A “Toolbox” for Forensic Engineers

96 Forensic Materials Engineering: Case Studies
Figure 4.5 Microstructures of four different types of cast iron. Linear magnification:
75¥. (A) Flake graphite (“gray”) cast iron. (B) Nodular (spheroidal
graphite) cast iron. (C) Chill (“white”) cast iron where carbon is present as Fe 3C.
(D) Malleable (blackheart) iron.
plane section, the “lettuces” look like separate flakes, but in three dimensions,
each group of flakes has grown from the same nucleus.
By making additions to the ladle of fractional percentages of either magnesium
or cerium (a rare-earth metal), the graphite separates in the form of
spheroids, quite separate from each other, in matrices that again can be
anything from the equivalent of pure, soft iron to a high strength steel. These
are called “nodular” or “spheroidal graphite” (S.G.) cast irons. 4
Figure 4.5A shows a typical microsection of a gray iron casting and Figure
4.5B shows a typical microsection of a ferritic SG iron, both as polished and
photographed at a linear magnification of 75¥. By making a casting solidify
in the mold very quickly, the carbon does not appear as graphite but instead
separates as iron carbide (Fe 3C), rendering the casting extremely hard and
brittle. Figure 4.5C shows such a structure, the section polished and etched
but again as a linear magnification of 75¥. If a casting with such a microstructure
is broken it exposes a silvery white, sparkling fracture, which is why
these types of casting are called “white” cast irons. For certain applications
where high hardness and maximum abrasion resistance are required, for
example, the nose of a plowshare or the surface of a roll for cold working
metals, castings are often made with chills inserted in the molds so that the
metal that solidifies in contact with them develops the iron carbide microstructure.