Wire injection for ductile iron lends flexibility to Gunite Corp.

Wire injection nodularization and desulfurization of tundish ladles at Gunite allows gray and ductile iron production from one cupola.

Gunite Corp., a 300-employee foundry, casts various parts for the heavy truck and trailer market. The shop melts 19,000 tons per month of gray and ductile iron for its green sand molding line. Gunite once had a steel casting operation that used four 7-ton electric arc furnaces (EAF). In 1989, steel was abandoned in favor of ductile iron, and the steel truck spoke wheels the firm specialized in were converted to ductile iron. Today, 80% of Gunite's business is gray iron, and 20% is ductile.

While a cupola continued duty for the gray iron operation, the base ductile iron was melted in the four EAFs once used for steel. Ductile treatment was done in the bottom-pour steel ladles using the sandwich method.

Search for an Ideal Method

The EAF melt shop, however, carried substantial overhead, with 15 employees and one supervisor forced to work five night shifts to avoid high power demand costs incurred by EAF melting. Without an alternative, the EAFs were in operation from January 1990 to July 1993, and ductile iron was produced by the tundish treatment.

In March 1992, foundry officials initiated trials to produce ductile iron using cupola iron containing 0.12-0.15% sulfur (S). In the first tests, the cupola iron was desulfurized to 0.05% using 9% Magnesium Ferrosilicon (Mg FeSi) alloy in the pouting ladles. In-mold nodularization was used to produce the ductile iron castings. This method saw some success, but some inherent problems forced officials to abandon the idea.

In the search for alternatives, Gunite reviewed the lime-spar desulfurization process, and considered it acceptable. But it did not provide the flexibility the foundry needed to quickly switch from gray to ductile iron production and back.

In the meantime, trials of cored wire treatment done in small pouring ladles were proving successful, and testing moved to heats of 6500 lb held in tundish ladles. Glitches in the process were gradually worked out, and in the first week of August 1993, six full heats were treated and castings poured. The results were satisfying enough for officials to shut down the EAF melt shop, and begin exclusively producing cupola melted base ductile iron.

The firm's 6500-1b tundish ladles were modified to accommodate 7000 lb and the new treatment method [ILLUSTRATION FOR FIGURE 1 OMITTED]. The tundish ladle covers were replaced with newly fabricated covers that feature openings at their centers for wire injection. No off-center pocket was required. The refractory dam at the bottom of tundish ladles were removed, and a major modification was made to connect the spout area to the body of the ladle for more effective treatment. This was done by creating a window of 8 x 30 in. in the alumina castable refractory wall.

Wire Treatment in Operation

Gunite alternates operations on two 40-ton cupolas, which deliver metal to two 45-ton holding furnaces, which also alternate operation. Thus, at any given time, Gunite is only operating a single cupola and holding furnace arrangement, and operators vary the charge materials depending on the type of iron desired.

For a ductile iron heat, the charge materials consist of flame-cut steel plate, slitter scrap, pig iron, ductile iron returns, ductile iron boring briquettes and 65% silicon carbide briquettes. No tin bundles are added. It takes about 2 hr to hit the correct base chemistries for ductile production, arid operating parameters are adjusted to achieve the desired tap temperature.

As soon as manganese (Mn), phosphorus (p), chromium (Cr) and copper (Cu) are lowered to acceptable levels, the treatment ladles are brought in for the iron. By this time, the S level in the iron drops to about 0.06%, and will continue dropping to about 0.04%.

Prior to tapping from the holding furnace, a portable alloying station is used to add FeSi, graphite and a proprietary flux to the treatment ladle, which is carried by a truck. The ladle opening is covered with an insulating material and the iron is tapped in from the holding furnace. The truck is placed on the platform of the weighing scale to check the amount of iron before going on to the wire treatment station.

The truck then places the ladle inside the wire treatment station and the ladle cover is adjusted to receive the wire [ILLUSTRATION FOR FIGURE 2 OMITTED]. This treatment area is enclosed and ventilated by a dust collector attached to a preexisting system that had been used for an EAF. The wire injector handles two strands of wire, which can be fed through one strand at a time, and at three different speeds [ILLUSTRATION FOR FIGURE 3 OMITTED]. Gunite uses 13-mm diameter cored wire containing 25% Mg powder blended with 75% FeSi, without any rare earths, and it is currently fed into the ladle at a rate of 180 ft per min. When the treatment is finished, the burning end of [TABULAR DATA FOR TABLE 1 OMITTED] the wire is cut to the desired length for the next treatment.

The truck then carries the treated iron to the pouring area, performing "rock n' roll" on the way to mix the semi-treated iron in the ladle spout with the rest of the treated iron. When it arrives at Gunite's SPO molding line, the treated iron is transferred to two 3500-lb teapot pouring ladles, which have been specially marked for ductile iron operation only. During the transfer, stream inoculation is made with 50% FeSi. Ladle temperature readings and a test coupon are taken from the first ladle, and a separate test coupon is taken from the second ladle to determine nodularity.

The single SPO molding unit produces molds at about 200 per hr, with two-on and three-on castings. Cores are set in the drag halves and a 2-in. diameter postinoculant block of FeSi fines is inserted just below the downsprue. Each mold is stamped for traceability.

This entire process, from treatment to the end of the pour, should not exceed 16 min, and is strictly monitored by inspectors with stopwatches. Prior to repeating the cycle, the treatment ladle is rolled over to slag-off in a slag box. This slag is reused in the cupola charge to raise the S in the iron. Two hours prior to switching the operation to gray iron brake drum production, the cupola charge is adjusted for gray iron.

Table 2. Typical Mechanical Properties of Treated Castings

Properties                             Ferritic-Pearlitic Grade

Tensile Strength, psi                  69,000-78,000
Yield Strength at 0.2% Offset, psi     51,000-57,000
Elongation %                           10.0-13.0
Hardness, BHN                          156-179

Quality Castings

The chemical composition of the treated ductile iron heats is presented in Table 1. The Mg residual was aimed higher to meet the operating requirements of 16 min of pouring time, as well as to compensate for other production variables. Gunite's operating personnel have been trained to review and adjust process parameters to control chemistries.

Table 2 presents the typical mechanical properties of the ductile iron castings. Up to 20% elongation results were obtained, and castings requiring higher tensile strengths are produced at Gunite with the addition of Cu in the base iron.

As for the ductile iron microstructure, the nodule count improved and the nodules are relatively more spherical with wire injection as compared with the old process [ILLUSTRATION FOR FIGURE 4 OMITTED]. Also, ferrite and pearlite contents were distributed more uniformly with this method [ILLUSTRATION FOR FIGURE 5 OMITTED].

Casting machinability is satisfactory, and foundry scrap was cut to 4% as soon as the process was standardized.

Operational Results

Since the replacement of the EAF-tundish ductile operation in August 1993, Gunite has made more than 13,000 treatments, while gradually tuning up the process to improve productivity and to dramatically improve overall foundry productivity. No capital investment was initially required, as existing facilities and equipment were used.

Substantial savings were realized in ceasing its five night shifts of EAF operations for two extended day shifts of ductile iron operation, and the cupola operation was continued uninterrupted. The supply of base iron was substantially increased, while the process provided the flexibility to produce both gray and ductile iron castings, using a single cupola and molding line.

Additionally, the quality of the firm's ductile iron castings improved, with better matrix uniformity and nodule count and structure.