Lessons Learned During Medium-Frequency Coreless Induction Melting.

This article examines production lessons learned at John Deere and Auburn Foundry and provides guidelines for charge make-ups to ensure efficient melt practices.

Foundries often learn best from their own experiences and those of other plants. Whether in coremaking, molding or melting, the lessons learned can be applied immediately in production to increase efficiency, quality and the bottom line.

This article looks at induction furnace charging sequence case histories from John Deere Foundry, Waterloo, Iowa, and Auburn Foundry (Plant 2), Inc., Auburn, Indiana. In both cases, current practices were re-examined to determine effectiveness and ultimately altered to increase melt quality and consistency and improve refractory life. The lessons learned by these foundries can be applicable across the industry for medium-frequency induction melters.

John Deere

John Deere's Waterloo plant uses three 16.5MW 20-ton medium-frequency coreless induction furnaces with continuous charge feed systems for melting. The foundry melts both gray (class 30 and 35) and ductile (65-45-12, 80-55-06 and 100-70-03) iron and produces agricultural and construction components from 150-2200 lb. The foundry is capable of melting 60 tons/hr. Figure 1 shows the initial gray iron charge sequence for the melting furnaces along with the respective melting temperatures for the materials.

Deere's problem began when the foundry was re-starting the furnace after a sinter cycle and relining with a boric-oxide silica refractory. It experienced a black foamy slag (carbon) in the melt that boiled up and bridged the furnace. After five heats, the ground detector shut the furnace down.

The foundry was forced to shut down melting and investigate what had happened. Looking from the top of the furnace, it was evident that the silica refractory taper section had undergone a chemical attack or severe superheating. The refractory had undergone severe erosion and the silica dry vibratable hot face had melted. The diagram in Fig. 2 shows the area of the furnace attacked.

Figure 3 shows how the taper was completely melted away. In fact, the red arrow (and the inset photo) is pointing to a ridge in the silica refractory hot face that had "stalagmites" growing up off of the ridge. These silica stalagmites confirmed that the molten metal was superheated to 3100F (1704C) to create liquid refractory. Also, the smooth surface and flow character of the refractory illustrating a ridge are evidence that a bridging condition existed in the melt.

Upon further investigation, the foundry found a localized penetrated zone of the taper section showing black carbon deposition against the safety brick. This black discolored penetration actually had reached beyond the brick to the taper grout on the coil. Through testing, it was confirmed to be carbon.

The foundry also took samples of the melted silica hot face immediately adjacent to the superheated back-up silica refractory in the taper section of the lining. In a cross-section view, white material called crystobolite was observed behind the melted hot face, which also is evidence of a superheat condition.

After the examination, the bridging condition in the initial meltdown was traced to the following key factors:

* excessive power;

* the type of charge used;

* timing and method of the alloy addition;

* the charge sequence.

The foundry had steel with a melting temperature of 2700F (1482C) at the bottom of the furnace, followed by pig iron that melted at 2000F (1093C). Then returns were added that melted at 2150F (1177C) and were followed again by steel. The foundry had created a condition for the pig iron to become molten first and pull the highest amount of power density into it. The result was a tremendous amount of stirring in a small amount of iron, which results in high abuse to the furnace refractory. To further aggravate the situation, the entire carbon raiser addition also was sequenced into this same charge zone. This ultimately provided the conditions necessary to create carbon boil and subsequent bridging condition in the bottom of the furnace.

To rectify these problems, Deere reviewed its process on the cold start and sinter start-ups and made the following changes to its melting process:

* the charge sequence was modified by adding returns before the pig iron. This resulted in a much more even melting of all charge materials;

* alloy additions were removed from the front of the charge and worked in after 50% of the charge was in the furnace for a more even melt distribution.

* the power input during the melt-in on the cold start and sinter start-ups and its normal melting process was reviewed and reduced during the initial melting of the charge. Once the charge begins to drop, the power is allowed to be increased up to the limits of the charge capacitance.

Auburn Foundry

Auburn Foundry Plant 2 melts Class 30 gray iron in three 10-ton coreless induction furnaces with 7000 kW medium frequency power supplies. The charge for this furnace is delivered by three dedicated 10-ton charge cars, and the furnaces are lined with silica boron oxide refractory. The foundry batch-melts 10-ton heats and duplexes (with each furnace poured completely empty) via a movable launder system into a 100-ton channel furnace holder with a 1500 kW inductor. Casting is done on one of 14 vertically parted green sand molding lines.

