Introduction to Precision Casting Process
In recent years, valve manufacturers have paid more and more attention to the appearance quality and dimensional accuracy of castings. Because a good appearance is the basic requirement of the market, it is also the positioning benchmark for the first process of machining.

The commonly used precision casting in the valve industry is investment casting, which is briefly introduced as follows:

( 1) Two process methods of investment casting:

①Using low-temperature wax-based mold material (stearic acid + paraffin), low-pressure wax injection, water glass shell, hot water dewaxing, atmospheric melting and pouring process, mainly used for carbon steel and low alloy steel castings with general quality requirements, The dimensional accuracy of castings can reach the national standard CT7-9.

②Using medium temperature resin base mold material, high pressure wax injection, silica sol mold shell, steam dewaxing, rapid atmosphere or vacuum melting and casting process, the dimensional accuracy of the casting can reach the precision casting of CT4~6.

( 2) Typical process flow of investment casting:



( 3) Features of investment casting:

①The casting has high dimensional accuracy, smooth surface and good appearance quality.

②It is possible to cast parts with complex structures and shapes that are difficult to be processed by other processes.

③The casting materials are not limited, various alloy materials such as: carbon steel, stainless steel, alloy steel, aluminum alloy, high temperature alloy, and precious metals, especially those alloy materials that are difficult to forge, weld and cut.

④ Good production flexibility and strong adaptability. It can be produced in large quantities and is also suitable for single or small batch production.

⑤ Investment casting also has certain limitations, such as cumbersome process flow and long production cycle. Due to the limited casting process available, its pressure-bearing capacity cannot be very high when used for casting pressure-bearing thin-shell valve castings.

Analysis of casting defects

Any castings will have defects inside. The existence of these defects will bring great hidden dangers to the inherent quality of the castings. Welding repairs to eliminate these defects in the production process will also bring a great burden to the production process. . In particular, as a thin-shell casting that withstands pressure and temperature, the tightness of the internal structure of the valve is very important. Therefore, the internal defects of castings have become a decisive factor affecting the quality of castings.

The internal defects of valve castings mainly include pores, slag inclusion, shrinkage porosity and cracks.

( 1) Air holes: air holes are produced by gas, the surface of the holes is smooth, and they are generated inside or near the surface of the casting, and the shape is mostly round or oblong.

The main sources of gas that generate pores are:

① The nitrogen and hydrogen dissolved in the metal are contained in the metal during the solidification of the casting, forming closed circular or oval pores with metallic luster on the inner wall.

②Moisture or volatile matter in the modeling material will become gas due to heat, forming dark brown pores on the inner wall.

③During the casting process of metal, air is drawn in and pores are formed due to unstable flow.

Methods of preventing stomatal defects:

①In the smelting, the rusty metal materials should be used as little as possible, and the tools and ladle should be baked and dried.

②The molten steel should be discharged at high temperature and poured at low temperature, and the molten steel should be properly sedated to facilitate the rise of gas.

③The process design of the pouring riser should increase the pressure head of the molten steel to avoid the entrapment of gas, and set up an artificial gas circuit to reasonably exhaust.

④The moulding material should control the water content and the gas volume, increase the air permeability, and the sand mould and sand core should be baked and dried as much as possible.

( 2) Shrinkage cavity (loose) : It is a continuous or incoherent circular or irregular cavity (cavity) generated inside the casting (especially in the hot joint), with rough inner surface, darker color, and metal The crystal grains are coarse, mostly in the form of dendritic crystals, which are gathered in one or more places and are prone to leakage during the hydraulic test.

The reason for the shrinkage cavity (loose) : the volume shrinks when the metal solidifies from the liquid state to the solid state. At this time, if there is not enough molten steel to replenish, the shrinkage cavity will inevitably occur. The shrinkage of steel castings is basically caused by improper control of the sequential solidification process. The reasons may be incorrect setting of the riser, excessively high molten steel pouring temperature, and large metal shrinkage.

