Characterising creep crack growth behaviour in austenitic steel weldments

表征奥氏体钢焊件的蠕变裂纹扩展行为

• 批准号: 2296231
• 金额: --
• 依托单位: Imperial College London
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2019
• 资助国家: 英国
• 起止时间: 2019 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2296231
• 关键词: Characterising; creep; crack; growth; behaviour

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.

这项工作的主要目的是提高对从焊接奥氏体钢焊接件中提取的C(T)试件中蠕变裂纹扩展行为的了解，特别是对这些试件中由残余应力和外加载荷组合而产生的裂纹驱动力的更好理解。电厂部件在高温下运行，在那里可能发生蠕变机制故障。一些部件可能包含裂纹状缺陷，这些缺陷可能会在蠕变和疲劳过程中扩展。这些缺陷通常在焊缝附近萌生和扩展。在断裂力学试件的实验室试验中，用稳态蠕变断裂力学参数C*表征蠕变裂纹扩展速率(CCG)是一项重要的工作。在大多数情况下，裂纹扩展特性是通过测试高约束(侧槽)紧凑拉伸C(T)试件来获得的。但是，为了获得焊接件不同区域(包括热影响区(HAZ)和焊缝金属)的CCG率，需要从多个焊接件区域提取包含材料的C(T)试件。例如，在热影响区有裂纹的C(T)试件也将含有大量的焊缝金属和母材。此外，从焊接(非应力消除)奥氏体钢焊接件中提取的C(T)试件已被证明含有足以影响试件在CCG试验中行为的残余应力水平。焊接件的问题尤其突出，因为其复杂的不均匀结构由许多不同晶粒度和显微组织的区域组成，材料性能的梯度可描述为未受干扰的母材(PM)、热影响区(HAZ)和焊缝金属(WM)。焊接件是高温部件失效的主要来源，由蠕变机制引起，通常由残余应力引起，并受材料脆化的影响。焊接残余应力在焊后热处理过程中或在高温设备中运行时会引起蠕变应变积累，导致热影响区出现应力消除或再热开裂现象，这是一个主要的工业问题。然而，预测焊接件失效的模型是有限的，尤其是在多轴应力状态下，焊接件及其单个部件的材料特性数据普遍存在不足。目前估计焊件性能的技术，包括焊接材料模拟压痕和冲压试验，只孤立地考虑WM/HAZ/PM的性能。因此，不能考虑组织不连续性、导致交互变形约束效应的局部性能梯度以及焊接残余应力的影响。此外，压痕和冲压试验产生复杂的加载状态，需要相当多的解释才能将其结果转换为等效的单轴试验数据。因此，在焊接件特性方面存在很大的创新空间。对拉伸焊接件进行高分辨率数字图像相关(DIC)原位测量，可以确定焊接件的弹塑性、蠕变变形和破坏特性，以及它们的交互/约束效应。通过DIC和机械试验测量的机械性能将提供验证焊件变形和断裂行为的有限元模拟所需的准确数据。