Steps to quality ductile iron: one foundry's procedures.

A retired foundry vice president of technology shares his recipe for producing maximum toughness ferritic ductile iron.

In producing ductile iron in coreless induction furnaces, charge component selection and other production considerations must be treated with extreme care to ensure that the chemistry and resultant physical and mechanical properties of the finished casting meet all customer specifications.

Cost-cutting measures or inadvertent veering from basic production rules in some foundries can result in the suffering of ductile iron casting quality. While virtually every foundry has found its own approach to producing ductile iron, following are the procedures determined years ago at Wagner Castings (Decatur, Illinois) as the best way to produce high-quality ferritic ductile iron castings with maximum toughness.

CHARGE COMPONENTS AND PURPOSE

Steel Scrap

A. Provides matrix density and tight fracture, and also nucleates austenite.

B. Allows sufficient addition of crystalline graphite to nucleate free carbon, reduce chill and maintain nucleation for a longer time.

C. Should be a minimum of 40% of the charge.

D. Should be clean (no rust) and of thin sections to dissolve faster.

E. Composition should be low-manganese (Mn), low alloys, low-phosphorus (P), and low oxygen activity. Purity shouldn't be so high that it doesn't contain trace elements such as vanadium (V), molybdenum (Mo), titanium (Ti), chromium (Cr), antimony (Sb), etc., which provide strength to the ferrite. It should be dry and free of oil.

F. The source should be known and purchased to a specification. The source must know what specification it was originally purchased to. It must be free of tin or zinc coatings.

G. Clean, thin and short pieces can be used for cover steel on top of the magnesium ferrosilicon (MgFeSi) treatment alloy.

H. Must be stored under cover and kept free of contamination. It can be pre-heated to 800F (427C) to assure dryness.

I. The addition to the melt should be made after the pig iron and sprue melt, and the bath is brought to 2650F (1454C). Add 50% of the steel charge and bring the temperature of the bath to 2650F.

J. Carbon (crushed electrodes) is added when enough steel has been added to make the bath carbon-hungry, and the temperature is at 2650F. Charge the other 50% of the steel charge on top of the carbon when it is added to the bath and bring the temperature to 2750F (1510C).

Pig Iron

The best form is Sorelmetal (F-1).

A. Provides a method to dilute and reduce Mn and alloys in the base iron to an acceptable level.

B. Provides a source of carbon that dissolves easily.

C. General composition must be low P 0.02% max, low sulfur (S) 0.015%, only a trace of Mn and low Si. Ti must be less than 0.04%.

D. The amount used should only be enough to dilute Mn to an acceptable level. (Normally, only a maximum of 20% of the charge.) About 40% of the charge can be used for the wash heat.

E. It should be clean and relatively free from excessive rust, holes, shrinks and blisters.

Sprue (Gates and Risers)

A. Provides low melting point for initial part of the charge. It should be clean and free from oxidation and burned-in sand.

B. A good charge composition is 50% sprue, 40% scrap steel, an amount of pig iron sufficient to lower Mn, silicon carbide (SiC) to deoxidize, and crushed carbon electrodes for carbon and nucleation of graphite.

C. Separate any alloyed pearlitic sprue [including copper (Cu)] from ferritic sprue to provide better control.

D. Sprue should be clean of adhering sand clumps. A coating of adhering sand can be beneficial to combine and remove iron oxide. Long and tangled sprue should be broken up to increase charge density.

E. A wash heat of ductile iron can be used to make a supply of sprue for a high-density charge material that is easy to melt clean and dissolve the steel scrap. Sorelmetal pig iron can be used in place of sprue only for the wash heat. Calculate the additional Si and Mn that is needed because the Sorelmetal isn't the same chemistry as the sprue.

Silicon Carbide Grain

A. Removes oxidation products of melt materials such as iron oxide (FeO).

B. Use the small size (100 mesh) grain because it dissolves rather than melts.

C. The amount added is 2-4 lb for every 1000 lb of charge. It is added with the last 50% of the scrap steel charge.

D. Include the chemistry of the SiC in the charge calculation.

E. Put only in the base charge of the furnace; don't put in any of the subsequent ladles because undissolved particles are abrasive.

F. Can be used for silicon addition, but wait 30 min before tap.

Graphite

A. Adjusts base carbon content and nucleates graphite (reduces chill).

B. Graphite must be at least 20% of the total carbon content of the charge.

C. Must be crystalline such as crushed electrodes.

D. Must be clean, dry and 1/8 in. x 8 mesh in size.

E. Add after sprue and pig iron are melted and after the molten bath is brought to 2750F and 50% of the steel charge is added. When the first 50% of the steel added is melted and the iron temperature reaches 2750F, graphite is added to the molten iron and the other 50% of the steel charge is added on top of the graphite. The SiC can be added with the steel charge.

