| Because of the global energy depletion and the greenhouse effecting, theultra critical and ultra supercritical units with a higher power generationefficiency will be the inevitable trend of thermal power generating units in thefuture. Supercritical unit operate in the harsh environments of high temperatureand corrosion gas, which requires the steels of supercritical unit has excellenthigh temperature strength and high temperature oxidation resistance.HR3C austenitic heat resistant stainless steel has been widely used insuperheater and reheater in the supercritical unit, owing to its excellent hightemperature mechanical properties and high temperature oxidation resistance.However the alloy will precipitate a large number of second phase, which have agreat influence on the mechanical properties and life materials, when HR3Coperates in high temperature conditions for a long time. So the relationshipbetween the microstructure evolution and mechanical properties of HR3C in the high temperature of operating conditions is the hot spot to research. Theresearch results have shown that HR3C will precipitate various types of secondphase in austenite,after aging treatment in600℃~750℃for a long time.With theincrease of aging time, the position, number, size, morphology and distributionof precipitated phase will change.The temperature of environment, compositionof alloy, load, medium, and working conditions are closely related to themicrostructure evolution.At present, the research on HR3C is still concentratedin precipitated phase during aging treatment in the temperature of500℃~800℃.However, HR3C mainly operate in the actual service conditions of temperaturealternating and internal gas flow of the cyclic stress,which demand HR3Cmaintain enough strength and microstructure stability to ensure its safety.So thehigh-temperature creep test under high temperature of700℃is more vital to theactual working conditions, which provides more guidance of the use of HR3C.The high-temperature tests with different stress of HR3C austenitic heatresistant stainless steel are conducted at a temperature of700℃in thispaper.The microstructure, precipitated phases and the crack extensionmechanism of HR3C after high-temperature creep test were studied by OM，XRD，EDS and TEM in this paper.The research results show that tiny CrNbN,the main precipitatedphase,distributed in the matrix of solid solution sample; After thehigh-temperature creep test in700℃, a large amount of the embrittlement phase(Fe,Cr)23C6were rapidly precipitated along grain boundaries in the HR3C heat-resistant steel. With the extension of creep test time to5986h, theprecipitated phase of (Fe,Cr)23C6were transformed into sigma phase and Gphase. CrNbN will precipitate in the grains continuously, but their size is stableduring the high temperature creep test.The sigma phase begin to precipitate inthe grains at1489h.According to the high-temperature creep test curve of HR3C at700℃,thecurve should be divided into two characteristics: When the stress is higher than110MPa, the stress is the main factor affecting the crack and fracture mode istransgranular fracture. However,when the stress is less than110MPa, the mainfactors of crack of HR3C is brittle (Fe, Cr)23C6precipitates and fracture mode isintergranular fracture.The precipitated phase along the grain boundaries of deformed grains ismore than those of grains in the matrix.The grain size also effect the precipitatedphase. In general, the phase more easily precipitated along the grain boundariesof small grains.The grain size and the number of different grains in therecrystallization microstructure affects the creep life of HR3C heat resistant steel.A certain number of large size grain in the sample’s surface can suppress theinitiation and propagation of crack.So a certain number of large size grain in thesample’s surface can improve the high-temperature life of HR3C heat resistantsteel. |
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