Effect Of Heat Treatment Process On Microstructure And Mechanical Properties Of COST-FB2 Steel

Coal-fired power generation will be the main form of power generation for a long period in our country.620? ultra supercritical power generation technology has the advantages of high efficiency and low energy consumption.The construction of ultra supercritical power plant at 620? degrees centigrade is of great significance for our country to master the advanced power generation technology and improve the comprehensive national strength.The COST-FB2 steel is chosen as the key steam turbine rotor material in the 620? USC power plant of our country.The large rotor forgings are required to be imported.The research of COST-FB2 steel is still in the initial stage,and it is not yet capable for industrial production.The study of heat treatment process and annealing process has certain reference value and guiding significance for the actual production of large steam turbine rotor forgings.In this study,the effects of heat treatment on microstructure and properties of COST-FB2 steel and the effect of isothermal annealing process on microstructure homogenization of COST-FB2 steel and the strengthening and toughening mechanism of COST-FB2 steel has been experimentally investigated using OM,SEM TEM and mechanical properties tester.The main results obtained are as follows:(1)Elevating the quenching temperature,the strength and hardness test steel increased with the effect of precipitation strengthening was enhanced due to the amout of carbides precipitation after tempering increased when quenching from 1000? to 1200?.Grain coarsening leads to the decrease of fine grain strengthening effect and impact toughness decreased.(2)The strength and hardness of test steel decreased significantly due to ferrite transformation began to occur with cooling rate after quenching reduced to 1?/min.The finer grain size and the fewer carbides at grain boundaries improved the toughness of steel.When quenching cooling rate reduced to 0.5?/min,complete transformation structure consisting of ferrite could be obtained lead to the strength and hardness of test steel decreases rapidly.The network carbides precipitation at grain boundaries lead to toughness decreased.(3)The microstructure has little difference when tempering below 570?.The high strength and hardness due to the high density of dislocation.the recovery began to occur when tempering above 570?.Evaluating the tempering temperature,recovery phenomenon intensifies and the density of dislocation decreases.The lower density of dislocation,the lower strength and hardness would be.Prolonging the tempering time at 570?,the microstructure has little difference between tempering for 1h and 2h.The coarsing of carbides lead to degeneration impact toughness when tempering for 4h.Prolonging the tempering time at 700?,the density of dislocation decreased due to the coarsing of carbides and recovery of lath,resulting in the lower strength and higher toughness.(4)The cooling rate after tempering at 570?and 700?mainly affects the diffusion and aggregation of atoms during the precipitation of carbides.Enhancing the cooling rate leads to short diffusion time atoms.It's easy to obtain fine carbides.Oppositely,slowing down the cooling rate leads to long diffusion time atoms.The coarse carbides lead to the degeneration of mechanical properties.The cooling rate at 700? has a greater influence on the size of precipitated carbides.Tempering at 570 ? and 700 ? both used in the air cooling cooling method can get the best mechanical properties.(5)Complete ferrite transformation can be achieved by isothermal annealing of COST-FB2 steel at 700?or 730? to eliminate the structure heredity.A large number of precipitated carbides can hinder the growth of austenite grain size during austenitizing.It can obtained fine and homogenized martensite structure via property heat treatment or preparatory heat treatment and property heat treatment after isothermal annealing.