Analysis on the Effect of Sn on Crack Defects of CN7M Castings and Control Measures

Author of the article:JI Hongshuang1 , Li Qi2 , CAO Qiao1 , TANG Weiyang1

Author's Workplace：1. Jiangsu Wanheng Casting Co., Ltd., Yancheng 224500, China; 2. Intelligent Manufacturing College, Huanghuai University, Zhumadian 463000, China

Key Words：stainless steel; Stannum; crack; segregation

Abstract：In the production practice of valve steel castings, an unqualified master alloy bar was used to melt and pour the investment casting of CN7M material, leading to a large number of cracks in the castings. The metallographic analysis shows that the cracks spread along the grain boundary of austenite, and many precipitated particles are found near the cracks. Scanning electron microscopy and energy spectrum analysis show that the precipitated phase is rich in Sn, with the content reaching 0.05% ~0.06%. The content of the residual element Sn in steel species seriously affects the quality of castings. A high Sn content promotes segregation at the grain boundary, resulting in the precipitation of Sn-rich low melting point intermetallic compounds, thus reducing the intercrystalline bonding force and finally leading to a large number of cracks in the castings. Production demonstrates that cracks can be completely eliminated by using an appropriate intermediate alloy instead of the original CN7M rod material for batching, melting and casting and strictly controlling the Sn content below 0.01%.