T91钢组织退化行为及对高温持久强度的影响

Microstructure Degradation Behavior and its Influence on High Temperature Stress Rupture Limit of T91 Steel

通过原始T91管的高温时效试验和T91过热器爆管段的取样,分析T91钢在长期服役中的组织演变,包括亚结构、沉淀相等,研究组织退化对高温持久强度和硬度的影响。结果表明,T91钢在高温服役过程中的主要组织变化有:板条马氏体结构发生回复和再结晶,退化为等轴铁素体晶粒,位错密度降低,M23C6型碳化物粗化,Z相在老化末期析出;在等效600℃、0~10万h时效条件下,T91钢的显微组织和高温持久强度非常稳定。Z相析出以前就发生的M23C6粗化及板条亚结构退化为等轴晶是T91钢服役中持久强度降低和爆管的主要因素。T91显微组织退化导致持久强度和硬度的降低。

The microstructure evolutions, including changes of substructure, precipitates, and the effect of microstmcture degradation on the high tempera^xe stress rupture limit and hardness of T91 steel were investigated using the specimens taken from the thermal aging materials and exploded superheater tubes. Results show that during long-term exposure the tempered martensite transforms to an equiaxed sub-grain structure with low dislocation density and coarsen M23C6 carbides through recover and recrystalization, and the Z-phase forms at final aging state. It is found that the microstructure and stress rupture limit of T91 steel are very stable after equivalent ageing at 600℃ for 100 000 h. M23C6 carbide coarsening and lath microstructure degradation to equiaxed sub-grain structure before the formation of Z-phase lead to the significantly breakdown of stress rupture limit, which is the main cause for the tube bursting. The lowing stress rupture limit and hardness of T91 is caused by microstructure degradation.