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. Furthermore, they are prone to cracking due to high thermal stress.
When cracks occur in castings, not only is a significant amount of repair work required, but it can also result in dismantling, causing substantial economic losses.
The main causes of cracks in casting include the casting structure and the casting process, among others.
To prevent cracking, 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 structure, shape, large size, wall thickness and transitions.
To avoid casting defects such as shrinkage cavities and porosity, it is important to select appropriate process parameters.
The design of the casting system and riser of the casting must be well thought out, and if cold iron or other technological measures are used, the placement of the parts must be carefully considered to ensure that the internal structure of the casting is compact and to avoid concentration of tensions.
two . S Melting
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 such as nitrogen (N), hydrogen (H) and oxygen (O).
Using a low phosphorus steel master alloy can lead to good results.
3 . I insulation
By properly extending the retention 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 contraction in the sand mold. This helps prevent stress concentration caused by external forces.
4 . S fish
During the process of shaking the casting and cleaning the sand, it is imperative to avoid wetting the sand molds and castings during shaking. In addition, strong external force impact methods such as box collision are strictly prohibited to avoid interaction between external force and internal stress of the casting, which could cause cracking.
5 . cutting riser
Based on the casting conditions, it is important to select the appropriate hot cutting and casting process to ensure that the initial hot cutting temperature is not lower than 300℃.
When carrying out the operation, the gas cutting gun and oxygen blowing tube must be used with vibrational cutting.
It is important to cover the partition with asbestos fabric or carry out timely heat treatment in the oven for important parts after gas cutting.
For castings such as crowns and thrust blades that have complex structures, secondary thermal cutting is used with special measures taken in the process.
6 . P reheating
When treating defects in stainless steel castings, it is crucial to follow the principle of preheating.
During the blowing and welding process, the part must be preheated to a temperature above 108°C before the operation is carried out.
In the case of large crack defects, stress relieving annealing must be performed prior to treatment.
7 . I increase annealing
For important stainless steel castings, it is necessary to incorporate a final stress-relieved annealing process and strictly control holding time and furnace temperature.
The objective of this process is to reduce the new concentration of stresses generated during production, completely eliminate the internal stresses of the casting and prevent the occurrence of cracks.
