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Fe-Mn-Si-C steels are currently the most widely used type of steel for structural, building and automotive applications. Additionally, these steels also find important applications within welded structures, due to their low cost and versatility with regard to chemical composition and heat treatments because of the possibility to tailor their final mechanical properties. The production of a specific component made out of such steels generally involves continuous cooling heat treatments. Therefore, the way in which an optimum balance is achieved between the heat treatment parameters, the alloy composition and the desired microstructure for a specific application, is a vital step in the investigation of Fe-Mn-Si-C steels.

This research employed a novel physical deformation simulator to evaluate the effect of different processing condition parameters and cooling rates (C.R) and their influence on the variability of microstructure and mechanical properties of X-70 and X. 100 grade line pipe steels. A series of plane strain compression (PSC) tests were designed to simulate the thermomechanical controlled processing (TMCP) of plate rolling, with the influence of finishing temperature and C.R being of particular interest. A multipass deformation schedule at a strain rate of70 s' with constant delay times of 4 and 10 seconds were used in this work, and a total of two different deformation temperatures, that is 950 and 850°C, were used in the present, experiments, in which multipass isothermal and non-isothermal deformation took place, after which cooling commenced at one of three possible rates, 0.05, 6, or 15°C/s. A complete microstructural characterization of the as-received and processed material conducted using characterization techniques such as optical microscopy, scanning electron microscopy (SEM), and electron back scattered diffraction (EBSD) analysis, The results of the TMCP simulation with the PSC tests of HSLA steel showed that it was a very complex process with many parameters that need to be controlled throughout the process history. The key for optimum TMCP was the understanding of the microalloying behaviour at the different temperature and strain conditions. Grain refinement, which was the main aim in producing a high performing HSLA steel, could be achieved by the increasing the effective grain boundary per unit volume (Sv) in the prior austenite during the steel rolling stage. Higher S, values could be achieved by proper selection of rolling temperatures below the recrystallisation stop temperature (Ts%) with sufficient strain, The S, could be further optimized with the proper microalloying additions. Microalloying elements play an important role not only to increase the S, of the austenite during the rolling stage, but also to further stabilizes the ferrite nucleation and growth during the cooling stage. The analysed specimens also showed that the cooling rate after thermomechanical processing has a great influence on the final evolved constituents and ferrite grain diameter; where in most cases samples cooled at faster cooling rates even when applied to API X-70 steel grades with existing chemical composition, these cooling rates produced a refined non-polygonal and/or polygonal ferrite shape. The hardness and tensile testing were conducted on the thermomechanically processed specimens as well. These tests showed that the TMCP could enhance the performance of microalloyed steel when advanced parameters were used. This later statement was proven by the ability to enhance the X-70 steel mechanical properties by applying carefully selected TMCP and accelerated cooling parameters on the original composition.

This thesis covers two main topics- the development of Electromagnetic (EM) on-line microstructure inspection system for steel under controlled cooling and the investigation of using EM sensor to measure rail decarburisation depth off-line.First, through extensive Finite Element Modelling (FEM) the link between EM sensor output and steel microstructure has been found. Both zero-crossing frequency for real inductance and the peak- frequency for imaginary inductance are linearly proportional to magnetic permeability of steel which is an indicative for microstructure. Furthermore, the response of the complex H-shaped ferrite core sensor is found can be described by a simple analytical model of an air core sensor after normalization. In addition, the factors that might affect sensor performance are been investigated, including lift-off, rollers and industrial housing.Second, experiments were carried out both in the lab and at the service line of Tata Steel to check the sensor performance. Test results show the Multi-frequency Impedance Analyser (MFIA) system works very stable in real industrial setup with good performance in signal to noise ratio. It can successfully distinguish samples with different magnetic properties (paramagnetic and ferromagnetic).After that, the possibility to apply EM sensor in off-line rail decarburisation depth test is investigated. Both FEM simulation and experiment results show the decarburisation depth has a linear relationship with inductance. Also the EM sensor output has a good agreement with the predicted decarburisation depth (Fick’s law) and measured results from other methods (micro-hardness and visual test).

Surface treatments used to improve the life of a material known as peening are already extensively used in industry. The main aim of peening is to introduce compressive resiudal stress to the surface and subsurface of a metallic material, however literature also includes a number of microstructural and mechanical effects that peening introduces to a material when the compressive residual stress is established. The aim of this dissertation is compare and contrast the mechanical and microstructural effects of a current industrial peening method called shot peening, with three new increasingly competitive surface treatments. These are laser shock peening, ultrasonic impact treatment and water jet cavitation peening. The surface finish, and changes in microstructure, hardness depth profile, residual stress depth profile and plastic work depth profile of the four surface treatments are analysed. The effect of the peening parameters on the material is also determined, such as length of time of treatment, shot size, step size, direction of treatment, and irradiance per centimetre squared. The effect of peening on the residual stress depth profile of a gas tungsten eight pass grooved weld is also determined. Welding is a known region of early failure of material, with one of the factors affecting this being the introduction of tensile residual stress to the surface and near surface of the weld. An analysis to determine if peening the welded region alters the residual stress was carried out. In all experiments in this dissertation, the material that was used was austenitic stainless steel, as this material is highly used, especially within the nuclear industry. The results of this dissertation show that different peening types and peenign parameters produce a variety of surface, microstructural and mechanical effects to austenitic stainless steel. Peening of an aaustenitic stainless steel welded region results in teh near surface tensile residual stress to alter to ccompressive residual stress.