Lightweighting demands have been achieved by third generation (3G) Advanced High Strength Steels (AHSSs) by a means of increased strength. The challenge faced in doing so, however, is in ensuring that ductility and crashworthiness is efficiently retained. Key methods in which automotive research has been invested to achieve this strength-ductility balance is by microalloying to promote grain refinement, the introduction of precipitates, and the effective use of plasticity enhancing mechanisms. Specifically, the ability to tailor the stability of retained austenite during deformation has been crucial in manipulating the strength-to-ductility ratio of 3G AHSSs using the Transformation Induced Plasticity (TRIP) effect. On the other hand, dual phase (DP) (i.e: non-TRIP-assisted steels) continue to be most significantly manufactured due to their robust thermomechanical processing but are also compromised by their poor damage tolerance. Hence, considerable reports are available regarding the damage tolerance of DP steels, but the ability for the volume expansion associated with the austenite-to-martensite transformation to suppress damage evolution and enhance a steel’s local formability has not yet been thoroughly investigated. Nonetheless, the damage processes that lead to fracture in 3G AHSSs are complex. A full understanding of the underlying phenomena requires a careful assessment of the strain partitioning amongst phases, how the microstructure evolves with strain and how damage, in the form of voids and micro-cracks, nucleates and grows. This can only be accomplished by applying a range of methodologies, including microscopic Digital Image Correlation (µDIC), X-ray Computed Microtomography (µXCT), Electron Backscattered Diffraction (EBSD) and X-ray Diffraction (XRD), all of which can be tracked as deformation proceeds. This PhD thesis uses a novel post µDIC data processing technique to prove that a reduction in strain gradient, linked to the evolution Geometrically Necessary Dislocations (GNDs), at dissimilar phase interfaces is attainable with vanadium-microalloying and with use of the TRIP effect. A local strain gradient post µDIC data processing technique was developed and first applied on 3G DP steels to show that the microcompatibility between ferrite and martensite directly at the interface is considerably improved with vanadium-microalloying. This in turn microscopically explains this DP steel’s increased local formability/damage tolerance with vanadium micro-additions. Moreover, when applying this novel µDIC technique on two other 3G experimental steels of interest, an ultrahigh strength Quench & Partition (Q&P) steel and a continuous galvanizing line (CGL)-compatible Medium-Mn (med-Mn) steel, an even slower evolution of microstrain gradients at dissimilar phase interfaces was observed. This indicates that, although vanadium-microalloying can improve the damage tolerance of a DP steel, its ability to achieve the ultrahigh strengths is a direct result of the severe inhibition of dislocation motion at dissimilar phase boundaries. Eventually, at high strains, these local strain gradients cannot be maintained and results in premature damage nucleation. By comparison, at such high strains, distinct evidence of damage nucleation was not apparent in the 3G TRIP-assisted steels which is the result of a slow strain gradient evolution delayed by the effective use of TRIP. This finding triggered a further investigation into isolating the impact the rate of TRIP exhaustion has on damage development. By intercritically annealing this prototype med-Mn steel (0.15C-5.8Mn-1.8Al-0.71Si) with a martensitic starting microstructure, within a narrow temperature interval (from 665 to 710°C), it was possible to make significant changes in the steel’s rate of TRIP exhaustion without making considerable changes to its physical microstructure. This steel exhibits the largest true strain at fracture (ɛf = 0.61), meets U.S. Department of Energy (DoE) mechanical targets (28,809 MPa%), and shows sustained monotonic work hardening when intercritically annealed at an intermediate IA temperature of 685°C for 120s. In addition, this IA condition showed optimal damage tolerance properties as an abundance of voids nucleated during its tensile deformation, but their growth was suppressed by prolonging TRIP over a large strain range. There is reason to believe that the heterogeneous distribution of austenite and Mn throughout this 685°C IA condition