Simple Solutions That Work! Issue 13

DEGASSING MEDIA – ARGON VS NITROGEN The two most common inert gasses used for degassing molten aluminum are nitrogen and argon. Other gasses used in the past, such as chlorine and Sulfur Hexafluoride (SF6), while effective, have been limited or banned from use do to greenhouse gas effects and safety issues. That stated, there are foundries that use both chlorine and SF6 when combined in low concentrations with either nitrogen or argon. Before using such a mixture, it is advised to consult with experts and local regulatory officials. In this article, we will discuss only the degassing properties of pure nitrogen and pure argon. Both nitrogen and argon can be effectively used for degassing. They work by being dispersed into the melt using either a refractory based lance or rotary inert degasser (RID). Once the argon or nitrogen gas is bubbled into the melt, the gas attaches to the hydrogen in the melt and carries it to the surface where it is burned off. If the degassing process is effective, you can literally see flames at the surface of the melt. The key advantage of degassing with nitrogen is the raw material cost. However, nitrogen is less effective than argon for removing hydrogen from the melt. Both will do the job, but given the same degassing equipment, the degassing time required with argon will be less than when using nitrogen. To determine whether to use argon or nitrogen, the foundry should perform a cost evaluation comparing material cost, degassing time, labor, and production output. In most cases argon wins this battle as the labor savings and increased output using argon typically outweighs the savings in material costs using nitrogen as the inert degassing media. DEGASSING EQUIPMENT LANCE VS RID The key to effective degassing is creating an extremely fine bubble distribution with your degassing tool. Distributing small inert bubbles throughout the melt are much more effective at carrying hydrogen to the surface than large bubbles confined to one area of the crucible. The entire reason for using a Rotary Inert Degasser (RID) is to chop up the inert gas bubbles and distribute them across the melt. When this is understood, it becomes clear that using a RID is more effective than using a lance. However, there are many cases in which a RID cannot be used. For example, a small crucible in which a RID won’t fit or a furnace with restricted overhead access which restricts RID access. No need to panic. A lance can get the job done. It will take longer than with a RID and, if used on a larger crucible (500 lbs or more), it may need to be moved a couple of times during the degassing process. If a lance must be used, try to select a lance with a diffuser head rather than just a straight opening. The diffuser head dispenses the inert gas through several small openings creating smaller bubbles and greater inert gas coverage in the melt. Some lance manufacturers use a straight tube made of graphite or refractory material. The tube is then plugged on the bottom end with 1/8” holes drilled around the base to facilitate distribution of inert gas bubbles. The photo above shows an argon lance in a small crucible with hydrogen flames burning at the surface. To effectively degas this crucible, the foundry had to move the lance several times during the 20-minute degas session. The photo below displays a drawing of a Palmer Manufacturing RID unit in a crucible. Note the effective bubble distribution throughout the melt. RPT ANALYSIS OF THE MELT Knowing the relative hydrogen levels in your melt is essential to both DEFECT PREVENTION 37 Continued on next page