The casting of the valve body is an important part of the valve manufacturing process, and the quality of the valve casting determines the quality of the valve. The following introduces several casting process methods commonly used in the valve industry:
Sand casting:
Sand casting commonly used in the valve industry can be divided into green sand, dry sand, water glass sand and furan resin self-hardening sand according to different binders.
(1) Green sand is a molding process using bentonite as a binder.
Its characteristics are: the finished sand mold does not need to be dried or hardened, the sand mold has a certain wet strength, and the sand core and mold shell have good yield, making it easy to clean and shake out the castings. The molding production efficiency is high, the production cycle is short, the material cost is low, and it is convenient to organize assembly line production.
Its disadvantages are: castings are prone to defects such as pores, sand inclusions, and sand adhesion, and the quality of castings, especially the intrinsic quality, is not ideal.
Proportion and performance table of green sand for steel castings:
(2) Dry sand is a molding process using clay as a binder. Adding a little bentonite can improve its wet strength.
Its characteristics are: the sand mold needs to be dried, has good air permeability, is not prone to defects such as sand washing, sand sticking, and pores, and the inherent quality of the casting is good.
Its disadvantages are: it requires sand drying equipment and the production cycle is long.
(3) Water glass sand is a modeling process using water glass as a binder. Its characteristics are: water glass has the function of automatically hardening when exposed to CO2, and can have various advantages of gas hardening method for modeling and core making, but There are shortcomings such as poor collapsibility of the mold shell, difficulty in sand cleaning of castings, and low regeneration and recycling rate of old sand.
Proportion and performance table of water glass CO2 hardening sand:
(4) Furan resin self-hardening sand molding is a casting process using furan resin as a binder. The molding sand solidifies due to the chemical reaction of the binder under the action of the curing agent at room temperature. Its characteristic is that the sand mold does not need to be dried, which shortens the production cycle and saves energy. Resin molding sand is easy to compact and has good disintegration properties. The molding sand of castings is easy to clean. The castings have high dimensional accuracy and good surface finish, which can greatly improve the quality of castings. Its disadvantages are: high quality requirements for raw sand, slight pungent smell at the production site, and high cost of resin.
Proportion and mixing process of furan resin no-bake sand mixture:
The mixing process of furan resin self-hardening sand: It is best to use a continuous sand mixer to make resin self-hardening sand. Raw sand, resin, curing agent, etc. are added in sequence and mixed quickly. It can be mixed and used at any time.
The order of adding various raw materials when mixing resin sand is as follows:
Raw sand + curing agent (p-toluenesulfonic acid aqueous solution) – (120 ~ 180S) – resin + silane – (60 ~ 90S) – sand production
(5) Typical sand casting production process:
Precision casting:
In recent years, valve manufacturers have paid more and more attention to the appearance quality and dimensional accuracy of castings. Because good appearance is the basic requirement of the market, it is also the positioning benchmark for the first step 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 solution 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.
② Using medium-temperature resin-based mold material, high-pressure wax injection, silica sol mold shell, steam dewaxing, rapid atmospheric or vacuum melting casting process, the dimensional accuracy of castings can reach CT4-6 precision castings.
(2) Typical process flow of investment casting:
(3) Characteristics of investment casting:
①The casting has high dimensional accuracy, smooth surface and good appearance quality.
② It is possible to cast parts with complex structures and shapes that are difficult to process with other processes.
③ 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 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 piece 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 techniques that can be used, its pressure-bearing capacity cannot be very high when it is used to cast pressure-bearing thin-shell valve castings.
Analysis of Casting Defects
Any casting will have internal defects, the existence of these defects will bring great hidden dangers to the internal quality of the casting, and the welding repair to eliminate these defects in the production process will also bring a great burden to the production process . In particular, valves are thin-shell castings that withstand pressure and temperature, and the compactness of their internal structures is very important. Therefore, the internal defects of castings become the decisive factor affecting the quality of castings.
The internal defects of valve castings mainly include pores, slag inclusions, shrinkage porosity and cracks.
(1) Pores: Pores are produced by gas, the surface of the pores is smooth, and they are generated inside or near the surface of the casting, and their shapes are 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 inner walls with metallic luster.
②Moisture or volatile substances in the molding material will turn into gas due to heating, forming pores with dark brown inner walls.
③ During the pouring process of the metal, due to the unstable flow, the air is involved to form pores.
Prevention method of stomatal defect:
① In smelting, rusty metal raw materials should be used as little as possible or not, and tools and ladles should be baked and dried.
②The pouring of molten steel should be done at high temperature and poured at low temperature, and the molten steel should be properly sedated to facilitate the floating of gas.