compared to the other two enabled its suppressed TRIP kinetics and in turn improved damage tolerance. The impact that changes in stress-state, from a stress triaxiality of 0.33-0.89, has on microstrain partitioning, TRIP kinetics and damage evolution was tested on this med-Mn at its 685°C IA condition. With the machining of notches on tensile specimens, it was seen that a high stress triaxiality (0.74-0.89) accelerated the rate of TRIP, whereas the introduction of shear, through a misaligned notched specimen design, delayed TRIP kinetics. The change in mean stress imposed by the notches was deemed to have played an active role in TRIP exhaustion during the material’s tensile deformation. A unique electropolishing micro-speckle patterning technique was applied to show that the amount of strain that can be accommodated by the steel’s the polygonal ferrite-tempered martensitic regions are considerably impacted by external modifications in stress-state. While damages studies using different such notched tensile geometries revealed that once a critical void size is reached in this med-Mn steel, coalescence proceeds at an increasing, exponential rate up to fracture. It continues to remain a challenge to quantify the effects microstrain partitioning, TRIP kinetics and damage evolution separately, opening new avenues for future experimental and modeling investigations. / Thesis / Candidate in Philosophy / A lot of research up to now has been invested in the automotive industry to create steels that are lightweight, strong and show improved crashworthiness. The means by which this has been achieved is with the use of innovative processing routes to manufacture and implement Advanced High Strength Steels (AHSSs) in a vehicle’s body-in-white. Nonetheless, the constant global pressure to reduce greenhouse gas emissions has eventually driven research to a third-generation class of ultrahigh strength, lightweight AHSSs. These steels retain the weight savings of their second-generation counterparts but are more cost-effective to manufacture and can be adapted to current industrial line capabilities. Considerable work has been done to enable the manufacturing of 3G steels, yet the steel characteristics which underpin fracture, thereby affecting the crashworthiness of these steels, continues to be weakly understood. As such, at a microscopic scale, this thesis uses three different promising 3G AHSSs candidates to evaluate the impact their unique steel characteristics has on the ability to resist damage evolution and fracture.

The formation of bacterial biofilms on surfaces and their subsequent biofouling pose extensive safe and healthy concerns to a variety of industries. Biofilms are ubiquitous, and the biofilm state is considered the default mode of growth for the majority of the world's bacteria population. Once mature, biofilms are difficult to remove completely and have improved resistance against antibacterial agents. Given this, there has been significant interest to mitigate or at least manage biofilm formation on surfaces. One such method has been through the material design of surfaces, and to the interest of this study, through the development of antimicrobial stainless steels. Stainless steel is not an inherently antimicrobial material. Stainless steels alloyed with small amounts of either copper (Cu) or silver (Ag), both well-known natural antimicrobial agents, have been investigated since their initial development in the late 1990's onward. This class of materials have been proven to show significant antimicrobial effect over their traditional counterparts without compromising the characteristic mechanical properties of the stainless steels. However, most of the antimicrobial assessments for these materials documented within literature are conducted over a 24-hour timeframe and do not adequately account for the biofilm mode of growth. As so, this study aimed to assess how biofilms grow on this class of antimicrobial steels over a longer duration of growth and under growth conditions which more adequately modeled the biofilm mode of life. The same strain of Escherichia coli commonly used in antimicrobial surface testing, ATCC 8739, was grown on submicron-polished coupons of a ferritic Cu-alloyed stainless steel (1.50 wt. % Cu), an austenitic Ag-alloyed stainless steel (0.042wt. % Ag), and a standard 304 series stainless steel, used as a baseline. Following ASTM-E2647-13, the E. coli/SS coupons were grown using a drip flow