Industrial and Metallurgical Applications

Steel is basically an alloy of iron and carbon, consisting of an iron phase and iron carbides. Crude steel produced from iron contains an undesirable amount of oxygen and some sulphur. Manganese plays a key role because of two important properties: its ability to combine with sulphur and its powerful deoxidation capacity. When there is insufficient manganese the sulphur combines with iron to form a low melting point sulphide, which melts at hot rolling temperatures, causing a surface cracking phenomenon known as "hot shortness". Desulphurisation processes reduce the need for manganese in this respect. Some 30% of the manganese used today is still used for its properties as a sulphide former and deoxidant.

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 mechanical properties.