Casting defects, inspection and quality control
Domain 9 - Casting defects, inspection and quality control
This domain tests defect identification, causes and remedies, and the inspection methods used to accept or reject castings.
Common defects
- Blowholes: large, smooth, round gas cavities from sand moisture or low permeability.
- Pin holes: small gas cavities from dissolved gas or a wet charge.
- Shrinkage cavity: rough, irregular internal void at a hot spot.
- Hot tears: cracks formed during contraction because the mould or core restrains the casting.
- Cold shut and misrun: incomplete fusion or incomplete filling from low pouring temperature or slow filling.
- Sand and slag inclusions, scab, rat tail, buckle, swell, drop, metal penetration and hard spots.
Inspection and QC
Visual inspection, dimensional checking, ultrasonic testing, radiography, magnetic particle and dye-penetrant testing, pressure testing, hardness and chemical analysis. Sand testing, pattern maintenance and control charts prevent defects rather than detect them.
Worked example
A casting has a smooth, rounded cavity just under the cope surface. This is a blowhole, not a shrinkage cavity. Shrinkage cavities are rough and located at hot spots; blowholes are smooth and often near the top surface. The likely cause is high sand moisture or poor venting. Remedies are lower water content, better mulling, adequate vents and avoiding over-ramming.
Traps
Do not confuse a misrun with a cold shut: a misrun leaves a missing section, while a cold shut is an incomplete fusion line. A hot tear is not a shrinkage crack; it forms while the metal is hot and restrained. Radiography finds internal defects but not surface cracks. A single visual check cannot replace dimensional and NDT inspection.