The other 70% of the manganese is used purely as an alloying element.
These alloying uses depend on the properties of steel. Steel, as has been
noted, contains iron and carbon. At room temperature, iron crystallizes into a
body-centred cubic structure named alpha iron (ferrite). At a high temperature
(above 910 degrees C), the structure is transformed into a face-centred cubic
form, which is called a gamma iron (austenite). When the steel is cooled down
slowly, the carbon, soluble in austenite, precipitates as an iron carbide
called cementite, the austenite transforms to ferrite and they precipitate
together in a characteristic lamellar structure known as pearlite.
Manganese plays an important role as it lowers the temperature at which
austenite transforms into ferrite, thus avoiding cementite precipitation at
ferrite grain boundaries, and by refining the resulting pearlitic structures.
The strength and toughness of steel depend, first of all, on the grain size and
the volume fraction of pearlite contained. Alloying elements, including
manganese, also contribute some solution-hardening of the ferrite, but this
effect is limited compared to that of carbon, nitrogen, phosphorus and even
silicon. When the cooling process is accelerated by quenching, austenite
transforms into structures with high strength such as bainite and martensite.
Manganese improves the response of steel to quenching by its effect on
the transformation temperature. Manganese is also a weak carbide former. Both
properties are advantageous in heat treated steels specified by mechanical
engineers. Another important property of manganese, like nickel, is its ability
to stabilise the austenite in steel. Since manganese is not as powerful as
nickel in its ability to stabilise austenite, more of it is required to achieve
the same effect. However, manganese has the advantage of being much less
expensive. The effect of manganese in forming austenite can be reinforced by
combining it with nitrogen, which is also an austenite-forming element.
Manganese also increases the work hardening rate, used to significant
advantage, depending on the steel type and the end product, to improve