Estudo dos mecanismos de fadiga e análise de desempenho em mola para válvula automotiva em regime de alto e ultra-alto ciclo. / Study on the fatigue mechanisms and performance analysis in automotive valve spring under high-and-ultra-high-cycle regime.

AUTOR(ES)

Edison Marcelo Serbino

FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

05/03/2012

RESUMO

This paper presents a study on the valve spring fatigue mechanisms, carried out through analysis of mechanical behavior correlated to microstructural aspect. The samples were made of high strength steel Si-Cr, obtained by conventional (quenching and tempering) and unconventional (austempering) heat treatments, generating a single level of hardness (approximately 610 HV). The samples were subjected to bench test with various loading levels, controlled through shear stress amplitude fluctuation. This variation has promoted distinct fatigue regimes known as high and ultra-high cycle, for greater and smaller levels of stress amplitudes, respectively. The performance was measured by calculating the fatigue total life, survival probability, verification of the elastic force and residual stress relaxation. The samples were characterized by means of metallographic methods, chemical composition analysis, hardness/tensile strength testing and X-ray diffractometry techniques. The fracture surfaces were investigated using scanning electron microscopy techniques. The cracks initiation and propagation mechanisms were similar in both samples; despite the proposed austempering heat treatment produced a mixed microstructure with predominance of lower bainite, containing sparse regions of retained austenite. The bainitic microstructure, resulting of austempering, when compared with martensitic samples, generated by quenching and tempering, led to an increase in fatigue total life under higher levels of stress amplitudes, but tending to be the same as the stress amplitude deacresases and the number of cycles increases. On the other hand, there was no residual stresses relaxation in both samples, after a significant amount of cycles, so this phenomenon did not influence the total life.

ASSUNTO(S)

aço de alta resistência austêmpera austempering fadiga fatigue high strength steel mola válvula valve spring

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