Industrial and Metallurgical Applications
Manganese and Steelmaking
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
The other 70% of the manganese is used purely as an
alloying element. These alloying uses depend on the
desired properties of the steel being made. Steel, as
has been noted, contains iron and carbon. At room temperature,
iron crystallises 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 is its ability to stabilize
the austenite in steel, as does nickel. Since
manganese is not as powerful as nickel in its
ability to stabilize austenite, more manganese
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 hardenability rate, used
to significant advantage, depending on the steel
type and the end product, to improve mechanical
Manganese Content in Steel Today
The bulk of steel production results in multi-purpose low carbon steels containing from 0.15% to 0.8% manganese. A large proportion consists of low carbon steel sheets with less than 0.3% manganese, some with even less than 0.2% for extra deep drawing qualities. High strength steels with a yield strength over 500MPa, and representing 3 to 4% of the tonnage of steel produced, contain over 1% manganese. A large number of these are high strength low alloy steels (HSLA). They are low carbon, controlled-rolled steels containing higher manganese levels (1.0% to 1.8%), taking advantage of its beneficial effect on the austenitic transformation temperature to obtain a very fine ferrite structure. Micro-alloying additions help to refine the structure or to strengthen the steel through carbide or nitride precipitates which are evenly distributed in the ferrite matrix. These steels are widely used for oil/gas pipelines, shipbuilding and in transportation equipment in order to reduce weight.
Engineering steels comprise either HSLA or heat-treated
either pure chromium-manganese grades, or with nickel,
chromium, molybdenum, vanadium additives and often 0.6
to 0.8% manganese. A few grades containing 1.0% to 1.5%
manganese (as well as chromium or boron) are popular
with the automobile industry.
Stainless steels which represent less than 2% of total
world steel production make use of chromium and nickel.
They also contain about 1% manganese. There are also
manganese-stainless steels, where nickel is replaced
partly or entirely by manganese, giving a manganese
content of 4 to 16%. These are not yet produced in large
quantities, but they could develop in the future depending
on the evolution of the nickel price compared with manganese
and on the marketing effort devoted to them. Large amounts
were produced in India in the 1980s.
Any overview of high Mn steels must include Hadfield
steel, named after its 19th century UK inventor and
the first alloy steel ever invented. This steel contains
13% or more manganese. It has unique properties which
make it indispensable for applications in which great
toughness and wear resistance are required. Among these
can be cited gyratory crushers, jaw-crusher plates,
railway points and crossover components, teeth for earth-moving
equipment, etc. High manganese (10-12%) non-magnetic
steels are used for such products as retainer rings
for turbo alternators and collars on oil rigs. Grades
with a similar chemistry are used as cryogenic steels.
A high manganese stainless "memory" steel
has been developed.