Search

The unwanted corrosion of metal structures, objects and consumables has a significant global cost, which was estimated to be approximately $2.5T in 2017, which is equivalent to 3.4% of global GDP. Materials degradation by corrosion processes is an important issue in the nuclear industry. Since the advent of nuclear technology in the 1940s, radioactive waste has inevitably arisen, and this has resulted in strict controls being established to safely manage it over long periods. Extensive research into the storage and eventual disposal of such waste in deep, underground repositories has been undertaken around the world. This thesis describes the background to the generation of the various categories of radioactive waste and summarises the approaches that are taken globally to manage the waste. Metallic containers are a key component of the engineered barrier systems used to ensure that the human population is not exposed to harmful levels of radiation from radioactive waste. The materials of choice for waste packages range from so-called corrosion allowance metals, such as carbon steel and copper, to corrosion resistant alloys, such as stainless steel, nickel alloys and titanium. The corrosion behaviour of container materials, and the waste metals within the waste, has been an important area of study in the countries operating nuclear facilities. The environments to which the metal containers are exposed, range from atmospheric conditions during interim surface storage, and, after placement in an operating underground facility, to water-saturated anoxic conditions in contact with backfill materials, particularly cementitious materials or clay-type materials, such as bentonite, when the underground facility is finally sealed. It is necessary to obtain a thorough scientific understanding of the corrosion behaviour of container materials in these different environments, in addition to obtaining well-established values for corrosion rates in the expected conditions. This enables prediction of waste package durability over very long periods (>100 years). This D.Sc. thesis contains a summary of the author's previous work in this area and presents a set of over thirty published, peer-reviewed, corrosion science papers on the subject, which describe experimental studies undertaken by the author since the 1990s for a number of radioactive waste management organisations, both in the UK and overseas. The areas of study covered by the published papers presented here can be divided into two main categories, namely laboratory-based projects and in situ experiments. The main themes of the laboratory studies are: • Measurement of gas generation due to the anaerobic corrosion of ferrous materials, both in aqueous environments and when encapsulated in solid backfill materials such as cement and compacted bentonite, and characterisation of the arising corrosion products. • Investigation of the interactions between the corroding metal and the surrounding solid matrix, particularly bentonite. • The effect of -radiation on the corrosion of carbon steel in simulated waste storage and disposal environments. • Investigation of the possibility of expansive anoxic corrosion, caused by the formation of corrosion products on cast iron. • Electrochemical aspects of the anaerobic corrosion of steel • Galvanic corrosion of waste container metals and waste metals • Corrosion of steels in cement-based encapsulants. In situ studies have included the setting up, operation and analysis of several large-scale experiments in underground rock laboratories using a range of in-situ monitoring and analytical techniques. These have focused on: • Characterising the atmospheric corrosion of stainless steel radioactive waste containers, as a function of the prevailing environmental chemistry, using a full-scale prototype waste container. • The corrosion behaviour of copper, cast iron and carbon steel when exposed to real underground environments in Sweden and Switzerland and characterisation of the effect of corrosion on the surrounding bentonite. The overall aim of the laboratory and in situ the experiments was to determine corrosion rates, corrosion mechanisms and the characteristics of the corrosion products, so that the information derived could be used to support the preparation of operational guidance and safety cases for waste disposal facilities, for various national waste management organisations. The work presented in the papers within this thesis has led to the generation of new information and a deeper understanding relating to a number of different aspects of corrosion that are key to the safe storage and eventual disposal of radioactive waste, including: • The relationship between atmospheric environmental parameters during interim storage and the corrosion susceptibility of stainless steels. • The corrosion mechanism, corrosion rates and characteristics of the corrosion products formed during the long-term anaerobic corrosion of carbon steel in the most common backfill materials in a geological disposal facility, namely cementitious materials and compacted bentonite, and the effect of the corrosion products on the properties of the surrounding backfill materials. • The possible effects of -radiation on the corrosion behaviour of carbon steel under disposal conditions. • The mechanical properties of oxides formed during corrosion and the risk of expansive corrosion causing damage to waste containers under disposal conditions. • The characteristics of galvanic corrosion occurring between dissimilar metals in electrical contact in disposal environments. • The role of microbially influenced processes on the corrosion behaviour of container materials under realistic conditions during waste disposal. Finally, the thesis outlines possible future avenues of research to further develop the work presented in the published papers.

Cost-effective corrosion mitigation of offshore steel structures can be achieved by Thermal Spray Coatings (TSCs). These coatings, when comprised of Al, Zn and their alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. As a result of the cathodic polarisation and subsequent increase in localised pH, calcareous matter (comprised of Mg(OH)₂ and CaCO₃) deposits on top of steel decreasing its corrosion. The study of thermal spray coatings onto steel with defects, such as those caused by mechanical damage or erosion remains largely unexplored. Electrochemical and surface characterization techniques were used in this PhD thesis to investigate the corrosion protection provided by TSCs on carbon steel substrates with and without defects in artificial seawater. Laboratory results showed that the presence of defects accelerates the formation of a protective corrosion product layer on the TSA coating. It was also revealed by Voltammetry and Electrochemical Impedance Spectroscopy (EIS) that both calcareous deposits and aluminium corrosion products hindered diffusion of dissolved O₂. The influence of the composition of seawater on the corrosion mechanism of TSCs was also studied. It was observed how, in order to recreate marine environments, it is crucial to prepare electrolyte solutions containing at least Ca²⁺, Mg²⁺ and SO₄², as these revealed by means of polarization resistances values to have high impact on the formation of corrosion deposits and calcareous matter. Thus, affecting the kinetics of both anodic and cathodic reaction. Lastly, validation of laboratory results was achieved by performing electrochemical measurements in situ. From the results here presented, it is expected to improve the design of corrosion protection systems with TSCs for offshore structures. This also translates in reduced costs and improved safety in marine environments.

