Research On A Kind Of Austenitic Steel Used In The Boiler Of Ultra-supercritical Power Plant Above 700℃

In response to the global energy demand growth and climate change caused by the burning of fossil fuel,we must improve the efficiency of thermal power,thus reduce the coal consumption and CO₂ emissions.To improve the efficiency of thermal power unit,it is necessary to increase the boiler steam temperature and pressure.So more stringent requirements for the high temperature strength,high temperature oxidation resistance and processability of the key unit material are put forward.Austenitic heat resistant steel with good high temperature strength and stable microstructure has the potential to be the high temperature material used in thermal power unit above 700℃.JMatPro software was used to simulate the equilibrium phase diagram of the austenitic heat resistant steel based on Snicro25.Effect of C,Cr,Ni and W on the precipitation temperature and precipitation amount of MX,M₂₃C₆,Laves and Z phase was studied.These simulation results provide theoretical data for the study on the control of precipitates in austenitic heat resistant steel.For studying the effect of composition adjustment on precipitates,content of C,Cr,Ni and W was finely adjusted.Only reducing the content of C can not eliminate coarse MX phase.At the same time,the coarse Z phase precipitates.Laves,rather than M₂₃C₆,would precipitate on the grain boundary.The high temperature creep resistance reduced with all these changes.While adjusting the content of C,Cr,Ni and W all at Once,coarse MX phase can not be eliminated,but there is no M₂₃C₆ on the grain boundary and the amount of Laves phase decreases.Moreover,fine MX phase and Z phase precipitate during the creep process.All above changes in the microstructural morphology improve the high temperature creep resistance.L1₂ ordered Ni₃Al precipitates under 750℃/150MPa creep condition in the alumina-forming austenitic stainless steels（AFA steel）with 2.5 wt.%Al.AFA steel containing N shows better high temperature creep resistance than AFA steel without N.Using thermal mechanical processing to prepare AFA steel,its structure and high temperature creep resistance can be improved to a certain extent.The addition of Ti in AFA steel can effectively increase the amount of L1₂ ordered Ni₃（Al,Ti）,thus improves the high temperature creep resistance of heat resistant steel.When the content of C increases to 0.04 wt.%,fine disperse MX phase precipitates in the process of creep,which further improves the high temperature creep resistance of the heat resistant steel.750℃air oxidation experiments were performed on all test steels.SEM was used to observe the surface and cross section morphology of oxidation layer.It was found that AFA steel can form a compact Al₂O₃ oxide film in 750℃air.But addition of Ti will lead to internal oxidation,which leads to a double layer oxide film.High temperature oxidation resistance of all test steels is superior to Super304H.