Basic Research On The Key Technology Of 9-12% Cr Rare-earth Oxide Dispersion Strengthened Heat-Resistant Steels

"Increasing environmental governance and promoting green development to achieve new breakthroughs." The government work report reiterated the importance and urgency of the fully implementing ultra-ultra-low emissions and energy-saving upgrades for coal-fired power plants.According to the calculations,increasing a 10? in the service temperature,the thermal efficiency can be relatively increased by 0.25%to 0.30%.In the existing 620? high-parameter steam turbines,the development and selection of the heat-resistant steel have highlighted its limitations.With the extension of the unit commissioning time,the problem of insufficient high-temperature performance of the existing heat-resistant steel has gradually been exposed.The research and development of 630? high parameter steam turbine are mainly monopolized by foreign manufacturers.Therefore,it is the most urgent task to develop our own materials suitable for higher parameters(630?)units to achieve the carbon neutralization and ultra-low emissions.Based on the scientific and technological cooperation project of Sichuan Provincial Academy and University,"Research and development of new heat-resistant steel for key valve core forgings of 630? ultra-high parameter steam turbine"(18SYXHZ0069),this project adopts a variety of experimental analysis methods and relevant theoretical analysis.Aiming at 9-12%rare-earth oxide dispersion-strengthened heat-resistant steel applied at 630?,the problems of "poor hot workability,unable to maintain the designed high-temperature creep strength for a long time and high-temperature oxidation resistance unable to meet the higher service temperature" in the actual development and production process of new heat-resistant steel were systematically studied in theory and practice.Firstly,Y2O3 nano additive was added to the traditional 9-12%Cr martensitic heat-resistant steel,and the stable dispersion Y2O3 particles were used to solve the problem that the existing strengthening phase of carbon nitride was easy to coarsen at high temperature and led to the reduction of material properties.It is found that Y2O3 particles do Brownian motion in molten steel and only dissolve 0.2%in 1600?.The average yield of Y2O3 particles is 37.5%after VIM smelting and ESR refining.Numerical simulation of the microstructure,temperature field and solidification field of small electroslag remelting ingot in the laboratory shows that the columnar crystal of heat-resistant steel presents inverted "V" shape in the stable crystallization region of electroslag ingot,and the optimal melting rate obtained by numerical simulation of 500 kg industrial electroslag process is 180 kg/h.The thermodynamic calculation shows that the liquidus temperature is 15 10?,solidus temperature is 1310?.MX,M23C6 precipitation strengthening phase and Laves phase and Z phase precipitate in the solid phase in sequence.The order of equilibrium solidification is as follows:L?L+?-Ferrite?L+?-Ferrite+MX?L+?-Ferri te+MX+???-Ferri te+MX+??MX+?+?MX+y+M23 C6+?-Ferrite?MX+M23C6+?Ferrite?MX+M23C6+?-Ferrite+Laves?MX+M23C6+?-Ferrite+Laves+Z;In the process of non-equilibrium solidification,due to the enrichment and redistribution of a large number of solute atoms at the solidification front,the solidification range is larger than that of equilibrium solidification,and M23C6 is induced to form at the end of solidification.The order of non-equilibrium solidification is as follows:L?L+??L+?+??L+?+Z?L+?+Z+M2B?L+?+M2B+M23C6.According to the traditional heat processing technology and gleable thermal compression experiment,it can be seen that in the low strain rate area of low temperature and low strain rate,the softening mechanism of the 9-12%Cr rare-earth oxide dispersion-strengthened heat-resistant steel is dynamic recovery,and in the high temperature and high strain rate area,the softening mechanism of the heat-resistant steel is dynamic recrystallization.According to the heat working diagram,the best heat treatment parameters of the new 9-12%Cr martensite heat-resistant steel are T=1050-1100? and ?=0.03-0.3 s-1.The phase transformation behaviors of martensite in 9-12%Cr rare-earth oxide dispersion-strengthened heat resistant steel at different cooling rates were studied by Gleeble thermal simulator and ultra high temperature confocal microscope.The results showed that lath martensite firstly nucleates at austenite grain boundary and grows to the inside;with the decrease of temperature;lath bundles of martensite form at grain boundary;multiple lath bundles of martensite form lath group of martensite,which divides the original austenite grain into several small areas.Increasing the cooling rate can increase the nucleation rate of martensite,increase the number of austenite defects and prevent the interface migration of lath martensite.Subsequently,based on the traditional heat treatment process,a large amount of Nb(C,N)and VN are precipitated by adding secondary quenching(1000?).The precipitated second phase particles can pin the austenite grain boundaries,resulting in a significant decrease in austenite grain size(the grain size decreases from 30.4 ?m to 12.1 ?m)and a significant increase in impact toughness(impact toughness increase from 12 J to 24 J).By adding Y2O3 particles,the number and average size of nano strengthening phases in heat-resistant steel increase,and the contribution of precipitation strengthening increases.The addition of Y2O3 can also make more second phase particles pinning the grain boundary to prevent dislocation movement,resulting in higher initial dislocation density and finer sub grain structure of heat-resistant steel,thus improving the properties of heat-resistant steel.Finally,the hard Cr1-xAlxN/CrN/Cr gradient coating was deposited on the 9-12%Cr Martensitic heat resistant steel by direct current magnetron sputtering(DCMS)technology.The gradient coating not only improves the surface hardness of the heat resistant steel,but also significantly improves the high temperature oxidation resistance of the base material.Increasing the Al content in the coating(x=0.7)is beneficial to the formation of Al rich amorphous oxide layer.According to the oxidation kinetic model of coating,the internal diffusion of oxygen is the main limiting step.