Bubbling smaller Steel Melts
Nova Precision Casting installed a porous plug in a crucible lined induction furnace to treat steel melts with argon gas. SCOT BOYD, NOVA PRECISION CASTING CORP., AUBURN, PA.
T
he state of the art with induction melting makes it a convenient and cost-effective way to
melt steel. A present limitation is the inability to use refining techniques to effect gas content reduction. If care- fully selected melt material is coupled with consistent and rapid melting, an acceptable steel can be produced as a “dead-melted” metal, without excess gas molecules such as nitrogen and oxygen that can cause defects. But with this practice, there is a gas con- tent increase during melting because of exposure to the atmosphere. Steel foundries have attempted to modify large-ladle porous plug technology to the induction furnace in hopes of using argon gas bubbling to reduce the gas content of the steel. Tis has been done successfully and has been reported with fur- nace sizes of 1,000 lbs. or greater. Notably, these furnaces used a rammed lining that lends itself to the installation of the nozzle in the bottom of the furnace. In our case, a 90% alumina
preformed crucible was used to line a nominal 200-lb. furnace. Te crucible is backup lined with a dry magnesium oxide lining to the induction coil.
Seeking a Best Practice As furnace sizes decrease, the
tendency is to use preformed and fired crucibles to line an induction furnace. Te challenge then becomes how to install a porous plug when these preformed crucibles are used. Te earliest attempts at Nova Preci- sion Casting, Auburn, Pa., were to use crucibles that had the plug pre- attached to the bottom of a crucible. Te concept was simple enough, but the execution was problematic. Handling the crucible with the gas line extending out of the bottom was difficult when it came to placing it in the hole made through the bottom of the furnace shell. Also, preformed crucibles are backup lined with a dry material between the crucible and the coil refractory lining. Some refractory
In comparison to other
plastic was needed around the gas line to prevent the loose dry material from running out the bottom of the furnace shell. Tis plastic refractory was placed in the hole when setting the crucible but, for assurance, more was placed from the bottom of the furnace shell after the lining was complete and the furnace could be tilted up. Six crucibles were acquired for
testing. When they were run, the first lining failed within the first 10 heats, the second had a run of approximately 60 heats and the third and fourth both failed around the plug with less than 10 heats. With this record, the last two crucibles were abandoned.
A Custom Crucible Nova then approached the refrac-
shielding and gas removal techniques, the porous plug has the advantage of direct treatment of the metal.
tory crucible suppliers about obtaining crucibles with holes cast in the bottom to allow the placement of a standard gas plug. Special crucibles with preformed holes were subsequently obtained. Te smallest porous plug available was 1.5 in. Working from this size, we decided to use a 3-in. diameter opening through the bottom of the crucible. Te immediate diffi-
culty became how to handle the height of the standard beryllium porous plug, plus
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