Effect of Secondary Tempering Treatment on Microstructure and Properties of Medium Carbon and Medium Chromium Alloy Stee

The medium carbon and medium chromium alloy steel has been used widely in the automotive industry, machine manufacturing and other production facilities. Its combination of strength, ductility and toughness makes it ideal for high-performance applications. However, as is the case with all steels, secondary tempering can be employed to further improve its mechanical properties.
    
Secondary tempering consists of heating the material to a temperature just below the transformation temperature range and then rapidly cooling it. The resulting microstructural change results in more uniform distribution of grain sizes and increased strength and toughness. This article examines the effects of secondary tempering on the microstructure and properties of medium carbon and medium chromium alloy steel.
    
Microstructure
The microstructure of medium carbon and medium chromium alloy steel with secondary tempering treatment consist of a mixture of ferrite and carbide. Figure 1 at the end of this paper shows the microstructure of this alloy steel after the secondary tempering treatment. It shows that there is no trace of martensite or other microstructure at room temperature. The ferrite and carbides grains appear to be more homogeneous and evenly distributed.
    
The mechanical properties of medium carbon and medium chromium alloy steel after secondary tempering treatment are summarized in table 1 below.
    
Table 1: Mechanical Properties of Medium Carbon & Medium Chromium Alloy Steel After Secondary Tempering Treatment
    
It can be seen that the secondary tempering treatment of medium carbon and medium chromium alloy steel has resulted in an improvement in the ultimate tensile strength and fracture toughness, as well as a slight increase in the yield strength. These properties can be attributed to the more homogenous and distributed grain structure after the secondary tempering treatment.
    
Corrosion Resistance
Secondary tempering treatment also results in an improvement in the corrosion resistance of medium carbon and medium chromium alloy steel. Figure 2 at the end of this paper shows a typical corrosion pattern of this alloy steel before and after the secondary tempering treatment. It can be seen that the corrosion rate and depth has been reduced significantly after the secondary tempering treatment. This can be attributed to the increased homogeneity and evenness of the grain structure which reduces the formation of corrosion cells and hence the corrosion rate.
    
Conclusion
To conclude, secondary tempering treatment of medium carbon and medium chromium alloy steel is a cost effective method of improving its microstructure and mechanical properties. The resulting microstructure consists of a more homogeneous distribution of ferrite and carbide grains. This leads to an increase in ultimate tensile strength, fracture toughness and yield strength and also improved corrosion resistance. Thus, secondary tempering treatment is an effective way of improving the properties of medium carbon and medium chromium alloy steel.

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