For corrosion control in the petroleum industry the use of acid corrosion inhibitors is a necessity. In particular alkynol inhibitors are used in protecting oilfield steels. A future business need is the development of these inhibitors so that they may safely be pushed to higher operating temperatures, higher pressures, more sour oilwell conditions, and longer exposure times which is a present continuing trend. For these reasons improving the efficiency of inhibitors is paramount. Second, there is also a trend towards greener inhibitors that exhibit lower toxicity or at least improve the health and safety profiles of the toxic inhibitors in present use. The aim of this research is then two-fold, to address these two particular business drivers. A specific acid corrosion inhibitor, 4-ethyl-1-octyn-3-ol with a surfactant was found to display a mixed-type of inhibition which was more effective at higher temperatures. The inhibitor formulation commercially used in excess, was found to be susceptible to cause pitting in the underlying HS80 steel substrate which was identifiable through chronoamperometry as transient occurrences of noise. The mechanism leading to this is suggested to be violent microscopic ruptures of an overlying polymeric film that exposes regions of the underlying steel surface to corrosive attack. This polymer film was found to be persistent: as long as the established coating was not damaged the total inhibitor volume could be decreased after an initial film forming step and retain the same level of protection to the underlying steel substrate. This observation has the potential to represent a significant cost-saving as well as reducing the total amount of toxic chemicals that require handling. Given the centrality of the polymer film in imparting protection, key experiments on a dry coated specimen found the film to be roughly 3.5 nm to 10 nm thick and conductive to electrons. The polymer film could be perturbed and manipulated from the passage of electrons through it and this finding was leveraged to produce a program of polarisation scans directly onto the steel substrate which optimised the inhibitor film efficiency resident on it, providing an almost one order of magnitude improvement in corrosion protection at 83 ⁰C.

During operation within an Advance Gas Cooled Reactor (AGR) 9Cr-1Mo steel is subjected to high temperature CO2 rich gas. This leads to complex interactions taking place between the CO2 and the steel. The steel is subjected to simultaneous carburisation and oxidation changing both the chemistry and the physical properties of the steel. Experimental samples were exposed on behalf of EDF Energy to better understand the processes taking place and to better understand the factors which affect time to breakaway oxidation taking place. A range of microscopy techniques have been used to study experimental samples exposed to samples aged using accelerated exposure conditions including Scanning Electron Microscopy SEM), Focused Ion Beam Microscopy (FIB) and Transmission Electron Microscopy (TEM) to better understand the corrosion processes taking place under these conditions. Carburisation of the substrate takes place with carbides forming preferentially along grain boundaries or martensitic laths depending on substrate microstate. These carbides increase in complexity with exposure time with the carbide distribution favouring higher C containing carbides at later exposures (M7C3 vs M23C6). The increase in carbide volume throughout the spinel is associated with the internal oxidation characteristics of the steel, the porosity of the oxide scale and is linked with breakaway oxidation. Some of these substrate carbides have been shown to resist oxidation at the main oxidation front through the growth of a Cr rich shell, this allows the carbide to remain within the spinel for potentially thousands of hours. The porosity across the oxide scale changes with exposure time, and a mechanism of porosity development has been linked with the substrate carbide distribution and frequency. The oxidation behaviour of the steel has been shown to be dependent on both microstructure and carbide distribution, with both the internal oxidation zone and spinel oxide showing evidence of the prior microstructure and carbide distribution. Breakaway samples have been shown to have a thinner internally oxidised regions relative to the pre-breakaway samples. Predictions for time to breakaway have been made based off the growth rate of the IOZ across the fin tips and extrapolated to lower temperatures using the Arrhenius Equation and activation energy calculated from the fin tips. These predictions take the geometry of the fins into account, important as breakaway is always observed to initiate at the fin tips and not the main body.

This thesis aims to clarify the formation mechanisms of damage in steel bearings associated with white etching matter (WEM). WEM is nano-crystalline, equiaxed ferrite and is found bordering isolated 'butterfly' defects and extensive, networked white etching cracks (WECs). Recently, WEM formation has been attributed to severe plastic deformation (SPD), induced by various rubbing interactions which arise during rolling contact. However, it has not been assessed whether the morphologies of butterflies and WECs is consistent with this thinking. In this work, selected WECs and butterflies are explored, in 3D, via ion beam serial sectioning; with the positions of the cracks and WEM recorded using various electron microscopy (EM) techniques. The data was used to create 3D renderings, which provide insight into the formation of WEM. It was observed that WEM often lies predominantly along a single side of a WEC; which appears to contradict crack-face rubbing mechanisms. It is proposed that WEM can instead form due to SPD resulting from the concentration of in-service stresses, in the vicinity of crack-tips. Unevenly decorated WECs may then result from cracking along the boundary of the generated WEM. Conversely, WEM was observed to be thicker along mid-sections of the crack network, suggesting on-going WEM growth behind the advancing crack-tip. It is therefore proposed that WEM associated with cracks forms according to multiple SPD-driven mechanisms. For the investigated butterflies, a variety of crack and WEM morphologies were observed. It was not possible to ascribe the formation of the observed WEM solely to rubbing between the initiating inclusion and matrix; given the 3D shapes of the butterflies and an observation of untransformed material between the butterfly inclusion and its WEM 'wing'. Overall the observations appeared to be more consistent with a formation mechanism based on SPD driven by the concentration of in-service stresses around the butterfly inclusion. Selected 2D areas were also investigated with various techniques. In the case of WECs, the focus was cracks exhibiting an uneven decoration of WEM. The aim was to assess whether pre-existing microstructural differences were responsible for the observed WEM asymmetry. No such differences were found however, raising questions regarding the validity of crack-rubbing mechanisms. Wavelength dispersive spectroscopy (WDS) revealed that, contrary to expectation, WEM around WECs may be depleted or enriched in carbon, relative to the matrix material, despite dissolution of the matrix carbides. Conversely, butterfly-WEM was revealed to be consistently depleted. Carbon levels below the (carbide-free) matrix appear to imply expulsion of solute carbon from the WEM. It is suggested that WEM forms according to SPD-driven recrystallisation, leading to a relatively defect-free microstructure which initially expels carbon. It is also proposed that the varying levels of carbon observed are a consequence of variation in the accumulation of carbon-stabilising lattice defects, under continued rolling. Nano-indentation confirmed WEM is harder than the parent material, but no correlation was found between the hardness and carbon concentration; indicating the structural changes of the WEM are the main cause of its increased hardness, rather than supersaturation with carbon, as reported previously. Broadly, the results of this thesis reveal a high degree of variability in WEM, which warrants further investigation and needs to be accounted for in any future mechanisms which are proposed.

