Cost-effective iron inoculation; four foundries' perspectives.

Detailing the experiences of four leading iron foundries, this article examines cost-effective approaches to the post-inoculation of iron.

The inoculation of iron, according to the Ninth Edition of the Metals Handbook (published by ASM International), is the late addition of certain silicon alloys to molten iron to produce changes in graphite distribution, improvements in mechanical properties and a reduction of the chilling speed. Inoculation can occur while the molten metal is in the furnace or while it is in transit to the mold. If it occurs in transit, it is known as post-inoculation.

To post-inoculate iron, foundries choose from three points on the melt line to complete the task--in the ladle, in-stream or in the mold. With each post-inoculation point, a different chemistry is achieved within the melt.

Foundries determine their method of post-inoculation by analyzing their procedures, technology and final desired melt chemistry. But, for the most part, all plants desire the same result from inoculation--molten iron with increased nodularity and ferrite percentage, and decreased chill.

Due to individual differences in terms of furnaces, ladles and molding lines, each plant must develop its own practice to suit its needs. Following are the post-inoculation experiences of four foundries as presented at the 1997 AFS Casting Congress panel, "Cost Effective Post-Inoculation in Iron."

BRILLION IRON WORKS

Brillion Iron Works, Inc., a high-volume coreless induction melt shop in Brillion, Wisconsin, double inoculates its gray and ductile iron. The initial inoculation is achieved in the ladle during transfer into the final pouring vessel, while the second stage is completed via in-stream, pellet or downsprue inoculation.

The total mass additions of inoculant vary from 0.1-0.5% (with most jobs receiving 0.25-0.40%), and the typical ratio of ladle to late inoculation is 60/40-80/20.

Ladle Inoculation

Ductile iron is inoculated with foundry 75% calcium (Ca) bearing ferrosilicon (FeSi) and several types of specialty ductile iron inoculants with varied levels of Ca, aluminum (Al), barium (Ba) and rare earths. Its addition rates are at 0.2% for pearlitic grades, 0.3% for ferritic grades and 0.4% for thin sections. For gray iron, the inoculant is 50-63% FeSi containing strontium (Sr) or Ba. It is added at a rate of 0.1-0.4%, with 0.2% typical.

Once the iron is inoculated, Brillion's process controls follow with a 100% microscope check on ductile iron for nodularity and inoculation fade and an hourly check on gray iron with an ASTM W2 chill wedge. With gray iron, if the check is above standard, an additional 0.1% ladle inoculant is added until chill values are reduced to normal.

A maximum of 0.4% inoculant at 3/8-in. in size in the ladle is allowed, because additions above this percentage are rarely cost effective due to dissolution problems, ladle accumulation, filter plugging, dross defects and risk of over-inoculation induced shrink in the first iron out of the ladle. In addition, the foundry uses a minimum of 0.1% inoculant in gray iron when pour time exceeds 5 min.

Late Inoculation

Brillion uses three different late inoculation methods--in-stream, pellets and downsprue.

In-stream--It's the method of choice due to the low cost on units where dispensers can be mounted. This method is used on the foundry's Disa and BMD lines, but not on the manual-pour units. The foundry built its own in-stream units (Fig. 1) for the BMD lines.

In-stream units meter inoculant either by flow rate (grams/sec) or fixed mass (grams/mold). Both methods are calibrated to a target weight % addition of 0.05-0.2%, with 0.1% typical. Neither method is perfect as flow mass units add different mass depending on pouring time, and fixed mass units must be set to dispense in the shortest pour time expected on a given unit. But both methods work in practice.

The choice of in-stream inoculant material is also important. Some shops report success with resized conventional FeSi materials. Brillion prefers to use a specialty in-stream inoculant sized 0.2-0.7mm and containing high levels of manganese (Mn) and zirconium (Zr). The particle size and shape are less abrasive to dispenser components than a resized 75% FeSi, and the chemistry is optimized for rapid dissolution at lower temperatures. The 30% Mn content both lowers the dissolution temperature of the inoculant and promotes distribution and transport of trace inoculating elements due to the high solubility of Mn in iron. In addition, there is lower wear on equipment and nearly zero incidence of undissolved material defects in castings.

