Stainless steel castings such as ZG06Cr13Ni4Mo and ZG06Cr16Ni5Mo, etc., have poor casting performance due to their poor fluidity and large body shrinkage as well as linear shrinkage. Additionally, they are prone to cracks due to high thermal stress.
When cracks occur in castings, not only does it require a significant amount of repair work, but it can also result in scrapping, causing substantial economic losses.
The main causes of casting cracks include the casting structure and the casting process, among others.
In order to prevent cracks, the following measures are generally implemented in production:
1. Casting structure
When casting, it is essential to take into consideration various factors that affect the liquid and solid shrinkage of the casting, such as the structure, shape, large size, wall thickness, and transitions.
To prevent casting defects like shrinkage cavities and porosity, it is important to select appropriate process parameters.
The design of the casting’s pouring and riser system should be well thought out, and if using cold iron or other technological measures, the placement of the parts must be carefully considered to ensure the internal structure of the casting is compact and to avoid stress concentration.
2. Smelting
In the smelting process, it is advisable to reduce the content of harmful elements such as phosphorus (P) and sulfur (S), as well as gases and inclusions like nitrogen (N), hydrogen (H), and oxygen (O).
Using a low-phosphorus steel master alloy can lead to good results.
3. Insulation
By appropriately extending the holding time of the casting in the sand mold, the aim is to control the cooling temperature below 70°C, ensuring that the casting fully undergoes liquid and solid shrinkage in the sand mold. This helps to avoid stress concentration caused by external forces.
4. Shake out
During the casting shake-out and sand cleaning process, it is imperative to avoid watering the sand molds and castings during shake-out. Additionally, strong external force impact methods such as box collision are strictly prohibited to prevent interaction between external force and internal stress of the casting, which could cause cracks.
5. Cutting riser
Based on the casting conditions, it is important to select the appropriate hot cutting and pouring riser process to ensure that the hot cutting start temperature is not below 300℃.
When performing the operation, the gas cutting gun and the oxygen blowing tube should be utilized with vibrational cutting.
It is important to cover the partition with asbestos cloth or to perform timely heat treatment in the furnace for important parts after gas cutting.
For castings such as crowns and axial blades that have complex structures, secondary thermal cutting is used with special measures taken in the process.
6. Preheating
When addressing defects in stainless steel castings, it is crucial to follow the principle of preheating.
During the blowing and welding process, the workpiece must be preheated to a temperature above 108°C before the operation takes place.
In the case of large crack defects, stress relief annealing should be carried out prior to treatment.
7. Increase annealing
For important stainless steel castings, it is necessary to incorporate a final stress relief annealing process and strictly control the holding time and furnace temperature.
The purpose of this process is to reduce new stress concentration generated during production, thoroughly eliminate the internal stress of the casting, and prevent cracks from occurring.