In the present work, the microstructure, precipitation behaviour and corrosion resistance of a novel maraging stainless steel (Alloy P, Fe20Co15Cr5Mo0.016C) under a range of ageing conditions (as quenched, aged at 460°C, 500°C and 530°C) are investigated. The relationship between the microstructural evolution of the alloy matrix and ageing temperature was then evaluated, with a focus on subsequent effect on passivation and susceptibility to localised corrosion. The alloy possessed a lath martensitic microstructure with an average prior austenite grain size of 228 ± 2μm. A number of thin (100 hours) would result in the segregation of the matrix into Fe-rich and Cr-rich martensites, partially contributing to the strengthening of the alloy by solid solution and phase boundary effects. The passive film formed on the steel consisted of a bi-layer structure, including a Cr-rich compact inner layer made up of oxides and hydroxides of Cr3+ and a non-uniform porous outer layer made up of oxides and hydroxides of Fe and Co in their Fe/Co2+ and Fe/Co3+ oxidation states and Mo in its Mo4+ and Mo6+ oxidation states. Film thickness remained consistent with increasing ageing temperature at 4-5nm thick based on oxygen EDX mapping. This was due to the lack of availability of Cr from the alloy matrix, caused by clustering and segregation lowering the concentration of Cr in the adjacent alloy matrix below that necessary for the steel to passivate. This was observable as a change in the uniformity of the film from a compact, continuous structure in the as quenched state to a rough and discontinuous structure with cavities where passivation was unable to take place due to insufficient Cr being available. This is consistent with the thickness of the outer Fe-rich layer of the film remaining unchanged. Electrochemical analysis found the alloy became more susceptible to pitting and general corrosion with increasing ageing temperatures. Pitting occurred in the 530°C aged condition on the application of any anodic overpotential relative to OCP, while the as quenched state had a pitting potential of 450 mV vs Ag/AgCl ref and no observable surface corrosion. This was due to the passivation of the alloy being compromised as a result of the lack of available Cr in the alloy matrix. A comparative alloy based upon custom 465® did not exhibit a change in susceptibility to pitting as a result of ageing.

In July 1981, the last sections of the state-owned Bilston steelworks were unceremoniously shut, thus ending two centuries of hot metal production in the Black Country, the onetime workshop of the world. The devastating closure of this profitable facility occurred despite a decade-long grassroots defence campaign spearheaded by local rank and file workers. Using previously unexplored primary source material and oral testimony, this thesis provides a detailed analysis of the battle to save Bilston works. It explores how, in the midst of the 1970s steel crisis, an exceptionally diligent type of worker activist adapted traditional production practices to ensure the survival of the plant. With Bilston’s steelmen maintaining their uniquely profitable record, bungling industry officials conspired to marginalise their plant in order to justify a deeply flawed state-sponsored rationalisation programme. At the heart of this process were the activities of a senior and divisional management team who systematically rationalised the Bilston facility, whilst seeking to cynically undermine shop-floor solidarity. The thesis, therefore, highlights the ways in which management prerogative impacted the lives of steelworkers and their families. The work critically examines the actions of a small band of shop stewards who mobilised into a multi-union local action committee tasked with saving 2,300 jobs. A key focus here is their chosen strategic framework. As experienced activists, they initially recruited a cross-party coalition of political figures to convince sympathetic policymakers to absorb the facility into a medium-term operating plan. With the unfolding crisis prompting a less forgiving political landscape, Bilston’s enterprising shop stewards made a tactical transition, engaging in concerted collective direct action to persuade conservative union leaders to petition decisionmakers on their behalf. The thesis offers a critique of institutional behaviour, revealing how both the state and moderate steel unions undermined Bilston by repeatedly acquiescing to management prerogative. Abandoned by union and Government bureaucrats, the campaign eventually crumbled from within. The research identifies the ways in which ambivalent officials merely sat idly by as management undermined a profitable state concern before insidiously harassing its conscientious employees. The thesis concludes with an account of the legacy of the battle for Bilston works, demonstrating how redundant steelmen, politicised by their experiences, played essential roles in the post-industrial social, cultural and political culture of the town.

Advanced high strength steels undergo a series of microstructural transitions associated with damage, these lead to failure especially when hydrogen enters the lattice and gets trapped in defects. It is desired to design advanced steels highly resistant to hydrogen embrittlement (HE), while preserving a balance between their superior strength and ductility. For this purpose, a new 100Cr6 bearing steel is proposed with 0.3 wt.% vanadium, compared to a previous grade containing 0.5 wt.% vanadium, for which a new heat treatment is proposed. Mechanical testing was carried out on hydrogen-free and hydrogen-charged specimens to evaluate the potency of V4C3 traps to reduce the HE susceptibility in such steels. The nature of crack propagation and HE mechanisms in 100Cr6, 100Cr6+0.3V and 100Cr6+0.5V were assessed using hydro-hardness technique, which provides direct observation of hydrogen diffusion to cracks. The results showed that pre-charged 100Cr6+0.5V are anomalously weaker compared to 100Cr6+0.3V. Slow strain rate tests (SSRT) on notched round bar specimens have been carried out at a constant tensile crosshead speed of 0.005 mm min−1. Failure was more pronounced in vanadium-free specimens as V4C3 limited the transition from ductile to brittle fracture mode in 100Cr6+0.5V steel. Rolling contact fatigue (RCF) test was also carried out to study the propagation of cracks under rolling contact fatigue conditions. It was concluded that 100Cr6+0.5V steel has longer bearing life and higher resistance to hydrogen embrittlement than 100Cr6. However, microstructural analysis confirmed the presence of non-metallic inclusions (NMIs) in 100Cr6+0.5V steel and the disadvantageous effects on its mechanical properties were recorded. A crucial consequence of plastic deformation during RCF is the transformation-induced hydrogen desorption which occurs when retained austenite transforms into martensite. This mechanism can be investigated through studying the HE susceptibility of transformation induced plasticity (TRIP) steels. An intensive study was carried out to optimise the TRIP steel heat treatment aiming to achieve an ultimate tensile strength (UTC) of 1303 MPa. The microstructure showed lower bainite structure consisting of ferrite plates, film-like and blocky retained austenite. The most promising microstructure with respect to the expected UTC was obtained after bainitic isothermal tempering at 302 °C for 24 to 30 minutes. Thermal desorption analysis was used to study desorption of hydrogen from TRIP steel. The high activation energy of diffusion indicates that the hydrogen is trapped in retained austenite which is considered as irreversible trap. However, there was no evidence demonstrating the advantage of retained austenite on preventing embrittlement after saturation with hydrogen. As a result, it can be inferred that a finer and lower fraction of retained austenite can reduce the mass of trapped hydrogen minimising the susceptibility to HE in TRIP-assisted steels.

