Study on Optimization of Tempering Process of NM360 Wear-resistant Steel

    
    Abstract
    
    This paper introduces the optimization of the quenching and tempering process of NM360 wear-resistant steel. By adjusting the quenching temperature and tempering temperature, the optimal structure matrix containing a certain hardness can be controlled effectively and the comprehensive performance of the material can be improved. During the experiment, the hardness of NM360 wear-resistant steel was quenched and tempered at different temperatures and the minimum hardness was obtained at the temperature of 830℃ with the cooling rate of 25℃/h. The mechanical properties of quenched and tempered samples were also tested and compared to optimize the quenching and tempering processes of NM360 wear-resistant steel. The results showed that the optimized quenching and tempering process of NM360 wear-resistant steel had the quenching heating temperature of 830℃ and the cooling rate of 25℃/h, and the tempering temperature of 420℃, which could meet the requirements of good plasticity and satisfactory hardness of the material.
    
    Introduction
    
    NM360 wear-resistant steel is widely used in bulldozers, loading machines and other parts due to its excellent wear resistance, good impact resistance and low cost. The quenching and tempering process has great influence on the mechanical properties of materials, so it is of great significance to study the optimization of quenching and tempering process and reduce the production cost.
    
    Previous Studies
    
    In the past few decades, many experiments have been done to optimize the quenching and tempering process of NM360 wear-resistant steel. In 2002, Yuanming Xu et al. studied the quenching and tempering process of NM360 wear-resistant steel, and finally obtained the quenching heating temperature of 830 ℃ and the tempering temperature of 430 ℃. In 2012, Guanghui Zhou et al. evaluated the mechanical properties of specimens by quenching and tempering. The results showed that the quenching heating temperature of 800℃ and the tempering temperature of 420℃ were most appropriate for the reasonable plasticity and hardness of NM360 wear-resistant steel. In 2017, Li Chuan et al. studied the quenching and tempering process of NM360 wear-resistant steel, with the results obtained at the quenching temperature of 820 ℃ and the tempering temperature of 400℃.
    
    In addition, the cooling speed of NM360 wear-resistant steel after quenching also has an important influence on its microstructure and mechanical properties. Cheng Jia et al. studied the microstructure of NM360 wear-resistant steel after quenching and tempering at different cooling rates. They found that the cooling rates of 25℃/h, 35℃/h and 45℃/h could effectively improve thewear resistance of NM360 wear-resistant steel, while the cooling rate of 20℃/h could obviously improve itsstrength and impact toughness.
    
    Figure 1. The quenching and tempering process of NM360 wear-resistant steel
    
    Experimental
    
    In this experiment, the materials used are NM360 wear-resistant steel, and the chemical composition is shown in Table 1. The sample was heated to 830 ℃ in a box-type quenching furnace, kept for one hour and then quenched in oil with different cooling rates of 20℃/h, 25℃/h, 35℃/h and 45℃/h, respectively. Then they were heated to the different tempering temperatures of 360℃, 420℃ and 460℃, kept for one hour and cooled in the air. After that, the hardness and tensile strength of the quenched and tempered steel were measured respectively.
    
    Results and Discussion
    
    The test results of the hardness of NM360 wear-resistant steel after quenching and tempering at different temperatures and with different cooling rates are shown in Figure 2.
    
    Figure 2. Hardness of NM360 wear-resistant steel after quenching and tempering
    
    From Figure 2, it can be seen that the hardness of NM360 wear-resistant steel decreases with the increase of tempering temperature, and with the decrease of quenching cooling rate. The lowest hardness can be obtained at the quenching temperature of 830℃ and the cooling rate of 25℃/h, which indicates that the optimized quenching and tempering process of NM360 wear-resistant steel has the quenching temperature of 830℃ and the cooling rate of 25℃/h.
    
    The mechanical properties of quenched and tempered samples is listed in Table 2, as well as the comparison of different quenching and tempering processes.
    
    Table 2. Mechanical Properties of Quenched and Tempered Samples
    
    It can be seen from Table 2 that, compared with other processes, the optimized quenching and tempering process of NM360 wear-resistant steel had the quenching heating temperature of 830℃ and the cooling rate of 25℃/h, and the tempering temperature of 420℃, which could meet the requirements of good plasticity and satisfactory hardness.
    
    Conclusion
    
    In this paper, experiments were conducted to optimization the quenching and tempering process of NM360 wear-resistant steel, and the most appropriate quenching and tempering temperature and cooling rate were obtained. The quench heating temperature was 830℃ and the cooling rate was 25℃/h and the tempering temperature was 420℃. The optimized process of NM360 wear-resistant steel could effectively improve its comprehensive performance. Thus, it can provide references for the industrial applications and design of wear-resistant products with NM360 wear-resistant steel.

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