| Ultra-supercritical power generation is seen as an important way to realize the clean coal utilization,safe and stable operation is the basis to ensure clean,efficient and economical operation of ultra-supercritical units.With the development of China’s "3060" double carbon goal strategy,the deep peak regulation activities are becoming more frequent in the ultra-supercritical unit,the problem of ageing and damage for the high temperature component material is becoming more and more prominent,it is urgent to further study the deterioration of micro structure and mechanical property under complex service conditions after serving for a long time.P92 martensite heat-resistant steel and Inconel783 high temperature alloy and T23 bainitic heat-resistant steel are critical materials in ultra-supercritical units.In this paper,the performance degradation characteristics of P92 steel,Inconel783 alloy and T23 steel have been studied after ageing at elevated temperature,and the main work is summarized as follows:(1)The high temperature ageing tests of P92 steel at 650℃ for 29000h and 680℃ for 21000h were carried out.The ageing test results showed that the room temperature strength,high temperature strength and hardness of P92 steel decreased significantly during the 1000h ageing time at 650℃ and 680℃,and then remained at a certain level in an ageing time range of 1000h～29000h at 650℃ and however decreased significantly from 18000h ageing time at 680℃.Also,the plasticity of P92 steel at room and elevated temperature decreased quickly during the initial ageing time,and then remained at a certain level after that time.Laves phases had been precipited during the 1000h ageing time at 650℃ and 680℃ and the coarsening phenomenon was extremely obvious.The coarsening rate of Laves phases was sinificantly higher than that of M23C6 carbides.The recovery of martensite lath structures and coarsening agglomerations of Laves phases much more easily occurred in the aged samples of 680℃ than that of 650℃,which had been observed in the samples after 20000h ageing time at 650℃,and the same characteristics could be also observed.after 7000h ageing time at 680℃.The stability of the martensitic lath structures played a leading role in maintaining the strength of P92 steel.(2)The isothermal stress-rupture tests of P92 steel were carried out at 610℃,630℃ and 650℃,and the variable law between microhardness and creep fracture time was studied.The prediction model of creep rupture life based on endurance strengths and the monitoring model of creep rupture life based on microhardnesses at 610℃,630℃ and 650℃ were established,which could be used to evaluate the creep damage life of P92 steel in service.Besides,it was observed that the Laves phases were precipitated in the creep specimen aged at 610℃ for 1000h time,and the obvious coarsening agglomeration was found at 650℃ for 3915h ageing time.At the same ageing temperature.the recovery of the martensitic lath structures more easily occured in the creep condition than in the temperature ageing condition.(3)Based on the mechanical properties of P92 steel spencimens with different ageing time at 650℃ and 680℃.the law was obtained that the correlation between hardness and strength still basically satisfied the linear relationship during the ageing process.The static tensile strength prediction model based on the Brinell hardness was established,which could be directly used to assess the tensile strength of P92 steel under different service times(4)The ageing tests up to 20000 hours at 650℃ and 700℃ for domestic Incone1783 alloys in service for 35000 hours were carried out.Compared with the as-received alloy,the tensile strengths at room and high temperature for the samples both decreased to varying degrees aftre ageing,but the room temperature plasticity was reduced to an extremely low level.The elongation at room temperature for the samples aged at 650℃ and 700℃ was severely declined to 3.7%and 4.2%respectively,but the the elongation at high temperature remained almost unchanged while the reduction of area increased slightly.The Incone1783 alloy had a serious tendency to embrittlement in the room temperature and yet demonstrated a relatively good performance in the high temperature.The coarsening and evolution behaviors ofγ’ phase and β phase determined the strength and plasticity of the alloy.It was found that the coarsening sizes of γ’ phase and β phase precipitated in the sample of 700℃ageing were remarkbably higher than that of 650℃ ageing.After ageing for 20000h at 650℃ and 700℃,the impact toughness of Incone1783 alloy at room temperature continued to decrease to only 3.8J and 2.9J respectively from the unaged initial value of 26J,which showed that the brittleness of Inconel783 alloy in the room temperature would be significantly increased after aging at the high temperature.It was found that the impact toughness of Inconel 783 alloy at room temperature decreased as an exponential function with ageing time at 650℃ and 700℃.Based on the results,a prediction model of impact toughness of Inconel 783 alloy during ageing process at 650℃ and 700℃ was proposed,which could be used to monitor the deterioration state of the Incone1783 alloy.(5)The microstructure and mechanical properties of the T23 steel used in the final reheater tube at 569℃ steam temperature after about 78000h of actual service were studied.It was found that the tensile strengths of the front-fire tube side at the room and high temperature were lower than those of the back-fire side,and meanwhile lower than the requirement of ASME SA213 standard,and which made it impossible to continue service.The analysis showed that a certain amount of coarsened M6C type carbides transformed from M23C6 carbides were found in the grain boundartes and inside the grains,which was the main cause of the performance deterioration for the T23 steel.The research results could be applied to guide the life and safety management of high-temperature heating surfaces in the power station. |
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