In this thesis, a wide range of surface sensitive techniques have been used to characterise surfaces relevant to offshore steel structures, to provide fundamental understanding of steel-paint binding that prolongs coating lifespans and corrosion inhibition. Particular steel surfaces are characterised before and after almandine garnet abrasive blasting. Significant build-up and coverage of abrasive residue (almandine and calcium carbonate) is identified on the post-treatment steel, covering up to a third of the steel surface; Given the importance of the blasting materials on the surface, almandine garnet is also characterised for its surface chemistry and behaviour. By characterising both bare steel and almandine garnet, adsorption on `real' garnet-blasted steel substrates can be modelled. A dry ice-garnet mixed stream is also investigated to see whether abrasive residue could be reduced. It is found that a quarter of the steel surface is covered with blasting residue. However, cooling and moisture-condensation leads to corrosion spots formation. The adsorption of potential paint additives and components on S355 steel and garnet are determined using solution depletion isotherms. Quantitative data such as equilibrium adsorption constants, and estimation of monolayer molecular geometries are collected. The latter is further investigated through novel surface spectroscopy. In brief, most organics are found to have higher The steel surface is shown to be likely to corrode in offshore conditions with corrosion marine aerosols in a matter of a few hours. It is these corrosion products which will be substrates for coating/paint molecules. A `salt drop' corrosion study simulates relatively short timescale aerosol exposure (mins to hours). Surface chemical-environments are characterised. The corrosion products are found to be porous, inhomogeneous in chemical environment, and evolve through time, with adsorbed/occluded marine ions. Finally, as steel corrosion products are likely to adsorb marine salts, ion adsorption on almandine garnet is investigated. Cations of sodium, magnesium, and calcium are found to specifically adsorb. Numerical model co-fitting of data from different techniques successfully obtains the adsorption equilibria and constant, and the surface site density of almandine garnet. The study highlights a general need for more complete studies, using multiple adsorption experiments, for aqueous phase adsorption investigations on minerals in the future.

Tidal stream energy is a form of marine energy with significant advantages compared to other types of offshore renewables, albeit the levelised cost of energy is relatively high. Various types of foundations have been considered for prototype tidal stream generators, however the utilisation of Gravity Based Structures (GBS) seems like a promising approach to achieve cost reduction and increase the financial viability for commercial deployments. The generators are expected to be deployed onto rocky seabeds and the GBS (made of steel or concrete) need to ensure the overall stability of the structures. This research focuses on the investigation of the shearing resistance of the foundation – seabed interface materials by means of laboratory element testing. Interface tests of five different rock types (representative of sites with tidal energy potential around the U.K.) on steel and concrete were carried out under a range of normal stresses. The different properties of the counterface materials (i.e. steel, rock, concrete) allowed the investigation of the controlling parameters on the shearing resistance of steel on rock and concrete on rock interfaces. Tests results were interpreted to determine the effect of the applied normal stress, surface roughness, hardness and rock strength (UCS) on the shearing resistance of the interfaces. It was found that the relative ratios of some counterface properties (e.g. roughness, hardness) can be utilised to explain the interface shearing behaviour. A framework described by a power function has been developed to estimate the shearing resistance of rock on steel and rock on concrete interfaces, incorporating the effect of these counterface material properties.

While there is a strong link between microstructure and hydrogen diffusion and embrittlement in all metallic systems, the correlation between the two is poorly documented. It is, however, sorely needed as the current research shows that by quantifying the type and density of defects along with their associated segregation energy for hydrogen (commonly called trapping or binding energy) the effective diffusivity can be estimated, which is in turn correlated to the embrittlement susceptibility. Attempts to model hydrogen interaction with defects at continuum scales have historically been plagued by the large scatter in the binding energies resulting from the experimental challenges associated to their measurement and by a lack of experimentally verified scaling laws linking the density of microstructural defects such as dislocations and carbides to the corresponding number of hydrogen trapping sites. In recent years however, advances in first principles modelling have enabled the determination of trapping energies, enabling the identification of the correlations between the density of microstructural features and the resulting density of hydrogen trapping sites. Multiphase, high-strength steels, which are of particular importance due to their susceptibility to embrittlement and their ubiquity in applications where exposure to hydrogen is expected, are the main focus of this work. A combination of hydrogen charging, thermal desorption analysis and electrochemical hydrogen permeation was used in conjunction with continuum-scale hydrogen diffusion modelling to quantify the diffusion properties. The alloys studied were characterised using standard techniques including optical, scanning electron and transmission electron microscopy as well as X-ray diffraction. The studied microstructures were chosen such as to best isolate the effect of particular defects, namely vanadium carbides, grain boundaries, dislocations and retained austenite, on the overall rate of hydrogen diffusion. The results from this work include the identification of scaling laws for hydrogen trap densities associated with vanadium carbides, dislocations and grain boundaries. In addition, two new mechanisms have been incorporated into a multi-trap diffusion model. The first is the effect of grain boundary diffusion under the assumption of local equilibrium between the hydrogen at the boundaries and that in the surrounding grains, which may be used for future studies on hydrogen diffusion in alloys in which grain boundary diffusion is faster than bulk diffusion, in particular Ni-based alloys and austenitic steels. The second effect is the incorporation of a boundary condition at the ferrite -- austenite interface which allows for the diffusion of hydrogen into austenite against the concentration gradient. The boundary condition is able to describe an interface with an associated energy barrier for hydrogen diffusion from one phase to another and includes the effect of an interface energy barrier. It was also demonstrated throughout that diffusion modelling is complementary to experimental work and is a useful tool to separate the effect of different types of defects on the rate of hydrogen trapping and diffusion. The model has been shown to work on complex microstructures which include several different trap types and can furthermore treat bulk trapping in phases with higher H solubility such as austenite, whose effect has to date not been fully unravelled. The results presented here pave the way towards more accurate modelling of H diffusion problems in modern steels and other alloys. Examples include the tracking of H concentration in components exposed to H-rich environments, optimising heat treatments used for hydrogen release in the production of large casts and the heat treatment of welds which are prone to H-based cracking. Coupled with fracture-mechanics-based criteria the results can also be used to estimate the susceptibility to hydrogen embrittlement based on the information on the volume fraction and/or density of defects in the microstructure.

