超临界机组长期服役后P91钢微观组织演变研究

Microstructural Evolution of P91 Steel After Long-Time Service in Supercritical Units

摘要:

P91钢因优异的高温强度和抗蠕变性能，作为高温承压部件材料被广泛应用于超临界火电机组中。然而，长期在高温环境中服役会导致材料微观组织的退化，进而影响其力学性能及机组的安全运行。采用光学显微镜、扫描电子显微镜、电子探针显微分析仪和透射电子显微镜等多种表征技术，分析和对比了P91钢管道在超临界火电机组中长期（66 000—68 000 h）服役前后的微观组织变化，并探讨了微观组织转变的机理。结合P91钢显微硬度的变化情况，研究了微观组织转变对其力学性能的影响。研究结果表明，服役前后的P91钢管道金相组织均保持典型的板条马氏体结构，但服役后P91钢中马氏体板条内的位错密度下降，且部分板条出现宽化。在析出相方面，服役前的P91钢中主要存在富Cr的M₂₃C₆相、富V、Nb的MX相,而服役后的P91钢中除了原有的析出相外，还在原奥氏体晶界和板条边界发现了富Mo的Laves相。另外，服役后析出相的面积分数相比服役前增加了3.06%，但析出相的数量却减少了28.1%，这主要归因于M₂₃C₆相的Ostwald熟化和Laves相析出长大。析出相的粗化与聚集减弱了对位错和晶界的钉扎作用，导致板条合并宽化，以及亚晶粒的形成更加容易发生。服役后的P91钢，其平均显微硬度下降了20.70 HV_0.5，直接反映为力学性能的降低。一系列的微观组织的演变，是导致P91钢力学性能下降的主要原因。本研究为P91管道的长期安全服役提供了基础数据，具有非常重要的理论和工程应用意义。

Abstract:

In supercritical power plants， P91 steel is widely used for high-temperature pressure-bearing components due to its exceptional high-temperature strength and creep resistance. However， prolonged exposure to high temperatures can lead to microstructural degradation， adversely affecting its mechanical properties and the safe operation of the plants. This study utilizes optical microscopy， scanning electron microscopy， electron probe microanalysis， and transmission electron microscopy to analyze and compare the microstructural changes in P91 steel pipes before and after long-term service ranging from 66 000 to 68 000 h. The mechanisms underlying these microstructural transformations are discussed， and their correlation with changes in micro-hardness is evaluated to assess the impact on mechanical performance. The results indicate that both before and after service， the P91steel maintains a typical tempered martensite structure. Post-service observations reveal a reduction in dislocation density within the martensitic laths and a noticeable widening of some laths. Regarding precipitates， the as-received P91 steel primarily contains Cr-rich M₂₃C₆ phases and V/Nb-rich MX phases. After prolonged service， additional Mo-rich Laves phases were detected along original austenite grain boundaries and martensitic lath boundaries. The area fraction of precipitates increases by 3.06% after service， while the number of precipitates decreased by 28.1%， attributed to the combined effects of Ostwald ripening of M₂₃C₆ and the growth of Laves phases. The coarsening and aggregation of these precipitates diminish their pinning effect on dislocations and grain boundaries， leading to lath coalescence， widening， and enhanced subgrain formation. Consequently， the average micro-hardness of P91 steel decreases by 20.70 HV_0.5 after service， reflecting a significant decline in mechanical properties. These microstructural evolutions are identified as the primary factors for the deterioration of P91 steel’s mechanical performance after long-term service in supercritical power plants.