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Resurrecting chill wedges for ductile iron production: often forgotten as a means to measure ductile iron nucleation potential, foundries can turn to chill wedges as a simple tool to ensure melt quality.

To most iron foundries today, the value of chill wedges in cast iron production is in their ability to provide quick and inexpensive checks of the carbon equivalent in gray iron. Even with the adoption of spectrometers by many foundries for the chemical analysis of iron samples, small to medium-size gray iron foundries still rely on chill wedges for cost-efficient analysis.

By measuring the width of the chill on a solidified wedge sample, foundries can determine the relative carbon equivalent in the cast sample. If necessary, the width of the chill then can be adjusted by adding carbon and/or silicon to the melt to further specify the melt chemistry for the casting to be poured. By correlating the chill width to the section thickness of the casting being poured, a foundry can be assured of pouring carbide free gray iron.

Ductile Iran Problems Without Solutions

Iron foundries always have been cautioned against repeatedly melting returns without the addition of other materials to the charge such as iron and steel scrap, pig iron, graphite and/ or other ferrosilicon alloys. The resultant cast material from a melt without these additions would have little strength and contain both shrinkage and carbides, even though the chemical composition was still within acceptable range. This result is especially true in the electric melting process. While this phenomenon is not well understood, it is respected.

In the production of ductile iron, foundries experience another unexplaineable phenomenon termed "Monday Morning Iron." This is the term used to describe iron that has been held for too extended a period of time, resulting in a cast material similar to the one described in the previous paragraph. In addition to this, many ductile iron foundries have certain casting jobs that endure shrinkage defects that come and go, even though the chemistry certification remains the same from heat to heat and run to run. While the foundry often chases the iron pouring temperature and blames the iron pourer for these problems, the true reason for the defects never is determined.

When these situations arise, the ductile iron producer can and should resurrect the chill wedge. While the value of the chill wedge for carbon equivalent determination is well-known, its ability to measure the number of nucleation sites available in-cast iron has gone relatively unnoticed. This ability can warn ductile iron foundries against molten base iron that is not up the challenge of producing quality castings.

Chill Wedges for Ductile Iron

Since most ductile iron foundries have the equipment necessary to check metal chemistries, the chill wedge is not needed to check for carbon equivalent. Instead, the chill wedge is needed in ductile iron casting production to check for effective graphite nuclei in the base iron melt.

Some foundries believe it isn't critical to check for nucleation potential in the base iron because they use post inoculation, and post inoculations' only purpose is nucleation, However, if the base iron does not possess enough nucleation potential, then post inoculation will not have anything to enhance, resulting in low nodule count.

Ductile iron casters can use chills in various ways. They can be used to determine the base line for the length of time molten base metal can be held in the furnace at a given temperature without deterioration. Chill wedges also can be used routinely to insure that the base metal has not been superheated to an excessive temperature.

Chill Wedge Specifics

Chill wedges vary in size according to the width of the back. They may measure 0.25, 0.5, 0.75, 1 and 2 in. at the back, The angle at the sharp edge can vary from 11.5-34.5[degrees]. The chill wedge dimensions are specified in ASTM A 367.

Since carbon equivalent will affect the width of the chill (as measured across the width of the white zone of a broken chill), the size of the chill wedge must be determined by each individual foundry based on their casting mix.

Low silicon base irons may exhibit an all white chill unless an adequately large wedge test core is used. The width of the chilled iron should be at least 10% of the chill size. If it is less, a larger chill size should be used. If the width is greater that 50% of the chilled width, the chill will be inaccurate and a smaller size should be used. Once the chill size has been established, a baseline can be established.

During casting of the chill wedge sample, the wedge should be left in the sand core until it is a dull red and then knocked out and allowed to air cool until it is a dull orange and magnetic. It can then be cooled in water.

Experiences at Farrar

At Farrar Corp., a 140-employee green sand and nobake ductile iron and austempered ductile iron caster in Norwich, Kansas, the typical chill width for base iron is 0,125 in. using a 0.75 in. chill wedge. This is based on several observations to determine the effect of holding at an elevated temperature and at lower temperatures, as well as experiments with materials such as returns, pig iron and carbon to renucleate the metal.

The samples in Fig. 1 were taken with no changes in temperature and chemistry. It can be seen that as the temperature remains constant, the width of the chill increases as a function of time. Based on this knowledge, if metal is held in the furnace for a period of more than 15-20 mm at 2760F, it must be re-nucleated prior to tapping. If the temperature is lower, the loss of nucleation rate also is lower. At Farrar, metal has been held for 2.5 hr at 2550F and the width of the chill has only increased to 0.132 in.

In Fig. 2, metal was held for more than two hr at an elevated temperature while graphite was used to maintain the carbon level. As shown by samples C, D and E, the graphite helped to reduce the chill. However, the results are erratic and a small addition of pig iron or returns would help maintain good nucleation.

Farrar also has experimented with changes in charge make up and taken chill samples to determine if any affect in the chill wedge was evident. The charge make up was altered through a range of 14-21% pig iron, 25-38% scrap steel and 48-56% returns. The result: no affect in the depth of the chill was detected by changing any of the charge ratios.

Farrar also has used both pig iron and its own returns to bring back nucleation sites. Either option produces about the same results, as 0.71% of the metal is sufficient when reintroduced to the melt to bring back nucleation.

Time and temperature are the enemies of nucleation sites in ductile iron. Chill wedges can be an important tool to determine how much time and at what temperature metal can be held without losing the nucleation potential. Once the time line is established, then a procedure can be established to ensure that if the time is exceeded, a re-nucleation process for the metal will follow.

For More Information

"The Effect of Metallic Charge," James D. Mullins and Eugene C. Muatore, Ductile Iron Magazine, Issue 3 (1998).

"The Most Important Part of Ductile Iron Production-Inoculation," James D. Mullins, Sorelmetal publication #88 (1997).

"Chill Wedge Testing for the Control of Base Metal Quality," James D. Mullins, Sorelmetal publication #78 (1996).
Fig. 1

These chill wedge samples show that as the melt temperature remains
constant, the width of the chill increases as a function of time. As a
result, if metal is held in the furnace for a period of more than 15-20
min at 2760F, it must be re-nucleated prior to tapping.

Sample % Carbon % Silicon Temperature Minutes Chill
 (F) Held Width (in.)

 A 3.83 1.62 2760 1 0.120
 B 3.84 1.62 2760 15 0.169
 C 3.82 1.60 2760 25 0.221
 D 3.79 1.61 2760 40 0.383

Fig. 2

In these samples, the metal was held for more than two hr at an elevated
temperature while graphite was used to maintain the carbon level. As
shown by samples C, D and E, the graphite helped to reduce the chill.

Sample % Carbon % Silicon Temperature Minutes Chill
 (F) Held Width (in.)

A 3.82 1.60 2750 15 0.188
B 3.81 1.61 2750 55 0.125
C 3.80 1.60 2750 85 0.219
D 3.83 1.59 2750 115 0.250
E 3.82 1.60 2750 127 0.219


About the Author

Don Refiner has been the operations manager for Farrar Corp. foundry operations, Norwich, Kansas, for the last seven years. He has been in the foundry industry for 30 years.
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Author:Reimer, Don
Publication:Modern Casting
Date:Dec 1, 2002
Words:1471
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