Phase Diagram Calculation And Phase Transition Of High Aluminum Ferritic Heat Resistant Steel

Due to the poor thermal conductivity and high cost of austenitic heat resistant steel,ferritic heat resistant steel has become the primary choice for ultra-supercritical generator boiler steel.Traditional ferritic heat-resistant steels to improve the high temperature oxidation resistance and other properties through Cr2O3 film.However,it is found that excessive Cr in steel will affect the high temperature creep.When Al element is added to the steel,its generated compound is dispersed in the matrix as the reinforcing second phase,which is beneficial to further improve the creep property of the steel at high temperature.Therefore,this paper studies the specific effects of Al and Ni content on steel microstructure and properties.?1?Using the Thermo-Calc software to fine-tune the composition of the existing T92steel.The fine adjustment is mainly focused on the addition of Al and the adjustment of the Ni content of the modified additive,so as to design four sets of high-aluminum steels.The effects of Al and Ni content on the equilibrium precipitation phase and precipitation phase composition of steel are studied by simulating the thermodynamic equilibrium process of steel with different content of Al and Ni.The results show that the change of Al and Ni content has a great influence on the precipitation temperature and precipitation quantity of M₂₃C₆ carbide,Z phase and?phase in the steel,and has little effect on the precipitation temperature and precipitation quantity of Laves phase.The change of Al and Ni content to Laves Phase,M₂₃C₆ type carbide,Z phase composition element type and content have little effect,and have a great influence on?phase composition element type and content.?2?The thermal expansion behavior of experimental steels were measured by the material phase change analyzer,the phase transition temperature of experimental steels are obtained.The different heat treatment process parameters for the four sets of experimental steels are determined,and phase change of experimental steel during the heating,cooling and tempering process are studied.The results show that the equilibrium phase transition temperature of experimental steels increases with the increase of Al content.When 1 wt%of Al element is added to the steel and the Ni content is reduced from 3.8 wt%to 1.4wt%,the lath martensite structure in the steel is all changed into needle-shaped martensite structure.?3?The isothermal constant strain rate thermal compression experiment on experimental steels are carried out by Gleeble-3800 thermal simulator.The effects of Al,Ni content,deformation temperature and strain rate on hot deformation behavior,thermal deformation activation energy and constitutive equation are studied.The thermal activation energies for the four sets of experimental steels are 531.825,458.873,492.812and 430.874 kJ/mol,respectively.The results show that the addition of Al and the increase of Al content significantly reduce the processing difficulty of ferritic heat-resistant steel by comparing with T92 steel.?4?High temperature oxidation experiments of experimental steels at 650°C and750°C for 168 hours are carried out by box type resistance furnace.High aluminum ferritic heat resistant steels of different Al and Ni content's the oxidation properties and phase composition of scale at high temperature are studied.The micro structure for the four sets of experimental steels oxide layers are observed and analyzed.The results show that in the constant temperature environment of 650°C and 750°C,the four groups of experimental steels show obvious oxidation weight gain in the initial stage of oxidation?within 12h?.In the late oxidation stage?after 12h?,the oxidation weight gain phenomenon of steels are less obvious.Secondly,the surface of the 1Cr9Al3Ni4WNbB steel has less iron oxide burrs,which indicates that the area are oxidized is small and the oxidation resistance is the strongest.