The fatigue life of bearing steel is a crucial indicator of its intrinsic quality. When delving into the factors that influence the fatigue life of bearing steel, it becomes evident that merely reducing the oxygen content does not significantly enhance its fatigue life; rather, attention must also be paid to the content of oxides and sulfides.
Firstly, nitrides have a non-negligible impact on the fatigue life of bearing steel. Although increasing nitrogen in steel leads to a decrease in the volume fraction of nitrides, this is due to the reduction in the average size of inclusions. The presence of tiny nitride particles directly affects the fatigue life of bearing steel. In particular, nitrides formed by the Ti element, due to their low specific gravity and ease of floating, tend to form multi-angled inclusions in steel. These inclusions can easily cause localized stress concentrations, leading to fatigue cracks. However, when the oxygen content in steel drops below a certain level, the nitrogen content increases, improving the size, type, and distribution of non-metallic inclusions. This results in nitride particles being distributed in a dispersed state, becoming a favorable factor for enhancing the strength and toughness of bearing steel.
Secondly, oxides are also significant factors affecting the fatigue life of bearing steel. The oxygen content in steel is closely related to the content of oxide inclusions; the lower the oxygen content, the fewer oxide inclusions. However, the presence of oxides disrupts the continuity of the metal matrix, and their coefficient of thermal expansion is smaller than that of the bearing steel matrix. This makes them prone to stress concentrations when subjected to alternating stresses, becoming the source of metal fatigue. The lower the plasticity of the inclusions and the sharper their shape, the greater the stress concentration and the more significant the impact on fatigue life.
Furthermore, the presence of sulfides in bearing steel cannot be ignored. Almost all sulfur content in steel exists in the form of sulfides, which can effectively surround oxides, reducing their impact on fatigue life. However, the influence of the number of inclusions on fatigue life is not absolute; it also depends on the nature, size, and distribution of the inclusions. Sulfides are dispersed in a fine state and mixed with oxide inclusions, affecting the strength and toughness of steel.
It is noteworthy that expert Xiao Jimei mentioned in the article that inclusions in steel are a brittle phase; the higher their volume fraction, the lower the toughness. The larger the size of the inclusions, the faster the toughness decreases. However, when the inclusions are small in size and uniformly distributed, the toughness may not decrease but may even increase. This viewpoint was verified in fatigue tests comparing two batches of steel, A and B. Although the purity of steel A was worse than that of steel B, the consistent size and uniform distribution of its oxide particles made steel A a long-life material with a high fatigue limit. In contrast, steel B contained large particle inclusions with uneven distribution, making it a short-life material.
In summary, the factors affecting the fatigue life of bearing steel mainly include the content and distribution state of nitrides, oxides, and sulfides. In actual production, these factors should be comprehensively considered, and the smelting and heat treatment processes should be optimized to reduce the content of inclusions and improve their distribution state, thereby enhancing the fatigue life and overall performance of bearing steel.
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