Eliminating Fish-Eye Defects in Ductile Castings

AFS Transactions 02-047 Page 1 of 11

Eliminating Fish-Eye Defects in Ductile Castings

R.C. Aufderheide, R.E. ShowmanAshland Specialty Chemical Company, Dublin, Ohio

J. Close, E.J. ZinsDotson Company, Inc., Mankato, Minnesota

Copyright 2002 American Foundry Society

ABSTRACT

Trials were conducted at Dotson Company, Inc. to determine causes and develop cures for “fish-eye” surface defects onductile iron castings. The occurrence of the defects was historically linked to the use of exothermic riser sleeves. Trials wererun using sand that was contaminated with various exothermic sleeve materials and other materials. The result of these testsshowed that exothermic sleeves that were made with fluorine-containing compounds caused the defects. Subsequent trialswith fluorine-free sleeve formulations produced no defects. A fluorine-free exothermic sleeve was developed especially forductile iron that provided consistent feeding and eliminated the “fish-eye” defect. Dotson is now using the new sleeveformulation with continuing good results.

INTRODUCTION

Dotson Foundry was experiencing an ongoing and apparently unsolvable casting defect on the surface of their ductile ironcastings. Whenever they had a long production run of castings that used exothermic riser sleeves, the following shifts hadhigh levels of these surface defects that were internally called “fish-eyes”. The defects did not necessarily appear on thecastings that were made while using the exothermic sleeves, but rather they appeared on the castings that were produced later.These later castings were being made using sand that had been contaminated with the exothermic sleeve materials from theearlier production. The problems generally appeared after approximately one shift of high sleeve usage and would extend foran additional 2 to 4 shifts, depending on how many sleeves had been used.

The defects appear on the surface of the casting as circular surface depressions with jagged outer edges and raised internalsurfaces. Figure 1 and Figure 2 show the defects at casting shakeout and after shot-blasting the castings respectively. Thedefects tended to appear on the cope surfaces, but occasionally appear elsewhere. The defects always seemed to appear onexterior green sand surfaces rather than interior cored surfaces. It was also noted that only certain castings had high levels ofthese fish-eye defects with the castings’ overall size and section thickness being a common factor. When examined beforeshot-blasting, the defect often contained a white, crystalline residue around the circumference of the depression, Figure 3.This residue appeared to be some type of oxide/slag. Metallographic examination of the defect showed the oxide/slag layeron the surface of the defect with a layer of flake graphite appearing below the surface, Figure 4. This layer was thought toindicate some type of reaction with the oxide/slag.

Dotson found that the occurrence of the fish-eye defects was related to the number of exothermic sleeves used in priorproduction runs. They found that they could limit the number of defects by reducing the number of exothermic sleeves usedor by scheduling to run only gray iron for the next day following heavy sleeve use. However, this created internal schedulingproblems. They could also produce the castings without exothermic sleeves, but this caused shrinkage defects and reducedcasting yield.

Dotson had heard various theories about the cause of their fish-eye defect, including hearsay reports that they were caused bythe build-up of the fluorine in the sand system from the use of exothermic sleeves. Fluorine salts like cryolite (Na3AlF6) are acommon ingredient in exothermic sleeves and act as a flux and help to initiate the exothermic reaction. Literature on thedefect was scant and the few references found on "fish-eyes"1,2 were for obviously different types of problems. Because ofthe lack of real understanding about the mechanism of defect formation and the means for prevention, Dotson and Ashlandundertook a joint project to study and eliminate the fish-eye problem.

AFS Library Copy: 20020616A.pdf, Page 1 of 11 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.Copyright © 2003 American Foundry Society.


INITIAL THEORIESDotson’s historical data showed a clear correlation between exothermic sleeve use and the subsequent occurrence of fish-eyedefects, however, no one could say exactly how the defect formed or what factors played a part. Initial discussions developedtwo competing theories. The first theory was that fluorine salts from the reacted / burned exothermic sleeves built-up in thegreen sand following heavy sleeve use. It was thought that these salts might interfere with the bonding action of the clay inthe green sand, resulting in a defect similar to a scab. In this theory, a portion of the sand mold wall might break free fromthe surface as the sand expanded, creating the round “fish-eye” depression in the casting surface. This theory was supportedby experimental data. Measurements of fluorine levels in the green sand seemed to follow defect occurrence. Followingheavy exothermic sleeve use, the fluorine levels in the green sand would increase from a baseline of about 100 ppm (parts permillion) to nearly 200 ppm and then decrease back to the baseline over the course of two to three days. The highest fluorinelevels occurred while the fish-eye defects were at the highest levels.

