For the silica sol casting of automotive parts, most of the main components are made of stainless steel precision castings. This material has poor casting performance, poor fluidity, large volume and linear shrinkage, high thermal stress, and is prone to cracking. Once casting cracks occur, not only do they require heavy repair work, but in severe cases, they may also be scrapped, causing significant economic losses. Casting cracks are caused by factors such as casting structure and casting process. In the production process, the following measures are generally taken to prevent:

1. Casting structure

During the casting process, factors that affect the liquid-solid shrinkage of castings should be fully considered, such as the structure, shape, large size, wall thickness, and its transformation. Suitable process parameters should be selected to prevent defects such as shrinkage, porosity, and porosity in castings. The design of the casting riser system should be reasonable, such as using cold iron and other process measures. Its layout should be reasonable, ensuring the density of the internal structure of the casting while avoiding stress concentration.

2. Smelting

During the smelting process, the content of harmful elements such as phosphorus and sulfur should be minimized as much as possible, and the content of gases and inclusions such as N, H, O should be minimized as much as possible. By using low phosphorus steel intermediate alloys, good results can be achieved.

3. Insulation

The main purpose of sand casting by appropriately extending the insulation time is to control the unboxing temperature below 70 ℃, to ensure that the casting can fully complete the liquid and solid shrinkage in the sand mold, and to avoid stress concentration caused by external factors.

4. Falling sand

During the sand drop process of casting, it is strictly prohibited to clean and use strong external forces such as the box hitting and shaking the sand during the stamping of water sand molds and casting boxes, in order to avoid the interaction between external forces and the internal stress of the casting causing cracks.

5. Install the riser

According to the condition of the casting, choose the appropriate hot cutting pouring riser process to ensure that the starting temperature of hot cutting is not less than 300 ℃. During the operation, perform vibration cutting on the gas cutting gun and oxygen blowing tube. After gas cutting important parts, the diaphragm should be covered with asbestos cloth or heat treated material in a timely manner. For castings with complex structures and special measures, secondary thermal cutting is used.

6. Preheating

The defect treatment of stainless steel castings should follow the principle of preheating. During the blowing welding process, the workpiece should be preheated to above 108 ℃ before operation. If larger crack defects are found, stress annealing should be carried out first before processing.

7. Increase annealing

Add the final stress relief annealing process to the main stainless steel castings, strictly control the insulation time and temperature. The purpose is to reduce new stress concentration during the production process, more thoroughly eliminate the internal stress of castings, and prevent the generation of cracks.