bioreactor under low shear conditions at either ambient temperature or 37 ± 3 degrees C with a batch phase of 6 hours and a continuous phase of 48 hours up to 96 hours. Directly after harvesting, the coupons were analyzed with an Environmental Scanning Electron Microscope (E-SEM) under low vacuum with a water vapor environment. The effect of surface chemistry and alloy microstructure, surface roughness, rinsing the surfaces prior to inoculation and after harvesting, temperature, and growth duration on the resulting E. coli biofilms were all investigated in some capacity. Growth on the submicron finished surfaces indicated there were no significant differences between the biofilms grown on the three different steel compositions. Bacterial attachment appeared non-preferential to surface chemistry or alloy microstructure, suggesting that E. coli interacted with the surfaces effectively the same under the given growth conditions. To account for apparent randomness in bacterial attachment, it is hypothesized that the surface features of interest were on a size scale irrelevant to the size of single bacterial cells. To account for the lack of an observed biocidal effect from the Cu- and Ag-alloyed stainless steels, it is hypothesized that an organic conditioning film which developed on the surfaces from the fluid environment may have effectively inhibited the release of Cu and Ag ions from the steel surfaces. / MS / Bacteria frequently self-organize into what are commonly called bacterial biofilms, or an aggregation of bacterial cells that attach to a surface and which are embedded within a self-generated matrix of polymeric substances, such as proteins and polysaccharides. The biofilm state offers a lot of survival advantages to bacteria, and once biofilms form on a surface they are very difficult to remove. The formation of bacterial biofilms on surfaces and their subsequent biofouling pose extensive safe and healthy concerns to a variety of industries. There has been significant interest to stop or at least manage biofilm formation on surfaces. One such method has been through the design of surfaces, and to the interest of this study, through the development of antimicrobial stainless steels. Stainless steel is not an inherently antimicrobial material. Stainless steels which include small amounts of either copper or silver have been proven to show a significant antimicrobial effect over their traditional stainless steel counterparts without compromising the other desirable properties of the steels. However, most of the documented antimicrobial assessments for these materials have been conducted over a 24-hour timeframe and do not adequately account for the biofilm mode of growth. This study aimed to assess how biofilms grow on this class of steels over a longer duration of growth and under growth conditions which more adequately modeled the biofilm mode of life. This was done by growing a single strain of E. coli bacteria onto coupons of these stainless steel materials for either a 48-hour or a 96-hour timeframe within a low-flow, continuously-fed bioreactor. The coupons were visualized with an environmental scanning electron microscope to assess the effect of the material properties on the observed biofilms grown during this study. Overall there were little differences observed between the E. coli biofilms grown on the copper-containing stainless steel, the silver-containing stainless steel, and the standard stainless steel used within this study. Mirror finish smooth surfaces were needed in order to adequately visualize the steel coupons. The bacteria appeared to attach randomly without any preference for steel surface chemistry or other surface features. This suggested that under the given growth conditions the bacteria interacted with the smooth steel surfaces the same. To account for this randomness, it is hypothesized that the relevant surface features were significantly smaller than the size of single bacterial cells. E. coli cells are between 1 – 2 micrometers long and 0.5 – 1 micrometers in diameter. There was also no antimicrobial effect observed on the copper-containing and silver-containing stainless steels. To account for the lack of an observed antimicrobial effect, it is hypothesized that a conditioning film of carbon-based molecules formed on the surface of the steels from the liquid growth medium environment, preventing bacterial cells from being damaged by the copper and silver within the steel surfaces.