Increasing the share of scrap-based steel production is necessary to achieve CO2 emissions targets. However, the quality of recycled steel is compromised by contaminating elements, of which copper is the most pervasive. Copper from wiring and motors entangles with steel fragments during shredding and is not completely removed by magnetic separation. Beyond hand-picking, no commercial process exists for extraction, but copper in solution with steel segregates during hot rolling, causing surface cracking and defects that are unacceptable for high-quality flat products. This thesis characterizes copper in the global steel system, evaluates the energy requirements of possible extraction processes and presents experimental results to aid in the development of an efficient extraction technique. Copper contamination is currently managed by globally trading contaminated scrap to tolerant applications and by dilution with primary steel. An evaluation of copper in the global steel system is needed to develop long-term strategies, and this is presented in the first part of this thesis. The copper concentration of flows along the 2008 steel supply chain are estimated from a range of literature sources and compared with the maximum concentration that can be tolerated in all steel products. Quantities of final steel demand and scrap supply by sector from a global stock-saturation model are used to estimate the amount of copper in the future scrap supply, and the total amount tolerable. Assuming current scrap preparation continues, more copper will enter the steel cycle than can be tolerated by demanded products by 2050. This global constraint will set in sooner if primary production is cut to meet climate mitigation targets. Given the upcoming constraints, improved copper control is necessary. Various techniques for copper separation have been explored in laboratory trials, but as yet no attempt has been made to provide an integrated assessment of these options. The second part of this thesis presents a framework to define the full range of separation routes and evaluate their potential to remove copper, while estimating their energy and material input requirements. The thermodynamic, kinetic and technological constraints of the various techniques are analyzed to show that copper could be removed to below 0.1wt% (enabling the production of high-value flat products) with 5-20% of the melting energy in the electric arc furnace route. The above analysis reveals a promising and under-explored process route: preferential melting of copper from solid steel scrap, which could be integrated into conventional scrap re-melting with little additional energy. Previous investigations show removal of liquid copper is limited by its adherence to solid scrap. In the third part of this thesis, the individual and combined effects of several parameters (steel carbon content, initial surface oxidation and applied coatings) on the wetting behavior of liquid copper are observed with a heating microscope to understand if a process window to enable separation exists. The most significant factor was carbon content. On medium carbon steel substrates, copper spread rapidly, likely due to reduction of the oxide layer by carbon. Non-wetting copper droplets were observed on low carbon substrates in an inert atmosphere. This indicates a possible process window, but further investigation considering diverse, fragmented end-of-life scrap is needed. The scrap supply of all metals is expanding. The multi-scale, interdisciplinary method developed in this thesis could be applied to other metal systems to understand the constraints caused by contamination and identify key areas to develop efficient extraction processes, necessary to conserve resources and reduce CO2 emissions.

Miniature detonating cord (MDC), a typical linear explosive device, is primarily used in aircraft canopy severance systems. MDCs are typically flexible cylindrical cords with an explosive core and a robust sheath/cladding material. In practical applications, upon detonation, the MDC which is bonded to the inside of canopy propels the sheath outward at a high velocity, thereby penetrating or shattering the canopy transparency to clear an escape path. Conventionally, the materials for explosive core are pentaerythritol tetranitrate (PETN), cyclotrimethylenetrinitramine (RDX), cyclotetr-amethylenetetranitramine (HMX), or hexanitrostilbene (HNS). For decades, the sheath materials of MDCs were made by antimonial or non-antimonial lead, due to high density, good ductility, and relative ease of manufacturing. However, owing to the environmental and human health concerns regarding lead poisoning and the increasingly strict requirement in environmental legislations, the replacement of lead has been compulsory for recent development of new generation metallic sheath materials for MDCs in aerospace industry. The present study aims to study the feasibility and reliability of developing a costeffective and easy-manufacturing lead-free tin-based alloy, which is suitable as the sheath materials of MDCs for the clearance of aircraft canopies. The materials requirement was analysed and Sn-Cu based alloys were firstly chosen as potential candidates. The Sn-Cu alloys (0.3-1.0wt.%Cu) were prepared by casting and rolling. Microstructures of as-cast Sn-Cu alloys comprised Sn solutions with Cu6Sn5 intermetallic phase in the matrix. The as-rolled hypoeutectic Sn-Cu alloys (0.3-0.5wt.%Cu) offered the yield strength from 26.1 to 31.9 MPa, UTS from 30.1 to 34 MPa and elongation from 86.4 to 87.5%, which were found to be appropriate for the sheath. Particularly, the Sn-Cu alloys exhibited nonwork-hardening phenomenon under tensile stress, which could benefit sheath manufacturing and subsequent processing after assembly with high-energy explosive materials. Another achievement is understanding the deformation mechanisms and microstructure characteristics of the Sn-Cu alloy under rolling, which involves boundary formation, dynamic restoration, twinning, and recrystallization texture. A bimodal grain structure was well established after rolling, ascribed to the dislocation activities and dynamic restorations including dynamic recovery (DRV) and dynamic recrystallization (DRX). The Cu6Sn5 particle stimulated nucleation (PSN) was found as the major mechanism of DRX, which was also the dominant cause of forming of 〈001〉//RD oriented nuclei. Additionally, DRX nuclei are formed along the existing boundaries, resulting in a necklace structure via continuous dynamic recrystallization (CDRX). {301} and {101} twins were identified as additional significant microstructure features. Due to the microstructural inhomogeneity of Sn-Cu alloys subjected to rolling, alternative tin alloys were also developed for applications. The assessment of the mechanical properties of Sn-3Zn-xBi (Bi: 0-5wt.%) alloys processed by rolling were undertaken to explore their feasibility as sheath materials. Effects of Bi on the microstructure and mechanical properties of Sn-Zn were studied. Bi significantly refined the as-cast microstructure, altering the configuration of Sn-Zn eutectic from well-aligned Zn-rich needles to misaligned Zn-rich flakes. After deformation, secondary phases such as Zn-rich precipitates and Bi particles, and Bi solutes were critical to grain refinement because these secondary phase particles provided more sites for nucleation and more obstacles to growth of new recrystallized grains. Tensile results confirmed that Bi addition enhanced both strength and ductility of the rolled Sn-Zn-Bi alloys. The Sn-3Zn-5Bi alloy possessed superior strength and ductility (UTS: 84.4 MPa; Yield strength: 68.3 MPa; Elongation: 75.2%) due to its finest and most homogeneous equiaxed grains. The corrosion properties of Sn-3Zn-xBi (x=0, 1, 3, 5, 7 wt.%) alloys were investigated to explore the effect of Bi on the corrosion performance of the Sn-Zn alloy. Results indicated that the addition of 1 wt.% Bi increased the corrosion susceptibility of the Sn-3Zn alloy, mainly attributed to the coarsened and more uniformly distributed corrosion-vulnerable Zn-rich precipitates, while further increasing the Bi contents decreased the corrosion susceptibility of Sn-3Zn-xBi alloys due to the higher fraction of nobler Bi particles serving as anodic barriers. The Sn-3Zn-7Bi possessed the best corrosion resistance among all tested alloys. The role of Bi on the corrosion properties was considerably discussed. Finally, numerical performance simulations and proof tests were conducted using Ansys AUTODYN-2D to verify the reliability of the tin alloys for the cord sheaths. The cut depths of the cords sheathed by varied metals/alloys incorporating Pb, Pb-Sb, Sn, Sn-Cu, Sn-Zn-Bi, Al, Cu, and Ta were attained. Simulation results indicated that newly developed tin sheathed cords showed similar cut depths compared with lead. Proof firings demonstrated that the Sn-Zn-Bi sheathed cords fulfilled the requirement of cut depths against acrylic and aluminium targets, confirming the successful development of Sn-Zn-Bi for the MDCs in the specific application.

