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 × 10⁹ cycles. The chemical composition and the microstructure 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 the 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 origins of fatigue fracture detected on the fracture surfaces are typical of high cycle and gigacycle fatigue fracture. In the latter case, the detected fracture looks like a “fisheye” 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 accompanied by an increase in the number of origins of fatigue fracture 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 μm with a defect-containing structure associated with grain-boundary precipitates (presumably, carbides (Fe, Cr)₃C), which could have resulted 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. The interaction of the resulting cracks with longitudinal microcracks that originated from defects due to poor-quality mechanical processing of the gear led to the formation of centers of fatigue cracks, the development of which caused the final destruction of several teeth.