Mangalloy

Label on a bicycle frame indicating mangalloy

Mangalloy, also called manganese steel or Hadfield steel, is a steel alloy containing an average of around 13% manganese. Mangalloy is known for its high impact strength and resistance to abrasion once in its work-hardened state.

Material properties

Mangalloy is made by alloying steel, containing 0.8 to 1.25% carbon, with 11 to 15% manganese.^[1] Mangalloy is a unique non-magnetic steel with extreme anti-wear properties. The material is very resistant to abrasion and will achieve up to three times its surface hardness during conditions of impact, without any increase in brittleness which is usually associated with hardness.^[2] This allows mangalloy to retain its toughness.

Most steels contain 0.15 to 0.8% manganese. High strength alloys often contain 1 to 1.8% manganese.^[3][4][5] At about 1.5% manganese content, the steel becomes brittle, and this trait increases until about 4 to 5% manganese content is reached. At this point, the steel will pulverize at the strike of a hammer. Further increase in the manganese content will increase both hardness and ductility. At around 10% manganese content the steel will remain in its austenite form at room temperature if cooled correctly.^[6] Both hardness and ductility reach their highest points around 12%, depending on other alloying agents.^[1] The primary of these alloying agents is carbon, because the addition of manganese to low-carbon steel has little effect, but increases dramatically with increasing carbon content. The original Hadfield steel contained about 1.0% carbon. Other alloying agents may include metals like nickel and chromium; added most often to austenitic steels as an austenite stabilizer; molybdenum and vanadium; used in non-austenitic steels as a ferrite stabilizer; or even non-metallic elements such as silicon. ^[7]

Mangalloy has fair yield strength but very high tensile strength, typically anywhere between 350 and 900 megapascals (MPa), which rises rapidly as it work hardens. Unlike other forms of steel, when stretched to the breaking point, the material does not "neck down" (get smaller at the weakest point) and then tear apart. Instead, the metal necks and work hardens, increasing the tensile strength to very high levels, sometimes as high as 2000 MPa. This causes the adjacent material to neck down, harden, and this continues until the entire piece is much longer and thinner. The typical elongation can be anywhere from 18 to 65%, depending on both the exact composition of the alloy and prior heat-treatments. Alloys with manganese contents ranging from 12 to 30% are able to resist the brittle effects of cold, sometimes to temperatures in the range of −196 °F (−127 °C).^[8][9]

Mangalloy has been used in the mining industry, cement mixers, rock crushers, railway switches and crossings, crawler treads for tractors and other high impact and abrasive environments. It is also used in high impact environments like inside a shot peening machine. These alloys are finding new uses as cryogenic steels, due to their high strength at very low temperatures. Mangalloy is heat treatable, but the manganese lowers the temperature at which austenite transforms into ferrite. Unlike carbon steel, mangalloy softens rather than hardens when rapidly cooled, restoring the ductility from a work-hardened state. Most grades are ready for use after annealing and then quenching from a yellow heat, with no further need of tempering, and usually have a normal Brinell hardness of around 200 HB, (roughly the same as 304 stainless steel), but, due to its unique properties, the indentation hardness has very little effect on determining the scratch hardness (the abrasion and impact resistance of the metal).^[10] Another source says that the basic Brinnell hardness of manganese steel according to the original Hadfield specification is 220 but that with impact wear the surface hardness will increase to over 550.^[11]

Many of mangalloy's uses are often limited by its difficulty in machining; sometimes described as having "zero machinability."^[12] The metal cannot be softened by annealing and hardens rapidly under cutting and grinding tools, usually requiring special tooling to machine. The material can be drilled with extreme difficulty using diamond or carbide. Although it can be forged from a yellow heat, it may crumble if hammered when white-hot, and is much tougher than carbon steel when heated.^[1] It can be cut with an oxy-acetylene torch, but plasma or laser cutting is the preferred method.^[13] Despite its extreme hardness and tensile strength, the material may not always be rigid.^[1] It can be formed by cold rolling or cold bending.^[13]

History

World War I Brodie helmet, made from Hadfield steel

In 1816, a German researcher Carl J. B. Karsten ^[14] noted that adding fairly large amounts of manganese to iron would increase its hardness without affecting its malleability and toughness,^[3] but the mix was not homogeneous and the results of the experiment were not considered to be reliable.^[15] "and no one understood that the real reason why the iron mined in Noricum produced such superb steel lay in the fact that it contained a small amount of manganese uncontaminated by phosphorus, arsenic, or sulphur, and so was the raw material of manganese steel."^[16] In 1860, Sir Henry Bessemer, trying to perfect his Bessemer process of steel making, found that adding manganese and carbon to the steel after it was blown helped to remove excess sulfur and oxygen.^[3] Sulfur combines with iron to form a sulfide that has a lower melting point than steel, causing weak spots, which prevented hot rolling. Manganese is usually alloyed with most modern steels because of its powerful ability to remove sulfur, phosphorus and oxygen, which are all common impurities in steel.^[3] Mangalloy was the first alloy created from carbon steel,^[3] also called Hadfield steel after its British inventor, Sir Robert Hadfield, in 1882.^[10]

Hadfield was in search of a steel that could be used for casting which would exhibit both hardness and toughness, since ordinary carbon steels do not combine those properties. Steel can be hardened by rapid cooling, but loses its toughness, becoming brittle. Steel castings can not usually be cooled rapidly, for irregular shapes can warp or crack. Mangalloy proved to be extremely suitable for casting, as it did not form gas pockets called "blow-holes", and did not display the extreme brittleness of other castings.^[17]

Upon creating mangalloy, Hadfield tested the material, thinking that the results must have been erroneous. After performing several hundred tests, he realized that they must be accurate, although the reason for the combination of hardness and toughness defied any explanation at the time. Hadfield wrote, "Is there any case similar to this among other alloys of iron, if the term alloy may be used? No metallurgical treatise refers to them;...Possibly when the nature of the laws governing alloys is better understood, this will be found to be only one of other cases,...".^[18] Today it is known that manganese inhibits the transformation of the malleable austenite phase into hard brittle martensite that takes place for normal steels when they are quenched in the hardening procedure. The austenite of Hadfield steels is thermodynamically unstable and will transform into martensite when subject to mechanical impact thus forming the hard surface layer.

References