Effect of cooling rate on microstructure and hardness of medium manganese martensitic wear-resistant steel

    
    The medium manganese martensitic wear-resistant steel is a kind of superior high-strength and wear-resistant steel. With alloy composition optimization and cooling rate adjustment, higher mechanical properties and better wear resistance can be obtained. This paper mainly discusses the effect of different cooling rates on the microstructure and hardness of medium manganese martensitic wear-resistant steel.
    
    Medium manganese martensitic wear-resistant steel has a higher carbon content, usually between 0.60% and 1.20%, and a lower manganese content, usually between 1.00% and 1.80%. Mineralogical characterization reveals the presence of two trace elements, alloying elements and residual elements. In addition to these, a small amount of sulfur, phosphorus, oxygen and nitrogen is usually present.
    
    Under different cooling rates, the microstructure of medium manganese martensitic wear-resistant steel changes, and the hardness and strength also change significantly. Figure 1 shows the microstructure of medium manganese martensitic wear-resistant steel at different cooling rates. When the cooling rate is slow, the microstructure is a large columnar Ferrite and pearlite, with the pearlite distributed in the center and around the sides, forming a ferrite column structure. With the increase of cooling rate, martensite or bainite or martensite + bainite will be obtained, and the microstructure changes from dual-phase pearlite to single-phase martensite or bainite, or a mixture of martensite and bainite.
    
    At low cooling rates below 10°C/s, the microstructure is mainly composed of Ferrite, pearlitic and a small amount of carbide. Figure 2 shows the relationship between cooling rate and hardness of medium manganese martensitic wear-resistant steel. It can be seen from the figure that with the increase of cooling rate, the hardness of medium manganese martensitic wear-resistant steel increases accordingly and the hardness peak is at 10 °C/s. With the further increase of cooling rate, the hardness drops slightly, but is still higher than the original low-temperature hardness.
    
    Besides, the tensile strength of medium manganese martensitic wear-resistant steel increases with the increase of cooling rate, as shown in Figure 3. As the cooling rate increases, the size and uneven distribution of martensite are also increased, so that the hardenability increases. At high temperature and low cooling rate, the martensite nucleation and propagation are small, resulting in small martensite with small grain size, so that the strength is low and the toughness is high. At high cooling rate, the martensite nucleation and propagation are more sufficient, and more martensite with larger and more dispersed grains is obtained, so that the strength is high and the toughness is low.
    
    In conclusion, different cooling rates have a significant effect on the microstructure and hardness of medium manganese martensitic wear-resistant steel. The microstructure changes from ferrite-pearlite dual-phase to single-phase martensite or bainite, or martensite + bainite dual phase. The hardness of medium manganese martensitic wear-resistant steel increases with the increase of cooling rate, while the peak value appears at 10°C/s. The tensile strength of medium manganese martensitic wear-resistant steel is proportional to the cooling rate and the hardenability increases accordingly.

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