Nuclear power is a low carbon, high output energy source which is employed on a global scale. However, it is not without drawbacks; the generation of long lived radioactive waste has presented a problem since the operation of the first nuclear reactors in the 1940s. The treatment of radioactive waste materials is therefore of considerable importance to prevent further spread of radionuclides, deterioration of storage conditions and the potential for accidents. With these considerations in mind projects were undertaken investigating the use of new hydrogels for the decontamination of radionuclides and the use of foams for neutron poisoning applications. The development of a new hydrogel decontamination method for the removal of 137Cs and 90Sr radionuclides from stainless steel surfaces was explored and the significant number of synthesised gels characterised with full rheological profiles. Many targeted gel loadings were examined for the delivery of effective decontamination treatments; with solid encapsulation of radionuclides, high DFs (up to 95 for 137Cs and 24 for 90Sr on stainless steel substrates, exceeding literature precedents), no lateral spread of radionuclide contamination and a minimal volume of liquid waste reported. This shows excellent potential for industrial and research lab applications for the safe treatment and disposal of localised, highly contaminated surfaces with the need for minimal human interaction; thus greatly reducing potential dose. The ease of transport, storage and stability of these materials shows great promise for emergency implementation in the case of a 'dirty bomb' chemical, biological or radionuclide (CBRN) attack scenario, where deployment would be of minimal difficulty and high effectiveness without the risk of property destruction. The concept of neutron poisoning foams for accident remediation or emergency scenarios was investigated, with potential loading materials examined; building up to experimental studies of the feasibility of this technology. The experimental configurations and investigations into this concept have no prior literature precedent and address a key contingency need with full detailed assessment of the plausibility and potential materials which may be applied to this problem. This work concluded that a neutron poisoning foam is not a feasible delivery method.

Hydrogen embrittlement in ferrous and non-ferrous alloys is, and has been for over a century, a prominent issue within many sectors of industry. Despite this, the mechanisms by which hydrogen embrittlement occurs and the suitable means for its prevention are yet to be fully established. As hydrogen fuel becomes a prominent feature in modern concepts of a sustainable global energy infrastructure and nuclear power enters its renaissance, with commercially viable fusion plants on the horizon, hydrogen embrittlement is becoming an ever more pertinent issue. This has led to a considerable demand for novel alloys resistant to hydrogen embrittlement, notably within the bearings industry, where the commonly conflicting properties of high strength and hydrogen embrittlement resistance are required. This work investigates the mechanisms through which hydrogen embrittlement and irradiation damage occur in steels and nickel-based alloys respectively, with novel alloys designed for improved resistance. Through the engineering of secondary phases, optimised for helium and/or hydrogen trapping capacity, the novel alloys present the benefits of such trapping species with respect to embrittlement resistance. Such species have been studied in depth with respect to their interactions with hydrogen, establishing a novel mechanism of hydrogen embrittlement - the hydrogen enhanced dissolution and shearability of precipitates, leading to enhanced localised plasticity.

The phenomenon of hydrogen embrittlement in steel has been known for over 150 years. Hydrogen-resistant alloys have been developed to mitigate this effect and three types of alloys with optimised structures have been enhanced over the years: nickel alloys, stainless steels, and quenched and tempered martensitic low alloy steels. Nevertheless, those alloys are limited in terms of strength and ductility. The aim of the work presented in this thesis was to design bainitic alloys with hydrogen embrittlement resistance, and with a better combination of strength and ductility than conventional alloys. In the novel alloys, two microstructural features were produced to mitigate the damaging effects of hydrogen: 1. A percolating austenite structure, in which hydrogen diffusion is orders of magnitude lower than in bainitic ferrite. This feature was introduced to impede the ingress of hydrogen through the structure. 2. Iron carbide traps, which can form at the bainite transformation temperature. This feature was introduced to trap diffusible hydrogen and prevent it from causing damage. The alloys, designed with the aid of computer models and phase transformation theory, contained a volume fraction of retained austenite above its percolation threshold, theorised as 0.1, which was proven to form an effcient barrier to hydrogen ingress. The effective diffusivity of hydrogen, measured using an electrochemical permeation technique, was shown to decrease with increasing austenite fraction up to the percolation threshold. It was seen to plateau for austenite fractions comprised between 0.1 and 0.18, and to decrease further for fractions above 0.18. The compositions of the alloys were precisely selected to allow for iron carbides to precipitate during the bainitic transformation reaction. Until the present work, only alloy carbides V4C3, TiC and NbC had been reported to strongly trap hydrogen. The literature was very inconsistent regarding the trapping ability of cementite, with reported trap binding energies ranging from 11 to 66 kJ mol−1. The carbides produced in the alloys were identified as cementite. The cementite fraction was measured to be 0.001 ± 0.0001 for one of the designed alloys, which is the lowest ever reported carbide fraction in steel measured using a simple X-ray diffraction technique. Experimental thermal desorption spectroscopy data were used to determine the binding energy of hydrogen to cementite to be 37.5 kJ mol−1, suggesting that cementite is not a strong hydrogen trap. Further tests performed after room temperature hydrogen degassing displayed insignifcant amount of trapped hydrogen, thus confrming the reversible nature of cementite traps. The comparison of two successive transients using the electrochemical permeation technique confirmed that result. The influence of the heat treatments on the microstructures and on the mechanical properties of the designed alloys was extensively studied. The novel alloys met all the set requirements, and successfully outperformed conventional alloys in terms of strength and ductility. They did not meet the NACE TM0316-2016 standard requirement for operation in hydrogen-rich environments, likely owing to the inadequate trapping ability of cementite. Future work should focus on exploring the possible use of alternative carbides for hydrogen trapping in bainitic structures.

