Study On The Microstructures And Mechanical Properties Of Several Steels/Alloys For (Ultra) Supercritical Boilers

In order to improve the power generation efficiency and reduce CO₂ emissions of thermal power plants,(ultra)supercritical power generation technology with higher steam parameters has been developed all over the world.The increasing steam temperature and pressure make the service environment for boiler steels/alloys more complicated and rigorous.Therefore,higher requirements for their microstructure and performance stability are made.Microstructure of the steels/alloys for boiler would evolve with the long-term service under high temperature and pressure,further causing the change in mechanical properties of the materials.The degradation in the performance of these steels/alloys would thus affect the safety of thermal power plants and cause significant economic losses,even lead to casualties.To improve the stability and safety of the thermal power plants after long-term operation,it is necessary to analyze the evolution of the microstructures of the boiler steels/alloys and its influence on the relevant mechanical properties.Various analytical methods including scanning electron microscopy(SEM),transmission electron microscopy(TEM),back scattering electron diffraction(EBSD),selected area electron diffraction(SAED),convergence beam electron diffraction(CBED),energy dispersive spectrum(EDS)and the multiphase separation technology(MPST)developed by our group,combined with the thermodynamic calculation software(JMat Pro/Thermo-Calc)and finite element simulation software(ANSYS)are employed to analyze several typical steels/alloys with the increasing alloy content for(ultra)supercritical boilers:T91 ferritic/martensitic steel,Sanicro 25 austenitic steel,Fe-Ni-based Ni-23Cr-7W alloy and Ni-based 617 alloy.The main contents of this paper are as follows:(1)The microstructure and microhardness of a failure T91tube after service were investigated,and the cause of its failure was also discussed.The improper process of the perforated tube resulted in uneven wall thickness of the investigated failure T91 tube.The stress on the tube wall was simulated by ANSYS software.The influence of the stress on the amount and size of the precipitates and on the fraction of the low-angle grain boundary in the matrix,as well as the relationship between microstructure and microhardness were analyzed.Thermodynamic calculation was also applied to help analyze the cause for the failure of the investigated T91 tube.(2)The formation of Laves phase in Sanicro 25 austenitic steel after creep-rupture test at 700°C was explored,investigating its different formation mechanism.The effect of the Laves phase formation on other precipitates in the steel were discussed.The crystal structure of a metastable Si-enriched phase present in this steel was studied.(3)The evolution of the amount,size and morphology of the precipitates and its effects on the tensile strength,impact energy and impact fracture mode of the Fe-Ni-based Ni-23Cr-7W alloy after ageing at 700?for100/1000/7500h were investigated.A method for respectively quantifying the precipitates at grain boundaries and in grain interiors was developed.(4)The microstructure evolution in the Ni-based 671 alloy after ageing at 650/675/700°C for100/1000/7500 h was studied,especially the size,spacing and amount of the?'phase.The effect of the microstructural evolution on the microhardness of the alloy was also discussed.The phase analysis method and the relevant conclusions obtained in this paper are helpful to analyze the evolutions of the microstructure and mechanical properties of the steels/alloys for(ultra)supercritical boiler during the operation,and beneficial to improve the material quality,composition and processing technology and the localization of some steels/alloys.