Removal of inclusions from cast superalloy revert

• Main Author: Haruna, Yasushi
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/5342

It is known that the recycling of superalloy scrap causes the mechanical properties and the casting performance of reverted alloys to deteriorate. This is due to contamination of the alloy by inclusions. The contaminants affect the microstructure of the casting in terms of modification of MC carbide morphology and an increase in the amount of microporosity. Removal of such contaminants from the reverted superalloys is mandatory especially for the production of critical cast parts. The removal of TiN and HfO₂ has been investigated in this work. The computation of the precipitation conditions for TiN in IN100 and HfO₂ in Mar-M247 has been carried out using available thermochemical data, taking into account the segregation taking place during freezing. The formation reactions are: TiN(₈) T and Hf0₂(₈) (s) Hf + 2O, where Ti, N, Hf and 0 are dissolved in the liquid superalloy. The solubility product for TiN precipitation in IN100 is calculated as log K’tin = —2.62 at the liquidus temperature. The saturation solubility of nitrogen is lowered due to Ti segregation during freezing. The saturation solubility of oxygen for Hf0₂ precipitation in Mar-M247 is estimated as less than 1 ppm at the liquidus temperature, and that Hf0₂ particles mostly form by the reaction with the oxide crucible during remelting. The formation and the removal mechanisms of these inclusions are examined using DS and EB remelting techniques. It is concluded that the main factor for MC carbide morphology change in IN100 is the solidification conditions not the alloy’s nitrogen content. The total nitrogen content in the liquid determines the number of TiN particles that are found as nuclei in blocky carbides. In Mar-M247, it is confirmed that HfO₂ is formed by the reaction withA1₂O₃ crucible during multiple remelting. It is demonstrated that EB remelting is valid for TiN and HfO₂ particle removal. These inclusions are removed by a separation mechanism due to the interfacial tension forces between the particles and the liquid surface and not by a buoyancy mechanism. The saturation solubility of nitrogen for TiN precipitation in IN100 is determined to be approximately 6 ppm at the liquidus temperature which represents an aim-point for refining in recycling the alloys. Finally, the practical EBCHR conditions for removal of those inclusions are also discussed.