The valve body of seawater butterfly valve with nominal size of DN1200 is made of 00Cr25Ni7Mo4N, with a gross weight of 800 kg and a total weight of 1600 kg. The structure of the valve body is shown in Figure 1. In the production process, casting pores often appear at the top of the casting and below the riser, as shown in Figure 2.
For the casting pores found in butterfly valve castings, first identify their types, and then analyze the causes of their formation. Figure 3A shows the casting pores found after longitudinal cutting along the valve body, and Figure 3B shows the casting pores found on the top of the valve body during the turning of the top surface of the casting.
As shown in Figure 3a, some of the casting pores distributed on the longitudinal cutting surface of the valve body are spherical and some are needle shaped. The diameter of spherical hole is 2 ~ 4 mm, the diameter of pinhole is 1 ~ 3 mm, and the length is 3 ~ 20 mm. Some pinholes are open and connected with the outside world, and some pinholes are closed. The casting pore surface shown in Fig. 3 is analyzed microscopically, that is, the casting pore surface is photographed by scanning electron microscope, as shown in Fig. 4.
It can be seen from Fig. 4 that the inner surface of the casting pores is veined, not too smooth, with white globules and sunflower shaped objects. Through energy spectrum analysis of the white globules (Fig. 5), it is found that the oxygen element content in the globules is too high, indicating that the material is seriously oxidized. The globules are rich in Al2O3, and the Al element mainly comes from deoxidizer. The molding sand used for the modeling of the valve body is alkaline phenolic resin sand, which has a large gas generation temperature range and gas generation. When the humidity in the air is large, a large amount of water will be brought into the sand mixing and smelting process, so a large amount of gas will be generated during pouring. Combined with the morphology of casting pores and the characteristics of phenolic resin, it is speculated that the casting pores found after longitudinal cutting of valve body in Fig. 3a are invasive casting pores.
It can be seen from the energy spectrum analysis of the casting pore surface at the top of the valve body (Fig. 6) that a large amount of slag is distributed on the surface of the casting pore, which is mainly due to the fact that the slag remover is not removed during the smelting process. When the air humidity is high, a large amount of water reacts with the slag remover to produce a large amount of hydrogen. It is speculated that the casting pore containing a large amount of molten slag may be a reactive casting pore.
