Pinholes
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Pinhole defects are type of gas porosity. This casting defect consists of small sub-surface holes which can be seen in a casting fracture or on the casting surface after shot blasting, heat treatment or light machining. Two distinct types of pinhole can be found with substantially similar appearences but different formation characteristics. These are hydrogen pinholes and nitrogen-hydrogen pinholes associated mainly with resin-bonded cores.
Hydrogen pinholes are most likely to be formed by pick-up of hydrgen from moulding sand whereas the so called nitrogen pinholes are definately associated with nitrogen-bearing material such as resin-bonded cores and moulds.
