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Furnace design, research & material control head sessions.

Controlling the change of solid metal into a liquid (the melting process) lies at the heart of successful metalcasting. The theory and practice of this important process is to assure that melted metal completely fills the cavity of a mold. Numerous expert technical presentations explored the various avenues of the process, and the large audiences at the sessions attested to the strong interest in the art of melting.

As the metal demands of customers change, so do the required types of foundry melting equipment. Waupaca Foundry, adding ductile iron several years ago to its normal gray iron production, was able to retain all of its major equipment including vertical channel melting, automatic molding machines and pressure pour autopours. It found, however, that the inductor life of its autopours was short (several weeks) and expensive. Foundry officials determined that the autopours needed to be modified to allow them to pour treated ductile iron without the prolonged downtime of the required maintenance.

A joint project by the foundry and Duca Remanufacturing, Inc. to design a channel induction autopour furnace was described by W. Powell, Waupaca Foundry, Inc. He explained that base ductile iron was melted in a 30-ton channel induction furnace refitted with a teapot spout to minimize slag entering the treatment vessel. A round channel loop replaced the square loop to minimize channel buildup, and the metal was converted to treated ductile iron in a tundish treatment ladle. The treated iron is transported from the melters to the autopours by monorail. Each autopour feeds a flaskless molding machine.

Buildup from magnesium in the treated ductile iron reacted with oxygen and other metal oxides to form a high melting point oxide compound that accumulated on the refractory lining. The accumulation displaced metal, decreased the capacity of the autopour and closed the inductorthroat, causing temperatures to rise until the working temperature of the refractory was exceeded and failed.

The solution was to minimize or replace the formation of magnesium and calcium oxides by limiting oxygen in the vessel. This was accomplished by changing the atmosphere in the furnace from oxygen to nitrogen, which also increased the fade time by limiting atmospheric oxygen access to the furnace. Short pulsing power every six minutes increases stirring action in the bath to flush metal oxides.

Pour control repeatability problems were solved by pressure pouring molten metal into a launder equipped with a stopper rod. Routinely, some accumulated metal oxides were removed from the vessel through a top inspection hatch and every three months the top cover is removed for a more thorough removal of oxides from the lining. Rebuilt channel induction autopours, properly updated, are successful holding and pouring vessels for treated ductile iron.

According to G. Tosi, Politecnico di Milano, Italy, the use of rotary furnaces is growing rapidly in Europe and is projected to become a valuable melting tool for medium and small foundries in the U.S. In his evaluation of rotary furnaces in the production of cast iron, he noted that the furnace's benefits include simple, automated operation, reduced maintenance compared to current standard melting furnaces and economical, flexible production characteristics. The fuel source is oxygen and either methane or propane gas. Currently, furnaces in operation range from 1-25 tons.

The furnace operates around a horizontal axis and consists of a cylinder fitted with truncated cones at each end. The diameter and length vary as a function of required melting productivity. One cone houses the oxy-gas burner and the other contains the charging door. The furnace can rotate intermittently or continuously around its axis, and is inclined forward and backward for charging, slagging and refractory maintenance. The lining is usually 90% silica and 5-7% alumina. It can be charged on the horizontal axis using vibrating channels emptying into the furnace or tilted at a 45-degree angle to accommodate skips and hoppers.

The possibility of producing different types of cast iron makes the furnace suitable for limited batch foundries and its low dust emission levels add significantly to operational economy.

The melting costs for the unit are comparable to a cupola and are said to be 30-40% less expensive to operate than an induction furnace. Tosi explained that the quality of the metal product obtained is good and in certain cases, such as in the production of ductile iron, the low sulfur content of the bath due to the absence of pollutants from the combustibles is an advantage.

In his presentation on the dealuminization of cast iron, B. Tiwari, General Motors Research Lab., noted that the presence of Al in cast iron can cause serious gas hole defects in castings. The presence of Al is due to its introduction to the ferrous scrap stream or the use of large amounts of ferroalloy. It easily combines with oxygen, chlorine and nitrogen to form |Al.sub.2~|O.sub.3~, AlN and Al|Cl.sub.3~, all relatively insoluble and less dense than the molten iron. Thus, chemical dealuminization is an attractive approach to reaching a target aluminum composition of 0.01% where Al no longer produces gas defects.

R. Perkul, ABB Metallurgy, Inc., said the main advantage of the electric arc furnace (EAF) is that it will melt any kind of scrap efficiently and effectively in contrast to an induction furnace, which requires scrap to be closely monitored. He noted that the single electrode direct current EAFs are making some inroads on the use of alternating current units but that the 50% cost premium is still a limiting factor despite the advantages the furnace has over the AC EAF. He noted that electricity consumption can be reduced by half, the stirring effect is greater and they require less maintenance.

Perkul said the hollow electrode can be used to charge the furnace. He also noted that the carbon electrode in the DC EAF acts as the cathode in contrast to the AC EAF, in which the three electrodes are the anodes. New thyristors now allow the use of larger rectifiers and he described the "skin effect" of the single cathode as contributing to a stirring action as opposed to the splashing effect caused by the three carbon arcs of the AC units.

K. Sueker, Halmar Robicon Group, and J. Svoboda, Process Metallurgy International, Inc., said the single arc DC EAF, when used as a ladle furnace, produces uniform heating and eliminates "hot spots." Argon is fed through the electrode to reduce formation of impurities and to heat the slag cover.

As the foundry's chief raw material, monitoring of scrap charge material is key to foundry melting operations and economics, according to panel experts P. Simon, Paul Simon Co.; A. Jensen, Miller Compression Co.; P. Meyst, Iron Casting Research Institute; and J. Ward, Navistar International.

Noting that ISRI scrap guidelines are excessively broad, the panelists pointed out that scrap control specifications should detail physical characteristics, chemistry, inspection procedures and delivery requirements. Written procedures should be part of the scrap procurement process and should specify size, thickness, coating materials (paint, galvanizing, tin, terne), lead contamination, sealed scrap (shock absorbers, crimped pipe), residual components like cement-filled pipe and alloy content. Radiation inspection also should be part of an inspection plan that carefully documents scrap shipments, and communication between the user and the supplier should be encouraged.
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Title Annotation:CastExpo '93: 97th AFS Casting Congress, Chicago
Publication:Modern Casting
Date:Jun 1, 1993
Previous Article:Sand treatment dominant topic.
Next Article:Good decisions at shop floor & office determine success.

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