- Defecting design of pattern
- Improper melting of metal
- Defects in mould
- Due to moulding material
- Improper cooling of mould
- Defecting gating system
- Improper venting in mould
Below we examine some common casting defects and how to determine if a finished piece is of high quality:
Blow – The blow is a relatively large cavity or hole produced by gases which displace molten metal’s form.
Scar – Due to improper permeability or venting, a scar is a shallow blow. It typically occurs on a flat surface.
Blister – A blister is a shallow blow like a scar, but with a thin layer or metal covering it.
Core Shift
Core Shift – A core shift is a defect due to the buoyancy of molten metal where the core of a cast moves from where it was originally intended to be.
Mold Shift – A mold shift is caused by the mold cope itself shifting sideways relative to the drag. This results in a step or indent in the final cast at the parting line.
Buckle
Buckle – A buckle is a long, broad shallow depression, that occurs in the surface of flat castings. It extends in a straight line across the entire flat surface.
Rat Tail – A rat tail is a long, shallow, angular depression in the surface and resembles a buckle, except it’s not shaped like a broad vee.
Pin Holes – Pin holes are small gas holes either at the surface or just below the surface. When these are present, they occur in large numbers and are fairly uniformly dispersed over the surface.
Wash – A wash or cut is a low projection on the drag face of a casting that extends along the surface, decreasing in height as it extends from one side of the casting to the other end.
Hot Tears – Hot tears are hot cracks which appear in the form of irregular crevices with a dark oxidized fracture surface. They arise when the solidifying metal does not have sufficient strength to resist tensile forces produced during solidification.
Shrinkage – A shrinkage cavity is a depression in a casting that results from the volume contraction that occurs during solidification.
Swell – A swell is a slight, smooth bulge usually found on vertical faces of castings, resulting from liquid metal pressure. It may be due to low strength of mold because of too high a water content or when the mold is not rammed sufficiently.
Shrinkage defects[edit]
Shrinkage defects can occur when standard feed metal is not available to compensate for shrinkage as the thick metal solidifies. Shrinkage defects will have jagged or linear appearance. Shrinkage defects usually occur in either the cope or drag portion of the casting.[4] Shrinkage defects can be split into two different types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are open to the atmosphere, therefore as the shrinkage cavity forms, air compensates. There are two types of open air defects: pipes and caved surfaces. Pipes form at the surface of the casting and burrow into the casting, while caved surfaces are shallow cavities that form across the surface of the casting.[5]
Closed shrinkage defects, also known as shrinkage porosity, are defects that form within the casting. Isolated pools of liquid form inside solidified metal, which are called hot spots. The shrinkage defect usually forms at the top of the hot spots. They require a nucleation point, so impurities and dissolved gas can induce closed shrinkage defects. The defects are broken up into macroporosity and microporosity (or microshrinkage), where macroporosity can be seen by the naked eye and microporosity cannot.[5][6]
Gas porosity[edit]
Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools.[7] Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal,[8] which reduces strength in that vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas porosity.[6] In aluminum castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity.[9] For casting that are a few kilograms in weight the pores are usually 0.01 to 0.5 mm (0.00039 to 0.01969 in) in size. In larger casting they can be up to a millimeter (0.040 in) in diameter.[8]
To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as argon[10] or carbon dioxide,[11] or under a flux that prevents contact with the air. To minimize gas solubility the superheat temperatures can be kept low. Turbulence from pouring the liquid metal into the mold can introduce gases, so the molds are often streamlined to minimize such turbulence. Other methods include vacuum degassing, gas flushing, or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form a dross that floats to the top. For instance, oxygen can be removed from copper by adding phosphorus; aluminum or silicon can be added to steel to remove oxygen.[7] A third source consists of reactions of the molten metal with grease or other residues in the mould.
Hydrogen is produced by the reaction of the metal with humidity or residual moisture in the mold. Drying the mold can eliminate this source of hydrogen formation.[12]
Gas porosity can sometimes be difficult to distinguish from microshrinkage because microshrinkage cavities can contain gases as well. In general, microporosities will form if the casting is not properly risered or if a material with a wide solidification range is cast. If neither of these are the case then most likely the porosity is due to gas formation.[13]
Blowhole defect in a cast iron part.
Tiny gas bubbles are called porosities, but larger gas bubbles are called blowholes[14] or blisters. Such defects can be caused by air entrained in the melt, steam or smoke from the casting sand, or other gasses from the melt or mold. (Vacuum holes caused by metal shrinkage (see above) may also be loosely referred to as ‘blowholes’). Proper foundry practices, including melt preparation and mold design, can reduce the occurrence of these defects. Because they are often surrounded by a skin of sound metal, blowholes may be difficult to detect, requiring harmonic, ultrasonic, magnetic, or X-ray (i.e., industrial CT scanning) analysis.
