Simple Solutions That Work! Issue 16

54 Contact: BRAD HOHENSTEIN blh@porositysolutions.com Proper purge gas flow is a key to effective degassing. A low flow gas regulator with a glass cylinder and floating ball, as shown in Figure #5, is typically used for setting the purge gas flow. The units of the gas flow regulator are typically displayed in Standard Cubic Feet per Hour (SCFH) with a range of 0 to 50 SCFH. The gas flow setting required for the foundry can vary depending on the depth that the lance will be operating in the crucible. It is a good idea to start with a low flow during insertion (around 10 to 15 SCFH). The final flow of 15 to 30 SCFH will be adjusted once rotation of the lance is initiated. • Lance Placement in the Mel t If possible, the RID degassing Lance should be lowered into the center of the crucible or ladle to ensure maximum dispersion of purge gas bubbles throughout the melt. When degassing a large furnace, it may be required to degas in multiple areas of the melt. • Lance Rotation Speed The lance rotation speed is typically controlled by an air flow valve connected to the air motor of the RID unit. CAUTION: To prevent injury from spraying of molten aluminum, rotation of the lance must not start until the lance is fully submerged. After the lance is fully submerged, rotation of the lance is started. Typically, the lance rotation is set between 200 and 500 rpm. • Fine Tuning the Gas Flow and Lance Speed Determining the final purge gas flow and lance speed settings require fine tuning of each parameter. The lance rotation speed should be set to spread the bubbles in a wide as possible area without creating a vortex around the shaft. Once the metal starts flowing back down around the shaft, the lance rotation speed should be slowed. The gas flow should be set so that no rolling of the metal is noted on the surface of the melt. The arrow in Figure #6 points to harmful rolling of the surface melt caused by excessive purge gas flow. Figure 6. Fine tuning of both the lance rotation speed and purge gas flow rate is now made until fine bubbles are distributed across the melt surface. Once fine tuning is set, the parameters should be recorded for the foundry degassing procedure. • Degassing Time The last step in developing the foundry degassing procedure is setting the required degassing time. This can be determined by taking incremental Reduced Pressure Test (RPT) samples throughout the degassing process and measuring the density of the samples. Once the density of the RPT samples approach the book density of the alloy, the effective degassing time can be set, and the final foundry degassing process published.