P92钢时效的Laves相演化行为

LAVES PHASE EVOLUTION IN P92 STEEL DURING AGEING

对P92钢进行650℃,0~8000h时效实验,采用SEM-BSE和定量金相技术对Laves相的体积分数、平均直径和数量密度进行定量,研究了Laves相的析出和粗化动力学特性,评估了Laves相演化对蠕变强度的影响.结果表明,Laves相在时效0~2000 h内析出,优先在晶界析出长大,最终析出时的体积分数约为0.95%.Laves相粒子在时效3000 h后明显聚集粗化,粗化速率明显快于M23C6碳化物的原因是由于晶界扩散机制的作用.Laves相在1000~3000 h内具有最佳的析出强化效果,超过3000 h后由于显著粗化使沉淀强化作用明显下降.由于体积分数小和热稳定性差,Laves相的强化作用远小于M23C6型碳化物.

ASME P92（9Cr-0.5Mo-1.8WVNb） is a key material for the main steam pipe and header with larger diameter and thick wall in ultra-supercritical（USC） plant,because of its low thermal expansion coefficient,good corrosion resistance,good fabricability and especially its high creep rupture strength.The Laves phase（Fe2M） precipitates in service and plays complicated and controversial role in affecting and/or determining the creep strength of the P92 steel.The fine Laves phase particles may contribute to precipitation strengthening and decrease the creep rate in the primary and transient creep region,however,the subsequent coarsening of Laves phase reduces the precipitation strengthening.Thus,in order to provide a systematic and definite understanding of the creep properties,it is necessary to investigate the precipitation and coarsening behavior of the Laves phase in P92 steel.In this work,the Laves phase parameters of P92 steel,including volume fraction,mean diameter and number density,were measured using SEM-BSE and quantitative metallography methods during aging at 650 ℃ for 0~8000 h.The precipitate and coarsening kinetics were investigated based on the quantification of Laves phase in P92 steel.Furthermore,the martensitic lath stability during aging was observed by OM and TEM.Lastly,the influence of Laves phase evolution on the creep rupture strength was estimated from the change of Orowan stress during aging.The results indicate that SEM-BSE is a suitable method for measurement of Laves phase precipitates,and can achieve significantly statistical data when characterizing large particles comparing with the EFTEM,so that evaluate the kinetics of precipitation and coarsening of Laves phase.The Laves phase precipitates at grain boundaries preferentially during the 0~2000 h of aging and its final volume fraction is around 0.95%.Obvious coarsening of Laves phase is observed after aging for 3000 h and its rate is much greater than that of M23C6 carbides.Grain boundary diffusion may play significant role in much rapider coarsening of Laves phases than that of M23C6 carbides.The Laves phase has the most precipitate hardening in the P92 steel aged up to 1000~3000 h and this hardening would drop remarkably due to its fast coarsening after aging for 3000 h.The contribution of Laves phase particles to creep strength is much less than that of M23C6 carbides.The P92 steel has a sub-microstructure with clear lath and high density dislocations after aging at 650 ℃ for 8000 h due to the stable M23C6 carbides on sub-boundaries.