Wear and Oxidation Behaviour of Austenitic Stainless Steel in a Waste Gate Turbocharger

Abstract

Austenitic stainless steels are used in waste gate systems of turbochargers for their good mechanical strength at elevated temperatures. A key component of the waste gate system is the bushing, which guides and supports a rotating shaft under dynamic conditions. In these operating conditions, wear occurs which limits the service life of a bushing. Moreover, high alloyed austenitic stainless steels are used for conventional bushings, which results in high costs. For these reasons, alternative austenitic stainless steel alloys are considered. This investigation, in collaboration with Mitsubishi Turbocharger and Engine Europe B.V, focuses on the material selection of an alternative stainless steel alloy for the bushing application based on wear and oxidation behaviour.

In this work, conventional cast austenitic alloys were compared to alternative wrought austenitic alloys. The oxidation behaviour was determined using thermogravimetric analysis (TGA) and X­ray diffraction (XRD). Wrought materials showed better resistance against oxidation and better thermal stability than cast materials, which was attributed to the formation of a uniform Cr2O3 layer. In contrast, the interdendritic carbides in cast materials acted as sites for oxidation. The wear behaviour was characterized by hardness testing, pin­-on-­disc testing and engine testing. Worn surfaces were analyzed using white light interferometry, scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The presence of interdendritic carbides of the type MC, M7C3 and M23C6 in cast materials resulted in high hardness, which provided better resistance against adhesive wear. However, with the introduction of oxidation in the engine test, wrought materials outperformed cast materials due to the formation of stable layers consisting of oxidized wear debris. This layer acted as a solid lubricant and effectively reduced adhesive wear.

From these results, it is concluded that oxidation can have a beneficial effect on the wear behaviour of austenitic steels. Additionally, wrought alloys are suited to replace conventional alloys, which improves the service life and reduces the costs of bushings in future turbochargers.