Hadfield Austenitic Manganese Steel
Manganese is a powerful austenite stabilizer as indicated by the Fe-Mn phase diagram (see Figure 8.2). Hadfield manganese steels containing 10 to 14% Mn and 1 to 1.4% C are austenitic at all temperatures. They are extremely wear resistant and are used in ore grinding and for teeth on earth-moving equipment. These steels work harden rapidly and as a consequence are difficult to machine. Parts are almost always cast to final shape.
Maraging steels develop high hardness by precipitation hardening of a very-low-carbon martensite. They contain about 18% Ni, 8 to 12% Co, and 4% Mo with very low carbon (<0.03%), as well as titanium (0.20 to 1.80%), and aluminum (0.10 to 0.15%). Their primary use is in tools and dies. Because the carbon content is so low, the martensite, which is formed by austenitizing at 850°C and cooling, is soft enough to be machined. Finished tooling can then be hardened by aging at 480°C for 3 hours. Whereas tools made from conventional tool steels have to be austenitized, quenched, and tempered after machining, maraging steels need only be heated to a moderate temperature to age and can be furnace cooled. This avoids oxidation, distortion, and cracking that often occurs during conventional heat treatment. The main disadvantage is the high cost that results from the high Ni, Mo, and Co contents.