Stainless steel castings like ZG06Cr13Ni4Mo and ZG06Cr16Ni5Mo etc.
This type of material has poor casting performance, poor fluidity, large body shrinkage and linear shrinkage as well as high thermal stress, which is prone to cracks.
Once the casting cracks, it will not only have a large amount of repair work, but also may be scrapped, causing significant economic losses.
The main causes of casting cracks are the casting structure and casting process etc.
The following measures are generally adopted in production to prevent:
1. Casting structure
When casting, it is necessary to fully consider the structure, shape, large size, wall thickness and transition of the casting and other factors that affect the liquid and solid shrinkage of the casting.
It should select appropriate process parameters to prevent casting defects such as shrinkage cavity and porosity.
The pouring and riser system design of the casting should be reasonable,
If cold iron and other technological measures are to be used, the placement of the parts must be reasonable, not only to ensure the compactness of the internal structure of the casting, but also to avoid stress concentration.
In 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 a low-phosphorus steel master alloy, good results can be achieved.
By appropriately extending the holding time of the casting in the sand mold, the main purpose is to control the opening temperature below 70°C to ensure that the casting fully completes the liquid and solid shrinkage in the sand mold and avoid stress concentration caused by external forces.
4. Shake out
In the process of casting shake out and sand cleaning, it is strictly forbidden to water the sand molds and castings during shake-out, and it is strictly prohibited to use strong external force impact methods such as box collision to avoid the interaction of external force and internal stress of the casting to cause cracks.
5. Cutting riser
According to the casting conditions, it selects the appropriate hot cutting and pouring riser process to ensure that the starting temperature of hot cutting is not less than 300℃.
During operation, the gas cutting gun and the oxygen blowing tube adopt vibration cutting.
Cover the partition with asbestos cloth or enter the furnace for heat treatment in time for important parts after gas cutting.
For castings such as crowns and axial blades that have complex structures and special measures are taken in process, secondary thermal cutting is used.
When dealing with defects in stainless steel castings, the principle of preheating must be adhered to.
During the blowing and welding process, the workpiece must be preheated to above 108°C for operation.
If large crack defects are found, stress relief annealing should be carried out before treatment.
7. Increase annealing
For major stainless steel castings, it is necessary to add a final stress relief annealing process, and strictly control the holding time and the furnace temperature.
Its purpose is to reduce the new stress concentration generated in the production process, more thoroughly eliminate the internal stress of the casting, and prevent cracks.