POROUS PLUG WITH A GAS CONNECTION & GROUND FAULT
This inside view of the furnace shows the porous plug in position, ready for the reline with an alumina crucible.
The bottom view of the furnace shows the gas and ground fault connections.
Te 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. Te 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 avail- able 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 shield- ing 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. How- ever, the porous plug does not yield enough argon cover on the top of the bath to prevent all atmosphere interac- tion, and the gas cloud is subject to cross drafts over the furnace. From past investigations, the argon drip pro- vides the best top shielding by using a relatively massive amount of argon for covering. Te 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 plac- ing a ground fault detect wire. Te normal manufacturer recommenda- tion is to ram a wire mesh in the space between the crucible and the
coil refractory lining. Te wire mesh approach presents serious installation problems with properly placing it. Tis 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 cru- cible 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.
Tis article was presented at the Steel Founders Society of America (SFSA) T&O Conference in December 2013.
August 2014 MODERN CASTING | 39
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