Forensics

Casting Failures

Forensic Services Newsletter

The progress of technology can give the impression that premature failures will become a thing of the past. In respect of castings, the techniques of vacuum degassing and computer simulation of hot metal flow paths, combined with advances in ultrasonic and radiographic testing, would lead one to think that serious casting defects cannot occur in today's world.

However in the past year Forensic Services have linked machinery failures to casting defects of surprising grossness. In two cases, the defects were so severe that it was apparent the problem wasn't that they were unseen during manufacture, but that the manufacturers were determined to ignore them. Both machines were extrusion presses, one made in China and the other in Taiwan, and the following photographs illustrate how human nature remains the same, despite the march of progress.

The first indication of a problem with this Malaysian-located aluminium extrusion press was the detection of cracks as shown above. Note the apparent excellent surface finish of the casting. It is the practice to use body filler on the surface of a casting to hide imperfections, in the same way that a car workshop will repair a body panel.

When the casting was fractured open, as shown above, it became apparent that fatigue cracks had initiated at internal casting defects, one of which is shown below on the left.

The dismantling of the press revealed unpainted machines surfaces with defects that, as can be seen above, would have been very apparent to the manufacturing machinist, yet there was no attempt at repair.

The second extrusion press, installed in Jakarta for the manufacture of brass fittings, is shown above on the left. A layer of non-metallic filler is shown on the right, to improve the surface appearance.

The fractured portion of the press is shown above. The force of the fracture caused recessed surfaces to apparently disintegrate, as shown on the right, and it became apparent that after casting the surfaces had not been fettled to remove the casting sand, which had simply been painted over, as if the sand was casting.

The disintegration of the sand revealed pieces of wire and welding rod, used to keep the sand in place. If a casting is not fettled the metal surface cannot be inspected and defects will remain unseen. For this casting there was not even a distinct line of separation between metal and sand, as can be seen below on the left. As for the first example, dismantling of the machine revealed gross defects on machined surfaces, as can be seen below right.

Needless to say, for each loss the identification of the casting defects and their contribution to failure was relevant to whether liability was accepted. Forensic Services is able to provide expert metallurgical opinion on the causes of failures involving castings of all sizes.

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