FeCrAl ferritic alloys are highly oxidation and corrosion resistant alloys but have limited creep strength at high temperatures. The incorporation of a stable rare earth oxide such as yttrium oxide in a FeCrAl alloy increases the in service temperature of the alloy up to 1100°C. Oxide Dispersion Strengthening (ODS) is achieved by the Mechanical Alloying (MA) process where the yttrium oxide is added to the alloy powders, and is forced into solution with the matrix powder. MA is a time consuming and expensive process but it results in high number density of fine nanometer size particles that provide excellent creep resistance at high temperatures. One such alloy is PM2000 alloy made by Plansee™ GmbH A more cost effective processing route for the inclusion of oxides in the matrix is the Rapid Solidification Powder (RSP) process where the oxide powder is added to the gas atomised powder from the melt of the alloy to form an ODS alloy but with a much lower number density of larger particles. The alloy still has excellent oxidation and corrosion resistance but limited high temperature stressed applications. Kanthal. APMT is one such alloy that is commercially available. Conventional joining techniques that produce a melt zone cannot be used to join ODS steels as the nanoparticles tend to agglomerate and slag off to the surface weakening the weld. Friction Stir Welding (FSW) is a relatively new solid state joining technique that preserves the nanoparticle distribution in the matrix. In this work 4 different types of welds of ODS steels, using FSW were studied. The welds were of a combination of either MA produced PM2000 and/or RSP produced APMT alloys. The FSW of butt configuration as-rolled PM2000 were studied. Fragments of the polycrystalline cubic boron nitride (PCBN) tool were observed in the thermal mechanically affected zone (TMAZ). On the application of post weld heat treatment (PWHT) these fragments were partially broken down due to a phase change in PCBN and TiN grains were observed around the fragment with the surrounding matrix being rich in Ti. The FSW of as-extruded plates of PM2000 has revealed an area in the TMAZ with an unsuitable grain structure, the grain structure being controlled by the high number of nucleation sites for new grains during recrystallisation. In both types of weld above, coarsening of the nanoparticle distribution was observed in the TMAZ with extensive agglomeration of the particles observed in the top part of the welds. This was attributed mainly to the flow of the material creating more 'encounters' between particles rather than the heat created by the friction of the tool. The FSW of APMT has revealed that the wormhole type voids observed in the TMAZ, originated from a feature on the tool surface, as the voids were formed in a spiral along the welding direction. Finally in the dissimilar metal FSW between APMT and as-extruded PM2000, a distinct region of PM2000 was observed in the APMT half of the weld. Diffusion due to concentration differences were observed by Fe, Mo and Cr as expected. However diffusion of Ti from PM2000 towards APMT had resulted in the formation of Ti(C,N) rich particles after the application of PWHT at the prior boundary between APMT and PM2000. The higher C and N concentration in APMT had resulted in diffusion towards the PM2000 and the capture of Ti present in PM2000 in particles before Ti could diffuse out toward the APMT.

