Cast impellers are key components in large-scale equipment in industries such as shipbuilding, power, pharmaceuticals, pumps and valves, and transportation, playing a crucial role in the overall performance of the entire equipment. At present, there are still certain difficulties in the casting process of complex integral impellers in production, so research on impeller casting technology has always been a hot topic in the casting industry. Therefore, in response to the current situation that the processes applied in casting production are far from meeting the quality requirements of impeller castings, how to produce complex impellers that meet technical requirements through reasonable combination and research of casting processes under the existing equipment and technical conditions of enterprises has become an increasingly important issue that impeller process research and casting production departments need to solve.

The Process Characteristics of Stainless Steel Precision Casting

Stainless steel has higher precision casting mechanical properties than cast iron, but it does not have better casting performance than cast iron. Due to the high melting point of stainless steel precision casting, poor fluidity of molten steel, easy oxidation of molten steel, and large shrinkage, the linear shrinkage is about 1.8-2.5%, and the body shrinkage rate is about 10-14%. To prevent defects such as cold shut, insufficient pouring, shrinkage porosity, shrinkage porosity, sand sticking, and cracks in steel castings, more complex process measures than cast iron must be taken:

1) The structure of the pouring system should strive for simplicity and the cross-sectional size should be larger than that of cast iron. Generally, dry or hot casting molds are used; Due to the poor fluidity of molten steel, in order to prevent cold shuts and insufficient pouring in steel castings, the wall thickness of steel castings should not be less than 8mm; Appropriately increasing the pouring temperature, as high pouring temperature leads to high overheating of molten steel, while maintaining the liquid state for a long time can improve fluidity. However, if the pouring temperature is too high, it can cause defects such as thermal cracking, coarse grains, sand sticking, and porosity. Therefore, the pouring temperature of large and thick walled castings is generally about 100 ℃ higher than their melting point; Thin walled, small, and complex shaped castings have a pouring temperature of approximately the melting point temperature of steel+150 ℃.

2) Because stainless steel has a much larger shrinkage rate than cast iron in precision casting, in order to prevent shrinkage porosity and shrinkage defects in castings, measures such as cold iron, risers, and subsidies are mostly used in the casting process to achieve sequential solidification. In addition, to prevent shrinkage, porosity, cracks, and porosity defects in stainless steel castings, sharp and right angle structures should be avoided, wall thickness should be uniform, coke should be added to the core, sawdust should be added to the mold sand, and oil sand cores and hollow cores should be used to improve the permeability and yield of the core or sand mold.