Pellets--Brillion uses 75% FeSi pellets, which it sets in prints in the pouring basin on manual pour units that are not suited to in-stream. All ductile and some chill-sensitive gray iron jobs also are pellet inoculated. Solid cast pellets produce less dross, fewer inclusion defects and better graphite enhancement per gram than pressed, sintered or glued granular inserts. The basic principle is that surface oxide to volume is lowest for a cast insert.

Downsprue--Where cost or physical placement of pellets is a problem, a granular material addition of 0.1% in-stream inoculant may be used.

Both the ladle and late inoculation are necessary due to base iron fade. Without an initial ladle addition, the chill tendency of the base iron at extended pour times may be so great that most or all of the late inoculant Would be consumed just to overcome chill, leaving none for graphite enhancement during solidification. If a large enough late addition was made to overcome base iron chill tendency and still promote graphite in 15-min.-old iron, it would cost more than the combination approach and run the risk of over-inoculating the first molds poured.

Design of Production Inoculation Practices

The bottom line for Brillion in post-inoculation is making better and more consistent castings using expensive materials and equipment at an equivalent cost to making average quality castings using higher masses of cheap materials. Its process is as follows:

1. Determine the minimum ladle addition needed to provide acceptable chill and graphite at the average pour time after inoculation or 2/3 of the maximum allowed pour time, with no late inoculation.

2. Determine the minimum late addition level needed to provide acceptable chill and graphite at the average pour time or 2/3 of the maximum allowed pour time, with no ladle inoculation.

3. Verify that the total mass of both inoculations is not high enough to cause over-inoculation of the first poured iron. If it is, reduce ladle inoculant and shorten the allowed pouring time.

This simple-sounding approach establishes the lowest total cost inoculation practice for Brillion's applications. When both inoculation steps are working, the result is production casting microstructure, properties and machinability that are better and more consistent than required. This is clearly not the cheapest approach, but is cost-effective in the context of the foundry's total equation of costs, risks and quality benefits.

WAUPACA FOUNDRY

Waupaca Foundry, Inc.'s, plant #4, a vertical channel induction melter in Marinette, Wisconsin, post-inoculates its ductile iron in various stages at transfer points and on the pouring line. The plant starts out with a charge made up of 50% returns, 25% steel and 25% pig iron, and then uses a standard 5% MgFeSi alloy with a small amount of cover steel. Once the iron is tapped, it proceeds through three stages of inoculation.

Ladle Inoculation--Waupaca's treatment ladle also is its transfer ladle. As the iron emerges from the launder, a wire is injected into the molten iron. This wire contains inoculant grade FeSi and is added at a rate of 0.1% per lb of iron.

In-stream Inoculation--Next, the iron (just prior to the iron entering the mold) undergoes in-stream inoculation. A 20 x 100-mesh inoculant grade FeSi is used.

In-mold Inoculation--The final step, which takes place on most of the castings it produces, is the addition of inoculant on the filter core in the gating system of the Disa molds. This is considered the biggest inoculation bang for the buck.

For these three methods, the foundry uses different types of inoculant--granular additions, wire addition, in-stream addition, in-sprue addition, filter sandwich, formed pellets and sized chunks--that vary in size and weight from 0.5-1.5 in. and 20-40 grams.

Inoculation Procedure

Waupaca's inoculation focus is on the in-mold placement. Figure 2 illustrates the foundry's typical placement of the FeSi chunk. The angular configuration allows for a nice fit on top of the filter core. Because filters are used in most job applications, extra gating work is not required.

[Figure 2 ILLUSTRATION OMITTED]

Approximately 67% of the foundry's work uses some type of a core, which allows the filter and the chunk to be set into the core setter and automatically placed into the mold. This is the most efficient means of placing the needed items in the mold without extra time or labor.

For those castings that do not have cores, the chunk must be set by hand with the filter. For the cored and uncored molds, the placement of the filter and the inoculant can be completed without interrupting the machine cycle or using extra manpower.

The chunk has more surface area to start to dissolve at initial contact with the molten iron, and, when compared to loose material, it does not wash away. The result is a full inoculation effect of the material.

