LAVES PHASE EVOLUTION IN P92 STEEL DURING AGEING

Abstract

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.