| The further increase of service temperature in the fossil fuel-fired power plant drives the rapid development of heat resistant alloys as the leading power generation technology.This would be conducive for the clean and efficient utilization of coal resources and the strategic goal of energy conservation and emission reduction.Expensive austenitic steel and Ni-based alloys often exhibit the overwhelming rupture strength,they are still not pertinent due to the low thermal conductivity,high thermal expansion,and infeasibility to manufacture thick sections in comparison to 9-12 wt.%Cr martensitic heat-resistant steel(MHRS).To improve the efficiency of power generation and reduce the coal consumption,the temperature of steam transported in the martensitic heat-resistant steel tube/pipes has to increase to 630-650℃,which,however,brings a great challenge to MHRS.To take this chanlenge,the MHRS must be redesigned with the new alloying strategies based on the insigt of alloying’s influence.Therefore,the hardening contribution from the main alloying elements(Cu,W,Co and Mo)was quantified by both experiments and the physical-based quantitative hardening model.Due to the dominant role of carbide on stabilizing the microstructures during the creeping,the influence of W on its ripening kinetics was both measured and numerically simulated.In addition,we measured the creep rupture properties of the commercial P92 steel and novel G115 steel at elevated temperatures and carefully characterized the evolving microstructure during the lengthy creep.Finally,we can draw the following conclusions.(1)The alloying strategy and relative strengthening mechanism of 9Cr MHRS involving W(tungsten),Mo(Molybdenum),Co(Cobalt)and Cu(Copper)were attentively studied.First,the solid solution hardening(SSH)model was established on the baseof the size and elastic misfits between the solute and the solvent atoms,which reveals the negligible SSH role of Cu atoms.This was confirmed by the experimenetal measurements on the hardess of W/Mo/Co/Cu alloyed 9Cr samples with three different levels.Second,it was found that the hierarchical martensitic boundaries could be strongly pinned by the extensive refined M23C6 particles via the W/Mo addition,and by fine ε-Cu particles via the addition of 1wt.%Cu during the tempering and lateral ageing.The precipitation behavior of M23C6 carbides was hardly affected by the Co and Cu addition.Third,three strengthening models,including the additive model,the multi-scale model and the dislocation and obstacle model,were all compared with the measured hardness during the ageing,leading to the conclusion that the hardening resulting from dislocation,precipitate,solid solution and grain boundary are independent and can be added linearly.(2)During ageing at 650℃ for up to 3000h,the size of M23C6 carbides was greatly refined via the W addition thus improving the hardness during the aging.The numerical simulations that can successfully predict the coarsening kinetics of carbide in the three steels during the ageing requires much enhanced diffusivity of solutes(Cr,W and Mo)along the boundaries and the proper choice of carbide/matrix interfacial energy that depends on the size and the alloying composition of carbide.It was found that the W addition could reduce the interface energy at the carbide/α-Fe interface,resulting in the reduction of nucleation barrier and promotion of the extensive precipitation of fine carbides.Distinguished from the Gaussian size distribution of M23C6 particles in P92 with fine prior austenite grain(PAG)size of 25.4 μm,a novel bimodal size distribution was revealed in coarse-grained BS and 9Crl W added steels both having PAG sizes of about 250 μm.The two-cell approach can better simulate the ripening kinetics of bimodally sized carbide particles during the ageing than the single-cell one,because the competition between the coarsening of carbide at HAGBs and LAGBs is considered.Both experimental and simulation results suggest that the bimodal size distribution of carbide in the coarse-grained steels results from the insufficient presence of HAGBs available for the nucleation of carbide,and the bimodal size difference is becoming greater after the ageing at 650℃ for 3,000 h because the growth of carbide at HAGBs could suppress that at LAGBs due to the competition of solute atoms being partitioned.(3)We investigated the creep rupture performance and the microstructural evolution of P92 and G115 steels during the lengthy creep,i.e.,at 630℃ for up to about 14000 h and 650℃ for 40000 h for P92 and G115 s respectively.G115 steel exhibits the overwhelming advantage of creep strength that it can withstand the rupture strength of 100 MPa for up to 38802 h at an excellent rupture ductility,including 13%total elongation and 62%reduction in area.The measured rupture life is about 10 times as P92 and also the highest at the similar strength level among all the conventional MHRS for boiler components.Such a great advantage of longterm creep rupture strength of G115 over P92 steel mainly arises from the stronger Orowan hardening of fine precipitates,mainly ε-Cu and M23C6,which also sustains high dislocation hardening during short-term creep.The rapid strength decay after the long-term creep is ascribed to the coarsening of ε-Cu and Laves,at which,however,the grain refinement due to recrystallization could compensate for the strength loss of precipitates;meanwhile the dislocation and solid solution hardening kept stable.Moreover,high number density of fine precipitates in G115 effectively suppressed the void swelling during creep for up to 38802 h,resulting in the necking rupture,unfortunately,the brittle rupture took place in P92 at the applied stress of 100 MPa for 14434 h. |
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