O desenvolvimento e utilização de novos materiais, mais leves e com propriedades mecânicas superiores aos atuais, se mostram extremamente importantes devido à redução de peso e consequentemente redução na emissão de gases poluentes que poderiam gerar. As ligas de Fe-Mn-C com elevados teores de Mn (20-30%) representam um desenvolvimento muito recente de aços austeníticos, que, através dos seus mecanismos diferenciados de deformação reúnem elevada resistência mecânica com grande ductilidade. Essa nova classe de materiais estruturais possibilita uma efetiva redução de custos na produção através do reduzido tempo de processamento (sem a necessidade de tratamentos térmicos especiais e de processamentos termomecânicos controlados). A soldagem é, atualmente, o mais importante processo de união de metais usado no setor industrial. Dentro da variada gama de processos de soldagem existentes, a soldagem por fricção e mistura mecânica (SFMM, em inglês: Friction Stir Welding - FSW) se destaca por ser um processo de união no estado sólido que apresenta uma série de vantagens sobre as tecnologias convencionais de soldagem por fusão. Do ponto de vista metalúrgico, uma das suas principais vantagens se manifesta justamente na junção de materiais dissimilares, visto que o grau de mistura de composições e as transformações de fases entre materiais incompatíveis podem ser minimizados. Outra vantagem é que há um refino de grão no cordão de solda comparado com a microestrutura fundida que se forma nos processos convencionais. Este trabalho teve como objetivo produzir em escala laboratorial os aços de alta liga ao manganês com efeito TRIP, avaliar o impacto da velocidade de rotação da ferramenta na soldagem por fricção e mistura mecânica e avaliar a microestrutura e propriedades mecânicas das juntas soldadas. A microestrutura das juntas soldadas caracterizou-se pela presença apenas da zona de mistura e do metal base, além da formação de \'anéis de cebola\' na zona de mistura, esta não mostrou sinais de transformação martensítica induzida por deformação e sofreu recristalização dinâmica para todas as velocidades de rotação investigadas com a formação de grãos refinados e com morfologia equiaxial. Os corpos de tração fraturaram todos nos metais de base, mostrando que as propriedades mecânicas da zona de mistura foram superiores à do metal base e que a variação de aporte térmico alcançada com a velocidade de rotação da ferramenta não comprometeu a qualidade das juntas soldadas. / The development and application of new light materials with superior mechanical properties is extremely important to weight reduction in vehicles and consequently reduction of greenhouse gases emission. The Fe-Mn-C steels with high Mn (20-30%) are a recent development of austenitic steels, which, due to their different mechanisms of deformation, possesses high strength and high ductility as well. In addition, this new type of structural steel allows an effective reduction of manufacturing costs due to its reduced processing time (it does not require special heat treatments and controlled thermo mechanical processing). Welding has been one of the most important processes for joining metals. Among the available welding processes, friction stir welding (FSW) is notable for being a solid state process with great advantages over the conventional welding methods. In the mettalurgical point of view, welding dissimilar materials is a significant advantage of FSW over the other process. The main reason is the reduction of mixture of material and phase transformations between the incompatible materials in the weld. Moreover, grain refinement is another advantage from the process. The present study aimed to produce laboratorial scale high Mn steels with TRIP effect, investigate the impact of tool speed ont the microstructure and mechanical properties of friction stir welded joints. The microstructure of the welded joints exhibited only the stirred zone (SZ) and the base material (BM), besides the presence of ´onion rings´ within the stirred zone. The SZ exhibited no signs of martensite suggesting that dynamic recrystallization have occurred for all the speed tested. Moreover, the grains in the SZ had equiaxial morphology and were significantly refined. The fracture of the tensile specimens occurred in the base material, bringing to light that the welding process was beneficial to the mechanical properties. Furthermore, the variation of heat input achieved with the speed did not compromise the quality of welded joints.

