服役230000h的X20马氏体耐热钢主蒸汽管道的组织和性能

Microstructure and property in X20 martensitic steel main steam pipe exposed at elevated temperature over 230000 h

以热电厂服役时间长达230000 h的主蒸汽管道X20马氏体耐热钢为研究对象,通过室温和服役高温的力学性能测试、组织以及亚结构和析出相的SEM、TEM观察、电解萃取相XRD物相分析等,研究了耐热钢长期服役的力学性能和显微结构变化。结果表明:服役钢除室温冲击性能下显著下降外,其他的力学性能退化并不显著。服役后材料的组织仍为典型的马氏体,但马氏体组织粗化明显,出现板条碎化,板条界移动,位错密度下降,碳化物粗化等现象。服役前后材料中碳化物形态和种类出现明显变化,耐热钢中主要碳化物M23C6长期服役后粗化显著,原始态中存在的亚稳态碳化物M2C及M6C和部分MX溶解,析出了Laves相和Z相。

The mechanical properties and corresponding microstructure were examined extensively for X20 martensitic steel main steam pipe from a thermal power plant after 230000 h long-term service. Tensile, impact and hardness tests were conducted both in room temperature and service temperature. The results show that the degeneration of mechanical properties of the steel is not obvious except the impact property decreases seriously under room temperature. Microstructural observations by OM, SEM, TEM present that the microstructure is still martensitic but some creep degradations can be seen. The martensitic lathes are slipped, merged and fragmented; the density of dislocations decreases and dislocation free regions have emerged, subgrain boundaries are formed for dislocation accumulated, recrystallization is also found close to boundaries or sub-boundaries, the main carbides M23 C6 in the steel are coarsened seriously. The precipitates are extracted by electrolytic method and the phase composition were analyzed by XRD, and the results show that the phase transformation takes place during long-term service, as Laves phases and Z phases are precipitated with metasable MaC as well as M2C and part of MX re-solved.