Study On Microstructure Evolution And Critical Elements Segregation Of 9Cr3W3CoB Steel During Aging

Martensite heat-resistant steel is deemed as the most common structural material in Ultra Super-Critical(USC)power plants for its advantages such as high thermal conductivity,low thermal expansivity,and excellent anti-fatigue performance.Increase of the steam temperature and pressure is essential to realize the efficient utilization of fossil energy sustainable development.Service temperature of commercial 9-12%Cr heat-resistant steel restricts the increase of operating parameter of USC power plants,and thus the 9Cr3 W3CoB steel with better creep property was developed.However,the regime of 9Cr3W3CoB steel is complex,and much uncertainty still exists about the segregations of W,Co,B,Cu and their effect on the secondary phases.Hence,it is difficult to increase the mechanical properties of 9Cr3W3CoB steel through composition optimization.Under this background,9Cr3W3CoB steel was adopted as the objective of research in this thesis,the impact of microstructure evolution on mechanical properties was analyzed,and the thermodynamic behavior of precipitates and the segregation of critical elements under thermal activation were explored.The main results are displayed as follow:The effects of aging time and temperature on the evolution of microstructure and the thermodynamic behavior of Laves phase in 9Cr3W3CoB steel during aging were investigated,in which the relationship between microstructure evolution and variation of mechanical properties was elucidated.The experimental results indicated that 9Cr3W3CoB steel possessed excellent microstructure thermostability.With increasing the temperature,the aging process of steel was accelerated,the number density of Laves phase decreased while the growth rate of Laves phase increased.It was found that instead of dissolving,Laves phase nucleated beside M23C6 carbide and enwrapped several carbides inside during the growing process when aged at 800?.The growth processes of Laves phase and M23C6 carbides were a competitive procedure,and the coarsening of M23C6 carbides was prior to the growth of Laves phase at 750?,while the growth of Laves phase was prior to the coarsening of M23C6 carbides at 800?.The toughness of 9Cr3W3CoB steel dropped quickly during aging up to 2000 h,Laves phase was the main reason that caused the decrease of toughness.Taking 9Cr3W3CoB steel with 150×10-6 B and without B addition as the research objectives,the distribution of B in the steel and the thermodynamic behavior of BN nitride and M3B2 boride were investigated,and the effect of dissolved M3B2 boride on BN nitride was assessed.B was found to generally exist in the form of M3B2 boride(M=Fe,W,Cr,V and Nb)on BN nitride in the 9Cr3W3CoB steel.The uneven distributed M3B2 borides could dissolve during prolonged isothermal exposure,where the dissolution process was accelerated by increasing the aging temperature.It was indicated that the W-rich M3B2 type borides were thermodynamically metastable at high temperature.Their temporary existence postponed the nucleation and growth of Laves phase.The restrain effect of M3B2 borides would be gone when they all dissolved back to the matrix.Due to the low content of N and the existence of M3B2 borides,only a few BN nitrides were distributed along PAGBs in the as-treated steel.The dissolved M3B2 borides during the isothermal exposure led to the precipitation of BN nitride,while BN could not precipitate during aging in the sample without M3B2 borides.The nucleation and growth of BN nitride were thermodynamic processes affected by both aging time and temperature.Its number density and size increased with increasing the aging temperature.BN nitride with sharp corners distributed both at PAGBs and in the matrix during aging at 800?.The BN particles with sharp corners together with Laves phase were detrimental to toughness of the steel.The segregation of the critical metal elements(Cu?W?Co)in 9Cr3W3CoB steel and the effect of W and Co on Laves phase were investigated.Experimental results showed that Cu tended to form subsphaeroidal Cu-rich cluster which distributed evenly in the matrix during aging.With prolonged the aging test,the concentration of solute Cu decreased and the segregation of Cu was intensified.During aging,W mainly participated the precipitation of Laves phase and the coarsening of M23C6 carbide;Co always distributed in the matrix and would not enter in any precipitate during aging.Laves phase could precipitate during aging at 800-880?.W and Co contributed to cut down the incubation period and increased the solution temperature of Laves phase.Thermo-mechanical treatment was adopted to optimize the microstructure.The microstructure of 9Cr3W3CoB steel before and after TMT was characterized,and the effect of TMT process on the precipitation of secondary phase was investigated.TMT process could eliminate borides and make B go back to the matrix.Besides,it also contributed to the grain refinement,and decreased the size of M23C6 carbides in the astreat steel.Moreover,TMT process could increase the number density of precipitates and suppress their growth.