The formation mechanism of flaw detection in wind power gear steel

Abstract

In order to address the frequent occurrence of ultrasonic testing defects in wind power gear steel, this paper investigates the causes of these defects. Through defect observation and comprehensive sampling analysis of the refining process, the types of defects and their stages of formation have been clarified. Coupled with thermodynamic calculations, the formation mechanism of defect inclusions was determined, and process improvement measures were proposed. The results indicate that the main cause of the testing defects is large-sized inclusions, which typically exceed 400 μm in size. These large-sized inclusions are formed by the aggregation of micron-sized liquid inclusions of CaO-MgO-Al₂O₃-SiO₂. This type of inclusion originates from the high SiO₂ content of CaO-MgO-Al₂O₃-SiO₂ inclusions produced during the VD process. Analysis reveals that the excessively low [Al] content in the molten steel during the VD process is the fundamental reason for the formation of high SiO₂ content CaO-MgO-Al₂O₃-SiO₂ inclusions. Due to the low [Al] content, the [Si] in the molten steel reduces (Al₂O₃) in inclusions, resulting in high SiO₂ content CaO-MgO-Al₂O₃-SiO₂ inclusions. As the [Al] content in the molten steel increases again, the SiO₂ in the inclusions is partially reduced; however, high SiO₂ content CaO-MgO-Al₂O₃-SiO₂ inclusions still remain. Increasing the [Al] content in the molten steel during the VD process to 0.01 % − 0.014 % can effectively prevent the entry of [Si] into the inclusions when the [Al] content is too low, and it can also reduce the high [Ca] effect on the molten steel caused by slag-steel balance when the [Al] content is excessively high. This is a reasonable measure to reduce the occurrence of high SiO₂ content CaO-MgO-Al₂O₃-SiO₂ inclusions.