A “Toolbox” for Forensic Engineers

32 Forensic Materials Engineering: Case Studies
Figure 2.3 Casting shrinkage in a section cut from a wheel hub.
give no clue as to its mechanical strength or structural condition yet have
disastrous consequences on performance in service. Perhaps the best example
is the selection and heat treatment of steels to develop the desired combination
of hardness, strength and toughness to meet the service requirements
of a vast number of diverse engineering products, ranging from cutting tools,
hammers and dies for working other materials, to all manner of structural
fabrications and commodities used in the modern world. Few tests can be
carried out on a finished product without damaging it or destroying parts
beyond use, so if it meets the dimensional specification, is certified as having
been made from the correct material and exhibits the appropriate surface
finish, all that can be done in the way of quality control is nondestructive
testing. However, such tests did not prevent a serious accident caused by the
failure of a vital steel pin shortly after being put into service; the pin should
have been heat treated to give a tough core and a hard, brittle outer skin, but
instead it had received the wrong heat treatment, rendering it hard and brittle
throughout. Consequently, the pin failed the first time it was subjected to a
shock load that would easily have been withstood if the part had received the
correct treatment.
The pouring of molten metal into molds is widely used for the manufacture
of complex shapes, as well as being the first stage in the manufacture
of wrought products. However, casting can lead to a variety of defects and
some are extremely difficult if not impossible to eliminate entirely in a large
product, shrinkage in particular. Figure 2.3 is the cross section of a wheel
hub for a trailer made by casting liquid steel into a sand mold. The mold
should have incorporated a reservoir of liquid of sufficient size and should
have been so placed as to feed further liquid as the casting solidified in order