In oil and gas transportation systems, austenitic nickel alloy filler metals are deposited on low alloy steel forgings, to form dissimilar welded joints. An example is the joint between forged hubs and linepipes used in subsea operations. A buttering technique is used, allowing the steel forging to be post weld heat treated (PWHT) before an offshore closure weld is carried out. Many years of successful service has been achieved; however high profile failures have occurred within the fusion zone of the ferritic forging and nickel alloy buttering, due to hydrogen ingress. To investigate the embrittlement mechanisms, a programme of microstructural characterisation, environmental performance testing, strain measurements, and fractography was conducted. Two forging materials were compared: i) F22, a 2.25wt% chromium, low alloy steel, and ii) 8630, a low alloy forging steel with a higher carbon content than the F22. The forgings were buttered with Alloy 625; an austenitic weld metal. The effect of PWHT on microstructure evolution and fracture resistance was studied by applying heat treatments to the as-welded specimens. Slow strain rate notched bend tests, within a simulated marine environment, were conducted to rank the resistance to fracture. Strain evolution during PWHT was measured using neutron diffraction. Microstructural examination of the dissimilar welds clarified a tri-modal split in the mechanical properties between as-welded, heat treated, and ‘over-aged’ samples. Modelling of carbon diffusion across the fusion boundary revealed the extent of phase precipitation during PWHT. In the as-welded samples interfacial failure was dominant due to disbonding at the fusion boundary, whilst in ‘over-aged’ samples, failure occurred by linking cracks at M7C3 precipitates. A competitive failure mechanism was proposed: the optimal heat-treatment of dissimilar joints can be predicted by examining the competing mechanisms of heat-affected zone tempering, residual stress reduction, and diffusion-controlled precipitate formation.

No description available.

A method based on the kinetics of crystal growth has been developed and applied to the computation of three-dimensional microstructure in austenite-matensite steels. The model takes into account the phenomenological theory of martensite crystallography which can predict the shape and orientations of martensite precisely. The interaction energy based on the plastic work model is taken into account to compute the variant selection in an austenitic stainless steel and formation of martensite under externally applied stress. This method is programmed in C++ and visualized with an in-house software AcaVisual. This model is able to deal with every circumstance where the shape deformation due to martensitic transformation is defined, provided that detailed crystallographic data is available. To further investigate the processing-microstructure-property relationship in strong steel, a low carbon steel has been selected to study its microstructure and property evolution under various processing conditions. It is found that the interlamellar spacing increases as the number of pulses increases. The addition of energy change due to electropulsing to the interfacial energy is thought to be responsible for the increase in the spacing. It is noted that after a critical number of applied pulses, the interlamellar spacing remains invariant to the further electropulsing. Mechanical properties are examined as functions of the number of pulses and interlamellar spacing. It is shown that softening occurs after 1000 times of current pulses which is attributed to the spheroidisation of the lamellar structure. The present work also reports the experimental observation of electropulse-induced microstructural evolution in a ferritic-pearlitic steel at ambient temperature. Electropulsing treatment leads to the formation of new cementite plates aligned with the current direction. These effects are attributed to the reduction of the system free energy. It was thought that the enhancement of carbon diffusivity, the generation of vacancies, and the reduction of electrical resistance under electropulsing are factors that cannot be neglected.

No description available.

