Microstructural Evolution of P91 Steel After Long-Time Service in Supercritical Units
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 HV0.5 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.
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