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Recycling machined superalloy scrap, such as turnings and borings, is a highly desirable goal, yet it presents significant challenges for conventional remelting techniques. The scrap's high volume-to-weight ratio complicates handling and melting, particularly within vacuum melting processes. Furthermore, remelting in electric arc or induction furnaces can result in substantial burn-out or oxidation losses, potentially reaching up to 20% of valuable alloying elements. The fine nature of the scrap, often combined with surface oxidation, leads to low density and poor electrical conductivity, which hinders the efficiency of induction melting. Additionally, the conventional method of consumable electrode ESR is often impractical due to the difficulty in compacting the loose scrap into an electrode of satisfactory quality.
To overcome these limitations, a feasibility study was undertaken employing Electro Slag Remelting (ESR) technology, but utilizing a specially designed non-consumable, water-cooled electrode instead of the traditional consumable type.
The success of this approach hinges on the electrode design, which underwent significant development and optimization. The basic structure comprised two concentric mild steel pipes, where the lower tip section of the outer pipe was fabricated from copper, specifically machined from a solid rod to eliminate welds in the high-temperature zone. Water cooling, essential to prevent the electrode from melting, flowed in through the inner pipe and exited through the annulus between the pipes. This cooling system was designed for operation up to 5000A and 200kW, with a water flow rate of 12 m³/h calculated to minimize temperature rise and prevent vapor formation by achieving a target velocity of 3 m/s.
Several operational challenges necessitated specific design solutions. Conventional 'solid start' procedures, involving an arc to melt the initial slag, were found to cause electrode erosion and melt contamination. The adopted solution was to use a 'liquid start' technique, pouring pre-melted slag into the mould. However, during this liquid start, the cold electrode surface caused an insulating slag shell to form instantly, hindering process initiation. To counteract this, three Molybdenum (Mo) tips were attached to the electrode bottom; their refractory nature and lower thermal conductivity helped penetrate this initial chill layer and sustain the current flow, although the tips themselves experienced some erosion during operation. Another issue was the erosion of the exposed copper part of the electrode, which led to melt contamination. This was resolved by reducing the copper wall thickness from an initial 30mm down to 20mm. This reduction improved cooling efficiency, allowing a stable, protective slag skin (less than 2mm thick) to form on the copper surface during operation, which effectively prevented further erosion and reduced copper pickup in the melt to below 0.05 wt%.
A systematic experimental procedure was implemented to test the process. Machined scrap, compositionally similar to Nimonic 80A, was cleaned, magnetically separated, and preheated at 300°C for 4 hours to remove contaminants and improve its flow characteristics. The slag used consisted of 70% CaF₂ and 30% Al₂O₃, with an addition of 6-8% TiO₂; it was preheated to 850°C for 2 hours before approximately 6 kg was melted for the liquid start. The ESR operation itself was conducted in a 350 kVA AC ESR furnace equipped with a 150mm diameter water-cooled mould. The 90mm outer diameter non-consumable electrode was positioned, the molten slag was poured, and then the prepared scrap was fed into the mould through the annular gap between the electrode and the mould wall.
Process parameters were optimized based on process stability and the resulting ingot surface finish, settling at approximately 44V, 3.5kA, 150-160kW, and a scrap feed rate of about 1.0 kg/min (referencing Table 1 in the source text). It was observed that maintaining a small amount of unmelted scrap above the slag bath led to optimal, smooth feeding. A hot topping procedure, involving gradual power reduction, was applied at the end of the melt. Subsequent post-ESR analysis involved checking ingot soundness via radiography and examining macrostructure after etching. Chemical analysis was performed on both the starting scrap (using AAS/ICP) and the final ingot (using XRF/LECO). The ingots were then subjected to thermomechanical processing, including soaking at 1150°C for 1 hour and forging to a 3:1 reduction ratio, followed by a standard heat treatment cycle (solution treatment at 1080°C for 3 hours with water quench, then aging at 700°C for 4 hours with air cool). Microstructure was examined using Optical Microscopy and EPMA on both as-cast and forged/heat-treated samples. Finally, mechanical properties were evaluated through tensile testing on the forged and heat-treated material using an MTS machine, with subsequent fractography performed via SEM.
The study of Satyaprasad et al. (1996)” yielded predominantly positive results. Under optimized conditions, sound ingots with good surface finish were consistently produced, whereas non-optimal parameters led to surface defects and slag entrapment. The ingots exhibited a columnar macrostructure free from major casting defects. Chemically, a slight loss of aluminum and a corresponding pick-up of titanium were observed, attributed to slag-metal reactions influenced by the slag's TiO₂ content; however, the final compositions remained within the alloy's specifications. Importantly, impurity levels of sulfur and phosphorus were found to be very low in the remelted ingot.
Microstructurally, the as-cast ingots displayed a dendritic structure, which transformed into a recrystallized grain structure after forging and heat treatment. The presence of TiC and TiN precipitates, typical for this class of superalloy, was confirmed in both conditions. Mechanically, the tensile properties of the forged and heat-treated ESR material proved comparable to the minimum specifications established for the conventional wrought alloy. Examination of the tensile fracture surfaces revealed extensive dimpling, characteristic of a ductile fracture mode.
Recycling superalloy scrap using ESR with a non-consumable water-cooled electrode is technically feasible and can be successfully implemented. Success is contingent upon careful electrode design, incorporating effective water cooling, Molybdenum tips for process initiation, and an optimized copper wall thickness to promote protective slag skin formation, along with meticulous process control, including the use of a liquid start and optimized operating parameters.
The process demonstrated its ability to yield a final product whose chemical composition and mechanical properties are comparable to those of conventionally produced material. Consequently, this method holds potential for being highly cost-effective by upgrading low-value scrap into a high-value product. Furthermore, the process shows promise for scaling up to produce larger diameter ingots and for application to a diverse range of superalloy scrap types.