.L’objectif de cette étude était, en premier lieu, de mettre en place en Thaïlande un testd’adhésion par traction-écaillage sur une machine de traction classique (test« macroscopique »), de le comparer au test « microscopique » Grenoblois fonctionnant dansla chambre du MEB et de l’utiliser pour évaluer l’adhérence des calamines de process sur desaciers industriels. Deux paramètres ont été étudiés, la vitesse de déformation et la teneur desaciers en silicium. Il apparaît que l’écaillage des calamines au cours du test augmente quandaugmente la vitesse de déformation. Une vitesse de déformation élevée entraîne unedéformation au premier écaillage plus faible, donc une adhérence mesurée plus faible. Ceteffet est lié aux phénomènes de relaxation. On a pu alors montrer que la présence d’oxyde(s)contenant Si, situé(s) à l’interface avec le métal, augmentait l’adhérence. Les étudesd’oxydation dans la vapeur d’eau qui ont aussi été réalisées ont révélé que la présence desilicium réduisait la vitesse d’oxydation. En augmentant la teneur en Si, les couches defayalite et de wüstite s’épaississent ; par contre, les couches externes s’amincissent. Pour lesaciers contenant du cuivre, la vitesse d’oxydation est réduite quand la teneur en Cu estaugmentée. De la même façon, les couches internes sont plus épaisses et on observe uneaugmentation du nombre de précipités de Cu quand la teneur en cet élément augmente. / The purpose of this study was first to develop in Thailand a “macroscopic” adhesion testusing a conventional tensile machine, to compare it to the micro-tensile test used in Grenobleand sitting in the SEM chamber, and to use it for measuring adhesion of scales grown duringprocessing on industrial steels. Parameters affecting the test, i.e. strain rate and Si content ofsteels were investigated. The results showed that spallation of scales during strainingincreased with increasing tensile strain rate. A higher strain rate resulted in lower straininitiating the first spallation and lower mechanical adhesion of scales, which could beexplained by a relaxation effect. Oxide containing Si existed at the steel-scale interface andpromoted adhesion of scales. Oxidation studies were also performed, and the behaviour inwater vapour of steels with different contents of Si and Cu was investigated. Increasing Sicontent tended to decrease oxidation rate. It also resulted in the thickening of the wüstite andfayalite layers which formed by internal oxidation. When Si in steel increased, theintermediate (FeO + Fe3O4) and outermost (Fe2O3 sitting on Fe3O4) layers formed by externaloxidation were thinner. For Cu containing steel, increasing Cu content tended to decrease theoxidation rate. It also decreased the innermost and intermediate layers and resulted in moreCu precipitates along steel-scale interface.

O desenvolvimento e utilização de novos materiais, mais leves e com propriedades mecânicas superiores aos atuais, se mostram extremamente importantes devido à redução de peso e consequentemente redução na emissão de gases poluentes que poderiam gerar. As ligas de Fe-Mn-C com elevados teores de Mn (20-30%) representam um desenvolvimento muito recente de aços austeníticos, que, através dos seus mecanismos diferenciados de deformação reúnem elevada resistência mecânica com grande ductilidade. Essa nova classe de materiais estruturais possibilita uma efetiva redução de custos na produção através do reduzido tempo de processamento (sem a necessidade de tratamentos térmicos especiais e de processamentos termomecânicos controlados). A soldagem é, atualmente, o mais importante processo de união de metais usado no setor industrial. Dentro da variada gama de processos de soldagem existentes, a soldagem por fricção e mistura mecânica (SFMM, em inglês: Friction Stir Welding - FSW) se destaca por ser um processo de união no estado sólido que apresenta uma série de vantagens sobre as tecnologias convencionais de soldagem por fusão. Do ponto de vista metalúrgico, uma das suas principais vantagens se manifesta justamente na junção de materiais dissimilares, visto que o grau de mistura de composições e as transformações de fases entre materiais incompatíveis podem ser minimizados. Outra vantagem é que há um refino de grão no cordão de solda comparado com a microestrutura fundida que se forma nos processos convencionais. Este trabalho teve como objetivo produzir em escala laboratorial os aços de alta liga ao manganês com efeito TRIP, avaliar o impacto da velocidade de rotação da ferramenta na soldagem por fricção e mistura mecânica e avaliar a microestrutura e propriedades mecânicas das juntas soldadas. A microestrutura das juntas soldadas caracterizou-se pela presença apenas da zona de mistura e do metal base, além da formação de \'anéis de cebola\' na zona de mistura, esta não mostrou sinais de transformação martensítica induzida por deformação e sofreu recristalização dinâmica para todas as velocidades de rotação investigadas com a formação de grãos refinados e com morfologia equiaxial. Os corpos de tração fraturaram todos nos metais de base, mostrando que as propriedades mecânicas da zona de mistura foram superiores à do metal base e que a variação de aporte térmico alcançada com a velocidade de rotação da ferramenta não comprometeu a qualidade das juntas soldadas. / The development and application of new light materials with superior mechanical properties is extremely important to weight reduction in vehicles and consequently reduction of greenhouse gases emission. The Fe-Mn-C steels with high Mn (20-30%) are a recent