For most of the key parts of the turbine, stainless steel castings such as ZG06Cr13Ni4Mo and ZG06Cr16Ni5Mo are used. Such materials have poor casting properties, poor fluidity, large body shrinkage and line shrinkage, large thermal stress, and easy cracking. Once the casting has cracks, not only the amount of repair work is large, but it can also be scrapped seriously, causing significant economic losses. Casting cracks are mainly caused by casting structure, casting process, etc. The stainless steel castings manufacturer are generally used following measures 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, which affect the liquid and solid shrinkage of the casting, and select appropriate process parameters to prevent casting defects such as shrinkage and shrinkage. The design of the pouring riser system of the casting should be reasonable. If the cold iron and other technological measures are to be adopted, the place to be placed should be reasonable, and the compactness of the internal structure of the casting should be ensured, and the stress concentration should be avoided as much as possible.
In the smelting process, the content of harmful elements such as P and S is reduced as much as possible, and the contents of gases such as N, H, O and inclusions are reduced. Good results can be obtained by using a low-phosphorus steel intermediate alloy.
By appropriately prolonging the holding time of the casting in the sand mold, it is mainly to control the unpacking temperature below 70 ° C to ensure that the steel casting pipeline valve fully complete the liquid and solid shrinkage in the sand mold, and avoid stress concentration caused by external force factors.
4, falling sand
During the process of sand falling in castings, it is strictly forbidden to water the sand mold and castings when punching. It is strictly forbidden to use the strong external force impact method such as the collision box to fall sand, to avoid the interaction between the external force and the internal stress of the casting.
5, cutting riser
According to the casting conditions, the appropriate hot-cutting riser process is selected to ensure that the hot cutting start temperature is not lower than 300 °C. During operation, the gas torch and the lance are vibrated. After the important parts are cut, cover the partition with asbestos cloth or heat the furnace. For the castings with complex structure and special measures in the process of upper crown and axial flow vane, secondary hot cutting is adopted.
In the case of defect handling of stainless steel castings, the preheating principle must be adhered to. During the blowing and welding process, the workpiece is preheated to above 108 °C for operation. If a large crack defect is found, it should be first subjected to stress relief annealing and then treated.
7, increase annealing
Add a final stress relief annealing process to major stainless steel castings, and strictly control the holding time and the temperature of the furnace. Its purpose is to reduce the occurrence of new stress concentrations in the production process, more completely eliminate the internal stress of the casting and prevent cracks.
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