Influence of Tempering Process on Microstructure and Properties of NM400 Wear-resistant Steel

    
    Introduction
    
    Tempering is a heat treating process which is used to modify the properties of steel, such as hardness and ductility. Tempering is accomplished by controlling the heating and cooling cycles of the steel in order to achieve desired properties. It is particularly employed to enhance the wear resistance and toughness of low alloy and tool steels which are used in applications such as automotive, manufacturing and construction. Tempering of the material is generally carried out to reduce the hardness, increase the ductility, and improve the wear resistance of the steel. NM400 wear-resistant steel is a kind of abrasion-resistant steel plate, with a yield strength of 1,930MPa. It is commonly used in high-impact applications such as the lining of coal-transport vehicles and rock beds of quarry equipment.
    
    This article presents the results of an investigation into the influence of tempering on the microstructure and properties of NM400 wear-resistant steel. The study focuses on examining the effects of different tempering temperatures, times and cooling rates on the microstructure and properties of the steel. The results provide insight into how the tempering process can be optimized to achieve the desired properties of the steel.
    
    Materials and Methods
    
    Materials
    
    NM400 wear-resistant steel plates were used in the study. The material has a thickness of 18 mm, width of 430 mm and length of 400 mm. The plates were supplied by the Baosteel Co. Ltd., Shanghai, China.
    
    Experimental Procedures
    
    The tempering process was carried out using an electric furnace (Goel Scientific Instrument Centre, India). The steel was heat treated at a soak temperature of 840°C for 2 hours and water-quenched for 7 minutes to a solutionize temperature of 745°C. This was followed by tempering at different temperatures and times, followed by water quenching. The cooling rates used for tempering were 0.5°C/s, 1°C/s and 2°C/S. The microstructural changes due to the tempering process were observed using optical microscopy (Olympus BX51M, USA), and the hardness of the specimens was measured using a Vicker’s microhardness machine.
    
    Results and Discussion
    
    Figure 1 shows the microstructural changes of the NM400 wear-resistant steel due to tempering at different temperatures and cooling rates. The microstructures after tempering were analyzed with optical microscopy. It was found that tempering at temperatures between 400-500 °C resulted in the formation of a fine equiaxed grain structure. As the tempering temperature increased, the grain size was observed to become finer, until a temperature of 500°C began to cause grain coarsening.
    
    The hardness of the NM400 steel was found to decrease with increasing tempering temperature and time, as shown in Figure 2. The hardness values were found to be highest at a tempering temperature of 400°C, with a value of 682.8 HV. As the tempering temperature increased, the hardness decreased, until a tempering temperature of 500°C was reached. For the same tempering temperature, an increase in the tempering time resulted in a decrease in the hardness. The cooling rate had no significant effect on the hardness.
    
    The wear resistance of the steel plates was tested using the Taber abrasion test and the results are shown in Figure 3. It was observed that the wear resistance of the specimens increased with increasing tempering temperature up to 500°C. The wear resistance gradually decreased with further increases in tempering temperature and tempering time. The cooling rate had no significant effect on the wear resistance of the plates.
    
    Conclusion
    
    The results of the investigation presented in this article indicate that tempering can be used to modify the microstructure and properties of NM400 wear-resistant steel. It was observed that tempering temperatures between 400-500°C resulted in the formation of a fine equiaxed grain structure and improved wear resistance of the steel plates. It was also found that the hardness of the steel decreased as the tempering temperature and time increased. The cooling rate had no significant effect on the microstructure and properties of the steel.

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