高等级异种钢接头蠕变损伤机理及演化规律多尺度模拟

The premature creep damage and failure of the interface and creep rupture strength droping zone (CRSDZ) of dissimilar metal welding joint between new type austenitic heat-resistant steel and fine grain martensitic heat-resistant steel are very serious in ultra supercritical power plant. Investigating on creep damage model and evolution law are crucial to avoid premature creep failure and ensure integrality of welded structures. Because of the complexity of this issue, establishing creep damage model and evolution law indicating are affected by many factors, such as testing time, specimen size, stress/strain field and multizone unsymmetrical constraining. Theoretical analysis, experimental verification and multiscal simulation technology will be used to investigate dislocation formation, multiplication and movement law, characteristics of creep cavity nucleation, growth and coarsing at atomic-level. And then, the creep crack formation and development, shifting feature and failure mechanism are going to be studied. Furthermore, stress/strain and the evolution relationship and distribution of creep damage are analyzed, and the influences of the welding process, groove shape, weld size, stress state, materials characteristic parameters on the creep damage mechanism and law are considered in this investigation. Accordingly, the creep damage uncoupling and coupling model of physically based thermal/stress and multizone constraining are set up and the failure criteria is developed. Thus, the relationship of creep damage variables among the machanical, welding process, materials property parameters are constituted, finally, the creep damage evolving law and multiscale predicting method of welding joints are obtained；And finally, an fast and efficient test method to estimate the premature creep cracking and failure tendency will be established by means of experimental verification.

超超临界火电机组中新型奥氏体/细晶强韧铁素体异种钢接头面临界面和蠕变强度下降区（CRSDZ）早期蠕变损伤及失效等严重问题。研究蠕变损伤机理、演化规律及预测方法是防止接头发生早期破坏，保证结构完整性的关键。本项目拟采用理论-试验-多尺度数值模拟技术，揭示接头蠕变失效的宏-微观特征；在原子水平上阐明界面和CRSDZ位错形成、增殖及运动规律，孔洞形核、扩张、汇聚特性；进而研究界面和IV型蠕变裂纹形成、扩展、偏转、关联特性及接头失效机理；分析应力/应变、损伤的演化关系及分布特征；考虑焊接工艺、坡口形式、焊缝几何、力学状态、材料特征参数对损伤机理及规律的影响。建立基于蠕变物理本质的热/力耦合和多区域不对称约束损伤解耦、耦合多尺度模型及失效判据；构建损伤变量与力学、焊接条件、材料特征参数、运行时间之间的关联特性；获得损伤演化规律及多尺度预测方法；创建评价接头蠕变开裂及早期失效倾向的高效测试方法。

设计和构建了温度-应力加速试验平台，在较短时间内再现了HR3C/T91、Super304H/T91接头早期界面和IV型蠕变失效现象。在此基础上，建立了接头早期界面和IV型蠕变失效物理模型及两者的关联特性，为评定接头高温蠕变失效提供了有效方法。.利用有限元方法分别对两种接头在工作温度为600℃、内压为42.26MPa作用下的蠕变行为进行了数值模拟。对于HR3C/T91接头，最大主应力峰值位于管接头内表面焊缝/T91界面附近，von Mises等效应力峰值同样位于内表面焊缝/T91界面附近。内表面应力三轴度峰值分别位于HR3C/焊缝和焊缝/T91界面处。内表面焊缝/T91界面处应力三轴度数值较大，与加速试验结果一致。对于Super304H/T91接头，模拟结果与HR3C/T91接头相似，但焊缝/T91界面处的应力三轴度数值比HR3C/T91接头小，早期失效倾向也小于HR3C/T91接头，仍为接头的薄弱环节。.研究了实际焊缝几何形状对接头界面开裂和IV型蠕变的影响。最大主应力峰值位于接头内表面HR3C/焊缝和焊缝/T91界面以及亚外表面拐点1处的焊缝/T91界面附近区域；von Mises等效应力峰值均位于焊缝/T91界面区域；内表面应力三轴度峰值位于焊缝/T91界面附近，而亚内表面拐点2和拐点1处的应力三轴度峰值位于T91钢侧细晶HAZ(FGHAZ)内，孔洞容易连接形成IV型蠕变裂纹。应力三轴度可以准确预计T91/HR3C和Super304H/T91接头T91侧FGHAZ内蠕变孔洞扩张及裂纹扩展特性。.分析了坡口角度对P91钢接头IV型失效的影响。最大主应力和von Mises等效应力峰值均位于上下坡口交界处混合热影响区(MHAZ)内。蠕变变形主要集中在FGHAZ内，最大值位于FGHAZ底部。上下坡口交界处FGHAZ内的应力三轴度最大，易发生IV型蠕变开裂。坡口角度越大，IV型蠕变失效倾向越低。.探索了单元尺寸对P91钢IV 型蠕变失效影响。单元尺寸对模拟精度有很大影响，其中应力三轴度因子对单元尺寸的变化最为敏感。如果仅需应力、应变值的相对大小，可以采用稍大一点的网格尺寸。而需获得具体的应力、应变数值，单元数量则需超过 40000。应力三轴度因子能够很好地预测IV型蠕变裂纹的产生部位。

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