If the two metals that form the alloy are insoluble in one another, then they will
exist as two separate phases, often in alternate layers such as observed in
tin–lead alloys or cast irons, where the carbon is often found as minute tadpole
like shape (flakes) adjacent to the pure iron. These types of two-phase alloys
are extremely difficult if not impossible to shape by hot or cold working. Fortu-
nately, these alloys have a melting point well below that of the parent metals
and are very suitable to shape by casting into moulds. This is the reason why
iron 4.5% carbon alloys were called cast irons. These alloys have two important
limitations in that first, they are very brittle when subjected to impact loads, and
second, their corrosion resistance is inferior to pure metals or single-phase alloys.
Metals and alloys are commonly divided into two major types, Ferrous or Non-
Ferrous. The former are alloys of iron while the latter includes the remainder of
the metals including copper, lead, tin, silver, gold and aluminium and alloys of
these metals. Ferrous materials can be divided into three types as follows:
(1) Wrought iron
(2) Cast iron
(3) SteelWrought iron is essentially pure iron with particles of slag from the refin-
ing method still included in the structure. Wrought iron melts at 1535°C and
is relatively soft and ductile and, therefore, is able to be shaped by such tech-
niques as forging and hammering. As a result of these shaping processes, the
slag orientates itself in the direction of working (streaks of slag) and the end
result is a fibrous structure.
Cast iron is an alloy of iron and carbon with the latter ranging from 2% to
4.5%. The melting point of this alloy is between 1150°C and 1200°C, which
is considerably lower than wrought iron. The carbon is found in the cast iron
as graphite, which can take the shape of flakes (grey cast iron) or spheres
(spheroidal or ductile cast iron). In some instances, the carbon will be in the
form of iron carbide (cementite, Fe 3 C) and the alloy was called white cast iron.
Both forms of carbon make the cast iron a very brittle material and thus it was
impossible to shape it by forging or hammering. Fortunately, due to the lower
melting point of cast iron, it was possible to melt the iron and cast it into the
required shape.
Steels are alloys of iron and up to 1.7% carbon, although steels are not usu-
ally found with more than 1.2% carbon. The importance of steels is that their
mechanical properties are greatly influenced by their carbon content. As the
carbon increases in the steel, the ductility goes down while the hardness and
tensile strength go up. A further important consideration is that the hardness of
steels can be dramatically increased to even higher levels by a process of heat-
ing to above 800°C and quenching in water or some other fluid such as urine
130 Chapter 6