F. After the furnace temperature reaches 2750F, take a dip sample of the iron and pour into a spectrometer sample mold, pour a chill sample in a core sand mold and pour a cooling curve sample. Allow the chill sample to cool in the core for 1 min, then shake it out and air-cool for 1 min. Water cool, break, measure and record.

The wedge chill is described in ASTM A367 as Wedge No. 2. It is 1-1/2 in. high, 0.4 in. wide at the base and 4 in. long. The top apex has a slight radius to make the iron flow smoothly. Measure the width of white chill that penetrates from the apex. The chill test indicates the tendency for the iron to form carbides or shrink. The base iron Si must be less than 1.4%.

The spectrometer sample is 1-1/4 in. diameter and 1/4 in. thick. It is poured in a copper book mold for fast chilling of the sample. The sample must be fully chilled.

G. When the base iron in the furnace is correct in chemical analysis and the chill width is acceptable, the furnace is ready to tap.

Hold the iron at 2740F (1504c) until tap time. The furnace must be covered to reduce oxidation. During the hold period before tapping, place a chunk of clean scrap electrode (about 4 in.) on top of the molten iron and let it float until ready to tap. Remove the chunk of electrode just before tapping the furnace.

H. Before tapping the furnace after an extended hold period, take a dip sample and pour a wedge chill to indicate base iron nucleation, and a cooling curve sample to indicate solidification modes. If they show a change from normal, renucleate the furnace bath with a small chunk of sprue and bring the temperature up to required level.

Furnace Chemical Analysis

Using an average of three spectrometer burns, following are guidelines for producing as-cast ferritic ductile iron with maximum toughness.

A. Carbon          3.75-3.85% (calibrate
                   on chilled sample)
B. Silicon         1.25-1.35%
C. Manganese       0.20 max
D. Sulfur          0.01-0.015
E. Phosphorus      0.02 max
F. Chromium        0.04 max
G. Copper          0.10 max
H. Molybdenum      0.04 max
I. Vanadium        0.02 max
J. Titanium        0.04 max

TREATMENT LADLE COMPONENTS AND PURPOSE

Treatment Ladle Construction

A. Keep covered whether full or empty. This prevents heat loss and oxidation.

B. The height should be 1-1/2 times the diameter. There should be a 1/4 in. insulating layer placed between the lining and the steel shell to prevent heat loss. The lining consists of 80% alumina and shouldn't be more than 3 in. thick.

C. The bottom of the treatment ladle should be either:

1. Step bottom with a pocket large enough to hold the treatment alloy, preinoculant, any alloy needed and cover steel. The stream from the furnace is directed to the top of the step. This requires maintenance of the step so that the iron stream can't get under the alloy sandwich, cause it to pop up to the surface too soon and result in low recovery.

2. Cone bottom with the pocket being cone-shaped and in the center of the bottom. This makes it easier to place the alloy, but the shelf must be maintained to keep the iron from getting under the alloy.

3. The tundish cover provides better Mg recovery and much less heat loss. When tapping from a coreless furnace, the cover should allow easy removal to aid in keeping the reaction ring cleaned off the wall of the ladle. When properly designed and used, the tundish cover results in little fume and flash from the treatment reaction. The bottom contains a dam in the center that separates the alloy pocket from the tapping stream until the level of iron in the tundish basin is full and provides a choke to keep air out, fume in and allows the iron to flow over the top of the alloy sandwich. There are two holes in the cover; one which allows treatment alloy addition in one side of the dam. This hole is plugged prior to tapping into the other hole, which becomes closed by the iron choke.

Treatment Alloys

A. All additions are weighed and placed in the ladle pocket.

B. MgFeSi.

1. Magnesium 5-6%

2. Silicon 44-45%

3. Calcium 1.2-1.4% or 1.8-2.0%

4. Aluminum 0.8-1.0%

5. Barium 1-2.5% (produces more uniform distribution of high nodule count in thin and heavy sections, 1/4 in. to 2 in. sections.)

6. Cerium 0.3-0.5%

7. Size 3/8 in. x 8 mesh

8. Addition 0.09-0.12% Mg

C. Inoculating 75% FeSi content (provides preinoculation).

1. Calcium 1.10-1.2%

2. Aluminum 0.8-1.0%

3. Addition 2 lb per 1000 lb treated. Place on top of Mg alloy.

4. Cu and Mo as required for pearlitic.

5. Don't place in treatment ladle pocket until 15-20 sec prior to tap. This is done because the alloy may fuse and stick to the bottom of the ladle and cause poor recovery.

D. Cover Steel

1. Holds alloy sandwich down until 25% of the amount of the iron to be treated is on top of the alloy. This should occur within 5-7 sec. It may take 15-20 lb of steel.

