Abstract Since creep strength reduction of mod.9Cr–1Mo steel welds in long-term creep was confirmed experimentally, residual life prediction for Type IV creep failure has been an important theme in Ultra Super Critical (USC) plants after long-term service. The mechanism of Type IV creep failure within the fine-grained heat affected zone (FGHAZ) is twofold; an increase in number density of creep voids of a size comparable to FGHAZ grain size (5 μm), and strain softening in the last stage of creep life. In this paper, a creep damage analysis scheme considering the above two mechanisms is applied to welded joints with non-uniform weld metal materials, heat affected zone (HAZ) and base metal, such as those found in large uniaxial cross-weld specimens. A void mechanics model expresses the increase of number density of creep voids (creep void density) in terms of a strain for void nucleation, which is strongly influenced by the multiaxial state of stress. A critical value of creep void density corresponding to the initiation of a micro-crack is determined based on the results of a random-fracture-resistance model of grain boundaries by the authors of this paper. Both the creep void density distribution and the failure process with crack initiation, propagation and final rupture predicted by the analysis are compared with experimental results at 650 °C．

中文翻译：

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考虑空洞力学建模的 mod.9Cr-1Mo 钢焊缝蠕变损伤分析

摘要 由于长期蠕变中 mod.9Cr-1Mo 钢焊缝的蠕变强度降低被实验证实，IV 型蠕变失效的剩余寿命预测已成为超超临界 (USC) 装置长期使用后的一个重要课题。细晶热影响区 (FGHAZ) 内 IV 型蠕变失效的机制有两个；与 FGHAZ 晶粒尺寸 (5 μm) 相当的蠕变空隙数量密度增加，以及蠕变寿命最后阶段的应变软化。在本文中，将考虑上述两种机制的蠕变损伤分析方案应用于具有非均匀焊缝金属材料、热影响区 (HAZ) 和母材的焊接接头，例如在大型单轴交叉焊接试样中发现的那些。空隙力学模型根据空隙成核的应变来表达蠕变空隙的数量密度（蠕变空隙密度）的增加，其受到多轴应力状态的强烈影响。根据本文作者的晶界随机断裂阻力模型的结果，确定了对应于微裂纹萌生的蠕变空隙密度的临界值。将分析预测的蠕变空隙密度分布和裂纹萌生、扩展和最终破裂的破坏过程与 650 °C 下的实验结果进行了比较。根据本文作者的晶界随机断裂阻力模型的结果，确定了对应于微裂纹萌生的蠕变空隙密度的临界值。将分析预测的蠕变空隙密度分布和裂纹萌生、扩展和最终破裂的破坏过程与 650 °C 下的实验结果进行了比较。根据本文作者的晶界随机断裂阻力模型的结果，确定了对应于微裂纹萌生的蠕变空隙密度的临界值。将分析预测的蠕变空隙密度分布和裂纹萌生、扩展和最终破裂的破坏过程与 650 °C 下的实验结果进行了比较。