A “Toolbox” for Forensic Engineers

A “Toolbox” for Forensic Engineers 97
Figure 4.6 Microstructure of a sheared rivet (A) transverse section of a fracture
surface, and (B) parallel to the line of action of force showing deformation of the
grain structure.
Finally, there is a fourth group, the malleable cast irons, which have to
be cast as wholly white iron and subsequently heated for several hours at
temperatures where the iron carbide breaks down to form clusters of graphite.
Strength is determined by whether they are produced with a ferritic or a
pearlitic matrix after the heat treatment. Figure 4.5D shows such a structure,
where the graphite appears as the ragged black areas and the matrix is equivalent
to a medium carbon steel. A casting with this type of microstructure
will be strong, tough and reasonably ductile, hence the name “malleable” cast
irons. However, the application of such malleable castings has been largely
ousted by the SG irons which are cast directly into the molds and require
virtually no subsequent treatment.
As an analytical technique, metallography is relatively simple to undertake;
it consists of four steps:
• Selection of section
• Surface preparation
• Etching
• Examination under magnification
However, since metallography is a necessarily destructive operation, care
is needed in the choice of section (and the section itself becomes part of the
evidence and so must be conserved, for possible viewing by others). Care is
also needed to ensure that the chosen section will reveal material that will
be representative of the metal under examination. The actual position of