Page 197 - Materials Chemistry, Second Edition

184 3 Metals

creating a strengthening effect. That is, metal atoms have a lesser range of move-
ment due to the “glue” formed by interstitial carbon atoms. As a result, external
forces such as temperature and pressure will not as readily cause atomic movement
and surface/bulk deformation or fracturing. This is the reason why pure iron is not
particularly hard or physically durable, but steels are significantly improved in these
properties. Slow cooling of carbon-rich iron will yield a supersaturated solid solu-

tion. The carbon solubility in austenite decreases from about 1.7% at 1,150 Cto

about 0.7% at 715 C, causing the precipitation of the excess carbon in the form of
microscopic carbides or graphitic nuclei.
Supersaturated iron lattices yield a material known as cast iron, a ternary Fe–C–Si
alloy containing much higher carbon than steel, typically around 3–5 wt.% C. As the
name implies, these materials are cast from their molten states into molds to yield
the desired shapes. Due to the oversaturation of carbon present in these solids, cast
iron is not suitable for structural applications. However, cast iron is extremely
inexpensive to produce, making this material one of the most heavily used materials
in industry for the manufacture of tools, valves, and automotive parts. Cast iron
cookware has been employed for culinary applications since the late nineteenth
century. However, with the advent of nonstick coatings such as Teflon in the 1940s,
this application has largely been abandoned in favor of coated aluminum pans.
A useful form of cast iron known as “Duriron,” features a high silicon concentration
(13–16 wt.% Si, relative to standard cast irons with 1–3 wt.% Si), and is resistant to
strong acids and high temperatures.
There are a variety of cast irons, each differing in the nature of the carbon
impurity associated with austenite within the iron lattice. For instance, white and
gray cast irons contain cementite and graphite nuclei within the microstructure,
respectively. The graphitic suspensions may be present as flakes (gray cast iron), or
as spheres (ductile and malleable cast iron) depending on the cooling conditions
employed. For gray cast irons, the formation of iron carbide must be minimized in
order to prevent localized hard spots that would degrade ductility and machinability.
A number of dopants may be added to facilitate the preferential formation of
graphite rather than cementite. As we have discussed earlier, the excess carbon
precipitated from supersaturated iron will most often yield cementite. This is
especially intriguing, since the formation of graphite actually represents the low-
est-energy alternative for the Fe–C system. However, as a carbon-rich pure Fe/C
alloy is cooled, the localized density of carbon atoms is never enough to serve as a
nucleus for graphite formation. Rather, since the carbon is distributed throughout the
lattice, the intimate combination of iron and carbon atoms makes it relatively easy to
form Fe 3 C nuclei, relieving the supersaturation and lowering the overall energy of
the system. On the other hand, if a dopant is added to serve as a nucleation site, the
formation of graphite will occur due to more favorable thermodynamics.
It is proposed that the major nucleation mechanism in cast iron doping, known as
inoculation, is the formation of sulfide species upon the addition of strong sulfide
formers such as calcium, barium, cerium, or strontium. These sulfides possess lattice
parameters very similar to the graphite crystal structure, serving as substrates for