High manganese steel, also known as Hadfield steel, was invented by Robert Hadfield in 1882. Ordinary high manganese steel is commonly used in high manganese steel bucket teeth. The chemical composition (mass fraction) is: 0.9% ~ 1.5% C, 10% ~ 15% Mn, 0.3% ~ 1.0% Si, the content of S is not higher than 0.05%, the content of P is not higher than 0.10%, and when w (MN) / w (c) is about 10, it is easy to get a good strength toughness combination.
The room temperature structure of as cast ordinary high manganese steel is generally austenite plus carbide. After water toughening treatment at 1000 ~ 1100 ℃, a single austenite structure is obtained, which has high impact toughness and good work hardening performance. The hardness is 180 ~ 240hb, the yield strength is not less than 350Mpa, and the impact absorption energy of U-shaped notch is not less than 118j.
The alloying elements carbon, silicon and phosphorus in high manganese steel have important effects. It is found in production practice that the service life of high manganese steel is directly proportional to the carbon content. When the carbon content exceeds 1.0%, the wear resistance increases by 5% ~ 10% for every 0.1% C increase.
Carbon atoms are solid soluble in austenite, but when the carbon content is too high, it forms brittle carbide (Fe, Mn) 3C with iron and manganese, which is distributed in a network at the grain boundary, reduces the toughness of steel, and is very easy to form cracks, resulting in bucket tooth fracture. Therefore, the carbon content should be controlled at 0.9% ~ 1.2%. The silicon content of high manganese steel is often controlled below 0.9%. Phosphorus is a harmful element in high manganese steel. It has low solubility in austenite and is easy to form brittle phosphorus eutectic. Increasing phosphorus content will increase the cracking tendency of steel. When the phosphorus content decreases from 0.07% ~ 0.09% to 0.02% ~ 0.05%, the service life is prolonged by 35% ~ 40%.
Under the action of large impact load and stress, the metal surface of high manganese steel will produce work hardening, and its work hardening index is 5 ~ 6 times that of other steels, but the core maintains the original hardness and good toughness. A large number of studies show that high manganese steel will produce cold deformation under impact load, and the dislocation density will increase greatly. The delivery and stacking of dislocations and the interaction between dislocations and solute atoms have a significant strengthening effect; Moreover, the stacking fault energy of high manganese austenitic steel is low, and stacking fault is easy to occur during deformation, which can promote ε The formation of martensite and deformation twins improve the hardening effect
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However, the work hardening layer produced during the use of high manganese steel bucket teeth is generally less than 1mm, and the method of contact explosion can be used to achieve the purpose of deep hardening. For example, the explosion hardening of electric shovel bucket teeth is carried out by the blasting company of krivorog mining area in Russia, and the initial hardness of workpiece surface is increased from 180 ~ 220HB to 380 ~ 400HB, saving 2400t of high manganese steel every year. However, it is found that the work hardening peak of high manganese steel is in the sub surface layer, and the microstructure will initiate cracks due to embrittlement, resulting in the spalling of the worn surface.