Introduction to Valve Casting Process
Casting technology of valve
Overview and characteristics of foundry
It is called casting that liquid metal is poured into the mold cavity which is suitable for the shape of the parts, and then solidified to obtain the parts with certain shape, size and surface quality.
Overview of Foundry Technology
Casting has a long history, about 3000 BC, mankind has cast a variety of exquisite bronzes.
But for thousands of years, it was made by hand from clay, sand and other natural materials. The output of castings is very small. With the development of industrial revolution, the increase of mechanization and the increase of casting demand, pneumatic machine and synthetic molding clay sand process were used in the 1930s. With the development of the times, all kinds of modeling methods emerge as the times require. For example: cement sand mold appeared in 1933, cement flow sand mold appeared in 1967; cooling film coated resin sand shell mold appeared in 1944; thermal film coated resin sand shell mold appeared in 1955, furan resin self hardening sand mold appeared in 1958; CO2 hardening water glass sand mold appeared in 1947, and organic hardener water glass (organic resin water glass) process appeared in 1968. In recent years, there are many new ways to make mold by physical means, such as magnetic shot molding, vacuum sealing molding, lost film molding and so on.
Casting is widely used in machinery manufacturing, mining and metallurgy, transportation, petrochemical general equipment, agricultural machinery, energy and power, light industry and textile, civil engineering, power electronics, aerospace, international military industry and other departments of the national economy due to its advantages of small investment in processing infrastructure, large process flexibility and short production cycle, It is the foundation of modern large machinery industry.
two Casting has a long history in China, but the casting technology has been stagnant for a long time. Since the reform and opening up, China’s casting technology has made great progress, which is mainly manifested in three aspects: the mechanization and automation of molding and core making have been significantly improved; the self hardening chemical molding sand has replaced the dry clay sand and oil sand; the casting technology has changed from experience to science, such as Computer simulation design. This series of reforms play an important role in improving production efficiency, reducing labor intensity, improving production environment, improving internal and external quality of castings, and saving raw materials and energy.
- 1. Casting has a wide range of adaptability and great flexibility. According to product requirements and various working conditions, it can manufacture products of various metal materials, such as iron, carbon steel, low alloy steel, copper, copper alloy, aluminum, aluminum alloy, titanium alloy, etc. Compared with other molding methods, casting is not limited by the weight, size and shape of parts. The weight can be from a few grams to several hundred tons, and the wall thickness can be from 0.3 mm to 1 m. as long as the shape is within the scope of casting technology, it is very complex, or it is difficult to machine, or even difficult to make parts, they can be obtained by casting.
- 2. Most of the raw materials used in casting are from a wide range of sources and are cheap, such as scrap steel, sand, etc. However, due to the recent rise of the domestic foundry and steel industry, the prices of these raw materials have risen.
- 3. Through advanced casting technology, the dimensional accuracy and surface quality of castings can be improved, so that the parts can be cut less and no cutting. To achieve the effect of labor and material saving in product manufacturing, and save the overall production cost.
The problems in casting are as follows
- 1. Most of the foundry enterprises in China are backward in casting technology and low in mechanization, which leads to low dimensional accuracy, poor surface quality, high consumption of energy and raw materials, low production efficiency, high labor intensity, serious environmental pollution and poor enterprise benefits.
- 2. Foundry is a high-risk industry, but also a hard, dirty, tired work, the industry efficiency is poor, leaving no one. It is very difficult to find both technicians and specific operators, and the phenomenon of no successor is very prominent.
- 3. There are many casting processes, long process, difficult to control the product quality, high scrap rate, recent alloy prices and labor costs rise, casting costs rise sharply.
Casting process of nuclear power valve
Considering the particularity and requirements of nuclear power valve casting parts, in order to stabilize the casting process quality, there are strict regulations on casting process flow, material, process and quality control.
Casting process of nuclear power valve
Casting process plan
According to the product structure, size and technical requirements to develop casting methods. There are two kinds of manufacturing methods: sand casting and investment casting.
Sand casting: clay sand, resin sand (furan, alkaline phenolic, polyurethanes), sodium silicate sand [CO2 hardening method, vrhc vacuum CO2 replacement hot air hardening method (dehydration hardening), self hardening sand of dicalcium silicate, red mud and other powder hardeners, organic ester hardening].
