Effect of Nb Element on Microstructure and Properties of Low Alloy Wear-resistant Steel

    
    The wear-resistant low alloy steel is one of the most widely used agricultural materials, characterized by its unique performance and broad application range. Recently, alloying with different elements has been used to improve the wear resistance of the steel. The addition of niobium (Nb) to the steel composition has been proven to have outstanding effects in terms of increasing the wear resistance of the alloy. In order to investigate the effect of Nb on the properties and microstructure of wear-resistant low alloy steel, the wear-resistant properties of a low alloy steel with and without the presence of Nb were studied in this paper.
    
    The wear-resistant low alloy steel used in the present study is a 25Cr-1.25Mo-0.7Ni-Nb steel (hereinafter referred to as Nb steel), with a chemical composition summarized in Table 1. Three different wear-resistant low alloy steels were manufactured, the first being a 25Cr-1.25Mo-0.7Ni steel without Nb (hereinafter referred to as Ref), the second being a heat-treated 25Cr-1.25Mo-0.7Ni-Nb steel with a nominal Nb content of 1.2 wt.%, and the third being a heat-treated 25Cr-1.25Mo-0.7Ni-Nb steel with a nominal Nb content of 1.4 wt.%.
    
    Table 1. Chemical composition of Nb steel
    
    | Elements | C | Si | Mn | P | S | Cr | Mo | Ni | Nb |
    |----------- |--------------- |--------------- |--------------- |--------------- |--------------- |--------------- |--------------- |--------------- |--------------- |
    | (%wt.) | 0.14 | 0.25 | 0.88 | 0.012 | 0.015 | 25 | 1.25 | 0.7 | 1.2/1.4 |
    
    The microstructures of the three steels were characterized by optical microscopy, as shown in Figure 1. The Ref samples have a microstructure composed of only ferrite grains, while the Nb steels have ce-mentite lath-shaped particles in addition to ferrite and bainite grains. From Fig. 1b, it can be seen that the Nb steels have a much finer microstructure than the Ref samples, and the particles of cementite in the Nb steel are more evenly distributed than those in the Ref samples.
    
    The wear-resistant properties of all three steels were evaluated using a pin-on-disk abrasion test, as shown in Fig. 2. The wear-resistant performance of the Nb steel was found to be significantly higher than that of the Ref samples. The Nb beads significantly improved the wear resistance of the steel, with the 1.2 wt.% Nb content showing the highest wear resistance, and the 1.4 wt.% Nb content showing the second highest wear resistance. This suggests that the wearing properties of Nb steel can be improved by adjusting the Nb content of the alloy.
    
    Fig. 1. The microstructure of the wear-resistant low alloy steel (a)Ref, (b)Nb steel
    
    Figure 2.Abrasion wear test results for the wear-resistant low alloy steels
    
    The addition of Nb to the steel composition could improve the wear resistance of low alloy steel by increasing the volume fraction and uniformity of the pin-on-disk abrasion test. This was mainly due to the presence of fine lath-like cementite particles, which enhanced the hardness and strength of the steel matrix, as well as its ability to absorb energetic impacts during wear. Furthermore, the formation of bainite grains in the Nb steel provided improved wear-resistance properties as well.
    
    In summary, the addition of niobium in the wear-resistant steel was shown to possess outstanding effects in terms of improving the wear resistance of the material. Through the heat treatment of Nb steel, the addition of Nb increased the volume fraction and uniformity of the pin-on-disk abrasion test, and enhanced the hardness and strength of the steel matrix. The Nb steel also showed improved wear-resistance properties due to the formation of bainite grains. The properties and microstructure of the Nb steel can be effectively improved by adjusting the Nb content in the alloy.

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