MANGANESE
Manganese adds considerably to the tensile strength of steel, but this is dependent on the carbon content. High carbon materially adds to the brittleness, whereas low-carbon, pearlitic-manganese steels are very tough and ductile and are not at all brittle, providing the heat-treating is correct. Manganese steel is very susceptible to high temperatures and prolonged heating.
In low-carbon pearlitic steels, manganese is more effective in increasing ultimate strength than is nickel; that is to say, a 0.45 carbon steel with 1.25 per cent manganese is as strong as a 0.45 carbon steel with 1.5 per cent nickel. The former steel is much used for rifle barrels, and in the heat-treated condition will give 80,000 to 90,000 lb. per square inch elastic limit, 115,000 to 125,000 lb. per square inch tensile strength, 23 per cent elongation, and 55 per cent reduction in area.
Manganese when added to steel has the effect of lowering the critical range; 1 per cent manganese will lower the upper critical point 60°F. The action of manganese is very similar to that of nickel in this respect, only twice as powerful. As an instance, 1 per cent nickel would have the effect of lowering the upper critical range from 25 to 30°F.
Low-carbon pearlitic-manganese steel, heat-treated, will give dynamic strength which cannot be equaled by low-priced and necessarily low-content nickel steels. In many instances, it is preferable to use high-grade manganese steel, rather than low-content nickel steel.
High-manganese steels or austenite manganese steels are used for a variety of purposes where great resistance to abrasion is required, the percentage of manganese being from 11 to 14 per cent, and carbon 1 to 1.5 per cent. This steel is practically valueless unless heat-treated; that is, heated to about yellow red and quenched in ice water. The structure is then austenite and the air-cooled structure of this steel is martensite. Therefore this steel has to be heated and very rapidly cooled to obtain the ductile austenite structure.
Manganese between 2 and 7 per cent is a very brittle material when the carbon is about 1 per cent or higher and is, therefore, quite valueless. Below 2 per cent manganese steel low in carbon is very ductile and tough steel.
The high-content manganese steels are known as the "Hadfield manganese steels," having been developed by Sir Robert Hadfield. Small additions of chrome up to 1 per cent increase the elastic limit of low-carbon pearlitic-manganese steels without affecting the steel in its resistance to shock, but materially decrease the percentage of elongation.
Vanadium added to low-carbon pearlitic manganese steel has a very marked effect, increasing greatly the dynamic strength and changing slightly the susceptibility of this steel to heat treatments, giving a greater margin for the hardening temperature. Manganese steel with added vanadium is most efficient when heat-treated.