Quenching and partitioning (Q&P) is a novel processing route that can create microstructures that combine high strength, ductility and easy formability without the excess use of alloying elements by also keeping the production cost low. Therefore, this method is ideal to be applied to steels that are used in the automotive industry. The material is initially annealed at a high temperature to form austenite. Then it is quenched to a temperature below the martensite start temperature to form a controlled fraction of martensite. Second annealing follows, at a lower temperature than the first one, that stabilises the austenitic phase at room temperature. It has been found that the austenite grain size at the end of the first annealing (prior austenite grain size) has a significant impact on the processing method, the final microstructure and consequently on the final properties of the material. In this thesis, phase-field modelling was employed to investigate the role of prior austenite grain size on the quenching and partitioning process. Simulations of the processing were performed for three different prior austenite grain sizes (6, 25 and 67 μm). Microstructures consisting of about 10.5% austenite and 89.5% martensite were created by simulating the first quenching of the Q&P process. Then, the evolution of the microstructure and carbon distribution were investigated for partitioning at 400 ◦C for times up to 4000 s, and quenching to room temperature. The results gave insights into the effect of the prior austenite grain size on the morphology of the microstructure and the kinetics of carbon during partitioning. It was found that a finer microstructure was derived with decreasing PAGS. Moreover, partitioning becomes more efficient because of the shorter distance that carbon has to diffuse, and the even spatial distribution of austenite. Finally, wide grain size distributions of the austenite that is present during partitioning can lead to significant variations in the carbon content among the austenite grains of the final microstructure, especially for short partitioning times. This affects the efficiency of the partitioning process and has an impact on the mechanical stability of the austenite in the final microstructure and consequently on the TRIP phenomenon.

Carburized steel grades are widely used in applications where high hardness at the surface is required in combination with good core toughness as well as high fatigue resistance. The process of carburizing lower to medium carbon steel can generally provide this combination of properties and has been practised for several decades. Such steel is very essential in vehicle power-trains. The carburized 18CrNiMo7-6 pinions are ground to obtain high dimensional precision after the carburization, quenching and tempering heat-treatment process. During the grinding process, thermal damage is developed due to the high localised heat energy at the contact zone of the grinding wheel and pinion. The developed thermal damage is known as grinding-induced burn. The burns are observed when the temperature reaches above the tempering range (i.e. 210 ̊ C) and the intensity of the grinding-induced burn is in relation to the thermo-mechanical effects of the grinding process. The nital etch process is used to identify the burns using ISO 14104:2017 standard and the process detects using the discolouration developed on the burn site caused by the chemical attack. The information about the intensity of detected burns are not known in the nital etch method. However, the intensity of the grinding-induced burns can be measured using magnetoelastic parameter of Barkhausen Noise (BN) technique which functions in relation to the microstructure and stress state of the material. The present study aims to investigate microstructural features of grinding-induced burns of varying BN intensities, to evaluate the service life testing of the thermally damaged pinions and the effect of varying grinding parameters on the generation of grinding-induced burns. The microstructure characterization is done by optical microscope, Vickers hardness, scanning electron microscope, X-ray diffraction and correlate the obtained results with the BN signals. The increased pinion speed and reduced grinding cycles are observed in the favouring of increasing localized heat input at the contact zone of grinding wheel and pinion causing the BN signals to increase. 3 samples of varying intensities of grinding induced burns are detected using BN and are characterized. The obtained results gave a good correlation with the Barkhausen noise signals. As the heating rate at the grinding contact zone increased above the tempering range, the BN signals also increased due to the enhanced domain wall movement with the softer microstructure which is observed due to the retained austenite decomposition and carbon diffusion from the tempered martensitic phase. The further increase in temperature above the austenitization range led to the re-hardening burn. The freshly formed structure is brittle and hard untempered martensite at the surface surrounded by the softer tempering burn which might be detrimental during the pinion functioning. The axle test results of the tempered burn pinion observed the transformation of retained austenite to martensitic structure during the cyclic loading which eventually enhanced the surface properties by increase in hardness from 621 HV1 to 676 HV1 at 0.1 mm depth and generating -600 ± 6.6 MPa compressive residual stress due to the volume expansion. This transformation resulted in the grinding induced burn pinion to survive the axle test without failure.