Analysis of Deoxidation, Nitrogen Removal, and Transformation of Inclusions in GH4169 Alloy Processed from a Charge with a High Amount of Returns
摘要
This study investigated the refining behavior of GH4169 alloy processed from a charge containing 70 wt pct return materials under industrial-scale vacuum induction remelting (VIM) conditions using a 3 t furnace. Block return material (BRM) and chip return material (CRM) were compared to evaluate the stage-dependent variation of oxygen, nitrogen, and inclusions during melting and high-vacuum refining. Oxygen decreased mainly during melting, from 70 to 9 ppm in BRM and 13 ppm in CRM, and then remained at a low level during refining. In contrast, nitrogen removal mainly occurred during refining, decreasing from 70 to 64 to 42 ppm for BRM and from 80 to 63 to 38 ppm for CRM from melting to the end of refining. The average nitrogen-removal rate during refining was 8.8 ppm h−1 for BRM and 10.0 ppm h−1 for CRM, which was 7.3 and 2.9 times the corresponding melting-stage rate, respectively. Kinetic analysis showed that nitrogen removal during refining was predominantly controlled by mass transfer of dissolved nitrogen from the melt to the gas–liquid interface, with an apparent mass-transfer coefficient of 3.1 × 10−3 cm s−1. Inclusion characterization revealed that isolated oxides were scarce, whereas most inclusions were composite particles with MgAl2O4-type oxide cores serving as heterogeneous nucleation sites for Ti(C,N)-based phases. After refining, the spatial density and average size of inclusions decreased, and inclusions became concentrated in the 1 to 2 μm range. These results support the hypothesis that oxygen removal, nitrogen removal, and inclusion transformation in GH4169 alloy processed from a charge with a high amount of returns are strongly stage-dependent during VIM.