Metallurgical defects[edit]
There are two defects in this category: hot tears and hot spots. Hot tears, also known as hot cracking,[21] are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools. Proper mold design prevents this type of defect.[3]
Hot spots are sections of casting which have cooled down more slowly than the surrounding material due to higher volume than its surrounding. This causing abnormal shrinkage in this region, which can lead to porosity and cracks. This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal.[3]
- Blowhole is a kind of cavities defect, which is also divided into pinhole and subsurface blowhole. Pinhole is very tiny hole. Subsurface blowhole only can be seen after machining.
Blowhole Defect
- Burning-on defect is also called as sand burning, which includes chemical burn-on, and metal penetration.
Sand Burning Defect
- Sand inclusion and slag inclusion are also called as scab or blacking scab. They are inclusion defects. Looks like there are slag inside of metal castings.
Sand Inclusion Defect
- Sand hole is a kind of shrinkage cavity defect. They are empty holes after sand blasting.
sand hole defect
- Cold lap or also called as cold shut. It is a crack with round edges. Cold lap is because of low melting temperature or poor gating system.
Cold Lap Defect
- Joint flash is also called as casting fin, which is a thin projection out of surface of metal castings. Joint flash should be removed during cleaning and grinding process.
Joint Flash Defect
- Misrun defect is a kind of incomplete casting defect, which causes the casting uncompleted. The edge of defect is round and smooth.
Misrun Defect
- Shrinkage defects include dispersed shrinkage, micro-shrinkage and porosity. For large porosity on the surface, you could see them easliy, but for small dispersed shrinkage, you may see them after machining.
Porosity Shrinkage Defect
- Shrinkage cavities are also called as shrinkage holes, which is a type of serious shrinkage defect, you can see them easily on the rough surface of the metal castings.
Shrinkage Cavity Defect
- Shrinkage depression is also a type of shrinkage defect, which looks like depressed region on the surface of metal castings.
Shrinkage Depression Defect
- Elephant skin is a type of surface defect, which cause irregular or wrinkle shapes surfaces.
Elephant Skin Defect
- Veins defect is also called as rat tail, which looks like many small water flow traces on the surface of metal castings.
Veins Defect
- Rough surface, coarse surface is also a kind of surface defect. Normal rough surfaces can not be judged as defects, but too rough and uneven in surface will be a defect.
Rough Surface Defect
- Mismatch in mold defect is because of the shifting molding flashes. It will cause the dislocation at the parting line.
Mismatch Mold Defect
- Mechanical damage is because the damage during machining or delivery processes.
Mechanical Damage Defect
- Slag inclusion is also called as exogenous inclusion, entrapped slag.
Slag Inclusion Defect
- Raised mold defect. Because of the flotage of liquid metal, the mould flasks are raised, which caused the top part of casting become higher or thicker than lower part.
Raised Mold Defect
- Crack defects normally happen inside of metal castings. This defect will reduce the physical properties of metal castings.
Crack Defect
- Abnormal nodulizing or under-nodulizing defects. Because of many reasons, the spheroidization of graphite for ductile iron will be affected, therefore, caused the bad spheroidization rate. By metalloscope, you can see very few graphite balls, and many worm-like grahite.
Abnormal Nodulizing Defect
- Uneven hardness defect means the uneven hardness on the same surfaces. When machining to harder positions, the machining will become more difficult.
Uneven Hardness Defect
- Sand drop is also called as sand crush. The sand mold drops part of sand blocks, so they will cause the similar shaped sand holes or incomplete.
Sand Drop Defect
- Deformation will cause the oversized tolerance for flatness and straightness. This is very common defect for long castings, and flat castings with thin wall thickness. The reasons are the natural deformation during cooling process in sand molds, or in air, sometimes, the overly sand blasting also could cause this problem.
Deformation Problem
- After welding repair, even after machining or grinding, the welding marks will still be visual. As for unimportant casting surfaces, if the client allow welding repair, then these marks should be acceptable. But for high pressure-bearing positions, or the client has clearly forbidden any welding repair, then these marks will be taken as defects.
Welding Repair Marks
- Chill iron could effectively reduce the shrinkage for the key positions, so using chill iron is very common in iron foundries. However, the edges of chill irons could be clearly found by visual inspection. Some clients will not require to grind them if these marks do not affect the appearance. However, the clients could require the casting manufacturer to grind them just for better surface looking. Please clearly notice that these marks should not be judged as the casting defects. Refer to iron-foundry.com.