Methods to prevent shrinkage (loose): ①Scientifically design the casting system of the casting to make the molten steel solidify sequentially, and the first solidified part should be supplemented by molten steel. ② Set the riser, subsidy, and internal and external cold iron correctly and reasonably to ensure the order of solidification. ③When the molten steel is poured, the top pouring from the riser is helpful to ensure the temperature of the molten steel and the shrinkage, and reduce the generation of shrinkage holes. ④ In terms of pouring speed, low-speed pouring is more conducive to sequential solidification than high-speed pouring. ⑸The pouring temperature should not be too high. The molten steel is discharged at high temperature and pouring after sedation, which is beneficial to reduce shrinkage.

( 3) Sand (slag) : Sand (slag), commonly known as sand holes, is the appearance of incoherent round or irregular holes in the inside of the casting. The holes are mixed with molding sand or steel slag. The size of the One or more places, often in the upper part.

Causes of sand (slag) inclusion: slag inclusion is caused by the discrete steel slag entering the casting with the molten steel during the smelting or pouring process. Sand inclusion is caused by insufficient compactness of the cavity during molding. When molten steel is poured into the cavity, the molding sand is washed up by the molten steel and enters the inside of the casting. In addition, improper operation when repairing and closing the box, and the phenomenon of sand falling are also the reasons for the sand inclusion.

Methods to prevent sand (slag) from being generated : ① When molten steel is smelted, exhaust and slag should be exhausted as much as possible. After the molten steel is discharged from the furnace, it will be calmed in the ladle to help the slag float up. ② Try not to turn over the ladle of molten steel, but use a teapot bag or bottom pouring ladle to prevent the slag on the upper part of the molten steel from entering the casting cavity along the molten steel. ③Measures of castor slag should be taken when molten steel is poured to minimize the entry of steel slag into the cavity with molten steel. ④In order to reduce the possibility of sand inclusion, ensure the compactness of the sand mold when modeling, be careful not to lose sand when repairing the mold, and blow the cavity clean before closing the box.

( 4) Cracks : most of the cracks in castings are hot cracks, with irregular shapes, penetrating or non-penetrating, continuous or intermittent, and the metal at the crack is dark or oxidized on the surface.

Reasons for cracks: There are two aspects, namely high temperature stress and liquid film deformation.

High temperature stress is the stress formed by the resistance of the shrinkage and deformation of molten steel at high temperature. When the stress exceeds the strength or plastic deformation limit of the metal at that temperature, cracks are generated. Liquid film deformation is the formation of a liquid film between crystal grains in the process of solidification and crystallization of molten steel. With the progress of solidification and crystallization, the liquid film is deformed. When the amount of deformation and the deformation speed exceed a certain limit, cracks occur. The temperature range of thermal cracking is about 1200~1450℃.

Factors affecting cracks:

① S and P elements in steel are harmful factors for cracks. The eutectic between them and iron reduces the strength and plasticity of cast steel at high temperatures, leading to cracks.

② Slag inclusion and segregation in the steel increase stress concentration, thus increasing the tendency of hot cracking.

③The greater the linear shrinkage coefficient of the steel grade, the greater the tendency of hot cracking.

④The greater the thermal conductivity of the steel, the greater the surface tension, the better the high-temperature mechanical properties, and the smaller the tendency for thermal cracking.

⑤The structural design of the castings is not good in manufacturability. For example, if the fillet is too small, the wall thickness is too large, and the stress concentration is serious, cracks will occur.

⑥The compactness of the sand mold is too high, and the poor concession of the core hinders the shrinkage of the casting, which will increase the tendency of cracks.

⑦Others such as improper arrangement of pouring risers, too fast cooling rate of castings, excessive stress caused by cutting pouring risers and heat treatment will also affect the generation of cracks.

In view of the causes and influencing factors of the above cracks, corresponding measures can be taken to reduce and avoid the occurrence of crack defects.

Based on the above analysis of the causes of casting defects, finding the existing problems, and taking corresponding improvement measures, we can find a way to solve the casting defects, which is conducive to the improvement of the quality of castings.