CASTING FORGING DEFECTIVE SAMPLES

18.0. CASTING and FORGING DEFECTIVE SAMPLE:

We are our own well-equipped setup for casting and forging defective samples as per client-customized requirements. The flawed casting and forging samples are used as a demonstration process prior to examination.

CASTING DEFECTS:

A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: gas porosity, shrinkage defects, mould material defects, pouring metal defects, and metallurgical defects.

1. Shrinkage defects

Shrinkage defects can occur when standard feed metal is not available to compensate for shrinkage as the thick metal solidifies. Shrinkage defects will have a jagged or linear appearance. Shrinkage defects usually occur in either the cope or drag portion of the casting. 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.

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 macro porosity and micro porosity (or micro shrinkage), where macro porosity can be seen by the naked eye and micro porosity cannot.

2. Gas porosity

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. Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal, which reduces strength in that vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas porosity. In aluminum castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity. For casting that is 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. To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as argon or carbon dioxide, 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 mould can introduce gases, so the moulds 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 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. A third source consists of reactions of the molten metal with grease or other residues in the mould.

Blowhole defect in a cast iron part.

Hydrogen is produced by the reaction of the metal with humidity or residual moisture in the mould. Drying the mould can eliminate this source of hydrogen formation. Gas porosity can sometimes be difficult to distinguish from micro shrinkage because micro shrinkage cavities can contain gases as well. In general, micro porosities 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.

Tiny gas bubbles are called porosities, but larger gas bubbles are called blowholes 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 mould. (Vacuum holes caused by metal shrinkage (see above) may also be loosely referred to as 'blowholes'). Proper foundry practices, including melt preparation and mould 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.

3. Pouring metal defects

Pouring metal defects include misruns, cold shuts, and inclusions. A misrun occurs when the liquid metal does not completely fill the mold cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mold cavity, leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or cross-sections that are too narrow. The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mold cavities.

Misruns and cold shuts are closely related and both involve the material freezing before it completely fills the mold cavity. These types of defects are serious because the area surrounding the defect is significantly weaker than intended. The cast ability and viscosity of the material can be important factors in these problems. Fluidity affects the minimum section thickness that can be cast, the maximum length of thin sections, the fineness of feasibly cast details, and the accuracy of filling mold extremities. There are various ways of measuring the fluidity of a material, although it usually involves using a standard mold shape and measuring the distance the material flows. Fluidity is affected by the composition of the material, freezing temperature or range, surface tension of oxide films, and, most importantly, the pouring temperature. The higher the pouring temperature, the greater the fluidity; however, excessive temperatures can be detrimental, leading to a reaction between the material and the mold; in casting processes that use a porous mold material, the material may even penetrate the mold material. The point at which the material cannot flow is called the coherency point. The point is difficult to predict in mold design because it is dependent on the solid fraction, the structure of the solidified particles, and the local shear strain rate of the fluid. Usually, this value ranges from 0.4 to 0.8.

An inclusion is a metal contamination of dross, if solid, or slag if liquid. These usually are impurities in the poured metal (generally oxides, less frequently nitrides, carbides, or sulfides), material that is eroded from the furnace or ladle linings, or contaminates from the mold. In the specific case of aluminum alloys, it is important to control the concentration of inclusions by measuring them in the liquid aluminum and taking action to keep them to the required level.

There are a number of ways to reduce the concentration of inclusions. In order to reduce oxide formation the metal can be melted with a flux, in a vacuum, or in an inert atmosphere. Other ingredients can be added to the mixture to cause the dross to float to the top where it can be skimmed off before the metal is poured into the mould. If this is not practical, then a special ladle that pours the metal from the bottom can be used. Another option is to install ceramic filters into the gating system. Otherwise, swirl gates can be formed which swirl the liquid metal as it is poured in, forcing the lighter inclusions to the center and keeping them out of the casting. If some of the dross or slag is folded into the molten metal then it becomes an entrainment defect.

4. Metallurgical defects

There are two defects in this category: hot tears and hot spots. Hot tears, also known as hot cracking, 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.

Hot spots are sections of casting that have cooled down more slowly than the surrounding material due to higher volume than its surrounding. This causes 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.

FORGING DEFECTS:

Steel forging defects are not widely discussed because of a natural reluctance by the forging company to draw attention to them. There are many imperfections that can be considered defects, ranging from those traceable to the starting materials to those caused by closed die forging process or by post-forging operations. Defects can be defined as imperfections that exceed certain limits. In other words, there may be imperfections that are not classified as true “defects” because they are smaller than allowances in the applicable specifications.

The following listed information is about common and not-so-common defects of steel forgings that come from the actual closed die forging operations or from post-forging operations typical of many forge plants. The goal here is to acquaint the reader with these various defects, how they can affect forging performance, and how to eliminate them with future forging production.

1) Unfilled Section:

As the name implies in this type of defect some of the forging sections remain unfilled. This is due to the poor design of the die or poor forging technique. This is also due to less raw material or poor heating. This defect can be removed by proper die design, proper availability of raw material, and proper heating.

2)  Cold Shut:

Cold shut includes small cracks at corners. These defects occur due to improper design of the forging die. It is also due to sharp corners, and excessive chilling in forging products. The fillet radius of the die should be increased to remove these defects.

3)  Scale Pits:

Scale pits are due to improper cleaning of forged surfaces. This defect is generally associated with forging in the open environment. It is irregular deputations on the surface of forging. It can be removed by proper cleaning of the forged surface.

4) Die Shift:

Die shift is caused by misalignment of the upper die and lower die. When both these dies are not properly aligned the forged product does not get proper dimensions. This defect can be removed by proper alignment. It can be done by providing a half notch on the upper die and half on the lower die so at the time of alignment, both these notches will matched.

5)    Flakes:

These are internal cracks that occur due to improper cooling of forge products. When the forge product cools quickly, these cracks generally occur which can reduce the strength of the forge product. This defect can be removed by proper cooling.

6)    Improper Grain Growth:

This defect occurs due to improper flow of metal in casting which changes the predefined grain structure of the product. It can be removed by proper die design

7) Incomplete Forging Penetration:

This defect arises due to incomplete forging. it is due to light or rapid hammer blow. This defect can be removed by proper control on the forging press.

8)    Surface Cracking:

Surface cracking occurs due to exercise working on surfaces at low temperature. In this defect, So many cracks arise on workpiece. This defect can be removed by proper control of the working temperature.

9) Residual Stresses in Forging:

This defect occurs due to improper cooling of forged parts. Too much rapid cooling is the main cause of this type of defect. This can be removed by slow cooling of the forged part.