Due to its optimal balance of strength and toughness, AISI 8630M low alloy steel forgings are extensively used as structural components for subsea applications in the oil and gas energy sector. However, considering the high pressures and low temperatures conditions in deep water environments the oil and gas industry has continuously been demanding higher reliability levels on the structural integrity of large steel forgings in order to avoid in-service premature failures. In particular, special attention has been given to the CTOD fracture toughness as a critical parameter for structural design. Heat treatment is the last stage in the manufacturing sequence of large forgings and to certain extent, defines the metallurgical characteristics of final component. The cooling rate during industrial quenching treatment represents one of the most important processing parameters controlling the microstructure before tempering treatment. The research programme involved industrial-scale experimental heat treatments in which largescale forged segments with two different cross-sections (100 and 250 mm) were separately subjected to water, aqueous polymer solution and vegetable oil quenching and then tempered at 590°C, to evaluate the influence of cooling rate on the microstructure and mechanical properties produced under industrial conditions. Tensile, CVN and CTOD fracture toughness properties were measured at RT, -30 °C and 0°C respectively as per specification requirements. Microstructural evolution and fracture surfaces were evaluated by high resolution scanning electron microscopy. A CCT diagram was constructed by means of quenching dilatometry in order to validate the microstructural changes produced during industrial quenching. In summary, the present investigation, showed that irrespective of the cross section, the faster, intermediate, and slower cooling times between 800°C and 500°C (λ, t 8/5) were obtained by water, polymer and vegetable oil respectively. Kinematic viscosity may be the main variable controlling the cooling performance of the different cooling media evaluated due to changes in the thermophysical properties of the quenchants. In addition, the predominant microstructures for the different thickness-quenchant conditions were found to be associated with mixtures of tempered bainite and tempered martensite. This was evidenced by the fact that the majority ofthe industrial cooling curves fell within a similar microstructural region of the CCT (0.03 - 1˚C/s) dilatometric diagram which consisted of mixtures of martensite and bainite. Accordingly, the mechanical properties evaluated were similar among the different thickness quenchant conditions investigated. In this sense, all conditions evaluated showed strength and impact toughness properties well above the material specification limit for the selected forged component. It can be argued that the strength and impact toughness are controlled by changes in the distribution and size of carbide precipitates and packet substructure associated with the different fractions of tempered martensite and tempered bainite generated by changes in cooling rate. Accordingly, the higher strength and impact toughness values were observed at mixtures of tempered martensite (TM) and tempered bainite (TB) with proportions of 85% (TM) -15% (TB). The above due to partitioning effect of the acicular lower bainite on the austenitic grains in association with tempered martensite. Regarding the fracture toughness assessment, it can be argued that the yield strength variation between the selected specimens, along with the carbide size variation observed between tempered martensite and tempered bainite, were not large enough to induce significant changes leading to negligible variation in the final CTOD properties. Construction of CCT diagrams by means of quenching dilatometry has proven to be an effective technique to predict the microstructures industrially produced at the central part of large forgings, although the unavoidable effect of macro segregation must be considered for comprehensive analyses. Finally, in spite of the fact that vegetable oil provided slow quench rate compared with those of water or aqueous polymer quenchants, it is true that the mechanical properties produced by this bio-quenchant were similar to those produced by water and polymer quenching. As such, this finding indicates the possibility of implementing vegetable oil as an alternative quenchant in material specifications used in the production of large scale forgings made of low alloy steels, in particular when a balance between mechanical properties, dimensional stability (distortion) and reduced crack susceptibility is desired after quenching and tempering.

The stress-corrosion cracking of 0.08% C and 0.30% C steels in boiling nitrate solution has been investigated. The principal aim has been to assess the role of stress in stress-corrosion cracking. Slow strain rate tensile tests in boiling nitrate solution have enabled the stress required to initiate stress-corrosion cracks to be measured and compared with data from tensile tests in the absence of nitrate solution. Constant strain stress-corrosion tests in a hard-beam tensometer have been used in addition to conventional static stress-corrosion tests. The dependence of stress-corrosion propensity on grain size, strain rate and pre-strain has been studied. Thin film electron microscopy has been used to compare the dislocation arrangements in a steel susceptible to stress-corrosion cracking with those in a more resistant steel. Initial stages in the corrosion of mild steel in nitrate solutions have been studied by examination of thin foils in the electron microscope, both before and after their immersion in nitrate solution. The investigation has revealed no evidence for purely mechanical stages in stress-corrosion cracking and previous evidence for such stages has been shown to be capable of alternative interpretation. A cracking mechanism is proposed, by which failure occurs as a result of continuous electrochemical corrosion. The role of stress is attributed to that of opening up corrosion fissures, so as to make the corrodent available at the fissure tip.

