9-12%Cr马氏体钢蠕变-疲劳损伤机理的多尺度研究

9-12%Cr martensitic steel used in the ultra supercritical thermal power units will endure not only creep damage but also fatigue damage,and the damage evolution is related with time. With the service time becoming longer and longer, the damage will increase, and brings a serious threat to the units. In this study the Deborah number is introduced based on multiscale mechanics. The Deborah number is the ratio of the characteristic time of mesoscopic damage to that of macroscopic damage, and can reflect marcoscopic and mesoscopic damage evolution process. The P91 creep-fatigue tests and pure creep tests are carried out to study the relationship between marcoscopic damage and the Deborah number. A metallurgical microscope is used to monitor the micro-damage's forming and growthing online, and the mathematic expression of the Deborah number is deduced by using statistical mesomechanics theory based on the experimental data. The mathematic expression of macroscopic damage is also proposed by using the life fraction method and the creep-fatigue crack growth model which is obtained by regressing the experimental data. The relationship of macroscopic damag and the Deborah number can be obtained by numerical calculation, which will be compared with the experimental results. According to the error between them, the mathematic expressions of macroscopic damage and the Deborah number will be modified. Then these formulae will be applied to study P92 steel creep-fatigue damage mechanical, to reveal the influence of grain size and microdamage density on creep-fatigue damage, which will do good to the safety operation of steam pipe in ultra supercritical thermal power units.

超（超）临界火电机组用钢9-12%Cr马氏体钢在服役过程中既产生蠕变损伤又产生疲劳损伤，其损伤演化过程与时间相关，随着机组服役时间的延长，损伤也随之增加，对机组安全运行的威胁也越来越严重。本项目以多尺度力学为依据，引入反映宏细观损伤演化过程的Deborah数，通过P91钢蠕变-疲劳及纯蠕变试验研究蠕变-疲劳宏观损伤与Deborah数的关系；利用金相显微镜实时监测微损伤成核及扩展过程，从细观损伤统计力学出发推导出Deborah数的数学表达式，利用寿命分数法和裂纹扩展速率模型给出宏观损伤的数学表达式，通过数值计算研究宏观损伤与Deborah的关系；将数值分析结果与试验结果作比较，根据比较结果修正宏观损伤及Deborah数的数学表达式，并将其运用于P92钢蠕变-疲劳损伤机理研究，揭示晶粒尺寸、微损伤数密度等细观尺度变量对P92钢宏观蠕变-疲劳损伤的影响，为超超临界机组蒸汽管道安全运行提供保障。

超（超）临界火电机组用钢9-12%Cr马氏体钢在服役过程中既产生蠕变损伤又产生疲劳损伤，且两者交互作用下损伤比较复杂，不是单纯蠕变损伤与疲劳损伤的简单叠加，二者之间存在着某种交互作用，表现为蠕变损伤与疲劳损伤相互抑制或者促进；同时损伤演化过程与时间相关，随着机组服役时间的延长，损伤也随之增加，对机组安全运行的威胁也越来越严重。.本项目以P92钢和X12CrMoWVNbN10-1-1钢为研究对象，分别进行了600℃和620℃下纯蠕变和蠕变-疲劳交互试验，以及X12CrMoWVNbN10-1-1钢在620℃下疲劳裂纹扩展试验，依据试验结果及试样微观结构分析，开展以下研究内容：.（1）通过应变划分分别分析了不同应变范围与应力比R、高保载时间TH关系及各试验条件下的棘轮效应与滞弹性弛豫。.（2）引入蠕变-疲劳交互系数B及定义了疲劳对蠕变损伤影响系数μ，结合微观分析方法研究了疲劳对蠕变-疲劳交互作用下损伤的影响。.（3）定义机械功密度，基于能量方法提出蠕变-疲劳寿命预测模型，并试验数据验证寿命模型的适用性。.（4）提出蠕变-疲劳寿命P-△WP-Nf曲线，推导出蠕变-疲劳寿命可靠性分析模型，并以试验数据为具体实例论证该模型的合理性。.（5）研究载荷、应力比及频率对高温裂纹扩展的影响及裂纹扩展速率模型。.得出的结论如下：.（1）9-12%Cr马氏体钢在蠕变-疲劳交互作用过程中呈现出三个阶段：初始阶段、中间稳定阶段及失稳阶段；损伤以蠕变损伤为主，损伤表现为循环累积，呈现出棘轮效应。.（2）9-12%Cr马氏体钢蠕变-疲劳呈负交互作用，表现为蠕变促进疲劳损伤，但疲劳抑制蠕变损伤，且这种抑制作用随着保载时间、应力比的增大而减小。.（3）9-12%Cr马氏体钢蠕变-疲劳断裂均为延性韧窝型断裂失效，韧窝的形核及扩展为导致失效的重要原因。材料内部的异质物与基体发生脱离形核，且材料晶界滑移及变形形核，两者均在载荷持续作用下形成韧窝，疲劳载荷一方面使得蠕变韧窝前沿钝化，一方面减小晶格滑移变形，抑制了蠕变损伤的进行；反过来蠕变损伤累积导致材料特性下降，进而促进疲劳损伤。.（4）转子钢高温疲劳裂纹扩展速率与载荷水平F呈正相关，与应力比R负相关，与频率f正相关。