Gigacycle fatigue of the turbocharger gear wheel

Abstract

   The goal of the study is to elucidate the reasons for early fracture of the gear wheel teeth of a Cameron TA9000 turbocharger (1820 kW) after an operational load up to 1. 3 x 109 cycles.   The chemical composi­tion and the microstracture of the tooth metal were studied using the methods of metallography, microhardness and optical microscopy. The microrelief of fracture surfaces of operational fractures was studied using electron scanning microscopy. Analysis of chemical composition proved the steel grade of the tooth metal (DIN 31CrMoV9) declared by the manufacturer. Visual analysis of the fragments under study re­vealed numerous cracks present on the tooth contact surfaces. The fatigue fracture origins detected on the fracture surfaces are typical of high cycle and gigacycle fatigue fracture. In the latter case, the detected fracture looks like a "fish eye" exhibiting an area of?? structural heterogeneity with inclusions and pores in the center. The fracture probably developed from the first tooth fragment to the fifth one being accom­ panied by an increase in the number of fatigue fracture origins known to be attributed to an increase in the stress amplitude. Metallographic study showed the presence of a subsurface hardened layer with a thickness of 120 - 200 pm with a defect-containing structure associated with grain-boundary precipitates (presumably, carbides (Fe, Cr)3C), which can result from violation of the modes of heat treatment of the gear wheel. Formation of brittle intergranular cracks on the contact surface and their subsequent develop­ment in the entire depth of the subsurface hardened layer appeared to be the reason for a decrease in the strength and bearing capacity of the gear teeth.