Research on Casting Process of Ball Valve Castings for Hydropower Equipment

Abstract: The ball valve was modeled in 3D using Solidworks software, its structural characteristics were analyzed, and the casting process was designed. The casting process was simulated using Huazhu CAE casting simulation software, and the process was optimized based on the simulation results. The simulation results show that, in the casting process, the risers at the shaft holes on both ends of the ball valve are too small; the shrinkage feeding channel is uneven, and the distance between the shaft hole and the bottom riser is too great, resulting in shrinkage porosity at the upper corner of the shaft hole and between the shaft hole and the bottom riser. By adjusting the riser size, improving the shrinkage feeding channel, adding external cooling iron, and implementing other process optimization measures, the shrinkage porosity was effectively solved, resulting in a dense casting of high quality that met customer requirements during actual production.
 
Hydropower generation is energy-saving, environmentally friendly, sustainable, and renewable. It is an important part of China's clean energy and has made outstanding contributions to its economic development. The cast steel ball valve for a specific type of hydropower equipment produced by a factory has a complex structure and is difficult to manufacture. To ensure product quality, SolidWorks software is used to create a 3D model, and Huazhu CAE casting software is employed to simulate and optimize the casting process. The ball valve is made from WCB (equivalent to the domestic cast steel grade ZG230-450), and its main chemical composition is: C ≤ 0.30%, Si ≤ 0.60%, Mn ≤ 1.00%, P ≤ 0.0040%, and S ≤ 0.0045%. The approximate dimensions of the ball valve are 2,000 mm × 1,700 mm × 1,500 mm, and it weighs about 5 tons.


Figure 1 Dimensional drawings of ball valves
 

SolidWorks software is used to perform 3D modeling of the ball valve (Figure 2). Through the model, the hot spots of the casting can be observed more directly, and the parts that may cause defects can be analyzed, providing more accurate data for process design.


Figure 2 3D model diagram of ball valves
 

From Dimension Diagram 1 and 3D Model Diagram 2, it can be observed that the wall thickness of the ball valve is greater at the top and thinner at the bottom. According to the principle of sequential solidification, to facilitate modeling and subsequent processes, the riser is placed in a thicker and flatter area. Through specific calculations, the risers are configured as follows:

Figure 3 Ball valve process diagram (riser)
 

To ensure the sequential solidification of the casting, an external chiller is added to the partial casting area where the riser cannot effectively compensate for shrinkage (Figure 4). One external chiller is installed at the lower part of the shaft hole, and seven external chillers are installed at the intersection of the ball crown and the ball valve body. The dimensions of the external chillers are 160 mm × 140 mm × 90 mm.


Figure 4 Ball valve process diagram (external chiller)
 

The three-dimensional model of the ball valve is created using SolidWorks software, converted into an STL format file, and then imported into Huazhu CAE casting simulation software. The model is meshed with 12,532,212 elements, using WCB. The liquidus temperature is 1512°C, and the solidus temperature is 1469°C. The pouring temperature is 1580°C. The shrinkage rate is 5%, and the initial temperature of both the mold and the external chiller is 25°C. Figure 5 illustrates the solidification conditions at different time intervals during the process. It was observed that at 3700 seconds of solidification, an independent liquid phase forms between the shaft hole of the ball valve and the bottom riser. At 11,000 seconds, an independent liquid phase also forms at the upper corner of the shaft hole. The final simulation results indicate that shrinkage defects form in the independent liquid phase. The primary causes of shrinkage defects are as follows:

(1) The risers at the shaft holes on both ends of the ball valve are too small.

(2) The through holes at the shaft holes interrupt the shrinkage compensation channel from the riser.

(3) The distance between the shaft hole and the bottom riser is too great, making the shrinkage compensation distance inadequate.


Figure 5 Casting process simulation process diagram
 

Based on the simulation results and root cause analysis, the casting process was optimized (Figure 6).

(1) Change the shaft hole riser from a 290 mm × 350 mm blind riser to a 290 mm × 430 mm × 370 mm blind riser. 

(2) Optimize the through-hole structure of the shaft hole and enlarge the shrinkage compensation channel.

(3) Add an external cooling iron between the shaft hole and the bottom riser to form an artificial cooling end and extend the shrinkage compensation distance.

(4) Move the external cooling iron from the bottom of the shaft hole to the side of the entire shaft hole to prevent interference with the newly added shrinkage compensation channel and avoid introducing new defects.


(a) Process before the change (b) Process after the change
Figure 6 Comparison of the process before and after the change
 

The optimized casting process was simulated, and the results are shown in Figure 7.


Figure 7 Optimized casting process simulation diagram

As shown in Figure 7:
(1) At a solidification time of 1900 seconds, the additional external cooling iron effectively acts as an artificial cooling end, extending the shrinkage compensation distance of the riser.
(2) At a solidification time of 4100 seconds, the additional shrinkage compensation channel at the shaft hole prevents the formation of an independent liquid phase between the shaft hole and the bottom riser.
(3) At a solidification time of 11,900 seconds, the enlarged riser at the shaft hole provides sufficient molten steel for the upper corner of the shaft hole, preventing shrinkage.
 

Based on the simulation results, the casting process was finalized and implemented in actual production. Operating instructions for each stage were formulated to strictly control every step of the production process, ensuring the process was effectively implemented. The production process of the ball valve is shown in Figure 8. The ball valve casting produced using the optimized process features a dense interior, a smooth surface, and high quality. In line with customer requirements, ultrasonic (UT) flaw detection was performed on the rough-machined castings following GB/T7233.1-2009 "Ultrasonic Testing of Steel Castings Part 1: General Purpose Steel Castings," and the results met level 2 standards. No defects exceeding the standard were found, and the ultrasonic testing of the castings achieved 100% qualification. This demonstrates that the optimized process effectively eliminated shrinkage porosity, met customer requirements, passed acceptance smoothly, and earned customer recognition after delivery.
 

(1) The optimized simulation results indicate that process optimizations, including increasing risers at both ends of the shaft holes, adding shrinkage compensation channels at the through-holes of the shaft, and introducing external cooling iron, can effectively eliminate shrinkage defects.
(2) Based on the simulation results, the casting process was finalized, the production process was strictly controlled, and the procedure was effectively executed to produce dense castings of high quality that met customer requirements, verifying the process’s effectiveness.
(3) The application of SolidWorks 3D modeling and Huazhu CAE casting simulation analysis provides a valuable reference for designing and optimizing the casting process, replaces the traditional experience-based design model, reduces shrinkage defects in castings, and enhances product quality.