Efeito do shot peening na resistência à fadiga de contato de ferro fundido nodular martensítico

Abstract

Since it was formed in 2006 at UTFPR, the Materials, Tribology and Surface Group (GrMaTS) has been studying the contact fatigue resistance of ductile cast irons. One aspect of these studies is the effect of residual stress on the material endurance. Techniques as the use of a die to apply mechanical compression and also nitriding have been reported as an attempt to increase the compressive stress. In these work, the shot peening process was used as the inductor of residual compressive stresses. Firstly, the washer-like specimens were machined, quenched and tempered. The preparation of specimens surfaces for shot peening included grinding, sanding and polishing. Shot peening was carried out to achieve an Almen height of 0.364 Amm. Analysis of hardness, roughness and residual stresses, this latter measured by x-ray diffraction, were conducted for characterization of treatment effects, In order to evaluate fatigue resistance, the specimens were submitted to rolling contact fatigue tests under flood lubrication, with a maximum Hertz contact pressure of 3.6 GPa. The failures were analyzed by means of optical and scanning electron microcopies and exhibited to have a subsurface origin. The fatigue mean life was estimated by means of a two-parameter Weibull analysis. Comparisons of the tests results with literature showed that shot peened specimens presented a better performance than those submitted to quench, tempering and nitriding. However the endurance of specimens simply treatment of quenched and tempered was not over by those that were shot peened. This behavior was attributed to an excessively high shear stress generated by shot peening, which was not followed by a correspondent increase on hardness since the material, due to previous treatment, was already too close of its hardening limit.