Investment casting: also known as special casting. There are about 12 kinds of casting, such as lost wax casting, ceramic mold casting, metal mold casting, pressure casting, centrifugal casting, vacuum suction casting, magnetic casting, shell mold casting and full mold casting.
The casting method is determined according to the production conditions of the enterprise.
Sand casting process
1. The quality of molding sand (core sand) has a great influence on the casting quality, such as sand hole, air hole, sand inclusion, crack and so on.
Influence of molding sand properties on casting quality: molding sand should have the following properties:
- (1) Strength under the action of external force, its performance that is not easy to be destroyed is called strength. It is very important that this kind of performance will not deform and damage under the action of mold manufacturing, transportation, liquid metal impact and pressure, otherwise it will cause defects such as box collapse, sand flushing and sand hole.
- (2) Air permeability molding sand has the ability to make gas pass through because of the gap between sand particles, which is called air permeability.
- (3) The fire resistance molding sand can not soften and melt under the action of high temperature liquid metal. When the fire resistance is not enough, the sand particles will melt and stick to the surface of the casting to form a hard skin, which will accelerate the tool wear during cutting. In order to make up for the lack of fire resistance of molding sand, a layer of coating is applied on the surface of mold cavity.
- (4) After pouring, the lower the high temperature strength of the sand, the better the resiliency and the lower the mechanical resistance of the casting. On the contrary, the shrinkage is blocked, the internal stress is large, and even cracks occur. In addition, there are reusability, gas generation, for resin sand there are micro powder, loss on ignition, residual alkali of alkaline resin and so on.
2. Classification of molding sand:
According to different binders, molding sand can be divided into three types
(1) Clay sand (2) sodium silicate sand (3) resin sand (4) oil sand and grease sand. Resin sand technology is a great change in foundry technology. It uses “resin” as the cleaning agent, which makes the foundry technology to a higher level in all aspects. There are many kinds of resin sand technology, such as shell core, hot core box, cold core box, self hardening sand and so on. We adopt “furan resin self hardening sand process”. Furan resin self hardening sand process is not only suitable for simultaneous interpreting in large quantities, but also for single, multi variety and small batch production. Compared with traditional clay sand, it has the following advantages:
High dimensional accuracy, low surface roughness, saving energy, improving labor productivity, improving workers’ working conditions, high reuse rate of used sand, low environmental pollution and so on.
The process has been introduced into China since 1980, especially the production line investment, and the related technology and raw material problems have been solved. It develops very fast in our country, especially in machine tool, shipbuilding, heavy machinery, electrician and other industries. But it also has its own limitations. When pouring thin-walled carbon steel castings, because of its high temperature strength, it is easy to produce cracks. This is absolutely not suitable for nuclear power valves. At the same time, there is a problem of Carburizing on the surface of castings. It is better not to use this process for the production of ultra-low carbon stainless steel.
3. Modeling method:
Modeling methods are divided into manual modeling and machine modeling: manual modeling is suitable for single piece and small batch production. According to the characteristics of sandbox, it can be divided into two sandboxes, three sandboxes, pit and debox. According to the characteristics of the model, it can be divided into: whole mold, sand excavation, false box, movable block, parting mold, car plate, etc.
The formulation of casting process drawing is as follows
The first step of casting production is to determine the casting process plan and draw the casting process diagram according to the structural characteristics, technical requirements, production batch and production conditions of the parts. Casting process drawing is a kind of drawing that uses various process symbols and colors to draw the information needed for manufacturing model and mold directly on the part drawing. The drawing shall include: pouring position, parting surface, number, shape, size and fixing method of cores, machining allowance, draft angle and shrinkage rate, size and position of gate, riser and chill.
Selection principle of pouring position
The pouring position of the casting is the position of the casting in the mold during pouring, which has a great influence on the quality of the casting. The following principles should be considered in selecting the pouring position: the important parts of the casting and the parts prone to defects should be placed in the most favorable position.
- (1) The important processing surface or main working surface of the casting should face down or side, because the defects (such as sand holes, pores, slag inclusion, etc.) on the upper surface of the casting are usually more than those on the lower surface, and the microstructure is not as dense as that on the lower surface. If it is difficult for these planes to face down, try to keep them on the side. When there are several important machining surfaces of the casting, the larger surface should be facing down, and the machining allowance should be increased for the upward surface to ensure the quality of the casting.