Fusion welding processes commonly involve the localized input of intense heat, melting of dissimilar materials and the deposition of molten filler metal. The surrounding material undergoes complex thermo-mechanical cycles involving elastic and plastic deformation. This processing history creates large residual stress in and around the weld bead, which can be particularly detrimental in reducing the lifetime of fabricated structures, increasing their susceptibility to stress corrosion, fatigue and creep crack growth as well as reducing the fracture load. It is very important to have. a proper knowledge of the residual stress distribution in and around the weld region of structured components because knowing this allows their fitness to be assessed and the service life of critical components to be predicted. Characterizing weld residual stress fields either by measurement or finite element simulation is not straightforward because of the strain field complexity, inhomogeneity of the microstructure and the complex geometry of structural weldments. The residual stress distribution in a slot weld benchmark sample made from AISI 3 16L(N) austenitic stainless steel was analysed using the neutron diffraction at pulsed source. The presence of crevices and hydrogen containing super glue in the stress-free cuboids are some of the main issues effecting the neutron residual stress measurements. A residual stress of 400-450MPa was observed in first pass weld metal and in the HAZ of a three pass welded plate. The strain hardening behaviour of AISI 316L(N) steel around the slot weld was studied taking account of the asymmetric cyclic deformation and the typical strain rates experienced; inferences are drawn regarding how such effects should be modelled in finite element weld residual stress computations. The solution annealed material was tested under symmetric and asymmetric cyclic loading at both room and 550°C. During asymmetric cyclic loading, the 316L (N) material at room and high temperature was less strain hardened than in the same number of cycles of symmetric cyclic loading. At room temperature; the 316L (N) material deformed at fast strain rate showed higher strain hardening than at the slow strain rate. However, at high temperature (550°C); the 316L (N) material deformed at slow strain rate showed higher strain hardening than at the fast strain rate due to dynamic strain ageing. A mixed hardening model was to predict the strain hardening of the 316L (N) material at room and high temperature (550°C). However, the published mix~d hardening parameters were unsuccessful in predicting the strain hardening of the symmetric cyclic deformation at high temperature. Finally, the accumulated cyclic plastic strain resulting from the addition of each weld bead was studied using Electron Backscatter Diffraction (EBSD) and hardness measurements. The EBSD metrics showed a gradual increase of plastic strain and equivalent yield stress from the parent zone (approximately 0.02) to the fusion boundary (approximately 0.05-0.09). Although, in strain controlled cyclic loading, none of the EBSD metrics used were capable of assessing the plastic strain, below 58% cumulative plastic strain path. The quantified plastic strain (from the EBSD) and hardness analysis of the parent material indicates that the material deformed plastically. The EBSD derived plastic strain and equivalent yield stress correlate well with hardness, finite element prediction and von Mises equivalent residual stress.

Despite growing interest in the use of stainless steel in the construction industry and the development of a number of national and regional design codes, stainless steel is often regarded as only suitable for specialised applications. This is attributed largely to the high initial material cost associated with the most commonly adopted austenitic grades of stainless steel, as well as some conservatism embedded in current stainless steel guidance. A recently developed grade, known as lean duplex stainless steel (EN 1.4162), possesses higher strength than the common austenitic grades and has a lower cost, along with good corrosion resistance and adequate weldability and fracture toughness. The structural performance of lean duplex stainless steel remains relatively unexplored to date with only a few studies having been performed. The main aim of this study is to examine the structural behaviour of lean duplex stainless steel welded I-sections, and to assess the applicability of the current European stainless steel design guidance. As part of this research, a total of fifty two material tests, four stub column tests, eight 3-point and 4-point bending tests, eight continuous beam tests and nine shear buckling tests were carried out. The experimental programme was complemented by a parallel numerical investigation, in which finite element models were initially validated against the test results and subsequently used for parametric studies. These test and numerical results were used in conjunction with existing test data on stainless steel welded I-sections to characterise the basic material properties, assess the codified slenderness limits for cross-section classification, investigate the applicability of plastic design to indeterminate stainless steel structures, and establish new shear resistance design equations for stainless steel plate girders. Based on the findings, it was concluded that the present European design provisions can be safely applied to lean duplex but are rather conservative in some areas. To rectify this, modifications have been proposed for cross-section classification, plastic design and shear resistance calculations. These proposals, together with additional developments to the strain based continuous strength method of design, are suitable for incorporation into future revisions of Eurocode 3.

There is an increasing need - in various industries - for a reliable joint between aluminiwn and steel. Part I of this report is concerned with the development of a welding technique to accomplish this. The effects of varying the practical conditions of welding are discussed. The first part of the investigation resulted in the conclusion that a tin, zinc or aluminium coating in the steel provided suitable conditions for a satisfactory joint to be made by means of an inert-gas welding technique. In Part II, a study of the metallurgical considerations related to the use of these metals as buffer coatings, is reviewed. In addition, a section of the work has been directed towards establishing the relationship between bead geometry and weld strength.

