Effect of Sn Element on Crack Defects of CN7M Sand Casting

Pump and valve products are widely used in petroleum, chemical industry, hydropower station, nuclear power station and other fields. For different working conditions, different material grades need to be selected, such as CF8, CF8M, CF3, CF3M and other 18-8 stainless steel materials, which are widely used due to their good corrosion resistance and good sand casting performance. CN7M material is a kind of material with better corrosion resistance. It has good corrosion resistance in acetic acid, phosphoric acid, caustic soda and other media, especially in dilute sulfuric acid media. Therefore, it is applied to many pump and valve products under severe conditions.

(1) The content of Sn in the original purchased bar exceeds the standard, resulting in a large number of mesh cracks in sand castings; After the master alloy is used for self-proportioning and the Sn content is strictly controlled, the cracks of sand mold castings completely disappear.

(2) In the process of solidification and cooling, Sn precipitates at the austenite grain boundary and forms intermetallic compounds with low melting point, which increases the brittleness of grain boundary and reduces the intergranular bonding force. The internal stress generated by cooling shrinkage causes sand mold casting cracking.

However, due to the secondary utilization of a large number of scrap steel and the unreasonable refining process, there are a large number of harmful residual elements in stainless steel, such as Pb, Sn, Sb, As, etc., which is also an important problem to be solved in the field of iron and steel metallurgy. If these impurity elements cannot be effectively removed in the smelting process, solidification segregation will occur at the grain boundary during the solidification process of sand castings, reducing the intergranular binding force and causing cracks. Some studies have shown that the residual element Sn is very easy to produce solidification segregation in the chilling layer on the surface of sand mold castings, and promote cracking along the austenite grain boundary. Some scholars believe that the segregation of Sn can significantly reduce the thermoplasticity of steel from the perspective of thermoplasticity. This is because Sn at the grain boundary reduces the surface energy of the grain boundary, weakens the intergranular bonding force, and hinders the movement of the grain boundary, thus promoting a large number of cracks during hot processing. Although many scholars have done a lot of research on the influence of impurity elements on the formation of cracks in steel, the effect of Sn elements on the cracks of sand casting stainless steel is not sufficient. Therefore, CN7M stainless steel is used as raw material to further analyze the influence mechanism of Sn element on sand mold casting cracks with corresponding detection methods, and reasonable solutions are taken according to the research results to solve this kind of crack problem of sand mold casting.