Neutron-diffraction measurements for residual stress analysis in automotive steel gears

Standard production and machining of automotive components is still attractive, although it is not cost-effective if the life of the component has the highest priority. An important parameter to increase the fatigue life of these components is the beneficial introduction of residual stress, especially in the most loaded superficial layers. The aim of the present study is to investigate the residual stress in two steel gears produced from extruded bars cut and machined and then submitted to tempering and nitriding. The results have been compared with those obtained on sintered, nitro-carburised steel gears manufactured using the net-shape forming technique. They show a higher tensile stress level in the bulk of the component with higher carbon and chromium content, that is, a strong influence of nitriding elements. Consequently, in the nitrided layers, the calculated compressive stresses reach relatively high values in an aluminium- and carbon-rich sample. These stresses are sensibly larger than those found in the sintered and nitro-carburised gears, both in the surface and in the bulk.