A methodology of material characterisation and finite element model discretisation is presented for spot welded boron steel sheets, with the aim of predicting failure during quasi-static loading. The predicted load-displacement curves from the Finite Element model are compared with experimentally measured curves for lap-shear and cross-tension weld destructive geometries, and serve as model validation. During spot welding, the weld and surrounding material are exposed to a wide range of temperatures, from the melting point at the weld centre to room temperature in the base material. As a consequence, the weld exhibits varying microstructures with corresponding varying material properties which have a profound influence on its load bearing capacity and failure strength as a whole. In addition, boron steel spot welds exhibit unique hardness profiles, with high hardness values in the nugget and outlying base material, and a sudden drop in the area between these regions. This sudden decrease in material properties leads to further difficulties in modelling the failure of boron steel welds. The weld process inherently produces localised residual strains which also need to be accounted for in the model simulation, together with significant plastic strain redistributions resulting from the mechanical loading of the spot weld to its ultimate failure. The initial residual strains were measured in weld samples using neutron diffraction and were subsequently input into the FEA models. This thesis aims to quantify the varying material constitutive behaviour throughout the weld, required for the failure prediction. In particular, the following constitutive properties were extracted: the stress-strain response of certain weld regions, failure loci consisting of fracture strain versus stress state for the corresponding regions, and the residual stress distribution through the weld. Due to the small size of the weld, cutting test specimens directly from the weld is unachievable. To overcome this problem, specific weld and heat affected zone micro-structures were recreated onto practical tensile specimens through use of a Gleeble thermo-mechanical physical simulator. These specimens were subjected to the same thermal histories as specific points in the actual weld. From these tensile specimens, stress-strain curves relating to specific weld regions could be obtained. In a similar fashion, three additional destructive specimen sets were created to obtain failure loci. These failure loci give fracture strain as a function of stress state: specifically shear, uniaxial and plane-strain states. Due to the practical limitations in the accuracy of the Gleeble technique, deviations from the target microstructures were expected in the Gleeble samples. To gauge the extent of these deviations, a method of extracting reference material properties directly from the weld was required. Instrumented indentation offers such a solution, where the load and displacement of the indenter are measured and run through an algorithm to calculate the yield strength of the indented locations. These yield strengths are then compared with the yield from the Gleeble stress-strain curves to gauge the accuracy with which the weld microstructures were recreated. This technique serves to quantify the deviation of the Gleeble microstructures from the target material microstructures. It is common practice to discretise the weld into a small number of bulk regions during the design process, with material constitutive behaviour assigned to these discretised parts. In the new methodology, the extracted material constitutive behaviour is modelled as a continuously varying function of the distance from the weld centre. By performing appropriate interpolation, the data may be finely or roughly discretised. The data at a certain distance from the weld centre may then be assigned to the corresponding element in the finite element model. This means one may discretise the model by choosing the level of data interpolation refinement. The following results were observed in the thesis: • Residual strain distributions of boron steel spot welds, which have not been measured before, were presented. Clear correlations between hardness and residual stress distributions were seen. • A new application of instrumented indentation was attempted by verifying the accuracy of heat treated samples with respect to their target microstructures by comparing yield strengths. • The boron steel HAZ was characterised in a finer level of detail than seen in other literature works. • Through physical simulation, stress - strain and failure loci corresponding to certain HAZ areas were successfully extracted and used to model weld failure. • A new method of finite element model discretisation was presented, where material properties may be input as a relatively smooth function through the length of the model.

No description available.

A key challenge for the automotive industry is to reduce vehicle mass without compromising on crash safety. To achieve this, it is necessary to model local failure in a material rather than design to the overly conservative criteria of total elongation to failure. The current understanding of local fracture is limited to quasi-static loading or strain rates an order of magnitude too high for automotive crash applications. This thesis studies the local fracture properties of DP800 sheet steel at the macroscopic scale from strain rates of to for the first time. Geometries for three stress states, namely plane-strain, shear and uniaxial tension, were developed to determine a fracture locus for DP800 steel using optical strain measurement. These geometries were developed using Finite Element Analysis and validated experimentally for strain rate and stress state. Thermal imaging was used to determine the effect of strain rate on temperature rise and its associated effect on fracture. Fractography was used to examine the specimens’ failure modes at different strain rates. The geometries were applied to the advanced high strength steel grade DP800. Despite prior evidence from simple tensile test data, DP800 showed no significant variation in fracture strain with strain rate in all three stress states. Non-contact thermal measurements showed that the high strain rate tests ( ) were non-isothermal with temperature rises of up to being observed. As a result of this it is difficult to decouple the effect of strain rate from the effect of temperature and requires further investigation. The test geometries were also applied to the deep draw steel DX54 and the aluminium alloy AA5754 where a strain rate effect was observed. Both materials are significantly more ductile than DP800 whish exposed a limitation in the test procedures. At high fracture strains the stress state deviates from its intended value and can invalidate the test. Therefore, a method was developed for determining the validity of a test for each geometry and material from experimental data. The preliminary data from DX54 indicates significantly greater strain rate sensitivity across one order of magnitude than was observed in five orders of magnitude in DP800.

