Casting porosity of super duplex stainless steel butterfly valve body

Super duplex stainless steel (00Cr25Ni7Mo4N, French brand SAF2507) is widely used in nuclear industry, such as valve body, main coolant pipe and main pump casing. The nominal size of DN1200 seawater butterfly valve is large, and the thickness of each part of the valve body is quite different, and the research of super duplex stainless steel in China has just started. Therefore, there are many problems in the casting production of super duplex stainless steel valve body, such as common casting defects such as shrinkage cavity, porosity and casting porosity, which seriously affect the pressure bearing characteristics of the casting. Therefore, this study uses advanced analytical instruments to analyze and identify defects, and solves casting defects combined with traditional casting technology.

(1) Causes of casting porosity. ① The basic phenolic resin sand and curing agent used have large gas evolution at high temperature, and the minerals in the zircon powder alcohol-based coating decompose at high temperature to produce a large amount of gas, which enters the non solidified casting; ② From July to September every year, the humidity in the air in this area is very high, and the moisture content in the air is too much. A large amount of water vapor is brought into the process of electric arc furnace melting, pouring and sand mixing molding. At high temperature, the steam is decomposed into free oxygen and hydrogen, which is easy to be removed by the slag remover, but the solubility of hydrogen decreases greatly with the decrease of the temperature of the liquid metal, and it is too late to discharge and invade the liquid metal.

(2) The following measures are taken to effectively eliminate the casting porosity. ① In the process of resin sand casting production, the temperature and humidity of raw sand and recycled sand must be monitored at any time, so as to adjust the dosage of curing agent; ② During smelting, the proportion of new material and returned material shall be appropriate. Generally, the new material shall not be less than 70%. The special slag forming degassing agent for stainless steel shall be used to not only remove the slag, but also avoid the invasion of gas. At the same time, the smelting temperature and pouring temperature shall be controlled.