HOT TOPICS Issue # 12, 2005

Transcription

1 HOT TOPICS Issue # 12, 2005 Casting Defect: Doughnut defect 1. Defect code: B Name: Doughnut or Fisheye defect 3. Description: An annular depression in the casting in the shape of a half a doughnut. Slight deviations from this shape is also seen. This defect appears on green sand surface, never on chemically bonded core surfaces. When the defect shows up, it will often disappear as quickly as it appeared. 4. Pictures are at the end of this document. 5. Suggested mechanisms of defect formation (exact mechanism is still not understood) a) Green sand is contaminated with material that has high oxygen content. When this contaminant is mixed with moisture, and carbon from sea coal in the sand, it forms an explosive mixture. As the iron level rises above this point where contaminant is located, the heat from the iron sets off the explosion. The explosion sends off shock waves from the sand into the molten metal. If the metal is just close to the freezing temperature, the surface of the metal quickly freezes in the shape of an annular ring depressed into the metal. The explosion leaves a highly oxidizing material as residue in this depression. This excess oxygen affects the graphite shape in the vicinity. b) Contaminants mixed with the sand system (moisture carbon etc) produces localized high gas pressure, enough to cause a small blow. Under the right circumstances the depression occurs on the casting surface just as the metal is solidifying. The raised interior portion in the center of the gas depression develops as the eutectic expansion of the solidifying metal pushes metal up. This phenomenon is similar to the formation of a bb of metal that occurs inside a gas blow only this time the gas blow is only a surface depression and the bb of metal pushes up to the mold surface. c) In the case where the green sand that is contaminated with exothermic riser sleeves that contain fluorine, the fluorine may also reduce the surface tension of the metal providing a localized condition that lets lower gas pressure push the metal down forming the small surface depression. d) Clay balls have also been known to for this defect. Again, their presence can create a higher than normal localized gas pressure. (If clay balls are the main cause for this defect, the defect will be much more prevalent in the foundries than reported) e) The defect occurs only if the contamination responsible is mixed with green sand and present near the surface of the casting. Regulated experiments have shown that the defect is not created from contaminants placed directly on the casting surface. 6. All possible causes ruling out processes not contributing This defect is not affected by the variables in the following processes Gating Molding Process Melt and Pouring Cores Al Alagarsamy, Citation Corporation and Ron Aufderheide, Ashland Casting Solutions

2 Contaminant Contaminant exploding after exposed to heat Liquid metal Figure 1. Schematic view of defect formation on green sand. Close-up view The processes that affect this defect Design of casting One theory says that the fish-eye defects more likely to occur when castings sections are about 3/8 and up. Thin sections may freeze quickly before it is affected by the reactions with the contaminants. Sand Green sand Contaminants in the green sand system are the main cause of this defect. Certain exothermic sleeves un-reacted and reacted sleeve remnants mixed with moisture and carbon in the green sand. Exothermic sleeves that do not contain fluorine do not produce the defects. Also insulating sleeves do not produce the defect since they do not contain fluorine. There are sleeves with fluorine used in some foundries seemingly without this defect. Contaminants in the new sand trucks back hauling chemicals such as fertilizers, lime etc. (nitrates?) Some relate this defect to clay balls in the sand system, but that does not explain the rarity of this defect, since clay balls almost universally present in many foundry sand systems. 7. Process variables that should be controlled to avoid the defect Use sleeve material that is compatible with green sand and will not produce this defect Use adequate amounts of total new sand materials (core and molding sand) to dilute contaminants Ensure trucks that carry sand and bond do not carry chemicals Ensure effective screening of return sand. Continuous screening of a portion (10% throughput) of return sand through a fine mesh (8 mesh) screen will reduce contaminants in the system 8. Remedy Once the contaminants are in the sand system, the only way to eliminate this defect is to quickly dilute the system with new non contaminated materials. 9. Additional reading: Eliminating Fish-Eye Defects in Ductile Iron castings R.C. Aufderheide, R.E. Showman, Ashland Specialty Chemical Company and J. Close and E.J. Zins of Dotson Company AFS Transactions Solving Casting Problems with New Sleeve Technology Ronald C. Aufderheide and Ralph E. Showman Ductile Iron News,

3 Fig 2. Rare elliptical shape of the doughnut defect, prior to cleaning no riser sleeves used in this foundry Fig3. Round shape of the doughnut defect prior to cleaning, white residue can be seen. No riser sleeves used in this foundry Fig4. Close up view of the round defect 3

4 Figures 5, 6 and 7 Typical appearances of the defect after cleaning All these defects were found in a foundry not using any type of riser sleeves. 4

5 Fig8. Microstructure of the defects area. Graphite shape is affected by the residue Fig9. Scanning electron Edax analysis of the oxide layer of the defect area 5