Les alliages amorphes, ou verres métalliques, sont des matériaux relativement récents, datant au plus d’une cinquantaine d’années. Ils possèdent des propriétés mécaniques exceptionnelles (résistance, dureté, ténacité, énergie élastique stockée, …) sans commune mesure avec celles de la plupart des alliages métalliques cristallins. Leur fragilité apparente en chargement uniaxial constitue toutefois un frein majeur à leur application à grande échelle et donc à leur industrialisation. Par ailleurs, les études sur le comportement à la fissuration et à la rupture des verres métalliques massifs rapportées dans la littérature sont à la fois peu nombreuses et très diverses dans leurs résultats. Face à ces problématiques, ces travaux de thèse sur les verres métalliques à base de zirconium ont deux objectifs principaux. Le premier consiste à étudier leurs mécanismes de déformation plastique par des essais hétérogènes. L’essai brésilien (ou de compression diamétrale) se révèle, dans ce contexte, efficace pour atteindre des niveaux élevés de déformation plastique à l’échelle macroscopique. Celui-ci permet également d’étudier à plus petite échelle la formation et la propagation des bandes de cisaillement et de quantifier les champs de déformation au cours du chargement par corrélation d’image. Les résultats montrent une bonne reproductibilité à ces deux échelles et offrent une meilleure estimation des déformations intenses se produisant dans les bandes de cisaillement. De plus, une identification basée sur cet essai couplé avec un autre essai hétérogène (nano-indentation) dans le but de discriminer des paramètres élastoplastiques montre qu’une loi de comportement de type von Mises n’est pas pertinente pour ce matériau. Le deuxième objectif de ces travaux vise à caractériser l’influence des défauts cristallins, liés à leur procédé d’élaboration et présents dans la matrice amorphe de nos verres métalliques, sur l'initiation et la propagation des fissures mais aussi sur leur rupture par des essais de flexion. La mesure de la ténacité et l’analyse fractographique des éprouvettes montrent que ces défauts facilitent la pré-fissuration, mais entrainent aussi une fragilisation de nos matériaux. Cette dernière propriété, i.e. la résistance à la fissuration ou ténacité, s’avère alors un bon moyen pour discriminer les différentes qualités de synthèse de ces alliages amorphes. / Amorphous alloys or metallic glasses are relatively new materials, dating back to over fifty years. They exhibit exceptional mechanical properties (strength, hardness, toughness, stored elastic energy …), compared to those of most crystalline metallic alloys. Their apparent brittleness in uniaxial loading, however, is a major obstacle to their wide application and thus their industrialization. Studies on the cracking and fracture of these materials have so far been sparse and relatively contradictory in their results. The objectives of this PhD thesis work on zirconium base metallic glasses are therefore twofold. The first objective is to study their plastic deformation mechanisms by means of heterogeneous tests, namely instrumented indentation and diametral compression. The Brazilian test (or diametrical compression test) is shown, in this context, to be effective in achieving high levels of plastic deformation at the macroscopic scale. This test also allows to study, at a smaller scale, the formation and the propagation of shear bands and to quantify the strain fields during loading by digital image correlation techniques. The results show good reproducibility at these two scales and provide a better estimation of intense deformations occurring in the shear bands. In addition, a reverse analysis based on this test coupled with another heterogeneous test (nanoindentation) is carried out to identify elastoplastic parameters. This procedure shows that a von Mises yield criterion is not relevant for this material and that a Drucker-Prager model is capable of predicting the response. The second objective of this work is to characterize the influence of crystalline defects, linked to different casting processes, present in the amorphous matrix of our metallic glasses, on the initiation and propagation of cracks and on their fracture toughness. The measured fracture toughness and the fractographic analyses of the specimens show that these defects facilitate the pre-cracking, but result in an embrittlement. This latter property, i.e. the resistance to crack propagation or fracture toughness, then proves a good way to distinguish the different synthesis qualities of these amorphous alloys.