Microstructure development in a case-carburized bearing steel

Abstract

WIND turbines play a crucial role in the global transition towards a sustainable energy future. Maximizing energy production and ensuring a reliable operation is essential to harnessing the full potential of wind energy. Among the critical components, the main shaft bearings have for several years been a focal point due to their significant downtime. In this context, a tribochemical treatment called case-carburization has gained notable attention for enhancing the microstructure of these bearings, to improve their reliability. Case-carburization is a surface treatment technique capable of modifying steel to exhibit a combination of properties such as high fatigue strength, toughness, and wear resistance, that are essential for these bearings as they operate in high-load-bearing environments. In a multi-stage heat treatment process involving case-carburization as the initial stage, the microstructure development at each stage is affected by the final microstructure of the preceding stage. Therefore, a comprehensive understanding of the microstructure at every stage is crucial for assessing its impact on the final microstructure and its properties. This Ph.D. research investigates the microstructure evolution throughout a four-stage heat treatment: carburization, sub-critical isothermal treatment, hardening, and tempering. The second stage is where the sole difference lies with regard to the heat treatment parameters, and is performed along two different routes, also in industrial practise, called the "bainitic route" and "pearlitic route". One of the primary goals of this research is to understand the microstructure development during the different stages of the two heat treatment routes and to provide an understanding of the microstructural features that can potentially affect the properties/performance of bearings. Additionally, this research also aims to identify the specific stage at which these features form and to provide insight into their formation mechanisms to explore strategies to rectify or mitigate the formation of detrimental features in the microstructure....