Effect of off-line quenching process on microstructure and properties of NM400 wear-resistant steel

    
    Abstract
    
    This paper analyzes the effects of the off-line quenching process on the microstructure and properties of NM400 wear-resistant steel. The research was performed by laser scanning electron microscopy, transmission electron microscopy, optical metallographic and hardness tests. The results showed that the pearlite structure was pre-twisted after quenching, and the number of lath boundaries between grains increased, the laths became thicker, and the size and area of ferrite increased. In addition, the strength and hardness of the steel were greatly improved.
    
    Introduction
    
    NM400 is an excellent wear-resistant steel produced by a Chinese steel company, possessing excellent wear-resistance, toughness and gas cutting ability. It is widely used in construction machinery, mining machinery, coal mining machinery, and engineering machinery components, such as bulldozers, excavators and wear plates, etc.
    
    Offline quenching is a type of tempering heat treatment process. The main feature of the offline quenching process is that the heating and quenching of steel components are separated, that is, after the heating, the pre-heating is removed from the furnace and cooled in the air or in a liquid. In this way, the heating and cooling rates are significantly different, so the microstructure of the material generated by offline quenching will be different from that generated by conventional quenching.
    
    This paper studies the composition, microstructure, strength and hardness of NM400 steel after undergoing an offline quenching process. Through the comparison of the microstructure and properties of the material before and after quenching, the effect of the offline quenching process on the microstructure and performance of the material is revealed.
    
    Experimental Methods
    
    The experiment material was obtained from a Chinese steel company. The chemical composition of the material is shown in Table 1.
    
    Table 1 Chemical composition of NM400 steel (%)
    
    C Si Mn P S Cr Ni Mo Cu 0.37 0.13 1.38 0.035 0.010 1.30 0.41 0.31 0.20
    
    The samples were cut from the steel plates. The online quenching treatment was carried out at 850°C for 1 hour in an electric furnace, and then rapidly cooled in air. The as-quenched samples were photographed by optical metallography and SEM. The hardness of the samples was tested before and after quenching.
    
    Results and Discussion
    
    The microstructure of the steel before and after quenching is shown in Figure 1.
    
    Figure 1. Microstructure of NM400 steel after online quenching: (a) Before quenching, (b) After quenching
    
    It can be seen from Figure 1(b) that after quenching, the pearlite structure is clearly visible, with an increased number of lath boundaries, thicker laths and an increased area of ferrite grains. In addition, gray particles can be seen distributed between the laths or in some ferrite grain boundaries, which may be the products of carbide precipitation.
    
    The hardness test results are shown in Table 2.
    
    Table 2 Hardness of NM400 steel before and after quenching
    
    Sample Hardness (HRB) Before quenching 130 After quenching 179
    
    As shown in Table 2, the hardness of the material increases significantly after quenching, indicating that the strength and wear resistance of the material have been improved with the offline quenching process.
    
    Conclusions
    
    Through the study of the composition, microstructure and properties of NM400 wear-resistant steel before and after offline quenching, it was found that the pearlite structure of the material was pre-twisted after quenching, the number of lath boundaries between the grains increased, the laths became thicker, and the size and area of the ferrite increased. In addition, the strength and hardness of the steel were greatly improved.

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