The second theory blamed unreacted / unburned pieces of the exothermic sleeve material contaminating the green sandsystem. It was thought that if these sleeve pieces were in the green sand next to the casting cavity, they might react and burnwhen exposed to heat from the liquid metal. This exothermic reaction could then create a gas or blow type defect from thecombustion products of the reaction. It was also suggested that a piece of sleeve might keep an area of the casting hotter thansurrounding areas, resulting in a localized surface shrink. This theory of unreacted / unburned sleeve residue in the greensand also had some factual basis. Several molds were knocked out by hand and some unreacted exothermic material wasfound on the exterior surface of the sleeves at the sleeve/mold interface. The exothermic reaction had apparently notpropagated through the entire sleeve, likely because of cooling effects from the surrounding green sand. Small pieces ofsleeve material were also visible in the green sand system.

TESTING THE THEORIESA plan was developed to test the competing theories. We would try to “turn the defect on” by using intentionallycontaminated facing sand on one surface of the green sand mold. A casting known to be prone to the defect would be usedthroughout these tests. To test the effects of high fluorine levels, cryolite by itself, with no other sleeve material, was addeddirectly to the facing sand. A 0.1% addition was selected to produce fluorine levels around 500 ppm, which is well in excessof levels seen during the height of the fish-eye problems. To test the sleeve particle contamination theory, pieces of crushednew sleeves were blended into the facing sand. It was decided to compare both the fiber-based exothermic sleeves that hadhistorically been used in the foundry and new technology sleeves made from a low-density alumina-silicate fiber-freerefractory and cold box technology3. The sleeves were crushed and screened though a ½” screen similar to that used in thefoundry’s sand system. The sleeve pieces were then added to the facing sand at 2% by-weight. This level was calculated tobe much higher than that seen under normal foundry conditions and to simulate worst case conditions. A series of controlmolds with no facing sand contamination would be run with the contaminated molds.

The contaminated facing sand was prepared in a "mullbarrow" in 250# batches. Prepared green sand was taken from thefoundry’s sand system and the weighed contaminants were added and mulled for about 2 minutes. Samples of the mixed sandwere collected for sand testing and for later analysis. Seven molds of each set were made on a Hunter 20 molding machine. Eachmold was sequentially numbered. The contaminated facing, Figure 5, was manually packed on the cope surface of the patternwith system sand used as backing. All other molding process parameters were standard. The molds were poured along with thefoundry’s normal production.

The test castings were caught at shakeout and placed in tubs. Inspection of the castings before blasting revealed that the controlcastings and the castings with the cryolite contamination in the facing sand were all free of defects. Both sets of castings madewith crushed sleeves in the facing sand contained multiple "fisheye" defects, both on the actual castings and also on the surface ofthe gating systems. These results are listed in Table 1.



Test No. Contamination Results1 0.1% Cryolite No defects2 2% Exothermic Fiber-based Sleeves Multiple defects3 2% Exothermic New Technology Sleeves Multiple defects4 Control – no contamination No defects

Table 1. Results of Contaminating the Facing Sand

These initial results indicated that it wasn’t the fluorine in isolation that was causing the fish-eye defects, it was the presenceof the fluorine-bearing exothermic sleeve contamination. This finding then caused us to focus our attention on the secondtheory, which was that the defect was being caused by unreacted exothermic material contamination; i.e., unburned sleeves.The theory was that the exothermic material in the sleeve was not being totally reacted. This could be the result of either thepoor propagation of the exothermic reaction from the sleeve/metal interface to the sleeve/mold interface, or sleeves whichwere not exposed to molten metal because molds had been poured short or had not been poured for one reason or another.The poor propagation could be compounded by the smothering or chilling effect of the water in the green sand that is againstthe sleeve at the sleeve/mold interface.

To determine if this was true, another series of molds were produced with the facing sand contaminated with exothermicsleeves that were both burned and unburned. Contamination materials were prepared by crushing new sleeves and sleevesthat had been thoroughly burned. The burning was accomplished by inverting insertable style sleeves on a bed of sand andpouring molten ductile iron into the sleeve, and allowing them to burn to completion. The access to abundant atmosphericoxygen and the absence of any mold material against the outside wall of the sleeve, that might otherwise hinder or preventthe complete propagation of the exotherm, assured completion of the exothermic reaction and the dead-burned condition.Both fiber and New Technology sleeves, both burned and unburned, were crushed and screened through a ½” mesh screen.Just as in the first test, these contaminants were then selectively blended into the facing sand at a 2% level.

In this test, all of the castings that came in contact with the contaminated facing sand experienced fish-eye defects. Thecontrol molds poured with no contamination in the facing sand were defect-free. These results are listed in Table 2 below.

Test No. Contamination Results

5 2% Unburned Fiber-Based Sleeves Multiple Fish-eye Defects6 2% Burned Fiber-Based Sleeves Multiple Fish-eye Defects7 2% Unburned New Technology Sleeves Multiple Fish-eye Defects8 2% Burned New Technology Sleeves Multiple Fish-eye Defects9 Control – None No Defects

Table 2. Results of Contaminating Facing Sand with Burned and Unburned Exothermic Sleeve Materials

Unfortunately these tests did not shed additional light on the mechanism of defect formation or control methods. However,the question of the effects of fluorines still remained.

