Comparative study of evolution of residual stress state by local mechanical tensioning and laser processing of ferritic and austenitic structural steel welds.

Complex thermal stresses generated in welded structures are undesirable but inevitable in fusion welding. The presence of residual stresses can be detrimental to the integrity of a welded joint. In this research, redistribution of residual stress magnitude and profile was studied and compared in two multi-pass welded structural alloys (API X100 and 304L stainless steel) after cold rolling and laser processing. The residual stress field was studied by neutron diffraction using the SALSA strain scanner at their reactor neutron source at ILL, Grenoble. In addition to a complex distribution of residual stress state, multi-pass welds also forms dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. Dendritic grain structure is weaker and segregation of alloying elements may result in formation of corrosion microcells as well as reduction in overall corrosion prevention due to depletion of alloying elements in certain areas. The modification of as-welded residual stress state was done by cold rolling which was followed by laser processing to create a recrystallized microstructure to minimise segregation. The main objective of this study is to understand the suitability of this novel manufacturing technique to create a stress free weldment with recrystallised grain structure. Hardness evolution in the welded structures was scanned following welding, post weld cold rolling and cold rolling followed by laser processing. Hardness distribution in both the structural alloys showed a significant evidence of plastic deformation near the cap pass of the weld metal. Residual stress redistribution was observed up to 4 mm from the capping pass for ferritic steel, while in austenitic steel weld, post weld cold rolling was effective in modifying the residual stress redistribution throughout the entire thickness. Laser processing in both cases reinstated the as-welded residual stress distribution and resulted in softening of the strained area

Investigation of post-weld rolling methods to reduce residual stress and distortion

The mechanisms of post-weld rolling and how it reduces and eliminates residual stress and distortion are poorly understood. Finite element analysis was applied to two different methods of rolling: rolling the weld bead directly with a single roller and rolling beside the weld bead with a dual flat roller. The models showed that both rolling techniques were able to induce compressive stress into the weld region, which increased with rolling load. The distribution of stress was sensitive to the coefficients of friction between the workpiece and the roller and the backing bar. High friction coefficients concentrated the plastic deformation and compressive stress within the centre of the weld bead. Distortion can be eliminated by rolling; however, the experiments indicated that this was only achieved when applied to the weld bead directly

Effect of crack-like defects on the fracture behaviour of Wire + Arc additively manufactured nickel-base Alloy 718

The fabrication of large components using a high deposition rate, near-net shape process like Wire + Arc Additive Manufacturing (WAAM) is a promising option for many industries, due to the potential for reduction in material wastage and shorter lead times in comparison to conventional methods. Specialist materials like nickel-base superalloys, which are typically used in high temperature and corrosive environments, are particularly attractive options due to their high raw material costs. Although nickel-base Alloy 718 seems well suited to the process due to its good weldability, process-induced defects can arise from unfavourable deposition conditions and elimination of these defects may not always be possible. In WAAM Alloy 718 deposited under such conditions, crack-like defects with planar morphology and hot cracking characteristics were observed. These defects were observable using conventional non-destructive testing techniques and displayed directionality relating to the deposition path. The fracture behaviour of WAAM Alloy 718 containing these defects was “semi-stable” – a mixture of fracture instability and stable crack extension. The apparent fracture toughness of WAAM Alloy 718 containing these defects was found to be anisotropic, which can be attributed to the interaction of the notched crack with pre-existing defects. WAAM Alloy 718 displayed an apparent fracture toughness comparable to that of wrought Alloy 718 when notched perpendicular to the defects; but only half that of wrought when notched parallel to the defects. Therefore, careful consideration of defect orientation and their effects on mechanical properties is important in assessing the fitness-for-service of WAAM Alloy 71

Characterising the effects of high-pressure rolling on residual stress in structural steel welds

The large residual stresses which occur in welded objects are an unavoidable consequence of the non-uniform cycle of thermal strain inherent in most welding processes. Furthermore, the particular distributions of residual stress which are characteristic of welding can adversely influence several material and structural failure mechanisms, including fatigue fracture, elastic fracture and buckling. This thesis describes an experimental investigation into the use of localised high-pressure rolling of the weld seam for the purpose of residual stress reduction in steel welds. In preliminary experiments, it is demonstrated that the transient stresses which occur in an object while part of it is welded or rolled, can be inferred from strain measurements taken during the process. Furthermore, such measurements can be used to estimate the resulting residual stresses. Good agreement is observed between residual stress distributions found using this method and determined using neutron diffraction. The effect of rolling on residual stress in structural steel welds is then investigated using both of these measurement techniques. Rolling is shown to greatly reduce tensile residual stress at the weld seam, even introducing compressive stress when a greater rolling force is used. However, this is only the case when rolling is applied post-weld: by contrast, methods involving rolling prior to or during welding do not improve the residual stress distribution. It is proposed, on the basis of transient stress measurements, that this is because the deformation which occurs in a weld during cooling greatly exceeds its yield strain, and so any effect of high-temperature deformation on residual stress is subsequently erased. Other effects of rolling on the properties of a weld have also been studied. Using mechanical tests and microstructural analysis it is shown that while post-weld rolling causes work-hardening of structural steel welds, rolling the weld at high temperature results in refinement of the weld microstructure, also hardening it. The effect of roller geometry on residual stress and fatigue life of rolled specimens has been investigated: the induced residual stress distribution is relatively insensitive to the roller’s crosssectional profile, while the fatigue life is shown to be reduced by post-weld rolling. The implications of these findings for the practical implementation of weld rolling, along with many other applied aspects of the process are discussed. While rolling is undoubtedly a useful and highly effective tool for residual stress mitigation in welds, its secondary effects should always be carefully considered

The effect of pre-weld rolling on distortion and residual stress in fusion welded steel plate

Local rolling and other mechanical tensioning techniques can be highly effective at reducing residual stress and distortion in thin plate welds prone to buckling. However, the issues of high capital cost and low scalability currently prevent wider adoption of such processes. Pre-weld rolling aims to address these issues and can be applied easily to each component prior to fabrication. The results of an initial trial are presented, and indicate that post-weld distortion can be reduced by an average of 38% when correct rolling parameters are used. Finally, the mechanism by which prerolling acts to modify the state of residual stress around a weld line is discussed