Abstract
The nitrogen behavior of superalloy melt GH4169 during the vacuum induction melting (VIM) process was clarified by using different proportions of returned materials including block-shaped returned material, chip-shaped returned material, and pure materials to produce a high–purity superalloy melt and provide guidance for the purification of the superalloy melt. For the nitrogen removal during the VIM process, the denitrification rate in the refining period reached 10 ppm per hour on average, which is significantly higher than 1 ppm per hour on average in the melting period. The denitrification reaction of superalloy melt GH4169 under extremely low vacuum pressure is controlled by both the mass transfer of nitrogen in the melt and the chemical reaction of the liquid–gas interface. The nitrogen removal of superalloy melts during VIM occurs through the two methods of gasification denitrification and nitride floatation because the nitrides begin to precipitate in the liquid phase at 1550 °C. A higher nitrogen removal rate can be obtained by increasing the proportion of chip-shaped material or decreasing the proportion of block-shaped material.
Highlights
The Ni-based superalloy has been widely applied in the hot parts of aerospace turbine engines and chemical equipment [1]
The morphologies of carbonitrides in Inconel 718 superalloy were changed from cluster block or single octahedral in ingots to skeleton-like after calcium treatment in the electroslag remelting (ESR) process due to the modification of oxide inclusions by Ca-treatment resulting in a change in precipitation and growth conditions for carbonitrides [14]
The superalloy was smelted in a 3-ton vacuum induction melting furnace (VIMVIDP1000 manufactured by ALD in Hanau, Germany) with a certain ratio of returned materials (70%) and pure material (30%), and the total charging capacity of the alloy was about 2.3 ton per furnace
Summary
The Ni-based superalloy has been widely applied in the hot parts of aerospace turbine engines and chemical equipment [1]. Three cases, listed, were designed to investigate the behavior of nitrogen in superalloy melt during vacuum induction melting using different ratios of block-shaped and chip-shaped returned materials and pure materials. Block-shaped returned material (BRM) is mainly from the recycling of sprues, runners, risers, and scrapped superalloy machine parts. The superalloy was smelted in a 3-ton vacuum induction melting furnace (VIMVIDP1000 manufactured by ALD in Hanau, Germany) with a certain ratio of returned materials (70%) and pure material (30%), and the total charging capacity of the alloy was about 2.3 ton per furnace. The target composition was based on the median value of the standard chemical composition with the consideration of effectively reducing the contents of harmful elements of oxygen and nitrogen in the melt to below 10 ppm and 55 ppm, respectively. The formation of inclusions and degassing of the superalloy melt under vacuum atmosphere were analyzed by using thermodynamic software and relevant databases (FactSage 7.2, codeveloped by Thermfact/CRCT, Montreal, QC, Canada, and GTT-Technologies, Herzogenrath, Germany)
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