Effect Of Rare Elements On Microstructure And Properties Of Heat-resistant Steels

Austenitic heat-resistant steel is widely used in high-temperature service components such as coal-fired boilers and ethylene cracking furnace tubes due to its good high temperature oxidation resistance and high creep strength.However,the working environment of heat-resistant steel is more and more complicated and the service temperature increased with the production demand.Therefore,it has attracted more attention to improved the high-temperature comprehensive performance of materials,especially high-temperature oxidation resistance.In this paper,we designed several new austenitic heat-resistant steel by adding Y,Ce and Zr into the K-52 austenitic heat-resistant steel.The tensile fracture mechanism of austenitic steel was investigated by high tensile performance curve and high temperature tensile fracture morphology.The high-temperature oxidation resistance of the heat-resistant steel had been studied by using oxidation weight increasing method,which was depicted as the oxidation kinetics curves.The surface and cross section of oxidized samples were investigated by morphology observation,XPS diffraction and AFM surface roughness analysis.The structural composition of the oxide film at 1000?was discussed and the oxidation resistance mechanism of austenitic heat-resistant steel was revealed.The addition of Zr element to the experimental steel refined the grains and improved the high temperature hardness and high temperature tensile properties.Zr can improve the size and the distribution of fracture inclusions,the number of fractured dimples increased,the tearing phenomenon was not obvious.The yield strength and tensile strength of the steel were increased by 23.53%and 6.73%,respectively.The Zr improved the high temperature strength of the steel while reduced the sensitivity of the steel elongation to temperature at high temperatures.However,the effect of Zr was not obvious for the high temperature oxidation resistance of the experimental steel at 1000?.The rare earth element Y was added to the steel and the precipitated phase was mainly distributed along the grain boundary.Y changed the specific gravity of each oxide on the surface of the steel,which not only inhibited the conversion of Fe₂O₃ to Fe₃O₄,but also promoted the formation of MnO₂ in the Mn-rich oxide on the surface of the steel.Moreover,the"pinning"effect of the black oxide SiO₂ at the interface between the substrate and the inner oxide layer was strengthened.The bonding strength between the oxide and the substrate was improve and suppressed the oxide falling off.The oxidation resistance of the steel at 1000?was obviously improved.The rare earth element Ce was added to the steel and the inclusions were mainly precipitated in the crystal.Ce improved the high temperature hardness,the high temperature tensile and the oxidation resistance at 1000?of the steel.The addition of Ce to the experimental steel improved the morphology,size and distribution of the inclusions.The fracture was more uniform and the number of dimples increased,which made the fracture tend to be ductile fracture.When the addition amount of Ce in the steel was 0.36%,the yield strength,tensile strength and elongation were increased by 5.37%,11.69%and 22.82%.The rare earth Y and Ce elements were simultaneously added to the steel and the inclusions could precipitated in the grain boundaries and grains.The high temperature hardness was improved,but the high temperature tensile properties and the high temperature oxidation resistance were reduced at 1000?of the steel which added Y and Ce elements.For the high temperature tensile stretch of the steel,a large amount of inclusions were present in the fracture,accompanied by severe tearing.During the oxidation process,the formed oxide film was loose and porous,the oxide was seriously detached.