| Alkaline metal,chlorine and sulfur are known to be abundant in biomass,which easily volatilize under combustion and resulting molten alkaline salt slagging when compounds fluctuate with the flaming smoke and deposit on the surface of superheater.The slagging will lead to a serious attack of hot corrosion.Meanwhile,creep deformation will be resulted by the heat stress and vapour pressure after a long time operation.With the interaction of hot corrosion and stress at some extent,fracture and failure of thermotubes are caused.In order to better explain the properties of thermotubes,deformation behavior,mechanism of creep and transformation of microstructure of the components,the mechanical tests,SEM and EDS analysis were adopted using self-developed RDL05 creep and fatigue testing machine to carry out researches with different temperature,stresses,compositions of alkaline salt and time.Such factors are mainly imposed on the creep properties of 12Cr1 MoV alloys in hot corrosion environment.The following aspects are included in this thesis:(1)Compression creep tests of 12Cr1 MoV alloys had been conducted in chloride mixtures and air at different temperature to investigate the influence of hot corrosion on creep properties of alloys.Results shows that the protective oxide film is broken by active oxidation which is initiated from the reactions between high temperature chloride mixtures and the surface of alloys,leading to a serious corrosion effect on the alloy.When the temperature is above 700℃,the steady-state creep rates can be accelerated by hot corrosion effects after long term reactions.(2)Compression creep tests of 12Cr1 MoV alloys had been conducted in different salt environment.It could be concluded that there is an evident difference on the creep properties of alloys.The law of creep rates is described as2 4 2 4NaCl-Na SO NaCl NaCl-KCl Na SO(5)ε>(5)ε>(5)ε>(5)ε.Results also shows that time of 12Cr1 MoV on first stage of creep will be longer when there is an increase on the ratio of Na2SO4 in salt mixtures.(3)Compression creep tests of 12Cr1 MoV alloys with different applied stresses had been conducted.The creep mechanism of 12Cr1 MoV in chloride mixture is inferred as gliding and climbing of dislocations in NaCl-30%KCl at 650℃.With the increase of stresses,the attack by hot corrosion is more serious.Meanwhile,spheroidization of cementite and coarsening precipitates which discontinuously distributed on grain boundaries were clearly observed.(4)Compression creep tests of 12Cr1 MoV alloys had been conducted at different time to further analyze the initiation and propagation of defected structures in the inner parts of alloys.Results showed that grain boundaries are firstly chosen to be corroded by high temperature chloride mixtures,while some grains,ferrite and pearlite in severe spheroidization,are also attacked.When the compression load is sufficiently high for a long duration,the surface and cross sections will break preferably along those intergranular cracks in the direction of the compression stress,resulting the evident layer fracture on corrosive specimens.At the same time,porous sections are caused by the corroded grains,where stress concentration easily happen.The phenomenon will further expand the gap between each layers and lead to a faster creep deformation than alloys in air. |