development of austenitic steels, which, due to their different mechanisms of deformation, possesses high strength and high ductility as well. In addition, this new type of structural steel allows an effective reduction of manufacturing costs due to its reduced processing time (it does not require special heat treatments and controlled thermo mechanical processing). Welding has been one of the most important processes for joining metals. Among the available welding processes, friction stir welding (FSW) is notable for being a solid state process with great advantages over the conventional welding methods. In the mettalurgical point of view, welding dissimilar materials is a significant advantage of FSW over the other process. The main reason is the reduction of mixture of material and phase transformations between the incompatible materials in the weld. Moreover, grain refinement is another advantage from the process. The present study aimed to produce laboratorial scale high Mn steels with TRIP effect, investigate the impact of tool speed ont the microstructure and mechanical properties of friction stir welded joints. The microstructure of the welded joints exhibited only the stirred zone (SZ) and the base material (BM), besides the presence of ´onion rings´ within the stirred zone. The SZ exhibited no signs of martensite suggesting that dynamic recrystallization have occurred for all the speed tested. Moreover, the grains in the SZ had equiaxial morphology and were significantly refined. The fracture of the tensile specimens occurred in the base material, bringing to light that the welding process was beneficial to the mechanical properties. Furthermore, the variation of heat input achieved with the speed did not compromise the quality of welded joints.

Orientador: Prof. Dr. Gerson Luiz Mantovani / Coorientador: Prof. Dr. Renato Altobelli Antunes / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, Santo André, 2018. / Recentemente, a indústria automotiva tem acelerado os esforços para melhorar a economia de combustível dos automóveis. Os aços de alta resistência possibilitam a redução de peso, garantindo a segurança e desempenho. A Microestrutura, propriedades mecânicas e comportamento de corrosão de juntas soldadas de dois aços de alta resistência (LNE500 e AHSS900) foram estudados. Os aportes térmicos utilizados foram de 0,72, 0,70, 0,47 e 0,31 kJ/mm utilizando soldagem a arco elétrico. A microestrutura da junta soldada foi analisada por microscopia eletrônica de transmissão, microscopia eletrônica de varredura, microscopia óptica e difração de raios X. Os resultados confirmam a influência do aporte térmico na microestrutura, fases cristalinas, tamanho de grão, precipitados e suas distribuições. O perfil de dureza (Vickers) revelou uma região de amolecimento localizado na zona afetada pelo calor (ZAC) com grãos finos para as juntas soldadas do metal de base AHSS900. Os resultados do teste de tração revelaram uma variação significativa do escoamento e da resistência à tração entre o metal de base e as amostras soldadas, com a última apresentando ductilidade e resistência reduzidas. O comportamento de corrosão foi estudado utilizando impedância eletroquímica, polarização potenciodinâmica e técnica de varredura por eletrodo vibracional (SVET). A combinação dessas técnicas indicou onde e como a corrosão aconteceu. ZAC e metal de base atuaram como ânodo e o cordão de solda atuou como cátodo. SVET revelou que a corrosão iniciou no cordão de solda e se espalhou pela superfície da junta soldada. A grande quantidade de locais ativos (óxidos) e o alto teor de Si no cordão de solda podem ter sido responsáveis pelo início da corrosão nessa região. As imagens de microscopia confocal mostraram que a ZAC e o metal de base corroeram mais rápido do que o cordão de solda. / Recently, automotive producers have been accelerating efforts to significantly improve vehicle fuel economy. High-strength steels have been proven to achieve weight reduction while meeting vehicle safety and performance requirements. The microstructure, mechanical properties and corrosion behaviour of gas metal arc welded joints of two high strength steels (LNE500 and AHSS900) have been studied. The welded joints were obtained using heat input of 0.72, 0.70, 0.47 and 0.31 kJ/mm. The microstructure was investigated by transmission electron microscopy, scanning electron microscopy, optical microscopy and X-ray diffraction. The results confirm the influence of heat input on the microstructure, crystalline phases, grain size, precipitates size and distribution. Vickers microhardness test revealed a softening region in the fine-grained heat affected zone (FGHAZ) for the AHSS900 welded joints. The tensile test results revealed a significant variation in the magnitude of yield and tensile strength between the base metal and welded samples, with the latter exhibiting reduced ductility and strength. Corrosion behaviour was studied using electrochemical impedance spectroscopy, potentiodynamic polarisation and scanning vibrating electrode technique (SVET). The combination of these techniques indicated that HAZ and BM acted as the anode and weld metal (WM) acted as the cathode of the galvanic couple. SVET showed that corrosion started in the WM and, then, it spread to the whole joint. The high amount of active sites (oxide inclusions) and the high Si content in the WM may be responsible for corrosion initiation. HAZ/BM corroded faster than WM, producing a depth difference, which was detected by confocal laser scanning microscope.

