Answer: The casting performance of stainless steel castings is poor, the fluidity is poor, the volume shrinkage and linear shrinkage are large, the thermal stress is large, and cracks are prone to occur. Once the casting is cracked, it will not only require a lot of repair work, but also may be scrapped in serious cases, resulting in significant economic losses. Casting cracks are mainly caused by casting structure, casting process, etc. The following measures are generally used to prevent them in production:
1. Casting structure
When casting, factors that affect the liquid and solid shrinkage of the casting should be fully considered, such as the structure, shape, large size, wall thickness and transition of the casting, and appropriate process parameters should be selected to prevent casting defects such as shrinkage cavities and porosity. The design of the pouring and riser system of the casting should be reasonable. If technological measures such as cold iron are to be used, the place where it is placed should be reasonable. It is necessary to ensure the compactness of the internal structure of the casting, and to avoid stress concentration as much as possible.
2. Smelting
During the smelting process, the content of harmful elements such as P and S should be reduced as much as possible, and the content of gases and inclusions such as N, H, and O should be reduced. By using a low-phosphorus steel master alloy, a very good effect can be achieved.
3. Insulation
By appropriately extending the holding time of the casting in the sand mold, the main purpose is to control the unboxing temperature below 70 °C to ensure that the casting can fully complete the liquid and solid shrinkage in the sand mold and avoid stress concentration caused by external factors.
4. Falling sand
In the process of casting falling sand cleaning, it is strictly forbidden to pour water into the sand mold and castings when boxing.
5. Cutting the riser
According to the casting conditions, select the appropriate hot cutting and pouring riser process to ensure that the initial temperature of hot cutting is not lower than 300 ℃. In operation, the gas cutting torch and the oxygen blowing tube use vibration cutting. After gas cutting of important parts, cover the partition with asbestos cloth or heat treatment in the furnace in time. For castings with complex structures such as upper crowns and axial flow blades, and special measures are taken in the process, secondary thermal cutting is used.
6. Preheating
When dealing with defects in stainless steel castings, the principle of preheating should be adhered to. In the process of blowing and welding, the workpiece should be preheated to above 108 ℃ before operation. If a large crack defect is found, stress relief annealing should be carried out first, and then the treatment should be carried out.
7. Annealing
For major stainless steel castings, a stress relief annealing process must be carried out, and the holding time and release temperature should be strictly controlled. Its purpose is to reduce the new stress concentration in the production process, eliminate the internal stress of the casting more thoroughly, and prevent the occurrence of cracks. (Editor / Jing Liwen)
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