Characterising creep crack growth behaviour in austenitic steel weldments

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The main aim of this work is to develop an improved understanding of creep crack growth behaviour in C(T) specimens extracted from as-welded austenitic steel weldments, with particular emphasis on developing an improved understanding of the crack driving force in these specimens resulting from the combination of residual stress and applied loads.Power-plant components operate at high temperatures where failures by creep mechanisms are possible. Some components can contain crack like defects which could grow by creep and fatigue processes. These defects usually initiate and grow in the vicinity of welds. Significant work has been performed to characterise the creep crack growth (CCG) rate with the steady state creep fracture mechanics parameter C* in laboratory tests on fracture mechanics specimens. In most cases, the crack growth properties are obtained by testing high constraint (side-grooved) Compact Tension C(T) specimens. However, in order to obtain CCG rates in different regions of a weldment (including the heat affected zone (HAZ) and weld metal), it is necessary to extract C(T) specimens that contain material from more than one weldment zone. For example, C(T) specimens with the crack in the HAZ will also contain significant amounts of weld metal and parent material. In addition, C(T) specimens extracted from as-welded (non-stress relieved) austenitic steel weldments have been shown to contain levels of residual stress that are sufficient to influence the behaviour of the specimens during CCG testing. Weldments are particularly problematic due to their complex inhomogeneous structures that consist of numerous regions of variable grain sizes and microstructures, with a gradient of material properties that can be described as the undisturbed parent material (PM), heat affected zone (HAZ) and the weld metal (WM) Weldments are the principal source of failure in high temperature components, caused by creep mechanisms that are generally caused by residual stresses and influenced by material embrittlement. Welding residual stresses can induce creep strain accumulation during post weld heat treatment (PWHT) or operation in high temperature plant, resulting in a phenomena known as stress relief or reheat cracking in the HAZ, which is a major industrial concern. However, models to predict weldment failure are limited and, of great concern, there is a general deficiency in material property data available for weldments and their individual constituents, especially under multiaxial stress states. The current techniques to estimate weldment properties, including weld material simulation indentation and punch tests only consider the properties of WM/HAZ/PM in isolation. Hence, the effects of microstructure discontinuity, local property gradients leading to interactive deformation constraint effects, and welding residual stress cannot be accounted for. In addition, indentation and punch tests generate complex loading states and require considerable interpretation to transform their results into equivalent uniaxial test data. Significant scope for innovation therefore exists in weldment characterisation. In-situ, high-resolution digital image correlation (DIC) measurements on tensile weldment specimens will enable the elastic-plastic and creep deformation and failure properties of weldment constituents and their interactive/constraint effects to be established. The mechanical properties measured from the DIC and mechanical tests will provide the accurate data required to validate FE simulations of weldments deformation and fracture behaviour.