Effect of Tempering Temperature on Microstructure and Properties of Nb Microalloyed NM500 Steel

The niobium microalloyed NM500 steel is a low-carbon ultra-hard steel which has excellent mechanical properties and wear resistance. It is widely used in industrial applications such as engineering structures and heavy-duty construction equipment. The mechanical properties of NM500 steel depend largely on its microstructure, which is directly affected by the heat treatment process. To improve the properties of NM500 steel, proper heat treatment is essential. One of the important processes for heat treatment of NM500 steel is tempering, which is done to refine the microstructure of the steel and reduce the hardness. In this paper, the effect of tempering temperature on the microstructure and properties of Niobium microalloyed NM500 steel is discussed.
    
The microstructure of Niobium microalloyed NM500 steel consists of martensite, bainite, ferrite and carbide, which are formed during the heat treatment process. Tempering is one of the important steps during the heat treatment process as it helps in refining the microstructure and improving the strength and wear resistance of NM500 steel. Different tempering temperatures have different effects on the microstructure and properties of NM500 steel.
    
At low tempering temperatures of approximately 150-250°C, the microstructure of the steel mainly consists of small lath martensite, a mixture of bainite and ferrite, and tempered carbides. This microstructure has superior strength and wear resistance, but poor toughness. At moderate tempering temperatures of about 250-460°C, microstructure mainly consists of large blocks and plates of bainite, small amounts of ferrite and tempered carbides. This microstructure has good strength, toughness and wear resistance, but low hardness. At higher tempering temperatures of 460-650°C, the microstructure primarily contains tempered ferrite and tempered carbides. This microstructure has relatively low strength but excellent ductility and toughness.
    
The overall mechanical properties of Niobium microalloyed NM500 steel depend on the tempering temperature used. At lower tempering temperatures, the yield strength, ultimate tensile strength and hardness of the steel increases, whereas the elongation and impact energy decreases. With increasing tempering temperature, these properties decrease slightly. This is due to the fact that the microstructure at higher tempering temperatures consists of softer ferrite, harder carbides and finer bainite, which can reduce the strength and hardness. The wear resistance of the steel increases at higher tempering temperatures due to the presence of hard tempered carbides.
    
In addition, different tempering temperatures also affect the microcrack formation in the steel. At lower tempering temperatures, microcracks form due to a greater temperature gradient between the centre and surface of the steel. However, at higher tempering temperatures, the tempering effect is more uniform, leading to fewer microcracks.
    
Showing the effect of tempering temperature on the microstructure of Niobium microalloyed NM500 steel.
    
Showing the effect of tempering temperature on the mechanical properties of Niobium microalloyed NM500 steel.
    
In conclusion, the tempering temperature plays a significant role in affecting the microstructure and properties of Niobium microalloyed NM500 steel. At lower tempering temperatures, the microstructure contains small lath martensite, which results in higher strength and hardness but lower strength, elongation and impact energy. At moderate tempering temperatures, the microstructure contains tempered ferrite and carbides, which gives good strength, toughness and wear resistance. Finally, at higher tempering temperatures, the microstructure contains tempered ferrite and carbides, which gives excellent ductility and toughness but relatively low strength. Therefore, it is important to select the proper tempering temperature for Niobium microalloyed NM500 steel to achieve the desired mechanical properties.

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