To reduce raw material charge costs, Auburn began adding customer-supplied borings to their charge in August 1999. While these borings undergo a centrifuge process before arrival to eliminate excess oil, they still are coated with cutting fluid when delivered.

The initial amount of borings introduced into the furnace represented 7.5% (1500 lb) of the total charge of 10 tons (eventually this was increased to 15% oily borings.) For Auburn, the makeup and positioning of the charge materials as they entered the furnace was critical. Depending upon the charge mix, the point at which everything is introduced into the furnace was critical to lining life.

Into each magnet-charged charge car, the foundry returns were placed in the back, the steel scrap in front of the returns and the borings on top of the steel in the front. With this order, the borings were introduced into the bottom of the furnace with steel scrap. Despite the foundry's initial beliefs, the melt did not present an oil burn-off or flame problem, and there was no increase in slag. Auburn did run into a problem when the borings were introduced at the middle or rear of the charge car. The result was premature lining failure occurred in the induction furnace in the form of elephant's foot wear in the taper section.

With the introduction of borings, the foundry realized an initial cost reduction of $87.80/charge. However, the supply of borings was limited and the charge amount was reduced to 1000 lb. As the study progressed and the supply of borings ran out, Auburn purchased ductile iron borings that met material specification from a scrap dealer.

Beyond the reduced raw material charge cost, Auburn also realized three unexpected benefits related to the introduction of borings that reduced operating costs.

1. Lining life started to increase. Without the borings, the average yearly throughput per lining was 4300 tons (430 batch heats). With the use of borings, the lining life increased to an average of 6200 tons (620 batch heats). The longest lasting furnace lining was an 8240 ton throughput (Fig. 4).

2. The total kilowatt hour per batch melted reduced from 510-550 without borings to 470-475 with borings.

3. The time required to melt each batch was reduced by up to 5 mm.

The second and third benefits amounted up to an additional 50 tons per day of melt at the old power consumption level.

Auburn believes that the oily borings play a role in increasing the lining life of its furnaces. Although further study is required, when the borings ignite at the bottom of the furnace at the beginning of the melt, the oxygen in the furnace is depleted and creates a non-oxidizing environment similar to melting in nitrogen gas. This environment reduces the corrosive nature of the molten metal toward the refractory.

This article was adapted from a panel presentation at the 2001 AFS Casting Congress.

Examining Charge Sequences for Coreless Induction Furnaces

A comparison of three theoretical charge sequences was performed for the melting of gray iron. This comparison is based on a charge comprised of 50% sprue, 40% steel, 10% basic pig iron, 30 lb/ton silicon carbide (SiC) and 25 lb/ton petroleum coke. The sprue is estimated at 30 lb/ton of Si[O.sub.2], and the steel at 20 lb/ton of FeO.

In sequence one, the steel is added first. After 50% of the steel is added, the petroleum coke and SiC are added and then followed with the rest of the steel. This is followed by the pig iron and then the sprue.

In sequence two, pig iron is added first. This is followed by the steel and then the sprue. The petroleum coke and SiC are added after 25% of the total charge is added to the furnace.

In sequence three, the sprue is first, followed by the petroleum coke and SiC. The steel is added next and then the pig iron.

The conclusions made based on each melt sequence were:

Sequence One--Adding the steel first to the furnace:

* raises the average temperature the lining experiences throughout melting, thus resulting in faster lining wear;

* creates a high FeO/MnO, low Si[O.sub.2] slag early in the heat. The result is a more basic slag, which promotes faster wear of the silica refractory lining.

Sequence Two--Adding the pig iron first to the furnace:

* lowers the average temperature of the melt when compared to sequence one;

* creates a high carbon (C)/low Si pool in the bottom of the furnace. As a result, the carbon begins to infiltrate the steel scrap above it, lowering the melting temperature of the steel scrap to closer to iron and reducing overall melt time;

* creates a slightly lower FeO/MnO slag early in the heat as compared to sequence one, but still has lower Si[O.sub.2] overall.

Sequence Three-Adding the sprue first to the furnace:

* offers the lowest average temperature during melting of the three sequences;

* creates a balanced high C and Si iron early in the heat to benefit both melting time and slag composition;

* produces a slag high in silica from the beginning to maintain an acid slag throughout the heat.

Based on these conclusions, sequence 3 provides the most efficient and refractory-benefiting environment for melting gray iron.

[Graph omitted]