Low-Inclusion Content for High-Grade Streel Material Used in Gear-and-Bearing Components

Commercial vacuum melting grew substantially in popularity in the 1950s in the demand for superalloys. The techniques remain popular in all high-performance metal due to their high level of control over composition, cleanliness, and homogeneity. The US Navy commonly uses Alloy 53 (AMS 6308) in safety critical applications such as helicopter rotors which is produced using a double vacuum melt VIM (vacuum induction melting) and VAR (vacuum arc remelting) process. Despite the advances of vacuum melting over the past century, parts made with Alloy 53 experience premature part failures due to the presence of non-metallic inclusions in the final product. Therefore, the US Navy will benefit from a cost-neutral technology that can reduce the presence of non-metallic inclusions and improve the overall quality of the vacuum melted material. The approach taken here focuses on the VAR process during which an electrode (produced by the VIM process) is remelted under vacuum by generating arc-discharges that super-heat the electrode surface. This process improves the solidification structure and further refines the material. During this process, low-density non-metallic inclusions nominally float on top of the melt pool and migrate to the edges where they are eventually removed. Arc behavior is a likely mechanism behind the persistent inclusion entrainment phenomenon of interest to the Navy. Furthermore, it is well documented that anomalous behavior of the arc distribution can create solidification defects such as the inclusions found in Alloy 53. To date, no technology exists to directly monitor the arc-distribution during VAR leaving a gap in the quality control. This is largely because a VAR furnace is inhospitable to common process monitoring tools (e.g. optical) due to a large metal furnace body and a high temperature plasma environment inside. To address this need, we have created a multifaceted approach to monitor and control the arc distribution during the VAR process to improve the consistency and quality of the final ingot. We use magnetic flux imaging to map the three-dimensional current distribution throughout the system. This technology enables monitoring of the arc-distribution over time. To enable the use of this technology we have performed a series of research-scale experiments and developed multi-physics simulations of the VAR process. By mounting electromagnets on the exterior of the furnace, we can create external magnetic fields to control the arc-distribution. Creating quality metrics with magnetic flux imaging enables the use of externally applied magnetic fields to correct deleterious arc behavior. Magnetic flux imaging and external control over the arc distribution will ultimately be used to create an ingot with improved micro-cleanliness to meet the goals of the US Navy to create a cost-neutral reduction in the inclusion content of Alloy 53.