Efeitos da nitretação por plasma na fadiga de contato de ferro fundido nodular temperado

Abstract

Researchers from Materials group of UTFPR have been studying the effect of compressive residual stresses on the rolling contact fatigue endurance of quenched nodular cast iron. Mechanical compression, plasma nitriding and shot peening are some of the techniques used so far. On the studies concerning nitriding (WOLLMANN, 2012) it was observed an atypical type of surface damage which demanded additional investigation. On the present study, nodular cast iron samples have been quenched and, tempered, nitrided with different nitrogen contents in the gas mixture and finally submitted to rolling contact fatigue tests. The conditions applied on the treatments and tests were similar to those used by Wollmann (2012), but with variations (5, 10 and 20%) of the nitrogen content in nitriding process. The aim of this work was to analyze the effect of nitrogen amount in the nitriding gas mixture on the aspect of the damaged produced during rolling contact fatigue tests. This information is crucial on upgrading nitriding process of nodular cast iron. Scanning electron microscopy, x-ray diffraction and microhardness were applied on the materials characterization. The results shown that nitriding was successful on producing residual stresses, but as a result of nitriding time and temperature there was also a significant drop of the material hardness. It was also confirmed that 20% of nitrogen is decisive on the superficial embrittlement, and, as an additional observation, it has been found an autocontamination mechanism of the tribosystem caused by particles originated by fragmentation of more heavily nitrided regions.