HOT TOPICS Issue 2, 2003

Transcription

1 HOT TOPICS Issue 2, 2003 Casting Defect: Surface Defects 1. Defect Code: D 231 -DI-GI-GS (Ductile iron, Gray Iron, in Green Sand) 2. Name: Scab, Expansion Scabbing 3. Description Usually a very rough metallic formation generally with sharp edges that is a raised attached skin on the casting surface, either the cope (top) surface, the drag (bottom) surface, or on the vertical walls of the casting. It will be attached to the casting surface in more than one point, but not a continuous bond. The scab may be easily broken free by a chisel or other type of tool exposing an underlying casting depression. 4. Pictures are at the end of this document. 5. Mechanism of defect formation When filling a mold cavity with molten iron, the upper portion, or cope side of the cavity, is exposed to the radiant heat from the iron (Fig. 1). This leads to a drying of the exposed sand, and a migration of the moisture from that surface sand back to some distance underneath the sand surface. With the high water content of this condensation zone (Wet Layer), there is a reduction in the sand mechanical properties. The moisture level in this zone is typically 2.5% higher than the base green sand, causing the wet zone total moisture to be over 5%. As the temperature of the dried surface layer rises, sand grains expand. If this expansion is hindered, the dry sand may buckle into the mold cavity creating a tensile stress between the Hot and Wet layers (Fig. 1B). If further thermal expansion is resisted, the buckled layer of dry crust (this crust layer can be thick; Fig. 1C) will break under compression. This break creates a path for molten iron to enter the cavity left behind by the separated layer of sand, creating a scab defect. Therefore, the tendency towards a scab or buckle -type defect is the relationship between the wet tensile strength, or wet layer strength, and the compressive stress, or load, that is on the dry surface sand layer. To combat this problem, the wet tensile strength needs to be strong enough so it will be able to withstand the tensile load set up by the compressive stress, and the scab or buckle will not occur. Furthermore, a decrease in the cushion (moisture absorbing materials that allow for a plastic deformation during silica expansion) of a sand system will help Timothy A. Dorn, Neenah Foundry Company, Neenah, Wisconsin, USA 1

2 create a larger compressive stress in the dry surface layer of sand, which will help promote scabbing tendencies. 6. Testing For Scabbing Tendencies The green strength of tempered sand, the usual condition of sand when tested in the foundry lab, does not adequately describe the mold strength. Heat from the molten iron creates different temperature gradients in the mold, resulting in a wide range of sand strengths throughout the mold cavity (Fig. 2). The Wet strength is the lowest strength in the mold, and subsequently a perfect place for defects to originate (scabs, buckles, etc.). The Wet Tensile test can help detect scabbing tendencies in green sand. The test simulates the latent heat of the mold filling with molten iron, which heats the sand. This results in green sand moisture being pushed back into the green sand mold, creating a Hot Dry zone and a Wet zone. A tension load is applied to this Wet zone where the green sand strength is the weakest in the mold and the ultimate strength of that zone is determined. This wet tensile strength is relative to scabbing tendencies in your sand system (low wet tensile strength high scabbing tendency). 7. All Possible Causes Design Pouring time too long, exposing the sand layer to heat for an extended time. Too much metal going through one gate, super heating the layer of sand. Inadequate corner radii, excessive free expansion resulting in rupture. Process Mold rammed too hard very high mold density increased sand grain contact high thermal expansion. Too high a silica sand content (greater than 88%) creating a high thermal expansion potential. Too high of pouring temperature increasing the rate of thermal expansion. Low wet tensile strength due to high moisture or low clay content of the sand. No enough cushion material in the sand to counteract the sand expansion. 8. Most Probable Causes Hard Mold Mold hardness greater than 95 ( B scale). High silica content in the sand above 88%. Low wet tensile strength of molding sand. Slow pouring. Too much metal through one gate. Low clay level with well mulled sand. 9. Corrective Actions/ Process Variables to Control to Avoid Defect Increase the green strength of the sand by: Increasing the clay/bond additions. Improve the mullor efficiency. Timothy A. Dorn, Neenah Foundry Company, Neenah, Wisconsin, USA 2

3 Reduce the compressive stresses in the Hot Dry Zone sand by: Improve the mold filling by pouring faster and evenly. Make certain there is no excess moisture in the green sand (high compactability). Increase the cushion in the sand which will help compensate for the expansion of the sand in this zone. Avoid mold hardness in excess of 95 ( B scale). Improve the venting and/or permeability of the sand mold. Maintain a pouring time (seconds) of the square root of the pour weight (pounds). Use the lowest pouring temperature allowable while avoiding cold laps and mis-runs. Timothy A. Dorn, Neenah Foundry Company, Neenah, Wisconsin, USA 3

4 A) Latent Heat Molten Iron Wet Zone (weakest strength) Tensile load set up between the Wet and Hot Dry Zones Compressive load set up in the Hot Dry Zone B) Hot Dry Zone A Scab forms C) Hot Dry Sand breaks due to compressive stresses Fig. 1 Scab Formation in a Green Sand Mold. Green Sand Warm Strength Molten Iron Wet Strength Hot Strength Fig. 2 Anatomy of a Green Sand Mold Strength Timothy A. Dorn, Neenah Foundry Company, Neenah, Wisconsin, USA 4

5 Picture 1 Picture 2 Picture 1 is of a cross section of a scab defect. Picture 2 is a top view of a scab defect. Picture 3 is of a large gray iron casting showing a large amount of scabbing defects. (Pictures 1-3 courtesy of International Atlas of Casting Defects, American Foundrymen s Society, Fourth printing, 1986) Picture 3 Picture 4 Picture 5 Pictures 4 and 5 are of production castings with the scabbing defects. References Bonding Mechanisms in Sand Aggregates, G.A. Smiernow, E.L. Doheny, and J.G. Kay, AFS Transactions Discussion of the Scabbing Tendencies of Green Sand, Dr. D. Boenisch, AFS Transactions Wet Tensile Strength in the Condensation Zone - Its Relation to Scabbing Tendency of Synthetic and Natural Bonded Sands, Dr. J.R. Van Eeghen, AFS Transactions Functional Properties of Na and Ca Bentonites in Green Sand Systems, E.E. Odom, AFS Transactions Timothy A. Dorn, Neenah Foundry Company, Neenah, Wisconsin, USA 5