Characterising creep crack growth behaviour in austenitic steel weldments

这项工作的主要目的是发展一个更好的理解蠕变裂纹扩展行为中提取的C（T）试样从焊接奥氏体钢焊接件，特别强调发展一个更好的理解，在这些试样中的裂纹驱动力的组合产生的残余应力和所施加的load.Power电厂组件在高温下运行，蠕变机制的故障是可能的。某些部件可能包含裂纹样缺陷，这些缺陷可能会因蠕变和疲劳过程而增长。这些缺陷通常在焊缝附近产生和发展。在断裂力学试样的实验室试验中，用稳态蠕变断裂力学参数C * 来表征蠕变裂纹扩展速率（CCG）已进行了大量的工作。在大多数情况下，通过测试高约束（侧槽）紧凑拉伸C（T）试样获得裂纹扩展特性。然而，为了获得焊件的不同区域（包括热影响区（HAZ）和焊缝金属）中的CCG率，有必要提取包含来自多于一个焊件区域的材料的C（T）试样。例如，在HAZ中具有裂纹的C（T）试样也将包含大量的焊缝金属和母材。此外，从焊态（非应力消除）奥氏体钢焊件中提取的C（T）样本已被证明含有足以影响CCG测试期间样本行为的残余应力水平。由于其复杂的不均匀结构，焊接件是特别成问题的，所述不均匀结构由许多具有可变晶粒尺寸和微观结构的区域组成，具有可以被描述为未扰动母材（PM）、热影响区（HAZ）和焊接金属（WM）的材料性质梯度。这是由蠕变机制引起的，蠕变机制通常由残余应力引起并受材料脆化的影响。焊接残余应力会在焊后热处理或高温设备运行过程中引起蠕变应变积累，导致热影响区出现应力释放或再热裂纹现象，这是一个重要的工业问题。然而，预测焊件失效的模型是有限的，并且非常令人关注的是，焊件及其各个组成部分的材料性能数据普遍不足，特别是在多轴应力状态下。现有的焊接材料模拟压痕和冲压试验等方法仅考虑了焊件的热影响区和粉末冶金区的特性。因此，微观结构的不连续性，局部性能梯度，导致相互作用的变形约束效应，和焊接残余应力的影响不能解释。此外，压痕和冲压试验产生复杂的加载状态，需要大量的解释，以将其结果转换为等效的单轴试验数据。因此，在焊件特性方面存在重大的创新空间。在现场，高分辨率的数字图像相关（DIC）测量拉伸焊件试样将使弹塑性和蠕变变形和破坏性能的焊件成分和它们的相互作用/约束的影响被建立。从DIC和机械测试中测得的机械性能将提供验证焊件变形和断裂行为的FE模拟所需的准确数据。

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.