The aim of the project is the assess the local mechanics of case-carburised steel grades and microstructures for roller bearing applications, and to link in-service mechanical performance to Timken industrial parameters and alternative production routes. Bearings are key precision components used in motion and load transmission in a wide range of equipment and industrial machinery. Rings and rollers for bearings, esp. those with larger cross sections, are manufactured using low-carbon steel grades containing enhanced levels of solute elements such as Mn, Si or Ni to attain high steel hardenability and solid solution strengthening effects. The as-manufactured 'quenched & tempered' (QT) microstructure comprises a metastable austenite phase (<15vol.%) embedded in a fine matrix of tempered martensite. During operation, the bearing is subject to Hertzian contact stresses of several GPa together with subsurface shear stresses, potentially leading to rolling contact fatigue (RCF) of the bearing. Bearing steels are therefore required to offer high strength and toughness, together with resistance to surface wear through life. The RCF tolerance is dominated by the characteristics of the fine martensitic matrix, and recently mixed martensite/bainite microstructures with various degrees of solid solution strengthening are being considered for bearings. The characteristics of the local surrounding matrix also affect the austenite stability, and can delay the undesired austenite decomposition during service, and consequently the detrimental change in bearing dimensions brought by the transformation.
Case hardening is used at industry to increase the surface hardness and wear resistance of engineering components. Unfortunately, during processing a high carbon content near-surface in the prior austenite phase at elevated temperature enhances the tendency to form coarse-grained martensite structures during the subsequent quenching in the carburised case, together with globular brittle carbides at grain boundaries. Consequently, case-carburised steels present a complex heterogeneous microstructure through thickness, transiting from (case) plate mixed plate/lath martensite (core), an increase number of (globular) carbides close to the surface depending on the carburization parameters, and a changing grain size, volume fraction, carbon content and local environment of metastable austenite.
There is currently a strong industrial need to prove through thickness the mechanics of case-carburised bearing steels, and correlate the mechanical behaviour to the local microstructure and how it was manufactured. In this project, we will be using upfront experimental capabilities at large-scale synchrotron sources to depth profile in situ the local mechanics (local plasticity and damage, residual stresses, work hardenability, phase stability and RCF-related crack phenomena) of case carburised QT microstructures in simulated working conditions. This proposed work goes well beyond current practice, where traditional metallography is preformed destructively in case-carburised bearing components. The synchrotron scattering work will be supported by in-house analytical electron microscopy (dislocation analysis, nano-sized carbide characterization) and nano-indentation. Finally, we will be assessing the impact of non-QT bainitic/martensitic microstructures, produced by either austempering or quenching & partitioning, on the bearing mechanics. The student will have the unique opportunity to design and perform in-situ synchrotron experiments, acquire distinctive electron microscopy skills, and work on steel metallurgy with a direct connection and impact in the bearing industry.