Methods for preventing cracks in stainless steel castings

A major advantage of stainless steel castings is that due to the high dimensional accuracy and surface smoothness of investment castings, mechanical processing work can be reduced. Only a small amount of machining allowance can be left in the parts with higher requirements, and some castings can be used without mechanical processing. During use, attention should be paid to preventing the occurrence of cracks. How should we specifically prevent it?

1. Casting structure

When casting, it is necessary to comprehensively consider the factors that affect the liquid and solid shrinkage of the casting, such as its structure, shape, size, wall thickness, and transition. Appropriate process parameters should be selected to prevent casting defects such as shrinkage and porosity. The design of the sprue and riser system for castings should be reasonable. If cold iron or other process measures are to be used, the placement of the sprue and riser system should be reasonable, ensuring the density of the internal structure of the casting while avoiding stress concentration as much as possible.

2. Smelting

During the smelting process, try to reduce the content of harmful elements such as P and S, and reduce the content of gases and inclusions such as N, H, and O. By using low phosphorus steel intermediate alloys, good results can be achieved.

3. Thermal insulation

By appropriately extending the insulation time of the castings in the sand mold, the main goal is to control the opening temperature below 70 ℃, ensuring that the castings fully complete liquid and solid shrinkage in the sand mold, and avoiding stress concentration caused by external factors.

4. Falling sand

During the process of cleaning sand from castings, it is strictly prohibited to water the sand mold and castings when boxing, and it is strictly prohibited to use strong external forces such as hitting the box to remove sand, in order to avoid cracks caused by the interaction between external forces and internal stresses in the castings.

5. Cutting riser

Select the appropriate hot cutting and riser process based on the casting conditions to ensure that the starting temperature of hot cutting is not lower than 300 ℃. During operation, vibration cutting is used for the gas cutting gun and oxygen blowing tube. Cover the partition with asbestos cloth or conduct furnace heat treatment promptly after gas cutting of important components. For castings with complex structures and special process measures such as upper crown and axial flow blades, secondary hot cutting is used.

6. Preheating

When dealing with defects in stainless steel castings, the preheating principle should be adhered to. During the blowing and welding process, the workpiece should be preheated to above 108 ℃ before operation. If larger crack defects are found, stress relief annealing should be carried out first before treatment.

7. Increase annealing

Add a stress relief annealing process to major stainless steel castings, strictly control the insulation time and furnace temperature. The purpose is to reduce the generation of new stress concentrations during the production process, more thoroughly eliminate internal stresses in castings, and prevent crack formation.

The above 7 points are about methods to prevent cracks in stainless steel castings. To do a good job in these crack prevention work, the main operating methods and steps in the machining process are necessary to avoid this situation to a greater extent.