Aplicação da técnica de remoção química na determinação de perfis de tensão residual em amostras de fadiga de contato submetidas a nitretação e shot peening

Abstract

Researchers from the Group of Materials, Tribology and Surfaces of the Federal University of Technology-Paraná have been studying, since 2005, the effect of introducing compressive residual stresses on the rolling contact fatigue resistance of nodular cast iron. Techniques such as mechanical compression, plasma nitriding and shot peening were applied as an attempt to increase the cast iron’s fatigue life, but no positive results were found. In previous works, measurements of residual stresses were restricted to the surface of the specimens. The lack of residual stress profiles limits the analysis of the surface treatments applied. For this reason, the present work had the primary objective of measuring and analyzing residual stress profiles of shot peened and nitrided specimens from previous works (Wollmann, 2012; Melo e Nunes, 2013), using a chemical removal method associated with X-ray diffraction. Among the nitrided samples from Wollmann’s (2012) study, some have previously been submitted to a process of diffusion of the nitrided layer. Additionally to the samples from previous works, nitrided and shot peened specimens of AISI 5150 steel were also analyzed to allow comparisons between stress distributions in both materials. Optical microscopy, microhardness and surface roughness were the applied techniques to evaluate the effects of each surface treatment. A Shimadzu XRD-7000 X-ray diffractometer was used for residual stress measurements. The material removal between subsequent measurements was done through chemical etching to prevent the generation of new stresses. In order to compare removal methods, mechanical removal was also applied in one sample of AISI 5150. Results showed that only chemical removal method was effective on measuring residual stress profiles through X-ray diffraction. It was also concluded that the surface treatments performed in previous works were unable to introduce compressive stresses as deep as the region of maximum shear stress, which is defined by contact conditions. Diffusion in ductile cast iron nitrided samples, as well as the presence of some alloying elements in the AISI 5150 steel were decisive to increase the depth of compressive stresses.