In this article we are going to learn about casting defects and what are the types of casting defects.
What is Casting Defects ?
Casting Defects : It is an undesirable irregularity that appears in the casting during the metal casting process. Defects in casting metal can occur for a variety of reasons or from a variety of sources. We’ll go over all of the major forms of casting faults in this article. Some flaws may be overlooked or tolerated, while others are unacceptable and must be corrected in order for the parts to work properly.
Types of Casting Defects
Sand casting , particularly, are subject to certain defects which in a well designed casting, are controllable by proper foundry technique, but are not wholly preventable. However , the common types of casting defects found, their causes and remedies are explained hereunder.
This is an external defects in casting caused due to core misplacement or mismatching of top and bottom parts of the casting usually at a parting line. Mis-alignment of flasks in another likely cause of shift.
These casting defects can be prevented by ensuring proper alignment of the pattern or die part, moulding boxes, correct mounting of patterns on pattern plates, and checking of flasks, locating pins, etc. before use.
Warpage is unintentional and undesirable deformation in a casting that occurs during or after solidification. Due to different rates of solidification different sections of a casting, stresses are set up in adjoining walls resulting in warpage in these areas. Large and flat
sections or intersecting sections such as ribs are particularly prone to warpage.
The remedy is to produce large areas with wavy, corrugated construction, or add sufficient ribs or rib-like shapes, to provide equal cooling rates in all areas. A proper casting design can go a long way in reducing the warpage of the casting.
A thin projection of metal, not intended as a part of the casting, is called the fin. Fins usually occur at the parting of the mould or core sections. Moulds and cores incorrectly assembled will cause fins. Insufficient weighting of the moulds, or improper clamping of flasks may again produce the fin.
The remedy lies on the use of sufficient weight on top part of the mould so that the two parts fit tightly together, and correct assembly of the moulds and cores used for the casting.
A swell is an enlargement of the mould cavity by metal pressure, resulting in localized or overall enlargement of the casting. This is caused by improper or defective ramming of the mould. To avoid swells, the sand should be rammed properly and evenly.
Blow holes are smooth, round holes appearing in the form of a cluster of a large number of small holes below the surface of a casting. These are entrapped bubbles of gases with smooth walls. Blow holes are caused by excessive moisture in the sand, or when permeability of sand is low, sand grains are too fine, sand is rammed too hard, or when venting is insufficient.
To prevent blowholes, the moisture content in sand must be well adjusted, sand of proper grain size should be used, ramming should not be too hard, and venting should be adequate.
A drop occurs when the upper surface of the mould cracks, and pieces of sand fall into the molten metal. This is caused by low strength and soft ramming of the sand, insufficient fluxing of molten metal and insufficient reinforcement of sand projections in the cope. The above factors are eliminated to avoid drop.
In some cases, particles of dirt and sand are embedded in the casting surface. This is caused by crushing of the mould due to improper handling, sand wash and presence of slag particles in the molten metal.
Dirt may be prevented from entering the mould cavity by proper fluxing and the use of dirt traps.
8. Honeycombing or Sponginess
These are external casting defects consisting of a number of small cavities in close proximity. Honey-combing is caused by dirt or “scurf” held mechanically in suspension in the molten metal, and is due to imperfect skimming in the ladle.
The remedy is to prevent the sand wash and to remove the slag particles present in the molten metal by the proper skimming in the ladle.
9. Metal Penetration and Rough Surface
This defect in casting appears as an uneven and rough external surface of the casting. The metal penetration between the sand grains occurs due to low strength, large grain size, high permeability and soft ramming of sand. Remedies involve removing the causes mentioned above.
10. Sand Holes
Sand holes are found on external surface or inside the casting. They are caused by loose sand washing into the mould cavity and fusing into the interior of the casting or rapid pouring of the molten metal. Sand holes are prevented by proper cleaning of the mould and careful pouring of the molten metal.
11. Pin Holes
Pinholes are numerous small holes, usually less than 2 mm, visible on the surface of the casting cleaned by shot blasting. These casting defects are caused by sand with high moisture content, absorption of hydrogen or carbon monoxide gas or when steel is poured from wet ladles or is not sufficiently gasified.
This defect of casting can be minimized by using good melting and fluxing practices, by reducing the moisture content of moulding sand and easing its permeability, and by promoting a rapid rate of solidification.
Scabs are a sort of projection on the casting that occur when a flows portion of the face of the mould or core lifts and metal underneath in a thin layer. In other words, liquid metal penetrates behind the surface layer of sand. Scabs can be identified as rough, irregular projections on the surface containing embedded sand.
These casting defects are caused by using too fine sand, sand having low permeability and moisture content, and by uneven mould ramming or intermittent or slow running of molten metal over the sand surface thereby producing intense local heating.
Mixing additives such as wood flour, sea coal, or dextrin, into the sand is one step which will eliminate such defects in casting.
13. Shrinkage Cavity
Shrinkage cavity is a void or depression in the casting caused mainly by uncontrolled and haphazard solidification or the metal. This may also be produced the pouring temperature is too high.
These casting defects can be eliminated by applying the principle of directional solidification in mould design and by judicious use of chills, padding, etc.
14. Hot Tears (Pulls)
They are internal or external cracks having ragged edges occurring immediately after the metal has solidified. Hot tears may be produced if the casting is poorly designed and abrupt sectional changes take place, no proper fillets and corner radii are provided, and chills are wrongly placed. Incorrect pouring temperatures and improper placement or gates and risers and hard ramming can also create hot tears.
Improved design, proper directional solidification, even rate of cooling, correct pouring temperatures, and control of mould hardness eliminate hot tears.
15. Cold Shut and Misrun
A cold shut is an external defects in casting formed due to imperfect fusion of two steams of metal in the mould cavity or unequal sections of pattern assembled together. These casting defects may appear like a crack or seam with rounded edges. A misrun casting is one which lacks completeness due to the failure of the metal to fill The mould cavity.
The reasons for cold shut or misrun may be too thin sections and wall thickness, improper gating system, damaged patterns, slow and intermittent pouring, poor fluidity of metal caused by low pouring temperature, improper alloy composition , etc. Use of hotter metal, frequent inspection and replacement of patterns and core boxes and proper design of the casting keeping in mind the fundamental principles of gating are some of the steps that may be used to eliminate cold shut and misrun casting defects.
16. Poured Short
When the metal cavity is not completely filled at one pouring, the defect is called poured short. Sufficient metal in the ladle at correct temperature will eliminate these defects in casting.
17. Internal Air Pocket
This appears as small holes inside the casting and is caused by pouring boiling metal or by rapid pouring of the molten metal in the mould. Faulty and poor quality of metal, and excessively moist sand may also create air pockets. Correct pouring temperature of the molten metal, right quality of metal and dry sand minimizes this defect to a great extent.
We discussed the undesirable irregularity that appears in the casting during the metal casting process known as casting defects. Defects in casting can be minimized if proper care and precautions are taken. Defects of casting creates a negative impact on overall production and hence proper knowledge should be there about types of casting defects to minimize them.
Check Out Other Important Topics
|IC Engine||Important PDFs||Boilers||Synergy Maritime Exam||Naval Arch||MEO Class 4|
|Interview Questions||Difference Between||Types of Pumps||Auxiliary Machines||Types of Valves||Home|