Success Stories

The chunk inoculation method has provided many benefits. Following are three examples of this method in use:

* While pouring a mold, the speed can decrease, forcing the foundry to stretch out to its maximum holding time. When this happens, a fade can occur in percent nodularity, with a nodule count around 100 nod per sq mm. But since the foundry was using a chunk on the filter, a marked improvement in nodularity and nodule count occurred with a typical microstructure of 97% nodularity and 200 nod per [mm.sup.2].

* By using the chunk, there is a slight increase in ferrite in the foundry's ferrite grade irons. With its base iron chemistry containing about 0.28% Mn and 0.14% Copper (Cu), the typical matrix in a 65,000 grade disc brake contains 15-20% pearlite. With the addition of the inoculant chunk, several ferritic castings have been converted to 5% pearlite--a much appreciated side benefit to its customers.

* The reduction and elimination of carbides or chill is a large success. Some of Waupaca's complex casting shapes have thin sections (0.12 in.) attached to larger areas. These act as radiator fins, cooling and chilling the iron rapidly, even at a fast pour rate of more than 10 lb/sec. This area of the castings is highly pearlitic and contains 5-7% carbides. But with the addition of the chunk inoculation, the foundry eliminated the carbides and reduced the amount of pearlite present.

GREDE-ST. CLOUD

Grede Foundries, Inc., St. Cloud, Minnesota, installed a new melt shop in 1995 and revamped its inoculation procedures. The ductile iron foundry, which melts in a coreless induction furnace and holds in a channel furnace, makes up its charge from ductile iron returns, pig iron and steel, and treats it with silicon carbide and graphite. The base iron is treated with a 5-ton pure magnesium converter mounted on a forklift, and the ductile iron is preconditioned with SiC to improve nodularity.

For St. Cloud, the magnesium converter alters the standard inoculation practice, as the foundry needs to add the rare earth elements normally added with the Mg Alloy. The rare earth elements combat the tramp elements (like lead) in the melt that degenerate graphite, in addition to stabilizing the iron from carbides and preventing a metamorphosis to white iron.

Normally with the addition of master alloys to a melt, rare earth is present in the pour ladle. But for St. Cloud, the converter takes the place of the master alloys, and the post-inoculation becomes the source of rare earth.

Grede-St. Cloud's post-inoculation practices, for the most part, have been developed in accordance with the Grede-Reedsburg, Wisconsin plant. The foundry's inoculation includes ladle inoculation and in-stream inoculation with the addition of rare earth.

Ladle Inoculation--The converter is duplexed into two manual pouring ladles. The iron is tapped into the ladles and 75% FeSi 3/8 x 12-in. mesh is added to increase the final silicon count.

In-stream Inoculation--As the iron is poured, a 0.2% per lb iron poured, 75% FeSi inoculant with 2% rare earths is added into the pouring stream. A hopper for the inoculant is mounted on the ladle with a limit switch to start when the ladle tilts. The inoculant is gravity fed into the stream as it enters the mold. The cooling lines are set up to give extended in-mold cooling of up to 6 hr to aid the heavy sections.

As a backup system to the in-stream pouring to insure the inoculation of its iron, St. Cloud uses a solid mold insert of 75% FeSi in 75% of its BMD-made molds. For castings with thin sections, the foundry always uses the in-mold inoculant.

St. Cloud's inoculation practices are determined by the rare earth elements added to the melt. The composition of this rare earth addition has undergone several changes while in experimentation at the Reedsburg facility, before being adapted at St. Cloud.

In early 1994, Reedsburg was experimenting with Mischmetal (a rare earth compound) and high cerium (Ce) as major components of its rare earth additions. The foundry, analyzing nodularity, nodule count, size and alignment, ran eight different trials with additions of Ce at 0.96-3.04% and total rare earth at 3.046% to 75% FeSi in 20 x 80 mesh.

The results of the trial defined the two foundries' rare earth inoculation efforts. Neither the high nor low additions of the rare earths affected nodularity adversely, but high Ce additions to the total rare earth resulted in higher nodule count. Reedsburg determined the Ce, at any level tested, to be the key element of total rare earth in achieving desired results.

Other trials were run with additions of bismuth (Bi) and the-Mischmetal to the total rare earth additions. Results show that both increase nodule count, but when the Bi reaches too high a level, the nodules explode.