Civilisation has depended on welded structures to facilitate production and improve the quality of life. Welds are used to create infrastructure upon which we rely, such as transportation, oil and gas piping, shipbuilding, bridges and buildings, and to produce the equipment that makes all of this happen. In short, the joining of two metals through welding has immensely contributed to our society. A critical factor in the strength of welded joints is the geometry of the joints, and for this reason a robust optimisation of geometrical parameters of welded joints has been conducted in order to establish the optimum and most robust design in the presence of variation amongst geometrical parameters. A parametric finite element analysis, using Python script, has been performed with the objective to investigate the effect of the welded joint parameters on the stress concentration factors under tensile and bending load. The results indicate that the parametric model, which is generated by Python script, can be used in a wide range of welded geometry, and has the capacity to reduce the time of computation. Additionally, an experimental study, including the geometrical identification of the welded joints, tensile test, hardness test and fatigue, has also been performed. In order to select the best optimisation algorithms, different optimisation algorithms and performance metrics with various types of problem were examined in this study. The results from this part show the accuracy of Circumscription Metric (CM) in comparison to Pair wise Metric (PW) - which is used widely in optimisation studies. Furthermore, the results show that the Fast Multi-objective Optimisation Algorithm (FMOGA-II) outperformed other optimisation algorithms used during this study. In this study, a new methodology for selecting the most robust designs from within the Pareto set has been developed. Finally, a traditional and robust optimisation of a butt welded joint has been performed by establishing a link between an optimisation software package and parametric finite element, the results of which show the ability of this approach to extract the robust optimal designs from the Pareto front.

S960 and S700 are two types of high strength low alloy steels (minimum yield strengths at 960 MPa and 700 MPa, respectively) developed recently by Tata Steel. These steels are typically used in heavy lifting equipment. This research examines the feasibility and characteristics of single pass autogenous laser welding (ALW), multi-pass ultra-narrow gap laser welding (NGLW) of 8 mm thick S960 and 13 mm thick S700 high strength low alloy (HSLA) steels and compared the characteristics of the welds with those of gas metal arc welding (GMAW). The work aims to understand the development of welding induced residual stresses, microstructures, microhardness, tensile properties, bending properties and Charpy impact toughness at different temperatures as produced by different welding techniques (ALW, NGLW and GMAW).Design of experiments and statistical modelling were used to predict and optimise laser welding parameters of S960 and S700 HSLA steels. The contour method was used to measure the 2D distribution of residual stresses of the welded specimens. X-ray diffraction was carried out to measure the surface residual stresses of the welded specimens. The main novel contributions include:(1) Development of welding procedures for ultra-NGLW of HSLA steels. The ultra-NGLW process was successfully applied to the welding of 8 mm thick S960 and 13 mm thick S700 HSLA steels with a very narrow groove (1.2-1.4 mm wide) using a moderate laser power (2-3 kW).(2) Resolving the melt sagging problem for single pass autogenous laser welding of thick section materials. Horizontal (2G) welding position was applied to successfully resolve the melt sagging problem when single pass flat (1G) position ALW was applied to welding a 13 mm thick S700 steel plate. Computational fluid dynamic (CFD) modelling was carried out to understand the dynamic force interactions in the weld pool and the factors affecting the formation of the weld bead profile.(3) Understanding the effects of heat input on the microstructures evolution and mechanical properties of the welded high strength steel joints. The much lower heat input for ALW of 8 mm thick S960 steel and ultra-NGLW of both 8 mm thick S960 and 13 mm thick S700 steels results in the generation of hard martensite in the narrow fusion zone (FZ) and heat affected zone (HAZ), which strengthened the welded joints but deteriorated the toughness of the welded joints. The strengthened narrow FZ and HAZ for both the ALW and ultra-NGLW of 8 mm thick S960 steels demonstrated almost the same tensile strength and elongation as the base material. A relatively high heat input for the ALW of 13 mm thick S700 steel results in the generation of bainite in the FZ, which has almost the same microstructure and hardness as the base material.(4) Understanding the effect of solid-state phase transformation on the residual stresses of the welded specimens. It was demonstrated that the solid-state phase transformation from austenite to ferrite, bainite and martensite changes the magnitude of residual stress in the fusion zone for the welded S700 steel plates. In addition, it also changes the yield strength of the FZ, which also has a significant effect on the welding residual stress. In summary, this work has resulted in a significantly enhanced understanding of the way in which the choice of welding process affects the properties of welded joints in high strength steels. Laser welding was found to offer strengthened welded joints. However, the laser welded joints presented low impact toughness. If the toughness of the laser welded joints can be improved, laser welding will be a promising technique for joining high strength steels.