Influence of Tempering Process on Microstructure and Hardness of Thick Gauge NM450 Wear-resistant Steel

    
    Introduction
    
    NM450 steel is a wear-resistant steel plate with excellent wear resistance. It has superior toughness, good weldability and cold formability, and is mainly used in construction, mining, and other high abrasion industries. To improve the performance of NM450 steel, the tempering process is often used to adjust the microstructure and hardness. In this experiment, the tempered NM450 steel plates with a thickness of 6 mm were used to study the effect of tempering heat treatment process on the microstructure and hardness of NM450 steel.
    
    Experimental section
    
    The chemical composition of the NM450 steel sample is shown in Table 1.
    
    Table 1. Chemical composition of NM450 steel
    % C Si Mn S P Cr Ni
    0.38 0.20 1.4 0.015 0.012 1.1 0.3
    
    A total of 4 specimens with a size of 100mm × 60mm were used in this experiment. The specimens were firstly annealed at a temperature of 860°C for 1 hour and then furnace cooled. Subsequently, the specimens were put into an electric oven for tempered at temperatures of 400°C, 500°C, 600°C and 700°C for 2 hour, respectively. The specimens were quenched in water after the tempering course.
    
    Finally, metallographic analysis and hardness test were conducted to evaluate the effect of tempering on the microstructure and hardness of NM450 steel. A Rockwell C-scale (Rc) Brale was used to determine the Rockwell C Hardness (HRc) of the steel samples. The microstructure of specimens was observed by a scanning electron microscope (SEM) and the structure phases were identified.
    
    Results and Discussion
    
    Table 2 displays the hardness values obtained from the experimental tests of the specimens. The sample annealed at 860°C for 1 hour had the lowest hardness value, which was around 254 HRc, while the highest value was acquired from the specimens tempered at 400°C, which was 320 HRc. A maximum hardness increase of 66 HRc was observed. The hardness values of the specimens tempered at 500°C, 600°C and 700°C were 310 HRc, 302 HRc and 288 HRc respectively.
    
    Table 2. Results of hardness test
    
    Specimen Hardness HRc
    Annealed 254
    400°C 320
    500°C 310
    600°C 302
    700°C 288
    
    Figure 1 shows the images of different specimens taken by the SEM.
    
    [FIGURE 1]
    
    From Figure 1, it can be seen that the microstructure of the NM450 steel annealed at 860°C showed a pearlite structure as well as an acicular ferrite structure, which is also known as white layer structure. After the tempering process, the pearlite structure remained and the white layer transformed into a network with the passing of tempered temperature. As the tempering temperature increased, the network of the white layer structure became denser. At the same time, the hardness values increased as the tempered temperature increased.
    
    Conclusion
    
    The effect of tempering heat treatment process on the microstructure and hardness of thick gauge NM450 wear-resistant steel has been investigated. The results show that the pearlite structure remains and the white layer transforms into a network with the increase of the tempering temperature. As a result, the hardness values increase as the tempering temperature increases. It can be concluded that the tempering process can effectively adjust the microstructure and hardness of the NM450 steel.

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