The paper contains results of experimental programme aimed at evaluation of fatigue resistance of laser welded 6 mm and 10 mm thick sheets of a low alloy structural S355 steel. High cycle fatigue tests were performed to verify fatigue strength of the sheets containing laser welds performed with optimised laser welding parameters. Surface of the sheets was in as-received conditions simulating real welding applications. The results are compared to each other and to fatigue S-N curve of the S355 material evaluated on quite small standard specimens. Endurance limit of the 10 mm thick laser welded sheet was by more than by 30% higher than that of the 6 mm thick sheet. In connection with this effect, all fatigue cracks in the 6 mm thick sheets initiated in the weld area unlike the 10 mm sheet, where cracks initiated both in the weld area and outside it indicating no or just minor unfavourable effect of the welding. However, fatigue strength and endurance limit of small material specimens were considerably higher even in comparison with the 10 mm welded sheets. The results are discussed considering, results of microstructure analyses, different values and distribution of residual stresses and also differences between surface conditions of the sheets and the standard specimens.
Laser surface hardening is an advanced method of surface treatment of structural steels with a great potential for wide industrial applications. The technology is quite new and so, investigations have to be performed in order to gain a comprehensive knowledge about effects on microstructure, hardness, surface properties of treated materials, but also mechanical an particularly fatigue properties. Concerning fatigue resistance of material treated with this technology, results and knowledge recently published in the literature indicate that fatigue resistance can be either reduced or increased, even considerably, depending on numerous parameters of basic material, laser hardening parameters etc. This contribution contains results of a partial study of effect of laser hardening of relatively small specimens on fatigue resistance of 42CrMo4 steel. Two different parameters of the treatment were used, namely two speeds of laser beam on the material surface at constant beam energy. Unlike the lower speed, when fatigue resistance was slightly reduced, higher speed of laser beam resulted in a slight increase of fatigue resistance and fatigue limit. The results are discussed considering an occurrence of residuals stresses. Key words: Laser hardening, residual stress, lifetime, fatigue, fracture, microstructure, surface
The paper deals with an investigation of surface laser hardening characteristics on contact and bending fatigue resistance of a 42CrMo4 steel, being often used for manufacture of gear wheels. The aim of the experimental programme was to simulate the complex service loading of fairly large gears by two separate investigations, namely contact fatigue performed on a special, so called analogon machine and bending fatigue performed on high cycle fatigue resonance machines using three-point-bend (3PB) fairly large specimens. It was shown that after optimising the methodology and parameters of the laser hardening technology, contact fatigue resistance is very good, comparable with very expensive and time consuming thermochemical high-depth surface treatments. Bending endurance limit can be increased. The role of residual stresses resulting from the laser treatment was shown as decisive.
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