Casting defects
Casting defects are qualitative defects in cast parts that require the casting to be discarded ( rejects ) or can be remedied with various post-treatment measures so that the defective cast part is usable or the defect can even be eliminated. Even if the greatest care is taken in the melting, casting, heat treatment and quality production technologies, there is a final risk that cannot be influenced in the production of castings, as this branch of production creates complex influencing conditions, the unfavorable effects of which can lead to quality defects.
Detection and types of errors
The errors are often only recognized as such during the sequence of work steps in the fettling shop, when the casts are ready for shipping. Depending on the application of the cast parts and the type and magnitude of the defects that occur, a certain defect level can be tolerated. This is defined in the delivery specifications of the cast consumer. The quality departments of the foundries decide in agreement with or with the casting consumer about the defects found, which can be roughly differentiated into volume and surface defects.
- Surface defects affect the casting surface directly (e.g. cracks, pores, imperfections, etc.) or the dimensional accuracy compared to the required dimensions (drawing) of the end product.
- Volume defects (cavities) can often be recognized without access to the surface inside the cast parts as voids , structural defects, non-metallic inclusions and the like only by means of ultrasound or X-ray testing.
If the casting is offset, the external impression of the casting is as if an attempt had been made to shear it off. The resulting protrusions on the "shear surface" (molding plane) are the same size. Such cast parts are created when the original mold is offset (model, model plate, cores ), when the mold is moved when inserting or clamping or when the molding box gives way before casting (blows, bumps) or during the casting process.
Defective mechanical properties of the casting material can only be discovered by means of destructive test methods before any damage occurs. The mechanical properties such as B. the strength properties ( tensile strength , yield point ) or the toughness of the material is often determined on the cast sample material and compared and certified with the target values of the material used and the construction. Surface hardness testing methods, from the values of which can be used to infer the strength properties, simplify this testing process and are also meaningful in many cases and are common in practice (for example, in the case of tempered cast steel, in order to be able to quickly check the success of the heat treatment).
See also
- For structural defects in cast iron ( gray cast iron , cast iron with lamellar graphite) see also white radiation .
- Gas bubble
- Blowholes : as a large contiguous cavity in unpowered accumulations of material in cast structures. The cavity surface appears rough and covered with dendrites . Cavities arise due to a thermal center in accumulations of material. They are formed in connection with the volume contraction when the cast part cools (e.g. approx. 6% in cast steel as a volume deficit that could not be compensated for by making up with liquid material).
literature
- Casting error atlas; Foundry-Verlag GmbH, Düsseldorf;
- Volume 1 - Classification, General Defects and Gray Cast Iron Defects;
- Volume 2 - cast steel, malleable cast iron, copper, aluminum and magnesium alloys (Copyright 1956 by Gießerei-Verlag GmbH, Düsseldorf).
- German standard DIN EN 1370 Foundry - Assessment of the surface condition; Beuth Verlag GmbH, 10772 Berlin