Defect Types Found During Non-Destructive Testing

Defect types found

The following is a non-exhaustive list of some of the defect types which can be found during the non destructive testing process.

 

CASTING DEFECTS

The following defects are present in the material before any further processing operations such as forging, or rolling have begun.  All casting defects are therefore inherent.

Porosity

This is formed by gas which is insoluble in the molten metal.  The gas is trapped within the metal when if solidifies and remains in the form of spherical or tubular cavities.

Airlocks

A cavity formed by air which has been trapped in the mould by the metal during pouring.

Blowholes

These are small holes in or on the surface of the casting.  They are caused by gas evolution from the decomposition of grease, moisture etc., but not from the mould itself.

For example, during the sand casting operation, moisture from the mould produces steam, this is normally forced through the mould due to the absorbent nature of the sand but sometimes the steam cannot get through to the outside and is forced back into the casting, blowing holes in the casting surface.

Non-metallic inclusions

Non-metallic inclusions are impurities such as slag, oxides, and sulphides, which exist in the molten metal and finally the solidified metal.

Pipe/shrinkage defects

This is a cavity in the centre of the ingot/casting caused by shrinkage during solidification.  A primary pipe is surface breaking; secondary pipes are those which exist sub-surface.  The top of an ingot casting is removed to get rid of the primary pipe.

Other shrinkage defects may occur in steel castings where there is a localized variation in section thickness.  Shrinkage defects are not normally associated with gas, but a high gas content will magnify their extent.

Interdendritic shrinkage: very small shrinkage cavities associated with dendrite solidification.

Segregation

Segregation is chemical heterogeneity, or the non-uniform distribution of the alloys or impurities.  Pure metals do not exhibit segregation.

In carbon steels, the elements which segregate are those that are either insoluble or form lower freezing point complexers, e.g., sulphur, phosphorus, carbon, manganese, and silicon.

 

 

Cold shuts

A cold shut is an area of lack of fusion which may be surface breaking or sub-surface in a casting.  Cold shuts may result from splashing, surging, interrupted pouring, or the meeting of two streams of molten metal coming from different directions.

Hot tears (cracks)

Cracks caused by non-uniform cooling resulting in stresses which rupture the surface of the metal while its temperature is still in the brittle range.  They appear as ragged lines of variable width and numerous branches.  The tears may originate where stresses are set up by the more rapid cooling of thin sections that adjoin heavier masses of metal, which are slower to cool.  Curved surfaces and corners tend to promote hot tearing

A wrought product is a worked product, primarily produced by hot working, e.g., forging or rolling, although cold working is possible in some areas.

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