Changes of cleanliness and carbide during vacuum preparation of stainless bearing steel
CHANG Li-zhong1, XU Tao2, SU Yun-long1, ZHANG Long-fei1, ZHU Chun-li1, SHI Xiao-fang1
1. School of Metallurgy Engineering, Anhui University of Technology, Ma'anshan 243002, Anhui, China; 2. Anhui Fukai Special Material Co., Ltd., Jixi 245300, Anhui, China
Abstract:In order to improve the cleanliness and refine the carbide structure of G102Cr18Mo high carbon stainless bearing steel, the process route of vacuum induction melting(VIM), vacuum consumable arc remelting(VAR) and forging with large forging ratio was adopted to research the influence of vacuum treatment and large forging ratio on chemical composition, gas content, inclusions distribution, secondary dendrite spacing and carbide size. The results show that during VIM process, the deoxidation ability of carbon decreases greatly with the increase of aluminum content. Even 0.003% aluminum has an obvious obstacle function to deoxidation of carbon. The oxygen content is greatly reduced due to the improvement of thermodynamic conditions such as high vacuum degree, high remelting temperature and reaction kinetic conditions during VAR process. After the primary VAR, the mass percent of oxygen decreases to 0.000 57% from 0.001 49%, reduced 61.7%, and after the secondary VAR, the mass percent of oxygen decreases to 0.000 50%. During VIM and VAR, the composition of inclusions changes little, the main inclusions are Al-Si inclusions, followed by Al2O3 inclusions, MnS inclusions, Mg-Al-Ca and Mg/Ca-Al inclusions. The inclusions of diameter over 20 μm are completely removed, there are only a few inclusions with a diameter of 10-20 μm and the inclusions are mainly composed of fine inclusions with diameter less than 5 μm after double vacuum smelting. The cleanliness of G102Cr18Mo steel is greatly improved. It is found that the morphology of secondary dendrite changes little from edge to core on the cross section of VAR ingot. The secondary dendrite spacing increases slowly from the edge to the core, and varies from 85 to 95 μm. Such fine dendrite structure benefits from low remelting speed. The VAR ingot is forged repeatedly and finally forged into a round rod with a diameter of 40 mm. Through metallographic examination, it is found that the maximum size of carbide particles is no more than 20 μm and the average size is 15 μm. No aggregated carbides are found. Low VAR speed and forging with large forging ratio are the key to refinement of carbides.
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