refractory facing over it, in the nor- mal distance from the furnace shell fl oor to the bottom of the inside of the crucible. To address this issue, a well in the refractory of the fur- nace shell fl oor 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. T is placed the crucible less than 1 in. higher in the furnace than was the normal practice. T e refractory used to fi nish place- ment and cover the plug is a special type specifi cally formulated with high gas permeability. Nova used Allied Mineral’s Dri‐Vibe 682A. T e 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. T e minimal thickness of only 1 in. of cover refractory proved suffi cient 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.
Production Details To give more perspective on operat-
ing 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. T e 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. T e gas fl ow is started when the furnace is powered on and left on until the heat is poured. T e fl ow rate does need to be varied by observation of the bubbles forming on the top of the bath. A typical fl ow is 7 ml per minute.
Benefi ts vs. Adverse Effects T e lower campaign life is prob-
ably 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 fi nal 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 con- sidered to be excessive or as a result of the argon gas fl ow. T e advantages of using argon
injection are unclear. T e lower need for aluminum deoxidation certainly indicates there is less oxygen being introduced to the metal. T e quantifi ca- tion of this eff ect was not undertaken.
CUSTOM ALUMINA CRUCIBLE FOR FURNACE RELINE
The crucible with a hole in the bottom is shown before its placement in the furnace. 38 | MODERN CASTING August 2014
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78