2. Use clean, thin-section steel. It melts fast.

3. Place on top of treatment alloy and 75% FeSi.

4. At the end of the tap, skim off top of ladle and place a clean slag collector and allow it to form a pancake. Remove slag collector prior to transferring to the pouring ladle.

5. The tap must be completed within 55 sec.

Treatment Ladle Maintenance

A. Maintain a cover to prevent excessive heat loss when full or empty.

B. Use a high alumina (80%) lining to withstand scraping and cleaning.

C. Keep reaction products cleaned off after each treatment.

D. Never allow iron to remain in bottom, always empty it out.

E. Keep lid clean and wide for a good mixing transfer stream.

F. Keep shelf or cone lip full size and sharp.

G. Keep alloy pocket clean, free from buildup and large enough to hold alloy and cover steel.

H. Keep tapping scale calibrated.

I. Ladle can be hung from a monorail crane or mounted on a frame for high-lift transport.

Treatment Ladle Size

A. Quantity treated should fill ladle so it can be skimmed off after treatment reaction stops, but still have enough freeboard to prevent excessive splash during the tap.

B. Reaction buildup ring should be high enough to be chipped clean after each treatment. If the ring is too low, cleaning may be neglected.

C. If small treatment batches are used, be careful to clean off ring. The reaction buildup ring will remove Mg from the treated iron and when it becomes large in size, can cause flake graphite to form.

D. At the end of the treatment, set a timer for 10 min. Mold pouring must be completed within 6 min from inoculation, leaving 4 min to pig the iron.

POURING LADLE COMPONENTS AND PURPOSE

Pouring ladle Construction

A. Must be covered when full or empty.

B. Crucible-type ladles promote better mixing and have lower surface area for heat loss or oxidation.

C. Can use a straight-side ladle or teapot spout.

D. Lining should be 2 in. thick. It is best to have insulation between the lining and shell to reduce heat loss.

E. Lining must be at least 60% alumina. Silica lining reacts with MgO and accelerates Mg fade rate.

Pouring Ladle Inoculation

A. Alloys

1. Must be weighed.

2. 75% FeSi

a. must be inoculating grade. b. calcium 1.10-1.2%. c. aluminum 0.8-1.0%. d. size 1/4 in. x 8 mesh. e. clean and free from fines; fresh. f. amount: equal to 0.50% Si addition. g. add with a dipper to the base of stream from the treatment ladle after 3 in. depth occurs in the bottom of the ladle. Add a smooth addition rapidly, then increase the transfer stream from the treatment ladle to promote good mixing. Any unmixed FeSi should be stirred with a bar.

3. A small addition of Cu can be added to the pouring ladle. This must be carefully weighed.

B. Procedure

Typical Diluent Metal Compositions (In Percent)

Element      Los Phosphorus Pig      Sorel F-1       1008 Steel

Silicon             0.95               0.15             0.08
Carbon             4.0-4.3             4.25          0.06-0.08
Sulfur              0.025              0.013            0.02
Phosphorus          0.03               0.013            0.02
Manganese         0.03-0.15            0.005          0.3-0.45
Aluminum            0.010              0.003            0.03
Chromium            0.005              0.029         0.02-0.04
Copper              0.04               0.019         0.02-0.05
Nickel            0.04-0.08            0.07             0.09
Vanadium            0.02               0.019            0.02
Titanium            0.02               0.008            0.02
Antimony            0.001         [less than]0.001       -
Arsenic             0.002         [less than]0.001       -
Boron               0.000         [less than]0.001       -
Lead                0.001         [less than]0.001      0.000
Molybdenum          0.001                -              0.001
Tin                 0.000              0.001            0.005
Zinc                0.001         [less than]0.001      0.001
Zirconium           0.002              0.002            0.005

1. When the pouring ladle is full, skim off and cover.

2. Pour castings, keel block, dynamic tear specimens.

3. Pouring rate must be 15-17 lb per sec. In some cases, it may be 11 lb per sec. Faster iron flow reduces the amount of subsurface dross stringers.

4. With nearly the last iron in the pouring ladle, pour a nodularity check sample, but don't use the last dregs of iron.

5. Finish pouring within 6 min from inoculation.

6. Stop pouring when the iron temperature reaches 2450F (1343C).

7. If a teapot pouring ladle is used, start by pouring off a 20-lb pig to flush the teapot spout of any excess FeSi that may have floated up into it or any iron that flowed into it without inoculant. Excessive FeSi can cause high Si content in the iron resulting in low toughness. Insufficient inoculation can cause low nodule count and poor nodule shape.

C. Finish

1. Pour any remaining iron in the pouring ladle into a pig mold. Don't leave any iron in the ladle.

2. Empty out and scrape off the ladle lining.

3. Place a cover on the ladle.

4. It is now ready to pick up another heat of iron.

5. Nodule count should be 150-275 for the 1/2-in. section.