Research On The Oxide Mechanism And Preventing Technology Of Oxidation For 9Cr-1Mo-V-Nb Steel

With the development of the units having high temperature and pressure,a new type of material is needed.Therefore,the technology of developing the heat resistant material with having better performance has become an important problem of building large units.9Cr-1Mo-V-Nb(T91/P91)steel are heat resistant steels developed during 1970s in US for boiler components such as steam piping,steam head,reheater,etc in fossil power plants.The steels have higher performance in terms of allowable stress, rupture strength,creep-resistant ability,fatigue strength,and heat conductivity.The steels also have better weld ability and corrosion-resistant ability with a moderate price.Up to now,T91 and P91 have been widely used in fossil plants of the advanced countries and regions such as US,Europe and Japan with good economic benefits achieved.The steels were introduced in China during the 10th 5-year plan period,and the researches on its localization and the productions of the steels have since then been initiated.The 10Cr9Mo1VNb is a newly localized type of steel imitative of T91 and P91.But there have been many problems with 10Cr9Mo1VNb in its research and production.The quality of tube made of 10Cr9Mo1VNb is still substantially lower than that of tube made in advanced countries with as to be applied in power plant.In order to fill the performance gap between 10Cr9Mo1VNb and T91/P91,this paper carries out investigations on theoretical understanding and practice concerning production process;tube quality and engineering applications of domestically produced T91 and P91 steel.The paper also conducts application researches on theories of its structure,strengthening mechanism,alloying mechanism.Creep rupture equation and extrapolation were developed based on the data conducted at three different temperatures,which can be used for life assessment of the operating tubes.The thermostatic and cyclic oxidation were performed at three temperatures in air and mixed Ar and H₂O.The thermostatic oxidation behavior of SA213T91(T91) steel in (Ar+10vol.%H₂O) and air atmosphere at 650℃was investigated.The oxidation behavior of T91 steel in(Ar+10vol.%H20) atmosphere at 600℃,650℃and 700℃was investigated.During oxidation for 10h the oxidation kinetics followed a parabolic law at 600℃,but two-stage parabolic law at 650℃and 700℃,in which the rate constant at the initial stage was higher than that at the second stage.With temperature up,the oxidation rate of T91 steel increased significantly.The oxidation activation energy was identified as 157.2 kJ/mol.The oxide scale with three-layer microstructure was composed of Fe₂O₃,Fe₃O₄ and(Fe,Cr)₃O₄ in the order from the top layer to the inner layer.Meanwhile the internal oxidation was also observed,the corresponding internal oxides were Cr₂O₃ and FeO.However,at 700℃,Fe₂O₃ neither outer layer nor internal oxides formed.The outer layer(Fe₂O₃) had a weak adhesion to the intermediate layer (Fe₃O₄),easily spalled during cooling from oxidation temperature to room temperature.The oxidation mechanism of T91 in(Ar+H₂O) atmosphere was discussed finally.The cyclic oxidation behavior of T91 steel in 10%H₂O+90%Ar at 600℃,650℃and 700℃for various time was also studied.The experimental results indicated that a duplex oxide scale with an outer layer of Fe₂O₃ and an inner layer of mixed(Fe, Cr)₃O₄ formed on T91 steel during cyclic oxidation.In those test,the spallation of oxide scale was observed during cyclic oxidation of T91 steel at 600℃,650℃and 700℃,which indicated that stress generated in the oxide layer during cyclic oxidation. Some through cracks generated in both inner and outer oxide layers.Parts of oxide scales spalled from substrate during the cyclic oxidation.The oxidation mechanism of T91 steel in 10%H₂O+90%Ar was discussed.The thermal stress was evaluated in two oxide layers to illustrate the spallation of the oxide layer.The calculation results showed that a higher tensile stress in the oxide layer was formed at the early oxidation stage than at the later oxidation stage during heating.This tensile stress resulted in the formation of cracks in the oxide layer.In order to preventing the scale spallated from the oxide layers,two methods were done in the paper,which were aluminide coating and Y₂O₃ coated.An aluminide coating containing Fe₂Al₅ intermetallic was prepared on T91 steel by hot dip aluminizing.A diffusion layer containing Al existed beneath this coating during annealing.Some penetrated cracks were observed in the coating.The cyclic oxidation behavior of aluminide coated T91 steel was studied in 10%H₂O+90%Ar at 600℃,650℃and 700℃for various time.The observations showed that the coated sample exhibited much better oxidation resistance than that of bare sample oxidized under the sample conditions.AFM observation revealed that the growth of was dominated by Al diffusion outward.However,with temperature increasing,the oxidation rate was accelerated.During the cyclic oxidation,more cracks formed in the coating and part of coating spalled from sample surface,which resulted in degradation of coating.The cyclic oxidation behavior of Y₂O₃ coated T91 steel was studied in 10%H₂O +90%Ar at 600℃,650℃and 700℃for 100 cycles.The research results indicated that after T91 steel coated by Y₂O₃ the oxidation rate was obviously lower than that of bare samples under the same oxidation conditions.Furthermore,Y₂O₃ coated sample exhibited approximate oxidation rate at 600℃,650℃and 700℃.For the coated sample surface the oxide phases on the sample surface was identified as Fe₃O₄ and Cr₂O₃ after oxidation.The yttrium promoted formation of Cr₂O₃ on the sample surface, which improved the oxidation resistance and spallation of oxide scale of T91 steel in in 10%H₂O+90%Ar on the sample surface under the test temperature.