Currently the foundry uses a 1.8-2.1% addition of total rare earth to its inoculants. No one element dominates this compound, but the foundry continues to test varying degrees of Ce, Bi and Mischmetal.

RELATED ARTICLE: Assessing Post-Inoculation at Briggs & Stratton

Briggs & Stratton Corp., a cupola melt ductile iron facility in Milwaukee, relies on traditional technology with its post inoculation practices. The foundry performs a ladle inoculation and then an in-stream air-injected inoculation at its Disa molding line.

Ladle Inoculation--The foundry inoculates with 75%, FeSi at 0.3% by volume into a bottom pour ladle.

In-stream Inoculation--The in-stream inoculation is accomplished by the air-injection of foundry 75% FeSi inoculant powder into the pour stream to the mold. For a typical 10 sec pour at the foundry, the inoculant powder is added at a rate of 0.5-1.5 grams/sec.

The pouring system the foundry uses on its Disa line is automated with the inoculation system. When the nozzle for the pouring system is opened, the injection machine for the powder is activated. The in-stream portion of the foundry's inoculation is a minor percentage of the entire practice.

Although Briggs & Stratton relies on older methods to perform its post-inoculation, it has researched the implementation of fully automated inoculation systems into its foundry. Tables 1-3 illustrate the foundry's cost analysis of three different methods of post-inoculation.

Table 1. Cost of Manual Addition Post-Inoculation

                      Detail

Direct Materials      75% FeSi
Indirect Materials    Addition cups
                      Protective equipment

Direct Labor          One man per shift
Maintenance           None
Capital Equipment     None

                      Cost ($)                      Annual Cost ($)

Direct Materials      4.88 per ton iron                530,888
Indirect Materials     125 per line per year               375
                      1500 per operator per year          3000
Direct Labor           240 per shift                   116,160
Maintenance
Capital Equipment
                           Total annual cost           650,423
                           Cost per ton poured            5973
Table 2. Cost of Automated Tube Post-Inoculation

                      Detail

Direct Materials      75% FeSi
Indirect Materials    None
Direct Labor          None
Indirect Labor        Setting controls by part #
                      Filling hoppers every shift
Maintenance           One hour daily maintenance
                      Monthly repairs-labor
                      Parts and materials
Capital Equipment     Automated feeder/hopper
                      Computer/PLC controls
Depreciation          Depreciation expense of hopper
                      Depreciation of computer/PLC

                      Cost                          Annual Cost ($)

Direct Materials       4.55 per ton iron poured        495,495
Indirect Materials
Direct Labor
Indirect Labor            10 per day                      2420
                          10 per shift per line         14,520
Maintenance               30 per day                      7260
                         250 per month                    3000
                         100 per month                    1200
Capital Equipment     90,000 initial cost
                        8000 initial cost
Depreciation             750 per month                    9000
                          67                               800

                             Total annual cost         533,695
                             Cost per ton poured          4901
Table 3. Cost of Automated Wire Feed Post-Inoculation

                      Detail

Direct Materials      Hollow core wire w/ 75% FeSi
Indirect Materials    None
Direct Labor          None
Indirect Labor        Setting controls by part #
                      Replacing spools
Maintenance           Daily maintenance feeders
                      Monthly repairs
                      Parts and materials
Capital Equipment     Automated wire feed system
Depreciation          Depreciation of wire feed system

                      Cost ($)                      Annual Cost ($)

Direct Materials       4.80 per ton iron poured        522,720
Indirect Materials
Direct Labor
Indirect Labor           10 per day                       2420
                         10 per line                      7260
Maintenance              20 per line per day            14,520
                         60 per month per line            2160
                         50 per month                      600
Capital Equipment     10,000 per line installation
Depreciation             83 per month                     1000

                            Total annual cost          550,680
                            Cost per ton poured           5057

Assumption with Tables 1-3: 2 shifts pouring 18 hr/day; on three pouring lines; 900,000 lb poured per day; 242 production days per year; $30/hr total labor rate; S/L depreciation-10 yr life; 75% FeSi costs 40.065/lb; and 7% material savings with automated system