High Manganese Austenitic Steels: Part One
The main advantages of high manganese inclusion can include increased ductility, very high toughness and much improved tribology characteristics in the materials resistance to abrasion.
Some of the key applications of high manganese austenitic steels include railway crossings and impact hammers due to an excellent work hardening rate.
Manganese increases the ductility of the metal and adds greatly to its toughness and resistance to abrasive action. Sir Robert Abbott Hadfield was an English metallurgist noted for his 1882 discovery of manganese steel, one of the first alloys steel. His invention was based on adding large percentage of manganese to molten iron whereby producing a steel that have good toughness and hardness while possess distinguished characteristics. After a number of experiments performed by sir Hadfield, a conclusion was made that a steel having good toughness and hardness while possess distinguished characteristics can be found when the Mn content is between 11-13% Mn and 1-1.3% carbon with a Mn\C ratio of 10:1.
Austenitic manganese steel has certain properties that restrict its use. It is difficult to machine and usually has a yield strength of only 345 to 415 MPa. Consequently, it is not well suited for parts that require close-tolerance machining or that must resist plastic deformation when highly stressed in service. However, hammering, pressing, cold rolling or explosion shocking of the surface raises the yield strength to provide a hard surface on a tough core structure.
Due to its unique service properties, with a full austenite microstructure at room temperature, has been receiving much attention, and it is used in a variety of applications such as railway crossings, crawler treads for tractors and impact hammers, because of its excellent work-hardening rate, high toughness and high wear resistance. Many efforts have been made to study the deformation mechanisms of Hadfield steel, with an attempt to correlate the excellent work hardening properties of the steel with its evolved microstructures.
Hadfield’s austenitic manganese steel is still used extensively, with minor modifications in compositions and heat treatment. ASTM Standard A-128-64 covering this steel allows composition ranges from 1.0 to 1.4%C and from 10 to 14%Mn. However, commercial alloys with manganese contents greater than 12to 13% are seldom used because of cost. Moreover, work hardening in a 1.15% C alloy reaches a maximum at 13% Mn. Hadfield steel is used usually austenitized to dissolve carbides and to produce homogeneous austenite, which is preserved by water quenching from above 1000°C. So it is stable, single phase, austenitic alloy which is annealed and quenched before use to retain all the carbon in supersaturated solid solution. Typical properties are 0.2% offset yield strength 379 MPa, ultimate tensile strength 965 MPa, elongation in 50 mm 50%, reduction of area 40%, as-quenched hardness 190 HB, hardness, at fracture, 500 HB.
Table 1: Standard composition ranges for austenitic manganese steel castings
Figure 1: Phase diagram of steel containing 13% manganese