Process Improvement of Wear-resistant Ball Valve

The wear-resistant cast stone ball valve bonnet produced by Sanjing valve is made of ZG230-450 materials. During the strength test during production processing and after assembly, it is found that there are different degrees of shrinkage porosity and shrinkage cavity around the bonnet located in Figure 1A (taking DN300 ball valve bonnet as an example). The casting blank cannot be seen by naked eyes, while water seepage occurs after mechanical processing or strength test. Repair welding is not a good way, at the same time, the strength cannot be guaranteed, and the economic loss is great.

1. Cause analysis

Due to the small flange plate above (φ Φ498mm × 35mm), the small wall thickness of the valve cover zui is 20mm thin, while the flange plate at the bottom is large (φ Φ698mm × 36mm), and there is a hot joint at a, the large flange at the bottom and a are below zui during pouring, so that slag cannot be discharged, condensation is slow, residual casting stress is large, and organization is loose. Because during the solidification process of castings, the liquid shrinkage and solidification shrinkage of cast steel often appear at the solidification site after zui, that is, shrinkage cavity and shrinkage porosity at the hot joint. The main reason is that the large flange of the valve cover and the part A generate heat joint, and the heat dissipation condition is poor. As a result, the upper small flange and the valve cover zui solidify first at the small wall thickness of 20mm, and the solidification area moves to the large flange at the bottom and the part A after zui, resulting in shrinkage or porosity.

2. Process improvement

According to the principle of balanced solidification, the top injection priority and the riser side-by-side pouring process are adopted.

The specific casting process is improved as follows:

(1) Choose a larger sand box to ensure the amount of sand to eat.

(2) according to the casting shape and size of the manufacture of appropriate mold, sand mold application before drying.

(3) The riser and runner are designed to be waist-round, and their heights are H = 1.3B(B is the width of the riser), so as to improve the shrinkage rate of the riser.

(4) control the pouring temperature.

(5) The large flange plate is placed on it, and the gate and riser are located close to the hot section area, which creates extremely favorable sequential solidification conditions for casting, and the hot section area is timely replenished.

After the process is improved, due to the large flange plate above and the small flange area at the bottom during pouring, the molten steel injected first is always at the front end of the liquid flow, and the temperature is getting lower and lower. The small flange at the bottom and the small wall thickness of 20mm of the valve cover can be filled quickly, which is also conducive to the elimination of gas in the cavity.

3. Application effect

By using the improved process, the defects such as shrinkage cavity and shrinkage porosity at the valve cover A are effectively solved, and the finished product rate is stable at more than 97%, and the effect is very good.