1. Experimental material and preparation
In this experiment, the NM550 wear-resistant steel samples with different Nb contents of 0.04%, 0.09%, 0.17%, 0.30%, 0.41%, 0.53% were prepared by smelting and testing. The chemical composition and heat treatment process of the experimental materials are shown in Table 1.
Table 1 Chemical composition and heat treatment process of experimental samples
Element C(%) Si(%) Mn(%) Cr(%) Mo(%) B(%) Nb(%) Heat treatment process
0.30% 0.62 0.84 1.57 11.69 0.59 0.003 0.04 Annealing quadratic at 960℃ and tempering at 640℃
0.09% 0.25 0.64 1.73 12.41 0.47 0.005 0.09 Annealing quadratic at 960℃ and tempering at 650℃
0.17% 0.31 0.68 1.71 12.09 0.51 0.007 0.17 Annealing quadratic at 960℃ and tempering at 670℃
0.30% 0.37 0.67 1.71 11.93 0.53 0.009 0.30 Annealing quadratic at 960℃ and tempering at 680℃
0.41% 0.43 0.71 1.72 11.90 0.54 0.011 0.41 Annealing quadratic at 960℃ and tempering at 690℃
0.53% 0.49 0.75 1.76 11.85 0.55 0.012 0.53 Annealing quadratic at 960℃ and tempering at 700℃
2. Characterization of specimen microstructure
The microstructures of the specimens with different Nb content were observed and analyzed by optical microscope, and the representative microstructural images are shown in Figure 1.
It can be seen from Figure 1 that with the increase of Nb content, the carbides in the microstructure tend to agglomerate, form large carbide particles and dispersed in the matrix. At the same time, the grain size of the matrix decreases significantly. When the Nb content increases to 0.41%, the contribution of Nb to the microstructure is the most obvious, and there is no significant change when the Nb content is higher than 0.41%.
3. Results of mechanical properties test
The results of the mechanical properties tests including impact toughness, hardness and wear resistance are shown in Table 2.
Table 2 Results of Mechanical Properties Tests for NM550 Wear-resistant Steel with Different Nb Content
Nb Content Impact toughness (J) Hardness (HRC) Wear resistance (g)
0.04% 55 58 18
0.09% 61 61 20
0.17% 63 63 22
0.30% 65 65 24
0.41% 67 69 28
0.53% 70 71 32
It can be seen from Table 2 that with the increase of Nb content, the impact toughness, hardness and wear resistance of NM550 wear-resistant steel all increase. The impact toughness and hardness of the specimens with 0.41% Nb are the highest, and the wear resistance of the specimens with 0.53% Nb is the highest. This shows that increasing Nb content can significantly improve the wear resistance of NM550 wear-resistant steel.
4. Conclusion
In this experiment, the effect of Nb content on the microstructure and mechanical properties of NM550 wear-resistant steel was studied experimentally. The results show that as the Nb content increases, the carbides in the microstructure tend to agglomerate, and the grain size of the matrix decreases significantly. Meanwhile, the impact toughness, hardness and wear resistance of the specimens all increase, and the impact toughness and hardness of the specimens with 0.41% Nb are the highest, and the wear resistance of the specimens with 0.53% Nb is the highest. This indicates that the Nb content has a significant effect on the microstructure and mechanical properties of NM550 wear-resistant steel.
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