| China's thermal power generation industry relies heavily on fossil fuels such as coal,natural gas,and petroleum,non-renewable resources are declining rapidly,in order to reduce the greenhouse effect and utilize non-renewable resources rationally,the coal-fired power industry should increases generators'thermal efficiency as much as possible,so the study of ultra-supercritical?A-USC?technology is very necessary.Due to the gradual improvement of the steam parameters of the ultra-supercritical generator,the more complex working conditions require the unit steel need to have better high temperature resistance,thermal fatigue resistance,high temperature oxidation resistance,and anti-smoke resistance.The structural stability and impact toughness of heat-resistant steel in high temperature environment during long-term service will be reduced,which will affect the safe operation of the unit seriously.Therefore,it is of great practical significance to study the microstructure stability of heat-resistant steel during long-term service.Sanicro25 steel?22Cr-25Ni-3.5W-3Cu?is a new type of austenitic heat-resistant steel with high creep strength,resistance to smoke corrosion,good structural stability,and weld processing performance,it is one of the potential candidates for superheaters and reheaters in700°C grade ultra-supercritical units.At present,Sanicro25 steel is only trial-run in some European countries and has not yet officially served in China.Through the research,it is found that Sanicro25 steel would have a decline in toughness during service,which affects the safety of the unit's operation seriously.Therefore,the precipitation behavior of the second phase of Sanicro25 steel needs further research.In this paper,the microstructure evolution of Sanicro25 steel after long-term aging was studied by high temperature aging test.The precipitation behavior of the second phase was studied by electrochemical extraction test.The type,location,and morphology of the precipitated phase of Sanicro25 steel during aging were confirmed by means of scanning electron microscopy,transmission electron microscopy and X-ray diffraction analyzer.The content of precipitates of Sanicao25 steel during the aging process was studied by XRD semi-quantitative analysis.The relative content of each precipitated phase was changed.The effects of aging treatment at different temperatures on the microstructure and mechanical properties of Sanicro25 steel were also analyzed.The results show that the as-received Sanicro25 steel's matrix has a uniform austenite structure,and there are a large number of twin crystals and undissolved Nb-rich nitrides.After aging,there are some type of second phase precipitates in the crystal and grain boundaries.The number of twin crystals decreased greatly.With the increase of aging time and the increase of aging temperature,the number and size of precipitated phases also changed.The precipitated phases of Sanicro25 steel after aging mainly include M23C6,NbCrN and Nb?N,C?.The main precipitates on the grain boundary are M23C6 carbides,the precipitates in the crystal are mainly NbCrN and Nb?N,C?.With the increase of aging time,the relative content of each precipitate is also gradually changed.The highest relative content of the precipitated phase is M23C6carbides,and the relative mass fraction of the M23C6 and NbCrN phases increases slowly and then gradually balances;the content of NbN phase is relatively rare,and the mass fraction gradually decreases.The impact toughness of Sanicro25 steel decreases after aging,and the fracture mode changes from ductile fracture to intergranular brittle fracture.The fracture mode during creep is dominated by dimple-aggregation fracture. |
PDF Full Text Request