Study On Creep Property And Oxidation Behavior Of 11Cr-6Co-2W-1Mo Ferritic Creep-Resistant Steels

FERRITIC heat-resistant steels have attracted strong interest in applications for boilers and turbines of ultra super critical power plants.The steels need to have better creep strength to increase the steam temperature and pressure,which contributes to improving thermal efficiency.In order to get close to or achieve the target level of 100 MPa for rupture after 105 hours at 650℃,it is necessary to double the creep strength compared with Grade 92,the creep strength of which is estimated 55 MPa at 650℃ by Ennis etal basing on the quantified Monkman-Grant relationship,and at the same time the resistance against steam oxidation must be improved.If the oxidation protection is to be achieved through alloy additions,it is necessary to increase the Chromium content in the steels from 9% to 11% or more.A new tempered martensitic 11% Cr steel with 6% Co which is mainly austenite former to avoidδ-ferrite was designed,and it is crucial for the carbon concentration of this 11%Cr steel to be reduced to a very low level,so as to promote the MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M23C6.The purpose of this paper is to investigate the oxidation behaviour and creep property of the 11Cr-6Co-2W-1Mo ferritic alloy.After tempering,the specimens were subjected to creep tests at 650℃,700℃and 750℃ for different testing stresses.The precipitation of LAVES phase takes place during creep at prior austenite grain boundaries and matensite lath boundaries from supersaturated solid solution of11Cr-6Co-2W-1Mo steels.However,the subsequent large coarsening of LAVES phases results in the breakdown in creep rupture strength observed in the long-term creep.According to the L-M curve,the creep rupture strength of test steel under the condition of 650℃/105 h is only 52 MPa,which is much lower than the target 100 MPa.Multi-stage oxidation kinetics is observed for 11Cr-6Co-2W-1Mo steels at 650℃ in air after 8875 h.The first slow oxidation stage corresponds to slow growth of a Cr2O3 layer,and the second stage is the result of the rapid diffusion of Fe ions passing through the established Cr2O3 layer to form(Fe0.6Cr0.4)2O3on the top of Cr2O3 layer.Unreasonably,Si O2 is the only product on the surface of11Cr-6Co-2W-1Mo steels in the wet air(20%H2O + air)at 650℃ after 7370 h.