Laser beam welding has gained a significant interest during the last two decades. The suitability of the process for high volume production has the possibility to give a strong advantage compared to several other welding methods. However, it is important to have the process in full control since various quality issues may otherwise occur. During laser welding of boron steels quality issues such as imperfections, changes in local and global geometry as well as strength reduction can occur. The aspects that need to be considered are strongly depending on alloy content, process parameters etc. These problems that can occur could be fatal for the construction and the lowest level of occurrence is wanted, independent of industry. The focus of this study has been to investigate the properties of laser welded boron steel. The study includes laser welding of boron alloyed steels with strengths of 1500 MPa and a recently introduced 1900 MPa grade. Focus has been to investigate weldability and the occurrence of cracks, porosity and strength reducing microstructure that can occur during laser welding, as well as distortion studies for tolerances in geometry. The results show that both conventional and 1900 MPa boron alloyed steel are suitable for laser welding. Due to the martensitic structure of welds the material tends to behave brittle. Cracking and porosity do not seem to be an issue limiting the use of these steels. For tolerances in geometry for larger structures tests has been done simulating laser welding of A-pillars and B-pillars. Measurements have been done with Vernier caliper as well as a more advanced optical method capturing the movements during the welding sequence. Results from the tests done on Ushaped beams indicates that depending on the geometry of the structure and heat input distortions can be controlled to give distortions from 1 to 8 mm, at a welding length of 700 mm. This means that important geometry points can be distorted several millimeters if the laser welding process not is controlled.

In Pressurised Water Reactors, the reactor pressure vessel (RPV) is considered a life limiting component due to the degradation of its mechanical properties. Nano-scale Cu-enriched precipitates are known to cause embrittlement in the form of increases in hardness and the ductile-to-brittle transition temperature. The effect of irradiation on the RPV is the dominant contributor to this embrittlement. This is due to the increased mobility of Cu from the high number of vacancies, and the matrix damage providing many heterogeneous nucleation sites. However, there are also thermal effects which may be difficult to separate from the irradiation effects. To understand the contribution of the long term thermal ageing to RPV embrittlement a series of weld and plate materials containing systematic variations of Ni and Cu has been thermally aged for times up to 100,000 hrs at 330 degrees C, 365 degrees C and 405 degrees C. Microstructural characterisation using Atom Probe Tomography has been performed. Complimentary Monte-Carlo simulations have been used to investigate the early stages of formation of Cu-enriched precipitates. Thermal ageing produces a high number density of nano-scale Cu-enriched precipitates. These nanometre precipitates have a Ni-Mn-Si rich interface which was found to be wider with increased precipitate size, lower ageing temperature and higher bulk Ni content. This interface reduces the interfacial energy of the Cu-enriched precipitates through a combination of the minimising of unfavourable Fe-Cu bonds and reduction in lattice strain. The matrix Cu levels after ageing for 90,000 - 100,000 hrs were found to be around 0.06 - 0.07 at.%, close to the expected solubility limits for Cu in Fe. The Fe content of the precipitates has been characterised and found to be higher at lower ageing temperature and for smaller precipitate sizes. Cu precipitation and solute segregation at dislocations were observed, particularly in the SG steels aged at higher temperatures where the supersaturation is lowest. Movies were produced from the Kinetic Monte-Carlo modelling (see accompanying DVD) and along with other analyses indicated that sub-critical Ni and Mn clusters may be active in the formation of Cu-enriched precipitates. This mechanism appears to occur in both the high and low Ni steels. Thus, the higher number density of larger precipitates observed with increased bulk Ni content is thought to be a consequence of the greater number of sub-critical Ni-Mn clusters providing more nucleation sites. A small number of grain boundaries were examined. Segregation of many solute species to them was observed, which is thought to result from a multi-element co-segregation process. Ni-Mn-Si precipitates were observed at grain boundaries and dislocations in the high Ni steels with high and low Cu levels. These are consistent with similar phases in found in irradiated high Ni steels. In the high Cu steels these particles were much larger and associated with Cu-enriched precipitates.

