The Research Of Pressure Evaluating And Reducing Method On Purification Of Low Carbon Steel And The Ni-Based Superalloy JG4246A

With the development of high-tech and science such as high-speed railway and aviation industry,the quality and performance of steel are required to be improved.However,even a few concentration of oxygen,sulfur or other gas impurities and inclusions in steel also seriously affect the plasticity and toughness,which hinder the application of steel in the high-tech industry,so further purification technologies are needed to be explored.In this paper,the physical simulation experiment were carried out,the influence of the quantity and float speed of microbubbles on the efficiency of removing inclusions from melt was analyzed.And then experiments of dissolved nitrogen flotation to remove inclusions from low carbon steel in vacuum induction melting process was conducted.Furthermore,the mechanism of removing inclusions by dissolved gas flotation was also studied.High temperature alloy is the key materials to be a variety of high-temperature components widely used in the field of aviation,aerospace,petroleum,chemical,ships and so on.Outstanding performance,such as high temperature strength,oxidation and corrosion resistance,good fatigue properties,fracture toughness and other comprehensive behaviors,make it possible for applications in harsh operating environments.However,novel purification processes are required to be explored for its wide variety of alloying elements.Based on the mechanism of remove inclusions by pressure evaluating and reducing method,the method applied to purify nickel-based superalloys in vacuum induction melting process was studied.This paper analyzed the feasibility of using magnesium vapor to generate microbubbles,then studied the solubility of magnesium in nickel-based high temperature alloys,and the influence of this process on the removal of inclusions.The main results are as follows:（1）The physical modeling experiment to study the mechanism of dissolved gas flotation was carried out.It was found that,in the process of pressure reduction,oversaturated gas in the melt generate a large number of microbubbles nucleated on inclusions,which will be carried to the surface of the melt along with the flotation of bubbles.According to the theoretical calculation,the fact that multiple microbubbles floating upward faster than a single bubble shortens the time for bubbles and inclusions floating to the surface of melt,and enhances the efficiency of inclusions removal.（2）The purification process by dissolved gas flotation in low carbon steel was conducted;The oxygen content in the steel before and after remelting was analyzed;The quantities and size of inclusions were calculated.It was found that when the initial nitrogen partial pressure is about 0.1 MPa,solubility of nitrogen is 474 ppm,T.[O]decreased from 15 ppm in raw material to 3 ppm after refining;The number of inclusions<2μm reduced obviously,most of inclusions（2⁵μm）were removed,and the number of inclusions>5μm were removed.According to the thermodynamic calculation,during degassing process,microbubbles tend to nucleate on the TiN and Al₂O₃ inclusions which exist as solid.During solidification process of steel,the SiO₂inclusion nucleated,and then MnO nucleated on the surface of SiO₂ and grew up.It is agreed well with the topography in experimental results.（3）The method of pressure evaluating and reducing method was also explored to purify Ni-based superalloys;The content of oxygen,nitrogen and sulfur and microstructure of inclusions before and after remelting was analyzed;The quantity and size of inclusions in alloys were counted.The results show that when the pressure in the furnace was 0.095⁰.1 MPa,the content of magnesium dissolved in high temperature alloys reached 0.138wt.%.T.[O]decreased from 3.36 ppm in raw materials to 1.51 ppm,T.[N]from 6.74 ppm to 0.33 ppm,T.[S]from 2.05 ppm to 1.02 ppm.54%inclusions less than 2μm in size were removed,61%inclusions between 2⁵μm in size were removed,and 50%inlcusions over 5μm in size were removed.