Investigation Of Inclusions And Purification In Alloy GH3625

Superalloys are generally used as the key service material for various types of aero engine and gas turbine, the requirement for their metallurgical quality is stricter, especially for trace harmful impurity element contents(O, N, S) and non-metallic inclusions formed by these elements. It usually requires the contents of impurity elements and non-metallic inclusions as low as possible. In present paper, we made a series of studies about inclusions and purification technology of alloy GH3625 ingot:Equilibrium phases of alloy GH3625 were calculated by using JMat Pro software. The effects of different contents of alloying elements C, O and N on the related precipitated phases were investigated, and then analyzed the precipitation regularity of each phase. When the alloy containing impurity elements O and N, it will form the oxide inclusions M2O3(Al2O3) above the liquidus temperature, and will generate nitride inclusions MN(Ti N) in the solid liquid phase zone. The contents of M2O3 and MN are controlled by O and N contents respectively. The precipitation sequence of non-metallic inclusions and primary carbides in alloy GH3625 is L+M2O3â†'L+M2O3+γâ†'L+M2O3+γ+MNâ†'L+M2O3+γ+MN+MC(Nb C).Alloy GH3625 ingot was prepared by the duplex-melting process(vacuum induction remelting and E electroslag remelting). The composition, impurity contents and microstructure were investigated before and after electroslag remelting. Composition of the ingot distributes more uniformly in each region. The difference of alloying elements contents(top and bottom, cross sections) is lower 0.1% after electroslag remelting. Sulphur and oxygen contents are reduced to 10 ppm and 7ppm respectively, and the removal rate of sulphur and oxygen is 58%, 59% respectively. Electroslag remelting increases secondary dendrite arm spacing, and makes MC carbide transform granular into Chinese script. Nb segregation is worst in GH3625 ingot by the buplex-melting process, especially the interdendritic segregates a lot of Nb C, Laves phases at the ingot center part.Non-metallic inclusions were investigated before and after electroslag remelting with the methods of original position observation and electrolytic extraction. The results show that non-metallic inclusions in alloy GH3625 are mainly(Ti, Nb)N and Mg O·Al2O3 spinel, which size is between 2 to 3μm. Nitrogen in the alloy is mainly derived from smelting raw material Cr and Mo. Oxide inclusions mainly come from Mg O crucible during vacuum induction remelting, while oxide inclusions are formed through slag system component entering the alloy matrix during electroslag remelting.For further reduction of oxygen, nitrogen and sulphur contents in master alloy GH3625, two vacuum induction remelting experiments(high vacuum without calcium and high vacuum with addition calcium) were carried out. Alloy composition and impurity contents were investigated. Under high vacuum induction remelting burning loss and enrichment of alloying elements are all in the range of standard composition. Deoxidation is excellent, O content is reduced from 17 ppm to 10 ppm by C-O reaction, but removal of nitrogen and sulphur is not obvious under high vacuum induction remelting. High vacuum induction remelting with addition calcium using Al2O3 crucible Ca-O and Ca-Al2O3(the inner wall of crucible) reaction is helpful to deep deoxidation, Ca-S reaction makes S content is greatly reduced, reducing nitrogen content is limited, it is far higher than N solubility in alloy GH3625. In Al2O3 crucible, adopting high vacuum melting technology and proper calcium addition amounts, oxygen, nitrogen and sulphur contents could be reduced to 6ppm, 60 ppm and 9ppm respectively.