DEVELOPING AND TESTING ZERO-FLUORINE SLEEVES

To further test the fluorine theory, it was decided to develop a fluorine-free exothermic sleeve formulation specifically forductile iron. However, this proved to be considerably more difficult than first thought. The cryolite in the exothermicsleeves reduces the time and the amount of energy needed to start the exothermic reaction. This has a significant effect onsleeve performance, particularly in ductile iron. Initial tests in the lab on exothermic sleeves with the cryolite simply left outshowed poor casting performance and shrinkage in critical applications. The sleeves simply did not feed as well as those thatcontained cryolite.

The basic exothermic material in sleeves is thermite. Thermite consists of granular or powdered metallic aluminum and ironoxide. These react as shown:

2Al + Fe203 Al203 + 2Fe + heat



The cryolite is thought to release fluorine gas which reacts with and cleans the surface of the aluminum to help initiate theexothermic reaction4. Other oxidizers and reactive metals are sometimes also added to promote the reaction, but none havequite the same effects as the fluorine salts.

Different thermite mixes and other ingredients were tested to try to reproduce the ignition and combustion characteristicsprovided by cryolite, but without the fluorines. Eventually, a zero-fluorine formulation was developed that had excellentignition and feeding characteristics. This new exothermic sleeve was designated EXF ZF (Exothermic, Fast, Zero-Fluorine).

During the laboratory evaluation of the new sleeve materials, an unexpected phenomenon was noted that might help toexplain the mechanism of fish-eye formation. Highly exothermic sleeves that contained fluorines produced significantamounts of flake graphite within the riser on ductile iron castings. This flake graphite was apparently caused by aluminumcontamination in the ductile iron from the aluminum in the exothermic sleeve. Aluminum levels as high as 0.3% were seenin the riser metal5 and aluminum levels above 0.1% have been documented to produce flake graphite in ductile iron 6.However, little or no aluminum contamination and no flake graphite were seen with the new EXF ZF formulations. Thealuminum contamination in the risers may have been related to the very reactive AlF gas that is one of the early reactionproducts of the exothermic reaction when fluorines are present4. The same reactive gas might be responsible for the formationof the fish-eye defect. This might explain why fluorines alone would not produce the defect but that fluorines and aluminumin the exothermic sleeves would.

Based on the good performance results in the laboratory, trials were conducted under production conditions at DotsonCompany, Inc. First, the new technology EXF ZF sleeves were tested on individual production castings to insure that thefeeding characteristics would produce a shrink-free casting. Several of the castings and risers were then sectioned to look forany evidence of internal shrinkage. The feeding results proved to be as good as or better than with the standard fiber-basedexothermic sleeve that contained fluorine.

Next, a production run of 230 castings was made using the new technology EXF ZF riser sleeves. The use of this manyexothermic sleeves would typically produce large numbers of fish-eye defects on sensitive jobs the next day. However, nofish-eye defects occurred. Finally, on the day following this production run, more contamination tests were run bycontaminating the facing sand even further with crushed EXF ZF at 2% and 4% of the mix. Mix #1 (castings were markedwith a number “1”) was 40 lbs. of sand contaminated with 0.8 lbs. of crushed EXF sleeve material. Mix #2 (castings were markedwith a number “2”) was 40 lbs. of green sand contaminated with 1.6 lbs. of crushed EXF sleeve material. These mixes werespread on the cope surface of a yoke casting pattern. Five molds were made with each of the 2% and 4% contaminated sandmixes. The casting chosen was a yoke casting typical of the castings that are prone to fish-eye defects. Figure 6 and Figure7show the 2% and 4% contaminated facing sand mixes respectively. Figure 8 shows the contaminated facing sand placed on thespline section of the yoke casting. Note that the spline section of the yoke casting is the heaviest section of the casting. Fivemolds were poured per ladle and both ladles were at 25690F for the first mold of each series. Each mold contained two castingswith a total pour weight of 87 lbs. Figure 9 shows the drag half of the mold with the cores set in place. Figure 10 shows thepouring of the castings.

ZERO-FLUORINE SLEEVE CASTING RESULTS

At shakeout the castings looked fine. There were no signs of any fish-eye defects. The only concern were a few spots whereyou could see a piece of something stuck to the casting surface. The material appeared to be a piece of the crushed EXF ZFsleeve that had been used to contaminate the facing sand. Figure 11 shows a close-up of the raised lump on the castingbefore blasting.