- (2) The large plane of the casting should face down. This is because in the process of pouring column, the high temperature liquid metal has strong heat radiation on the upper surface of the cavity. Sometimes the sand on the upper surface of the cavity is arched or cracked due to the rapid thermal expansion, resulting in sand inclusion defects on the surface of the casting. Obviously, the thicker the horizontal plane is, the easier the sand inclusion is on the upper surface.
- (3) In order to prevent the defects of insufficient pouring or cold shut in the large thin-walled part of the casting, the large thin-walled part should be placed in the lower part of the mold or vertical or inclined as far as possible, which is particularly important for the alloy with poor fluidity.
- (4) The hot spot where shrinkage cavity is easily formed in the casting is set at the upper or side near the parting surface at the pouring position, so that the riser can be directly placed at the thickness of the casting to make it solidify and feed from bottom to top in order to prevent shrinkage cavity.
- (5) It should be able to reduce the number of cores to facilitate the fixation and exhaust of cores.
Selection principle of parting surface
The selection of parting surface is also one of the important keys to the rationality of casting process. It is difficult to guarantee the quality of castings if the selection is improper, and the process of mold making, molding, core making, box closing and even cutting is complicated. Therefore, on the premise of ensuring the casting quality, the selection of parting surface should simplify the process as much as possible and save manpower and material resources. Practice has proved that the following principles should be considered in the selection of parting surface
- (1) The mold should have the least parting surface and be shaped or divided into one parting surface as far as possible. Because there is one more parting surface, more errors will be added to the mold, which will reduce the accuracy of the casting. If the casting has only one parting surface, the simple two box molding method can be used. It must be pointed out that the actual selection of parting surface should proceed from the reality. For some large and complex castings or castings with special requirements, sometimes more than two parting surfaces are used, which is beneficial to ensure the casting quality and simplify the process.
- (2) The parting surface should be selected in such a way that the number of cores and moving blocks is as small as possible, so as to simplify the process of mold making, molding and box closing.
- (3) All or large castings should be placed in the same sandbox as far as possible to ensure the accuracy of castings. If there are many machining surfaces of castings, the machining datum plane should be in the same sandbox with most machining surfaces.
- (4) In order to facilitate molding, core setting, box closing and inspection of casting wall thickness, the cavity and main core should be located in the box as far as possible. But the cavity of the lower box should not be too deep, and try to avoid the use of hanging core and large hanging sand.
- (5) The parting surface should be flat and straight to simplify the mold manufacturing and molding process.
- (6) Make full use of sandbox height.
The above principles are often contradictory to each other for specific castings, so it is difficult to fully comply with them. Therefore, in determining the pouring position and parting surface, it is necessary to consider comprehensively, pay attention to the main contradiction, and try to solve the secondary contradiction from the process measures.
Determination of process parameters
In order to determine the casting process parameters in the future, it is necessary to draw the casting process diagram
(1) Machining allowance the size of casting increased for machining is called machining allowance. Its size depends on the type of alloy, the size of casting, the production batch, the distance between machining surface and datum surface, the position of machining surface during pouring, etc. Due to the high pouring temperature, the surface of steel castings is not smooth enough, so the machining allowance should be relatively large; in mass production, the machining allowance can be small due to the mechanical modeling and complete process equipment; in single piece and small batch production, the machining allowance will be increased due to the large error of manual modeling. Table 1 shows the normal machining allowance of our company.
Table 1 machining allowance (mm)
Note: the lower limit of machining allowance value is used for mass production, and the upper limit is used for single piece and small batch production.
Whether the holes and grooves to be processed are cast or not depends on the size of the holes and grooves, the production batch, the type of alloy and other factors. Generally speaking, under the condition of single piece and small batch production, machining holes with diameter less than 25 mm for iron castings and 35 mm for steel castings can not be cast, because it is economical to drill holes directly during machining.
(2) Shrinkage rate after cooling, the casting size is smaller than the mold cavity size (i.e. the mold size) due to the alloy shrinkage. In order to ensure the proper size of the casting, the comprehensive shrinkage rate of steel shrinkage and sand mold shrinkage is pre placed during the manufacturing of the mold, and the size of the mold is enlarged in proportion. Therefore, the “scale” including alloy shrinkage should be used to draw the model diagram. The shrinkage of the alloy varies with the type of alloy and the size, shape and structure of the casting. Generally, the percentage of cast steel is about 1.5-3.0%.
