Bubbling Smaller Steel Melts
Nova Precision Casting installed a porous plug in a crucible lined induction furnace to treat steel with argon gas.
Scot Boyd, Nova Precision Casting Corp., Auburn, Pa., and Rick Boyd
(Click here to see the story as it appears in the August issue of Modern Casting.)
The 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 carefully 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 content 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. This has been done successfully and has been reported with furnace 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. The 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. The challenge then becomes how to install a porous plug when these preformed crucibles are used. The earliest attempts at Nova Precision Casting, Auburn, Pa., were to use crucibles that had the plug pre-attached to the bottom of a crucible. The 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 plastic was needed around the gas line to prevent the loose dry material from running out the bottom of the furnace shell. This 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 refractory 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. The 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.
The immediate difficulty became how to handle the height of the standard beryllium porous plug, plus refractory facing over it, in the normal distance from the furnace shell floor to the bottom of the inside of the crucible. To address this issue, a well in the refractory of the furnace shell floor was chipped out to a depth of approximately 1 in. and it was decided to use only 1 in. of cover refractory over the plug rather than the manufacturer’s recommendation of 2 in. This placed the crucible less than 1 in. higher in the furnace than was the normal practice.
The refractory used to finish placement and cover the plug is a special type specifically formulated with high gas permeability. Nova used Allied Mineral’s Dri‐Vibe 682A. The porous plugs are reused at the end of a lining campaign and only two were used for an entire year. With this approach, there was success with being able to bubble argon through the bottom of melts and accomplish it safely with little reduction in lining campaign life of the crucibles. The minimal thickness of only 1 in. of cover refractory proved sufficient for a 3-in. opening in the crucible and cover over a 1.5-in. nozzle. When relining, there never was evidence of spalling in the plug area with this practice.
To give more perspective on operating parameters, it must be noted that Nova Precision pours ceramic shells directly from the furnace and does not need superheat associated with ladle pouring. A very wide variety of metals are cast with normal tap temperatures ranging from 2,600 F to 2,900 F. The higher temperatures are the exception, and 3,000 F is never exceeded under any circumstances.
A typical campaign life before using porous plugs was 90 heats. When argon is bubbled, the campaign life of the lining drops to approximately 80 heats. The gas flow is started when the furnace is powered on and left on until the heat is poured. The flow rate does need to be varied by observation of the bubbles forming on the top of the bath. A typical flow is 7 ml per minute.
Benefits vs. Adverse Effects
The lower campaign life is probably associated with a reduction of alumina formation from deoxidation of carbon and low alloy steels with aluminum. It was normal practice to add small amounts of aluminum to these steels while melting in the charge. An addition was made when sparking from the metal bath was observed. When argon is bubbled, the practice of aluminum addition during melting is greatly reduced, and the final aluminum content had to be reduced to maintain minimal residual levels in the castings. Without the alumina formation, a more normal erosion of the side walls takes place. But, what was observed was not considered to be excessive or as a result of the argon gas flow.
The advantages of using argon injection are unclear. The lower need for aluminum deoxidation certainly indicates there is less oxygen being introduced to the metal. The quantification of this effect was not undertaken. The metal being poured with argon injection appeared to be more fluid. As a result, pouring temperatures for just about all alloys were reduced by 25 F without adverse effects.
The ability to reduce pouring temperatures may be the result of lower inclusion contents, which would be associated with the lower oxygen content. It also is clear that any bubble in the bath becomes a site to remove nitrogen and hydrogen contents of the metal. No gas analysis results are available for either hydrogen or nitrogen. Argon bubbling was done with duplex stainless steels without any noticeable reduction of final nitrogen contents.
In comparison to other shielding and gas removal techniques, the porous plug has the advantage of direct treatment of the metal. Argon drip technology or other furnace top introduction of argon addresses only the shielding aspects of melting. However, the porous plug does not yield enough argon cover on the top of the bath to prevent all atmosphere interaction, and the gas cloud is subject to cross drafts over the furnace. From past investigations, the argon drip provides the best top shielding by using a relatively massive amount of argon for covering. The porous plug can be used at a cost of only $0.025 per pound of metal melted with this small sized furnace vs. 10 times that when argon drip was used.
A side benefit of having a hole through the bottom of the furnace and crucible is associated with placing a ground fault detect wire. The normal manufacturer recommendation is to ram a wire mesh in the space between the crucible and the coil refractory lining. The wire mesh approach presents serious installation problems with properly placing it. This approach also does not allow the electrical contact of the liquid bath with the ground fault detect circuit until there is a failure of the crucible lining. A wire is brought through the hole in the bottom of the crucible from the contact point through the side of the crucible hole. It is rammed along with the refractory holding the porous plug. If there is no porous plug placed in a crucible-lined furnace, using a crucible with a hole to allow placement of a ground fault detect wire should be considered from a safety perspective.
This article was presented at the Steel Founders Society of America (SFSA) T&O Conference in December 2013.