The UK has committed to deep, long-term reductions in national greenhouse gas emissions as part of a global effort to address climate change. Material efficiency, reducing the material inputs per service output, has long been identified as a globally underexplored mitigation strategy. Previous studies show unrealised technical potential to improve the efficiency of steel use, a large contributor of industry emissions, in the UK. This thesis explores why these opportunities may be unrealised along the steel supply chain.

Grade 92 steel is a commonly used material for steam pipe and tube applications in the power generation industry. The advantages of Grade 92 steel include excellent creep resistance and sufficient corrosion resistance, which provide a long service lifetime expectancy for components. Grade 92 steel is typically heat treated by a normalisation process performed at approximately 1100°C followed by a tempering process at approximately 750°C. The resulting microstructure of Grade 92 steel is composed of a tempered martensitic matrix with secondary precipitates distributed both on lath and grain boundaries and within laths and grain interiors. Welds in thick-section steam pipes made from Grade 92 steel are typically fabricated by a gas tungsten arc root process followed by a multi-pass submerged arc fill process. Post weld heat treatment (PWHT) is then performed on as-fabricated welds, helping to relieve residual stress and stabilise the martensitic microstructure. The microstructures in the HAZs of these complicated welds have not to date been fully understood. There is a lack of systematic microstructural investigations to define the different regions of the microstructure across the HAZ as a function of the welding process.

The ageing state of the world's nuclear power infrastructure, and the need to reduce humanity s dependency on fossil fuels, requires that this electrical energy generating capacity is replaced. Economic factors, and its physical and chemical properties, make high purity iron-chromium binary alloys a strong candidate for use in the construction of the pressure vessels of the next generation of nuclear reactors. This relatively inexpensive metal retains the oxidation resistance property of so-called stainless steel alloys, and has demonstrated dimensional stability and low degradation under harsh experimental environments of temperature and radiation. In this work, we consider radiation induced interstitial damage to the atomic lattices of iron-chromium binary alloys using the atomistic modelling methods, Molecular Dynamics and Adaptive Kinetic Monte Carlo, simulating collision cascade sequences, and the migration of defects in the aftermath. Variations in chromium content does not effect the initial damage production in terms of the number of Frenkel pairs produced, but iron and chromium atoms are not evenly distributed in defect atoms with respect to the bulk concentration. In simulations conducted at low temperature, chromium is under-represented, and at high temperature, a greater proportion of interstitial atoms are chromium than in the lattice overall. The latter phenomena is most strongly pronounced in systems of low bulk chromium content. During the simulation of post-cascade defect migration, interstitials atoms are observed to form temporary clusters and vacancies align along adjacent lattice sites, with the two types of defect also migrating to annihilate by recombination. Calculating the energy spectra of cascade events corresponding to an example experimental configuration using the SRIM package, we investigated the evolution of lattice systems in which a sequence of multiple cascade events occurred, both with and without a physically representative time gap between events. These simulations gave us the opportunity to observe the behaviour of cascades in the proximity of damage remaining from previous events, such as the promotion of defect clustering when this occurs.

Hardfacing alloys are utilised in pump and valve components in pressurised water reactors (PWR's). They are designed to withstand wear and galling effects, which occur as a result of surface-to-surface contact, where surface roughness increases by localised plastic deformation, resulting in fracture and material transfer. Typically, alloys that exhibit suitable hardness and galling resistance are known as the Stellites; a Co-base alloy family. Whilst these Co alloys perform well in a hardfacing capacity, they suffer from neutron activation and subsequently decay, forming 60Co isotopes, which emit hazardous γ-rays, contributing to plant worker exposure. The present study was developed to characterise and assess the metallurgical properties of two candidate Co-free replacement alloys; stainless Fe-based alloys Tristelle 5183 and a derivative alloy, developed and patented by Rolls-Royce, known as RR2450. The alloys are produced as gas-atomised powders before undergoing hot isostatic pressing (HIP) into usable parts in-service. As part of this work, we have identified a novel, high-strength Fe-Cr-Ni silicide phase, which precipitates extensively within the RR2450 alloy after HIP consolidation, resulting in the formation of a triplex (austenite/delta- ferrite/silicide) matrix. The use of automated diffraction tomography (ADT) has allowed the crystallography of this phase to be determined as a trigonal R3 space group setting. A carbon atom, identified at a trigonal bipyramidal site along [111] within the silicide phase unit cell, indicates a carbon solubility of up to 1.2 wt% within this phase. HIP cycles were studied in situ using synchrotron X-ray diffraction (XRD), which revealed that the silicide phase decomposes within the metastable gas-atomised RR2450 powder by a eutectoid γ → delta + M7C3 transformation. The starting fraction of metastable delta-ferrite within the RR2450 gas-atomised powder is shown to directly influence the rate of transformation from γ to delta-ferrite during the HIP cycle. The wear resistance of the triplex RR2450 alloy at 300 °C is shown to be superior when compared to the austenitic Tristelle 5183. This is attributed to the high strength silicide phase, which is shown to offer a hardness up to 2 to 2.5 times greater than the austenite and delta-ferrite phases. By producing an elastic angular distortion of the unit cell, the silicide phase is able to withstand loading up to 1 GPa without yielding. The Tristelle 5183 alloy, which produces a lower fraction of silicide phase compared to RR2450, is reliant on the formation of stacking faults and a strain induced martensitic transformation to provide a high wear resistance. These transformations are shown to reduce substantially during wear testing at 190 °C, leading to a loss of high temperature wear resistance in the Tristelle 5183 alloy. Future work into developing silicide based Fe hardfacings is suggested, the microstructures of which can be tailored by controlling Si and Ni additions.