Chill Iron Marks
- Chill defects are also called as “casting chilling defects”. The surface of the castings with this defect will be extremely white, shiny and smooth. The left one of the parts on the following photo is showing this defect. The defective castings will be fragile and crispy, so during machining, some edges and tips will be broken. This defect was caused by the low temperature of sand molds, and prematurely out of sand molds, so the hot iron become chilled quickly. The proper annealing heat treatment to them could solve this defect.
Casting Chilling Defects
- Massive free carbide. On metallograph, you could see many fish-bone free carbide. This is a serious defect of cast iron material, normally happen on ductile iron. Because of inverse chilling defects and poor inoculation, there will be mass free carbide, which will cause fragile, poor welding property to ductile iron castings. High temperature annealing heat treatment could improve its quality.
Massive free carbide
- Internal sweating, or called as cold short, short iron. There are iron beans in the castings. This is because of unreasonable gating design, which caused some liquid iron became beans suddenly, then these beans were wrapped by other liquid irons. This is a surface defect, but will cause serous problem if they located in key positions.
Internal sweating, cold short, short iron
- Stripping defect. There is very thin iron skin on the surface of castings. Two layers. This is because of unreasonable gating system, which caused very thin air layers existed. This defect is a surface defect, so normally it can be grinded off. However, it should be discarded if it is not just on the surface.
Stripping defect
- Cast iron graphite floatation. This defect is a kind of material problem. It is caused because of low pouring temprature and high content of carbon element. This defect is very harmful, will cause material very fragile. On the broken surface, you can see the obvious black surface caused by this defect.
Cast iron graphite floatation
Types of casting defects:
These defects can be classified into three major categories.
1. Blow Hole:
It is a gas hole or gas cavity which is occurs at the surface or the internal part of casting. It is due to gas interrupted into the casting due to poor ventilation. Mostly it is occur at surface which makes a circular of oval shape gas hole. These defects can be seen after machining of surface. These defect cause due to improper venting system, excessive gas or moisture contain and due to low permeability of sand and high temperature of sand. It is also due to low pouring temperature of metal.
2. Scar:
Scar is generally occur at flat surface and it is due to improper venting or permeability of sand. The blow is covered by the thin layer of metal.
3. Scab:
This is also known as slag inclusion of sand inclusion. This is look like as slag inside the cast and very difficult to remedy because generally it does not occur at single point. This defect generally occurs due to defective mould or core, low binding strength of moulding material or sand. It is also due to low permeability of sand.
4. Penetration:
This defect occurs when fluidity of molten metal is high. When mould is porous and does not enough stronger to resist molten metal, this defect occur. The liquid metal penetrates the mould and mix with sand. This gives uneven casting surface.
5. Cold shut:
It is the basic defect which generally occurs. It is occur when the molten metal flow through two different paths. When the two stream of molten metal comes in contact and solidify before they mix completely, some bubbles form between them. This phenomenon is known as cold shut. It is also known as cold lap.
6. Mismatch:
It is defect due to mismatch of cope and drag. It is also known as shift. When the cope shift relative to drag known as mold shift and when drag shift known as cope shift. It is due to the bouncy of the molten metal, loose box pins etc.
7. Misruns:
Misruns is a phenomenon when the cast materials solidify before the proper filling of mould. This result presence of incomplete cavity filling. It is due to too thin wall thickness, improper gating system, slow pouring, damaged pattern and poor fluidity of molten metal.
8. Hot tear:
A hot tear or crack is generally not visible because the casting has not separated into fragments. This is occurring due to uneven cooling and improper position of chills. This is generally occur where abrupt section change. This is occur during solidification of casting when the solid portion have not sufficient strength to resist tensile force.
9. Shrinkage:
This defect generally arises during volume contraction during solidification. It is occur when the liquid metal does not sufficient to compensate the volume shrinkage. This is due to insufficient size of riser, improper direction solidification of casting, improper location of chills or in gates.
10. Buckle:
The formation of uneven line on the cast surface is known as buckle. It generate due failure of sand surface or expansion of sand surface due to heat of molten metal.
11. Pin hole:
Pin holes are similarly to blow holes but smaller into size. These are generally present at surface or sub surface and in large numbers. This is due to improper ventilation for gas and low permeability of moulding material.
12. Warpage:
It is the defect or deformation in a casting that occur during or after solidification due to different rates of solidification of different sections of casting, so stresses are set up in adjoining walls resulting in warpage these area.
13. Swell:
This defect generated due to insufficient strength of mould when metal flow to the mould due to liquid metal pressure, the wall of the mould cavity damage. It is also due to rigidity of the pattern is insufficient then it cannot withstand with the ramming pressure and the wall of cavity damage.
Today we have learnt about major casting defects and its causes. These defects can be remove by proper casting techniques. If you have any query regarding this article ask by commenting. If you like this article, don’t forget to share it on social networks. Subscribe our website for more informative articles. Thanks for reading it.