The work presented has the ultimate aim to increase dynamic mechanical properties by improvements in density and optimisation of microstructure of ultra high carbon PM steels by careful selection of processes, i.e. mixing, binding, alloying, heating profile and intelligent heat treatment. ThermoCalc modelling was employed to predict liquid phase amounts for two different powder grades, Astaloy 85Mo or Astaloy CrL with additive elements such as (0.4-0.6wt%)Si, (1.2-1.4wt%)C and (1-1.5wt%)Mn, in the sintering temperature range 1285-1300ºC and such powder mixes were pressed and liquid phase sintered. In high-C steels carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat-treated. To assist the breaking up of these continuous carbide networks, 0.4-0.6% silicon, in the form of silicon carbide, was added. The water gas shift reaction (C + H2O = CO + H2, start from ~500ºC) and Boudouard reaction (from ~500ºC complete ~930ºC) form CO gas in the early part of sintering and can lead to large porosity, which lowers mechanical properties. With the use of careful powder drying, low dew point atmospheres and optimisation of heating profiles, densities in excess of 7.70g/cm3 were attained. The brittle microstructure, containing carbide networks and free of cracks, is transformed by intelligent heat treatment to a tougher one of ferrite plus sub-micron spheroidised carbides. This gives the potential for production of components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains of up to 16 % were attained. Forging experiments were subsequently carried out for spheroidised specimens of Fe-0.85Mo+06Si+1.4C, for different strain rates of 10-3, 10-2, 10-1 and 1sec-1 and heated in argon to 700ºC, density ~7.8g/cm3 and 769 MPa yield strength were obtained.

For ecological reasons, the current main challenge of the automotive industry is to reduce the fuel consumption of vehicles and then emissions of greenhouse gas. In this context, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as TRIP steels. A combination of high strength and large elongation is obtained thanks to the TRansformation Induced Plasticity (TRIP) effect. However, improvement of the mechanical properties is still possible, especially by the refinement of the matrix. In this work, two main ways were followed in order to reach improved properties. The classical way consisting of the annealing of cold-rolled samples and an innovative way consisting of obtaining the desired microstructure by direct hot rolling of the samples. In the classical way, this refinement can be obtained by acting on the chemical composition (with such alloying elements like Cu and Nb). It was observed that complete recrystallisation of the ferrite matrix is quite impossible in presence of Cu precipitates. In addition, if the ferrite recrystallisation is not completed before reaching the eutectoid temperature, the recrystallisation will be slowed down by a large way. An innovative heat treatment consisting in keeping the copper in solid solution in the high-Cu steel was developed. Therefore, ferrite recrystallises quite easily and very fine ferrite grains (~1µm) were obtained. In the innovative way, the effects of hot-rolling conditions on TRIP-assisted multiphase steels are of major importance for industrial practice and could open new dimensions for the TRIP steels (i.e. thanks to precipitation mechanisms leading to additive strengthening). Impressive mechanical properties (true stress at maximum load of 1500 MPa and true strain at uniform elongation of 0.22) were obtained with a relatively easy thermomechanical process, the role played by Nb being essential.