-hardenable steels has been proposed, including the influence of surface roughness on fatigue strength. There is given a new formula for estimation of the mean and residual stress sensitivity.The limits of the concept have been shown and its practical use has been demonstrated exemplarily.
The applicability of surface wave velocity measurements as a nondestructive test method to investigations of deformation layers with preferred orientations and residual stresses has been examined at steel C70W2. The residual stresses in various directions parallel to the surface have been detected from changes of velocity caused by a stress-free annealing. From the anisotropy and the frequency dependence of velocity the existence of a surface layer with preferred orientations has been deduced and such characteristic parameters as preferred orientation and depth have been estimated. The results have been compared and completed with X-ray texture investigations and data from Barkhausen noise.
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