③ The process design of the pouring riser should increase the pressure head of molten steel to avoid gas entrapment, and set up an artificial gas path for reasonable exhaust.
④Moulding materials should control the water content and gas volume, increase the air permeability, and the sand mold and sand core should be baked and dried as much as possible.
(2) Shrinkage cavity (loose): It is a coherent or incoherent circular or irregular cavity (cavity) that occurs inside the casting (especially at the hot spot), with a rough inner surface and darker color. Coarse crystal grains, mostly in the form of dendrites, gathered in one or more places, prone to leakage during hydraulic test.
The reason for shrinkage cavity (looseness): volume shrinkage occurs when the metal is solidified from liquid to solid state. If there is not enough molten steel replenishment at this time, shrinkage cavity will inevitably occur. The shrinkage cavity of steel castings is basically caused by improper control of the sequential solidification process. The reasons may include incorrect riser settings, too high pouring temperature of molten steel, and large metal shrinkage.
Methods to prevent shrinkage cavities (looseness): ① Scientifically design the pouring system of castings to achieve sequential solidification of molten steel, and the parts that solidify first should be replenished with molten steel. ②Correctly and reasonably set riser, subsidy, internal and external cold iron to ensure sequential solidification. ③When the molten steel is poured, top injection from the riser is beneficial to ensure the temperature of the molten steel and feeding, and reduce the occurrence of shrinkage cavities. 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 taken out of the furnace at high temperature and poured after sedation, which is beneficial to reduce shrinkage cavities.
(3) Sand inclusions (slag): Sand inclusions (slag), commonly known as blisters, are discontinuous circular or irregular holes that appear inside castings. The holes are mixed with molding sand or steel slag, with irregular sizes and aggregated in them. One or more places, often more on the upper part.
Causes of sand (slag) inclusion: Slag inclusion is caused by discrete steel slag entering the casting along with the molten steel during the smelting or pouring process. Sand inclusion is caused by the insufficient tightness of the mold cavity during molding. When molten steel is poured into the mold cavity, the molding sand is washed up by the molten steel and enters the interior of the casting. In addition, improper operation during trimming and box closing, and the phenomenon of sand falling out are also the reasons for sand inclusion.
Methods to prevent sand inclusions (slag): ① When the molten steel is smelted, the exhaust and slag should be exhausted as thoroughly as possible. ② Try not to turn the molten steel pouring bag over, but use a teapot bag or a bottom pouring bag to prevent the slag above the molten steel from entering the casting cavity along with the molten steel. ③ When pouring molten steel, measures should be taken to prevent slag from entering the mold cavity with the molten steel. ④In order to reduce the possibility of sand inclusion, ensure the tightness of the sand mold when modeling, be careful not to lose sand when trimming, and blow the mold cavity clean before closing the box.
(4) Cracks: Most of the cracks in castings are hot cracks, with irregular shapes, penetrating or not penetrating, continuous or intermittent, and the metal at the cracks is dark or has surface oxidation.
reasons for cracks , namely high temperature stress and liquid film deformation.
High-temperature stress is the stress formed by the shrinkage and deformation of molten steel at high temperatures. When the stress exceeds the strength or plastic deformation limit of the metal at this temperature, cracks will occur. Liquid film deformation is the formation of a liquid film between crystal grains during the solidification and crystallization process of molten steel. With the progress of solidification and crystallization, the liquid film is deformed. When the deformation amount and deformation speed exceed a certain limit, cracks are generated. The temperature range of thermal cracks is about 1200~1450℃.
Factors affecting cracks:
① S and P elements in steel are harmful factors for cracks, and their eutectics with iron reduce the strength and plasticity of cast steel at high temperatures, resulting in cracks.
② Slag inclusion and segregation in steel increase stress concentration, thus increasing hot cracking tendency.
③ The greater the linear shrinkage coefficient of the steel type, the greater the tendency of hot cracking.
④ The greater the thermal conductivity of the steel type, the greater the surface tension, the better the high-temperature mechanical properties, and the smaller the tendency of hot cracking.
⑤ The structural design of castings is poor in manufacturability, such as too small rounded corners, large wall thickness disparity, and severe stress concentration, which will cause cracks.
⑥The compactness of the sand mold is too high, and the poor yield of the core hinders the shrinkage of the casting and increases the tendency of cracks.
⑦Others, such as improper arrangement of the riser, too fast cooling of the casting, excessive stress caused by cutting the riser and heat treatment, etc. will also affect the generation of cracks.
According to 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 out the existing problems and taking corresponding improvement measures, we can find a solution to casting defects, which is conducive to the improvement of casting quality.
Post time: Aug-31-2023