Microstructure and properties of Fe-Mn-C medium manganese steel treated by reverse phase transformation annealing

Medium manganese steel is a commonly used structural steel in engineering applications. It is an important engineering material mainly used for the production of car structures, bridges, shipbuilding, wind power and other parts due to its good combination of strength and elasticity. It is also known for its superior toughness and wear resistance. This is due to the presence of small amounts of manganese in its composition. The mechanical properties of medium manganese steel can be further improved by its heat treatment, such as reverse phase transformation annealing. The microstructure and properties of Fe-Mn-C medium manganese steel after reverse phase transformation annealing are discussed in this article.
    
Structure of Medium Manganese Steel
Medium manganese steel is made up of iron, manganese, and carbon. The chemical composition of Fe-Mn-C medium manganese steel is approximately 0.2–2.0 wt% C, 0.8–1.3 wt% Mn and balance Fe. The carbon is mostly present in the form of pearlite. The pearlite is composed of lamellar ferrite and cementite in which the cementite is present in the form of plates, laths or needles. The ferrite is a soft, ductile and malleable matrix which improves the ductility of the material. The cementite is a very hard and brittle intermetallic compound. It is responsible for hardening and strength of the medium manganese steels.
    
Treatment of Medium Manganese Steel
Reverse transformation annealing is a heat treatment process used for medium manganese steels. In this process, the steel is first heated in a furnace to austenitizing temperature and then subjected to a rapid cooling process. This results in the formation of tempering carbide particles which are also known as Widmanstätten carbides. The carbides reduce the graphitization of the cementite, thus improving the strength and wear resistance of the material.
    
Microstructure and Properties
Shows the microstructure of Fe-Mn-C medium manganese steel after reverse transformation annealing. As can be seen, the microstructure consists of a ferrite matrix with tempered carbides distributed in it. The tempered carbides have a characteristic Widmanstätten pattern which is caused by their ordered placement in the ferrite matrix. The tempering carbide particles are responsible for the improved mechanical properties such as higher strength, hardness, wear resistance and better wear behavior.
    
Microstructure of Fe-Mn-C medium manganese steel after reverse transformation annealing.
    
Table 1 shows the mechanical properties of Fe-Mn-C medium manganese steel after reverse transformation annealing. As can be seen, the tensile strength, yield strength and hardness are improved after the heat treatment. The wear resistance is also improved due to the presence of tempered carbides which reduce the graphitization of the cementite in the ferrite matrix.
    
TABLE 1: Mechanical Properties of Fe-Mn-C Medium Manganese Steel
    
Conclusion
In conclusion, the microstructure and properties of Fe-Mn-C medium manganese steel can be improved by its heat treatment, such as reverse phase transformation annealing. The process results in the formation of Widmanstätten carbides which improve the mechanical properties like tensile strength, yield strength and hardness. It also results in improved wear resistance due to the reduced graphitization of the cementite in the ferrite matrix.

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