高铬耐热钢在高温低应力下的蠕变性能研究

High Cr heat-resistant steel is used for the material of components of thermal power plants, which are exposed to elevated temperatures for extensive periods. The demand for service life of power plant and nuclear power plant etc is longer than 20 years. This requires structural materials can resist creep deformation at high temperature and low stresses. The creep strength at low stress obtained by extrapolation method based on the date of creep deformation at high stress is larger than the true creep strength. The creep strength degradation is decided by microstructure degradation and the change of creep mechanism. Therefore, the aim of this project is to clarify the effect of microstructure degradation,the creep mechanism, and the creep long-term constitutive equation in p91 and p92 heat-resistant steel at low stresses. The creep mechanism at low stresses is clarified by analysis creep behavior at stress change,observing creep microstructure and studying anelastic process based on the magnitude and the kinetics of the process. Creep behaviors of P91 and P92 heat-resistant after heat aging are investigated using the helicoid spring creep method. The strain resolution of this creep method is very high. Parameters of microstructure degradation is decided by microstructure observation after heat aging and creep test using the helicoid-spring creep method. Then, the quantitative relation of parameters of microstructure degradation and scaling factors of creep curves is established. Finally, the project establishes creep constitutive equation of high Cr heat-resistant steel considered microstructure degradation and change of creep mechanisms. The research results will be used to establish strength of Engineering Materials.

高铬耐热钢是热电厂主要高温部件的主选或更新换代材料。热电厂服役寿命长于20年，这就要求所用材料能够维持高温、低应力长程耐蠕变性。工程上采用外推高应力蠕变试验数据来预测低应力蠕变强度。但此方法导致了蠕变强度的过度预测，即发生了强度退化。蠕变强度退化与组织退化和蠕变机制转变有关。本项目研究p91/p92高温低应力蠕变下组织退化及其对蠕变性能影响、蠕变机制和蠕变本构模型。采用具有高应变分辨率的“螺旋弹簧蠕变”试验法，调查应力突变下耐热钢的蠕变行为，结合蠕变组织观察和滞弹性回复现象分析，揭示耐热钢高温低应力蠕变机制。另一面，对热时效后样品进行微组织观察和短程恒应力螺旋弹簧蠕变测试，确定组织退化因子，建立组织退化因子与蠕变曲线尺度因子的定量关系，揭示组织退化及其对蠕变行为的影响。在此基础上，建立考虑了组织退化和滞弹性回复的高铬耐热钢低应力长程蠕变本构方程。试验结果期望用于构筑低应力区的材料强度科学。

高铬（9-12Cr）铁素体耐热钢服役条件下的蠕变寿命远小于实验室外推法预测的蠕变寿命，即发生了“蠕变强度退化”。蠕变强度退化与低应力下“组织退化”和蠕变机制转变有关。低应力蠕变和蠕变进程中的组织演化耦合在一起使研究低应力下的蠕变性能变的尤其困难。本项目通过不同时间热时效，然后进行组织定量化观察，获得“组织退化”规律；对时效后样品进行低应力短程蠕变测试，获得组织一定条件下的蠕变性能。为了克服传统单轴拉伸蠕变测试法由于应变分辨率低很难获得低应力数据的问题，开发了具有高应变分辨率的“螺旋弹簧蠕变实验法”（低应力30MPa，应变<10-10s-1）用于服役应力下短程蠕变测试。研究成果如下。.1、建立了基于有限元仿真的“螺旋弹簧蠕变”实验评价法。.2、热时效引起组织演化，Laves相演化是组织演化的最主要特征：.1）热时效引起晶粒粗化，但粗化不明显；.2）未观察到Z-Phase；.3）未发现MX粗化和面积分数的变化，热时效对MX影响很小；.4）Laves相在热时效一段时间后析出，其单位面积数量先增加，1500h后逐渐减少，Laves相平均直径增长率明显快于M23C6， 即Laves相长大效应明显，球状的析出物比棒状的析出物稳定；.5）M23C6在热时效过程中逐渐粗化，化学稳定性影响M23C6的粗化；.6）时效10000h小时后，Laves相和M23C6粗化率明显放缓。.3. 低应力蠕变由位错粘滞性攀移控制。低应力下存在两个蠕变区域，35MPa以上应力指数等于4的蠕变区域，35MPa以下应力指数等于1的蠕变区域。.4. 析出物演化对蠕变性能有重要影响。应力小于35MPa时,影响低应力蠕变性能的析出物主要是Laves相，应力大于35MPa时影响低应力蠕变性能的主要析出物是M23C6碳化物。.5. 建立了长程蠕变本构方程，并提出了基于“K参数法”的长程蠕变寿命预测法。.实际服役应力下（30MPa左右）Laves相是影响其蠕变性能的最主要因素，在一定组织形态下有强化作用。基于此发现，项目负责人提出了“基于Laves相多组分设计的铁素体钢高温强化法”（申请2020年国家自然科学基金面上项目）。