Study On Sand-Sticking Mechanism Of Map In Casting Process Of Ultra-Supercritical Steel

Along with the decrease of fossil energy and the emission of pollutants,the development of ultra-supercritical power generation technology is becoming more and more important in our country.At present,the main preparation methods in the production of the key components of the ultra-supercritical steel steam turbine include integral forging,separate forging and integral casting.The study found that the integral casting method is feasible to produce the key components of the ultra-supercritical steel.However,from the production practice,there will be a large number of map sand phenomenon on the surface of the casting after the completion of production,and the formation of sand on the casting surface will affect the surface quality of the casting.Increase the production cost and increase the supply cycle,serious will make the castings directly scrapped.If enterprises want to solve the problem of sand adhesion on the surface of castings,it is necessary to clarify the mechanism of sand adhesion in the casting of ultra-supercritical steel steam turbine.Therefore,this thesis mainly tests the interface of experimental results by conducting relevant experiments and using SEM+EDS and XRD detection methods.The chemical reactions and phase changes of steel sand,steel coating,sand coating and steel coated sand at the interface were analyzed.Finally,the mechanism of producing map sand in the casting process of ultra-supercritical steel was obtained.The main research contents and conclusions of this thesis are as follows:（1）The test results of steel sand show that when the experimental temperature is 1873 K,the separation of steel sand becomes more difficult with the increase of holding time,and the liquid phase volume increases,which leads to the aggravation of sand adhesion defects.When the holding time is 90 min,the liquid phase of the steel sand interface increases with the increase of the experimental temperature,and the sand sticky phenomenon becomes worse.The results show that[Al],[Mn],[Nb]and other active elements in the steel are involved in the interfacial reaction of the steel sand,and react with the surface sand（SiO₂）and（FeO）to form the low melting point liquid phase system such as Al₂O₃-MnO-Nb₂O₅,which leads to the sand adhesion phenomenon.（2）The results of the steel coating experiment show that the chemical reaction of the steel coating interface is the same as that of the steel sand experiment no matter the experimental temperature or holding time is controlled,but the liquid phase of the steel coating interface is relatively small.It can be seen from the test results that the liquid phase generation on the steel coating interface will produce a small number of holes and cracks,which leads to the penetration of the liquid steel into the surface sand pores,and the direct contact of the steel sand and then the formation of sticky sand.（3）The results of the sand coating experiment show that when the experimental temperature and holding time are changed,the sand coating will not have chemical reaction,and there is no liquid phase at the sand coating interface,which is mainly composed of silicate,Al,Cr spinel,olivine and corundum（containing Cr,Fe,et al.）solid substance.（4）The results of steel sand-coating experiment show that the chemical reaction at the interface of steel sand-coating is the same as that in steel sand-coating experiment,controlling the experimental temperature and holding time.The amount of liquid phase produced increases with the increase of temperature and holding time,and the sand-sticking situation becomes worse after steel solidification.（5）To sum up the experimental results,the mechanism of map clay in the casting process of ultra-supercritical steel is as follows:firstly,the active elements[Al],[Mn]and[Nb]in the liquid steel react with the coating to produce liquid phase,resulting in a small number of holes and cracks at the steel coating interface,which increases the contact area between the liquid steel and the surface sand.Resulting in chemical reactions in steel sand to produce liquid phase and low melting point compounds,steel after solidification adhesion to the surface to form clay sand.