FATIGUE RESISTANCE OF LASER-TREATED CONSTRUCTION STEEL

In order to improve the mechanical properties, and thus the in-service behaviour, of mechanical components subjected to fatigue and wear type working conditions the automobile industry uses surface treatment techniques. Using the CO2 continuous wave LASER as a surface treatment method allows mechanical components, which are subsequently subjected to high loading levels, to be locally and preferentially hardened. The advantage of this technique lies in its flexibility. Such a tool could easily be integrated into a production line, not only allowing on-line treatment but also considerably reducing the component downtime and stock level. The results of the present study have shown that the fatigue endurance limit obtained for specimens treated superficially by laser was comparable to that determined for specimens subjected to a carbonitriding treatment. In order to attain such a result several parameters were optimised including i) the partition of die laser beam's energy, power and the test piece treatment time ; ii) pre-treatment preparation of the test piece surface ; iii) initial metallurgical state of the material to be treated. In order to examine and quantify the fatigue behaviour of the laser treated material, both metallurgical (structure, hardness) and the residual stress fields were analysed. For the quenched and tempered 42CD4 steel examined, a laser surface treatment resulted in fatigue properties which were similar to those obtained for specimens subjected to a carbonitriding treatment under industrial conditions.