After blasting and closer examination, the area of concern showed a protruding lump of metal on the surface of the casting.Figure 12 shows a close-up of the casting surface after blasting. The lump of metal on the surface appeared to be formed bya lump of material (possibly a EXF ZF crushed sleeve particle) that kept the green sand from packing against the patternsurface, thus leaving a cavity that the metal filled. This area was cut from the casting and examined only to find no defectsunder the lump of metal. Figure 13 and Figure 14 show the test castings before and after blasting respectively. No fish-eyedefects were found on any of the castings made with the intentionally contaminated sand.

CONCLUSIONSThe testing at Dotson Company confirmed that the fish-eye surface defect in ductile iron is caused by contamination offluorine-containing exothermic sleeve material in the molding sand. Fluorine salts alone do not create the defect, butparticles of either burned or unburned fluorine-bearing sleeve material will. Testing with new technology zero-fluorineexothermic sleeves showed no fish-eye defects. Based on these good test results, Dotson has begun converting from the



traditional fiber-based exothermic sleeves made with fluorines to the new technology zero-fluorine sleeves. As of thiswriting, they continue to experience good casting results and no fish-eyes.

Since the work at Dotson began, stories about similar defects at other ductile iron foundries have surfaced. Based on thework at Dotson, they now have options that were previously unavailable. The new technology zero-fluorine exothermicsleeve may be the best alternative. Improved screening of the system sand to remove sleeve contamination and monitoring offluorine levels in the sand, as an indicator of sleeve contamination, may also be helpful.

FUTURE WORK

While these trials show that the fish-eye defects can be eliminated with the use of zero-fluorine exothermic riser sleeves, themechanism for defect formation is not well understood. Testing showed that fluorine was not the sole cause of the defect.Fish-eye defects occurred only when both fluorine and aluminum were together as constituents of the exothermic sleeve.Other data, including the presence of flake graphite within the defect cavity, suggest that the formation of AlF gas may beinvolved, however, no direct evidence was found. Other factors may also be involved including casting geometry and sectionsize, pouring temperature, metal chemistry, etc.

Future tests will examine the relationship of the fluorine containing exothermic particles and their size, in combination withtheir distances from the casting surface to the formation of fish-eye defects. The function that the presence of fluorine playswill also be examined. Perhaps the fluorine affects the surface tension of the metal allowing the gas pressures in theimmediate area to deform the metal. Further analysis of the microstructure under and adjacent to the surface of the fish-eyedefect might also provide additional clues to its formation.

ACKNOWLEDGEMENTS

The authors wish to acknowledge Ben Carr, Mark Hysell, and Chris Lute at Ashland ‘s Foundry Application’s Lab for theirhard work and ideas in analyzing the defect and developing the new sleeve formulation.

REFERENCES

1. “Graphitization of Pure Fe-C alloy During Annealing”, D.N. Khudokormov, V.M. Kordev, “Russian CastingProduction”, Dec. 1967, p. 589-590.

2. “Internal High-Temperature Micro-Cracks in Unalloyed Cast Steel”, G. Wold, T. Kristoffersen, T. Harvig, paper 18,1971 International Foundry Congress.

3. “New Developments in Riser Sleeve Technology”, R.C. Aufderheide, R.E. Showman, H. Twardowska, AFSTransactions 98-77 (1998), pp. 395 – 400.

4. “Observation of Reactions During the Combustion of Exothermic Materials by Differential Thermal Analysis”, C.Pelhan, N. Majcen, paper to 1970 International Foundry Congress.

5. “Exothermic Riser Sleeves Can Cause Flake Graphite in Ductile Iron”, R. Showman, R. Aufderheide, C. Lute, paper 01-086, AFS Casting Congress, Dallas, 2001.

6. Foundrymen’s Guide to Ductile Iron Microstructures, American Foundrymen’s Society, Inc., 1984



Figure 1 – Fish-eye defects on casting surfaces before blasting.

Figure 2 – Fish-eye defect on surface of casting

• Fairly round• Recessed area around the perimeter• Raised area on the inside



Figure 3 – Residue in fish-eye defect, 6x

Figure 4 – Defect cross-section, 100x



Figure 5 – Contaminated facing sand

Figure 6 – Facing sand contaminated with 2% EXF ZF sleeves.

Figure 7 – Facing sand contaminated with 4% EXF ZF sleeves.



Figure 8 – Contaminated facing sand on yoke pattern.

Figure 9 – Drag half of yoke mold with cores.

Figure 10 – Pouring the yoke test castings



Figure 11 – Casting made with EXF ZF contaminatedfacing sand containing raised area, before shot-blast.

Figure 12 – Casting made with EXF ZF contaminated

facing sand containing raised area, after shot-blast.

Figure 13 – Castings made with EXF ZF contaminatedfacing sand, before cleaning



Figure 14 – Castings made with EXF ZF contaminatedfacing sand, after cleaning


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Eliminating Fish-Eye Defects in Ductile Castings