Preparation And Steam Corrosion Resistance Properties Of TiAlN Coatings On 10Cr-Mo-V Heat Resistant Steel

In order to reduce the consumption of fossil fuel sources,including coal and petroleum,it is an important strategy to increase the steam temperature and press in stream-power plants.X12Cr Mo WVNb Nl0-1-1（10Cr-Mo-V）heat resistant steel has great potential application in ultra supercritical power generator.Previous applications indicate that the steel can be used in a power generator,whose steam temperature reach 560oC.With imcreasing steam temerature,the steel suffers the damage in manner of delamination of the oxide layer and pitting corrosion.Therefore,the steel does not satisfy the condition of ultra supercritical power generator,whose steam temperatures reach 650oC and higher.In order to apply the steel at much higher temperature,TiAlN series coatings were prepared on the heat resistant steel.NiCrAlYY/TiAlN coating was prepared by ion plating technique,where the TiN transitional layer was replaced by NiCrAlYY bonding coating.Moreover,alloying element Cr is doped into the coating,and the NiCrAlYY/TiAlCrN coating is prepared to improve the coating/matrix bonding strength,high temperature steam corrosion resistance.Scanning electron microscope,energy dispersive spectroscopy and X ray diffraction were utilized to analysis morphologies,microstructures and compositions of the surfaces and cross sections.Coating/matrix bonding strength and thermal shock performance were characterized by scratching and water quenching tests respectively.Steam corrosion properties of the TiAlN series coatings were measured by weighing the mass variation during the steam corrosion process.The steam corrosion mechanisms were investigated according to the microstructure and composition evolution and themodynamics knowledge.Moreover,the influence of NiCrAlYY/TiAlCrN coating on the fatigue and creep properties of 10Cr-Mo-V steel were studied.Main research works and main conclusion are described as follows:1.TiN/TiAlN coating was prepared on 10Cr-Mo-V heat resistant steel by multi-arc ion plating technique.The thickness of the as-received TiN/TiAlN coating is in the range of 2～3mm.the surface finish of the coating is smooth and dense.Some bright particles with various sizes are deposited on the coating.The preferred orientation of the TiN/TiAlN coating is（200）and（111）.The ratio of Ti/Al in the TiN/TiAlN coating is 0.92,microhardness is 1875HV0.1and is 4.3 times that of the matrix.Coating/matrix bonding strength was characterized by scratching tests.The results of scratch tests indicate that cracking force of the coating is 14N and the critical bonding force L_C between TiN/TiAlN coating and the steel is about 27 N.Thermal shock performance was characterized by water quenching tests.When the TiN/TiAlN coating were quenched in water from 650oC for more than 50 times,the coating/matrix bonding strength degrade.Cracks,delamination and pores fomartion appear on the TiN/TiAlN coating.These damage lead to the the oxidation of the steel.Therefore,the TiN/TiAlN coating cannot be used to protect the steel from oxidation when the temperature reaches 650oC.2.To solve the weak bonding strength between TiN/TiAlN coating and the heat resistant steel,NiCrAlYY bonding layer is applied to replace TiN transitional layer according to routes of thermal barrier coating,meanwhile,TiAlN surface layer is retained.The thickness of the NiCrAlYY/TiAlN coating is about 5mm and the surface finish of the coating is smooth and dense.The bonding strength between NiCrAlYY/TiAlN coating and 10Cr-Mo-V steel is higher than that of TiN/TiAlN coating.The results of the scratch tests show that the bonding strength between NiCrAlYY coating and the steel is very high.Cracking and delamination of the NiCrAlYY bonding layer cannot be found during scratching.Consequently,the bonding strength of NiCrAlYY/TiAlN coating and the steel is greatly improved.The cracking force is 19 N and L_c reaches 30N.When the coating experiences the water quenching 100 times between 650oC and 20oC,TiAlN/TiAlN coating remain good bonding to the matrix,which indicate the coating still protects the steel.The steam corrosion behaviors of 10Cr-Mo-V steel with NiCrAlYY/TiAlN coating is improved one order of magnitude.Weight gains per unit surface of the NiCrAlYY/TiAlN coating in 600oC and 650oC steam for 60 hours are 2.5%and 11%of that of the steel.Weight gains per unit surface of the NiCrAlYY/TiAlN coating in 600oC steam for 140 hours and 650oC steam for 100hours are 4%and 14%of that of the steel.600oC and 650oC steam will react with TiAlN coating and cause the formation of Al₂O₃and Ti O₂ protective layer.These oxides can prevent the steam corrosion process between the oxygen atoms and the matrix and reduce much lower the steam corrosion rates.NiCrAlYY transitional layer supplies the good bonding with the matrix.The delamination and cracking in NiCrAlYY/TiAlN cannot take place.Therefore,the steam corrosion resistance of the steel is much improved.3.To further improve the steam corrosion resistance of the coating,alloying element Cr was doped in to TiAlN and NiCrAlYY/TiAlCrN coating was prepared on 10Cr-Mo-V heat resistant steel by multi-arc ion plating technique.The atomic percentage of Cr is 10.23%.The ratio of Ti:（Al+Cr）:N is about 1:1:1.Compared with NiCrAlYY/TiAlN coating,the bonding strength between NiCrAlYY/TiAlCrN coating and the steel is further improved,compared with NiCrAlYY/TiAlN coating.There is no cracking in the NiCrAlYY/TiAlCrN coating during scratching.The cracking force and L_c are 31N.The steam corrosion behaviors of NiCrAlYY/TiAlCrN coating were investigated at 650oC and 700oC.It is indicated that the steam corrosion resistance of NiCrAlYY/TiAlCrN is much superior to NiCrAlYY/TiAlN coating.The steam corrosion rate paremater is about two orders of magnitude lower than that of NiCrAlYY/TiAlN coating.Weight gains per unit surface of the NiCrAlYY/TiAlCrN coating in650oC steam for 100 hours is 28%of that of NiCrAlYY/TiAlN coating.The variation of weight gain vs corrosion time of NiCrAlYY/TiAlCrN coating in 700^oC steam is almost same at that of NiCrAlYY/TiAlN coating in 650^oC steam.Therefore,the NiCrAlYY/TiAlCrN coating can guarantee that the steel can resist 700^oC steam,which is 50^oC higher than that of NiCrAlYY/TiAlN coating.4.The influence of NiCrAlYY/TiAlCrN coating on the fatigue and creep properties of10Cr-Mo-V steel are primarily studied.When the maximum stress are lower than the yield strength,there is negligible influence of the coating on the tension-tension fatigue lifetime of10Cr-Mo-V heat resistant steel at room temperature and 650oC.The NiCrAlYY/TiAlCrN coating can protect the steel from oxidation during the creep,and consequantly improve the resistance to the creep deformation.