The Effect Of The Second Phase Particles On The Creep And Fracture Properties Of P92 Heat-resistant Steel

Ferritic heat-resistant steel has excellent high temperature resistance,oxidation resistance and creep resistance,and plays an important role in today's industrial field,especially in the process production level of high temperature pipeline walls and high temperature headers of ultra(super)critical thermal power units Play an irreplaceable role.However,with the increase in service conditions,the creep rupture life of heat-resistant steel at high temperatures is shortened,which poses a serious threat to the safety of critical units and power plants.Under normal circumstances,we will use the isothermal endurance extrapolation method to predict the creep endurance strength under low stress,but because the creep performance of heat-resistant steel has been degraded during long-term high-temperature service,large errors have occurred in the prediction of creep life.Related experiments results show that the coarsening of the second phase(Laves phase and M₂₃C₆carbide)is an important factor leading to the deterioration of the creep properties of ferritic heat-resistant steel.In this paper,thermal aging is used to obtain P92 heat-resistant steel samples with different second phase microstructures,high and low stress creep experiments are carried out.Reveal the effect of second phase particles on the creep properties of heat-resistant steel by quantifying the relationship between the microstructure characteristics of the second phase particles and the macroscopic creep properties The main research contents and conclusions are as follows:(1)The second phase particle structure characteristics of heat-resistant steel under different thermal aging treatmentsUse optical microscope(OM)and scanning electron microscope(SEM)to characterize the microstructure of P92 heat-resistant steel at 650?for 0?25000h.The results show that the structure of P92 steel without thermal aging treatment is martensite lath,and a large number of precipitates are dispersed in the original austenite grain boundary,martensite lath boundary and matrix,mainly M₂₃C₆carbides,and a small amount is MX phase;After thermal aging treatment,the martensite laths are coarsened and the structure remains basically unchanged.After thermal aging,a large amount of Laves phase precipitates and continues to coarsen as the thermal aging time increases,and the rate is much higher than M₂₃C₆carbide.(2)Effect of second phase particles on creep properties of heat-resistant steel at high temperature and low stress(650?,25.5MPa?71MPa)In order to study the influence of the second relative creep mechanism under low stress,the author carried out the creep experiment of multi-axis helical spring with normal stress of 25.5MPa,34.6MPa,44.3MPa and 71MPa for P92 heat resistant steel with different thermal aging(10,000h,15,000h and 25,000h)at 650?.The results show that:in the region of low shear stress(14.7MPa,20MPa),Laves phase plays an important role in inhibiting the movement of grain boundary dislocations,while in the region of high shear stress(25.6MPa,41MPa),M₂₃C₆carbides play an important role in inhibiting the movement of grain boundary dislocations.In addition,the main precipitate that affects the creep performance of P92 heat-resistant steel under the stress conditions of 14.7MPa and 20MPa is the Laves phase,and the main precipitate that affects the creep performance under the stress conditions of 25.6MPa and 41MPa is M₂₃C₆carbide.(3)The influence of the second phase particles on the creep rupture properties of heat-resistant steelP92 heat-resistant steel with different thermal aging(0?7200h)at 650?is subjected to uniaxial rectangular creep fracture experiments with normal stresses of100MPa,120MPa,140MPa,and 160MPa.The results show that the creep damage tolerance coefficient of the unaged and short-aged samples is between 2.5 and 5,and the creep rupture is caused by the movement of dislocations;After 3000 hours of thermal aging,the damage tolerance coefficient increases to more than 5,and the precipitation coarsening leads to creep fracture.After long-term thermal aging,the creep strength of P92 steel is significantly reduced.By establishing the creep fracture life prediction method of P92 heat-resistant steel considering thermal aging,the results consistent with the long-term ASME test and trend can be obtained.