Knowledge

1 Production and Manufacturing

1.1 Mold Making

To ensure stable product quality during batch manufacturing, this model is processed and controlled with full numerical control for high-precision machining, further enhancing the accuracy and stability of the model's dimensions. The model is also subjected to three-dimensional data detection to comprehensively inspect all its dimensions and shapes, ensuring its quality and precision. This is crucial for batch manufacturing of castings and can effectively prevent casting quality issues caused by model errors. The three-dimensional model is illustrated in Figure 9, and the three-dimensional detection data is shown in Figure 10.

 

According to the shape and size requirements of the valve housing, a reasonable block design was carried out for the valve housing, and the forming method of lower solid sample, upper core assembly and inner cavity main core was adopted. During the forming process of the lower solid sample, it is necessary to ensure the stability of the basic structure of the valve housing and the dimensional accuracy. The upper structure of the valve housing is complex. The upper part adopts the forming method of core assembly by blocks, which can more flexibly realize the forming of complex structures and ensure the accuracy of dimensions and shapes at the same time. This forming method requires precise control of the dimensions and positions of each core assembly to ensure that they can be accurately assembled into the complete upper structure of the valve housing.

1.2 Molding

Each link of the molding process is strictly carried out in accordance with the operation procedures of each process. In addition, the molding of this valve housing is difficult. To ensure the dimensional accuracy of the valve housing during batch production, three-dimensional detection of the cavity of the sand mold is carried out during molding to ensure the dimensional accuracy of the valve housing cavity. After the core is inserted, the inner cavity of the valve housing is narrow, which is not conducive to observation. Before pouring, an endoscope is used to check whether there is any sand or other debris at the bottom of the valve housing cavity to ensure the cleanliness of the cavity. The actual picture of the core insertion after demolding is shown in Figure 11.

 

 

1.3 Heat Treatment

During the box removal and shakeout stage of the valve housing, due to the low temperature, the workpiece is at a very high risk of cracking. Therefore, immediately after the box removal, closely monitor the temperature changes at the root of the riser, thick and large areas, and thin-walled regions, and promptly carry out the initial finishing process. At the same time, strictly follow the dedicated heat treatment process for heat treatment. Throughout the entire heat treatment process, no cracks occurred in the workpiece. The valve housing has many heat treatment processes, mainly including post-casting annealing, performance heat treatment, and stress relief annealing after welding. Before post-casting annealing of the valve housing, shakeout is required. Due to the large difference in wall thickness of the valve housing and its being a high-chromium martensitic steel, after casting and pouring, cooling to a certain temperature will produce martensite, which adds significant organizational stress to the valve housing on top of thermal stress. During the shakeout process, closely control the temperature at the root of the riser, thick and large areas, and thin-walled positions to avoid cracking. The performance heat treatment of the valve housing typically uses air-cooled quenching followed by high-temperature tempering. The quenching crack sensitivity of the material can be judged by a specific carbon equivalent Ceq, and the calculation formula for this carbon equivalent is:

It can be calculated that the carbon equivalent of the material is approximately 3.2% (without considering Co and B elements), and the crack sensitivity is very high. Considering the influence of Co and B elements, the crack sensitivity of CB2 material is even greater. By having good control over the uniformity of quenching and cooling during the valve body's performance heat treatment, the final cooling temperature and tempering, the occurrence of heat treatment cracks can be effectively avoided. When the valve body undergoes stress relief annealing after welding, the selection of annealing temperature and holding time is matched with the material's welding procedure qualification. No cracks occurred throughout the entire heat treatment process.

 

2. Product Quality Status

After going through the casting process design, model making, molding, smelting, pouring, heat preservation, box opening, shakeout, cleaning, heat treatment, and machining, no surface defects such as slag inclusion, porosity, or cracks were found on the surface of the main steam valve regulating valve shell castings. Meanwhile, magnetic particle testing (MT), penetrant testing (PT), and ultrasonic testing (UT) were conducted on the entire valve shell in accordance with technical requirements, and the results were satisfactory. Among them, ultrasonic testing only found 1 to 2 minor defects on each valve shell. All valve shells met the design requirements in terms of dimensions. Eventually, over 30 valve shells produced in batches successfully met the standards. The physical images of the valve shell products are shown in Figure 12.

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3 Conclusion

(1) In terms of solidification feeding, a reasonable casting process plan was designed. With the help of MAGMA solidification simulation software, various methods such as zoned and segmented chill iron cooling, multi-layer riser feeding, and a double-layer large open pouring system were adopted. This effectively solved the problems of porosity and slag inclusion in the valve shell, and the UT quality of the product was good.

 

(2) In terms of crack prevention and control, through the control of smelting, casting process, molding process and heat treatment technology, the crack problem of the valve shell was effectively solved.

 

(3) In terms of dimensional control and batch manufacturing, through DOE simulation under different boundary conditions, a linear shrinkage value that matched the enterprise was selected. The model adopted modern numerical control technology, and the molding used the core assembly forming method. The model and the molding cavity were also subjected to three-dimensional data detection. This high-precision manufacturing method ensured the accuracy of the model dimensions and the stability of product quality, laying a solid foundation for the smooth advancement of batch casting.

 

Ultimately, the feasibility of the casting process was verified through batch manufacturing. Each valve shell had good overall quality, with no visible slag inclusion, porosity, cold lap or crack defects on the surface. The non-destructive testing, chemical composition and mechanical properties all met the technical requirements. Batch manufacturing of 30 pieces was successfully achieved in one production run.

 

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