Welding Inspection & Metallurgy API ICP Self Study Notes

Alloys may also consist of more than one microstructural phase and crystal
structure. A number of copper alloys including some brasses are composed
of two distinct phases. Plain carbon steel is also a two-phase alloy. One
phase is a relatively pure form of iron called ferrite. By itself, ferrite is a fairly
weak material. With the addition of more than 0.06 percent carbon, a second
phase called pearlite is formed which adds strength to steel. Pearlite is a
lamellar (i.e. plate-like) mixture of ferrite and Fe3C iron carbide. As a result of
fast cooling such as quenching in non-alloyed steels and also with the
addition of alloying elements such as chromium to steel, other phases may
form. Rather than pearlite, phases such as (1) bainite or (2) martensite may
be produced. These phases tend to increase the strength and hardness of the
metal with some loss of ductility. The formation of structures such as bainite
and martensite may also be the result of rapid or controlled cooling and
reheating within certain temperature ranges often termed “quenching” and
“tempering.”