A study of the constitution, transformation and precipitation effects in ternary Fe-Cr-Ni alloys and quaternary Fe-Cr-Ni-X alloys containing Mo, Nb, Ti, and Si was carried out. A systematic approach was adopted so that the microstructural effects observed as a result of ageing the selected iron base ternary alloys could be directly compared to the quaternary alloys. A series of ageing curves were plotted for the six ternary alloys over the temperature range 400°-900°C and for the ten quaternary alloys in the temperature range 650°-850°C. Optical and electron metallography were used to study the transformation and precipitation of intermetallic phases and carbides in the aged microstructures. The transformation of delta ferrite to sigma phase in a duplex (gamma+delta) ternary alloy was studied. The transformation was preceded by the precipitation of a cellular structure which formed on the delta/gamma grain boundaries and consisted of M23C6 and new austenite. A complex transformation product, which resembled a region of imiscibility, then formed at the delta/(gamma + M23C6) interface, this product consumed the delta ferrite grain, eventually transforming to sigma phase and new austenite. The alpha' phase, more commonly referred to as 475°C embrittlement, was also observed precipitating in the delta ferrite grains in two of the iron based ternary alloys in the temperature region 400°-500°C. Overageing of the alpha' precipitates after 1000 hours at 500°G was accompanied by the precipitation of a rod like austenite. The elements 2%Mo, 1%Nb, 1/4%Ti and 1% and 2%Si were added totwo Fe-Ni-Cr base alloys. One alloy was an austenitic (20Cr,23Ni) and the other was a transformable alloy (18Cr, 7Ni). The quaternary element additions had no effect on the constitution of the austenitic ternary alloy. However the addition of Mo to the 18Cr, 7Ni ternary alloy caused the transformation of delta ferrite to sigma phase over ageing temperature range 650°-850°C. The addition of Si moved the constitution of the ternary alloy further into the gamma+delta phase field, a small amount of sigma phase was observed in the 2%Si quaternary alloy on ageing at 650°C for 1000 hours. Irradiation damage studies were performed using High Voltage Electron Microscope and Variable Energy Cyclotron. The effect ofthe quaternary alloying additions Mo, Si, and Nb on the voidswelling behaviour of a 20Cr, 23Ni alloy were studied using 46MeV Ni6 ions in the Variable Energy Cyclotron, irradiating to a total dose of 10dpa at 550°C. All additions reduced void swelling, the largest reduction was observed in the Si containing allov. The Insert A Thermal ageing showed the presence of M23C6 the amount of which increased with increasing ageing temperature. No delta ferrite was observed in these alloys.

In view of the significant advantages offered by semi-solid processing, such as reduction in number of intermediate processing steps and energy input, and the potential for improving component complexity, it is of paramount interest to develop indigenous technology for semi-solid forming of steels, especially nuclear grade steels. For adopting semisolid processing as an alternative method of manufacturing of steels, it is essential to study the amenability of the steel for the process, understand the fundamental mechanisms of micro structural evolution and evaluate the mechanical properties of the steel after processing. To achieve this goal, the present work attempts to appraise the amenability of a low-carbon variant of 18%Cr-8%Ni austenitic stainless steel (304L SS) for semi-solid processing. Among the many requirements of the feedstock in semi-solid processing, a key feature that makes it amenable for semi-solid processing is the unique microstructure containing solid spheroids in a liquid matrix, thereby enabling thixo-tropic behaviour in the alloy. To understand the micro structural evolution in the steel, during major steps of semi-solid processing (partial melting, soaking and solidification), several experiments are carried out by varying the key parameters such as temperature, soaking time and cooling rate. Experimental results are analyzed in details to specify the effects of these parameters on the microstructure of semi-solid processed steel. The analysis indicates different phase transformation sequences during solidification of the steel from its semi-solid state. On the basis of experimental results, mechanism for micro structural evolution during partial melting and subsequent solidification of 304L SS is proposed. The effect of soaking time on the size and shape of the solid globules is analyzed using the theory of anisotropic Ostwald ripening. The semi-solid processing parameters, such as soaking time and temperature, are found to have significant influence on the globule distribution, globule shape, ferrite distribution and dislocation density, which in turn govern the tensile behaviour and mechanical properties of the steel after processing. Semi-solid processed 304L SS exhibits lower yield strength, ultimate tensile strength and higher strain hardening in temperature range 303–873K compared to as-received (rolled and subsequently annealed) 304L SS. However, semi-solid processed steel shows higher uniform elongation and fracture strain compared to the as-received steel. A pronounced effect of semi-solid processing is also found on the high temperature plasticity and dynamic recrystallization pattern. This work demonstrates the amenability of 300 series austenitic stainless steels for semi-solid processing. The investigation provides the significant insight into the mechanism of micro structural evolution in austenitic stainless steels during semi-solid processing and the important information on the mechanical properties and plastic flow behavior of the semi-solid processed steel. The results give crucial inputs for the optimization of processing parameters for obtaining the desired property in the product, and also for deciding the potential industrial application of the process.

The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.