Efeito de tensões residuais compressivas na fadiga de contato por rolamento de um ferro fundido nodular com matriz de dureza elevada

Abstract

This work analyzes the effect of compressive residual stresses, obtained by shot peening, on rolling contact fatigue resistance of a quenched ductile cast iron. This material presents similar mechanical behavior when compared to some steels, but with the benefits of being cheaper and lighter. In the other hand, when subjected to cyclic contact loadings, it presents shorter fatigue life than steels. Means of improving the performance of ductile cast irons are important specially to make them feasible for the manufacturing of parts that might fail in this situation. Previous works showed improvement on contact rolling fatigue life for some materials due to the presence of compressive residual stresses on the surface of the components. However, there is no consistency of results about residual stresses for this application. To achieve the proposed objective, shot peening process was used to produce a compressive residual stresses profile on the surface of quenched ductile cast iron samples. This material was obtained from continuous casting process and presented graphite count of 210 nodules/mm2. Quenching and tempering reached a hardness value of 607 ± 14 HV50. Residual stresses were measured using X-ray diffraction and chemically clearance of layers. Contact fatigue tests were performed on polished samples with ball-on-flat mode and fatigue lives were calculated using two-parameter Weibull probability distribution. The testing load induce a 2.2 GPa contact pressure. This load was chosen, considering the material properties and ensuring that the maximum shear stress point was inside the compressive residual stresses region produced by shot peening. Another group of quenched and tempered samples which were not processed by shot peening was tested to compare the results. It was observed that the effect of compressive residual stresses on fatigue life of samples was reduced approximately 47%. Through fractography analysis, it was possible to notice that the fail started close to the maximum shear stress region in both conditions. The residual stresses profile did not modify the failure mode. However, the compression caused by the shot peening process could have modify the contact principal stresses, which resulted in a faster cracks nucleation and growth.