In the drive towards higher strength alloys, a diverse range of alloying elements is employed to enhance their strength and ductility. Limited solid solubility of these elements in steel leads to segregation during casting which affects the entire down-stream processing and eventually the mechanical properties of the finished product. The work presented in this PhD shows that segregation of alloying elements during casting, particularly aluminium, leads directly to banding in the final product. It has been demonstrated that no significant homogenisation is possible in this alloy within practical time constraints of the industrial thermo-mechanical process. A through-process model was developed to design a thermo-mechanical treatment aimed at reducing the effects of segregation on the formation of banding. A new polynomial function for calculating the local phase transformation temperature (Ae3) between the austenite + ferrite and the fully austenitic phase fields during heating and cooling of steel is presented. Material was produced both with and without banding and used to study the effect upon the mechanical properties. The banded steel variants show a significant reduction in tensile strength for a similar level of ductility compared to non-banded variants. In situ measurement under uniaxial loading using high-energy synchrotron diffraction allowed direct quantification of the impact of the mechanically induced transformation of metastable austenite on the work- hardening behaviour. The results reveal that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding and the experimental evidence demonstrates that the austenite to martensite transformation increases the work-hardening rate of the ferrite phase and delays the onset of Stage-III hardening until the yield point of austenite. The increase in work-hardening rate (and thus work required) supports a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding which offsets the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Steels with a high degree of banding do not show this extra contribution due to the more dominant anisotropic effect of martensite bands on the work-hardening of ferrite coupled to increased mechanical austenite stability as a result of increased carbon content. A list of revisions as requested by the examiners is produced on pages 18 and 19 of the thesis for examination. Abstract: In the drive towards higher strength alloys, a diverse range of alloying elements is employed to enhance their strength and ductility. Limited solid solubility of these elements in steel leads to segregation during casting which affects the entire down-stream processing and eventually the mechanical properties of the finished product. The work presented in this PhD shows that segregation of alloying elements during casting, particularly aluminium, leads directly to banding in the final product. It has been demonstrated that no significant homogenisation is possible in this alloy within practical time constraints of the industrial thermo-mechanical process. A through-process model was developed to design a thermo-mechanical treatment aimed at reducing the effects of segregation on the formation of banding. A new polynomial function for calculating the local phase transformation temperature (Ae3) between the austenite + ferrite and the fully austenitic phase fields during heating and cooling of steel is presented. Material was produced both with and without banding and used to study the effect upon the mechanical properties. The banded steel variants show a significant reduction in tensile strength for a similar level of ductility compared to non-banded variants. In situ measurement under uniaxial loading using high-energy synchrotron diffraction allowed direct quantification of the impact of the mechanically induced transformation of metastable austenite on the work- hardening behaviour. The results reveal that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding and the experimental evidence demonstrates that the austenite to martensite transformation increases the work-hardening rate of the ferrite phase and delays the onset of Stage-III hardening until the yield point of austenite. The increase in work-hardening rate (and thus work required) supports a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding which offsets the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Steels with a high degree of banding do not show this extra contribution due to the more dominant anisotropic effect of martensite bands on the work-hardening of ferrite coupled to increased mechanical austenite stability as a result of increased carbon content.

The innovation of crucible steel, a high-carbon, homogeneous, slag-free steel, is regarded as a milestone in the history of the development of ferrous metallurgy. Associated in popular literature with the making of swords, particularly in the Early Islamic period, crucible steel, also known as wootz, possesses exceptional properties of hardness and strength. While much is now understood about its metallurgical composition and structure, little is known of its origins and spread. Few archaeological sites have been uncovered and to date pre-industrial production of this alloy is only known from Central Asia and South Asia. Previous studies have largely focused on individual sites in isolation from wider regional patterns of ferrous metallurgy. As a refining process of iron, it is argued here that crucible steel has a symbiotic relationship with the smelting technologies that produced the raw material for refining. This thesis explores the value of assessing crucible steel production within its wider landscape, cultural and technological context by presenting the evidence from Northern Telangana, India. Historical sources and recent archaeological field surveys have shown that Telangana has a rich metallurgical past, including the manufacture of crucible steel. Despite this, little archaeological work has been conducted in the region to elucidate the nature, scale and diversity of the metallurgical technologies that underpinned its production. Following a major reconnaissance survey in 2010 by the Pioneering Metallurgy Project, the present study tackled the assessment of the large body of field data and the recording of the technological waste assemblage collected. By combining detailed morphological analyses of the collected materials and contextual information recorded during field survey, a better understanding of the techno-cultural role of crucible steel was gained. Technological variations were identified across the survey area and the inter-relationship between iron smelting and crucible steel was assessed. The study reveals that crucible steel was embedded within a long-established local and regional tradition of iron smelting and concludes that it represented the intensification of a pre-existing iron processing industry. The evidence points to a widespread crucible steel production industry with varying degrees of site specialisation, indicating that it was perhaps more common than the few isolated sites commonly referred to in the literature suggests. The comparison of the material evidence with other production sites in Central and South Asia also revealed close parallels to the latter suggesting that they belonged to the same regional manufacturing tradition.

Stainless steels have been used for decades in nuclear power stations, particularly in pipe and tube applications. For these applications, welding is often required to join sections of components or pipes/tubes due to their large sizes and lengths. These components may be required to withstand the harsh operating conditions inside nuclear power stations (high temperatures and pressures) for durations of more than 25 years. During long term ageing at high temperatures, the microstructures of materials are known to evolve and change as a function of time at temperature. These changes must be characterised and understood to ensure the safe and economic operation of nuclear power stations. This thesis investigates five different stainless steels and their response to long term isothermal ageing in the context of advanced gas-cooled reactors. The five different materials include three base materials: A wrought type-321 stainless steel and two cast Nb-stabilised stainless steels. The other two materials are two welds made from welding each cast steel to the wrought type-321. The materials were aged at both 650°C and 750°C for up to 15,000 hours in an inert atmosphere. All five materials were characterised using optical microscopy, scanning electron microscopy, focussed ion beam imaging and transmission electron microscopy. A displacive form of ferrite was observed in the wrought type-321 material after ageing for 1000 hours at 750°C, which was found to increase in volume fraction during long term thermal ageing. Sigma phase was also detected during ageing at both temperatures along with secondary intergranular Ti(C,N) particles in the wrought material. An extensive sigma phase transformation took place in both of the cast materials, with a large proportion of delta ferrite transforming to sigma phase gradually during ageing. This sigma phase transformation was accelerated in both cast materials at 750°C compared to at 650°C. The sigma phase regions displayed a tendency to crack after longer ageing times, which is a significant concern with regard to the mechanical properties of these alloys during long term thermal ageing. Both welds exhibited a large scale martensitic transformation during long term ageing, which grew gradually inwards to the centre of the weld bead from the base materials. The martensitic regions were localised, with several small regions being present in the same parent austenite grain. The martensitic regions maintained a consistent orientation relationship with the parent austenite grains, along with being chemically indifferent to the parent austenite. The martensitic regions were found to increase in size at longer ageing times at both temperatures. The welds both also exhibited sigma phase precipitation after 1000 hours at 650°C and 500 hours at 750°C with particles found to increase in size at later ageing times with the general size of the sigma phase particles consistently being larger at 750°C than at 650°C when compared at the same ageing time.