(3) Draft angle in order to make the mold (or core) easy to take out from the mold (or core box), the vertical wall perpendicular to the parting surface must have a certain slope when manufacturing the mold, which is called draft angle or casting angle. The size of draft angle depends on the height of vertical wall, modeling method, model material and surface finish.
4. The shape and size of the core head have a great influence on the processability and stability of the core in the mold assembly.
The core head can be divided into vertical core head and horizontal core head. The horizontal core with single fulcrum is also called cantilever core.
Vertical cores generally have upper and lower cores, but short and thick cores may not have upper cores. The height h of the core head mainly depends on the diameter D on the core head. The core head must have a certain slope a.
The inclination of the lower core head should be smaller and the height should be higher, so as to increase the stability of the core;
The inclination of the upper core head should be larger, and the height should be smaller, so that it is easy to close the box.
The length of the horizontal core mainly depends on the diameter D of the core and the length of the core.
In order to lower the core and close the box easily, a certain slope a should be left at the end of the core seat on the mold. The hanging wall core head must be long and large to balance and support the core and prevent the core from sagging or being lifted by liquid metal.
There should be a 1-4mm gap (s) between the core head and the core seat to facilitate the assembly of the mold.
Casting properties of alloy
Pure metals are rarely used in foundry production, but various alloys are generally used. In addition to the mechanical properties, physical and chemical properties, the casting properties of cast alloys must also be considered. The casting properties of the alloy mainly include fluidity and shrinkage segregation, which are very important for obtaining sound castings.
Fluidity of alloys
When pouring metal, the liquid metal can fill the mold, which is the basic condition to obtain complete shape, accurate size and clear contour castings. However, in the process of filling, the liquid metal is accompanied by crystallization due to heat dissipation. At the same time, the resistance of the mold to the liquid metal and the back pressure of the gas in the cavity also exist, which hinder the smooth filling of the liquid metal. If the fluidity of the metal is insufficient and the flow is stopped before the metal fills the mold, the casting will have insufficient pouring or cold shut defects.
The fluidity of alloy refers to the fluidity of liquid metal itself and the ability of filling mold. The better the fluidity of alloy, the stronger the ability of liquid metal to fill the mold. Therefore, the fluidity of alloy is often summarized as the ability of liquid metal to fill the mold.
Alloy fluidity is one of the important casting properties of alloys. The better the fluidity is, the easier it is to cast a thin and complex casting with clear outline. At the same time, it is also conducive to the floating and elimination of non-metallic inclusions and gases in liquid metal, and it is easy to supplement the shrinkage of liquid metal during solidification. Therefore, the fluidity of the alloy must be considered in the casting design and casting process.
Shrinkage of alloys
1. Shrinkage of alloy and factors affecting shrinkage
(1) Shrinkage is the phenomenon that the volume and size of castings decrease during solidification and cooling. Shrinkage is the physical property of casting alloy itself, and it is the basic cause of many defects (such as shrinkage cavity, shrinkage porosity, crack, deformation, residual internal stress, etc.) in casting. In order to obtain a perfect casting with compact structure and shape and size in line with the technical requirements, it is necessary to study the regularity of shrinkage. The structure of liquid metal near melting point is composed of atomic clusters and holes, and its atomic spacing is much larger than that of solid metal. During the cooling process of metal pouring into the mold until solidification, the volume of liquid metal decreases due to the decrease of temperature, the decrease of the number of holes and atomic spacing. After the metal crystallizes, the holes disappear completely and the atomic spacing of the metal is further reduced. As the metal continues to cool after solidification, the atomic spacing will be reduced until room temperature. It can be seen that metal cooling from pouring temperature to room temperature goes through three interrelated shrinkage stages
- ① The liquid shrinkage is from the pouring temperature to the solidification starting temperature (liquidus temperature);
- ② The solidification shrinkage is the shrinkage from the start temperature to the end temperature (solidus temperature);
- ③ The shrinkage of solid state is from the end of solidification to room temperature.
The liquid shrinkage and solidification shrinkage of the alloy are the volume shrinkage of the alloy, which is usually expressed by the volume shrinkage rate. They are the basic causes of shrinkage defects. Although the solid shrinkage of alloy is also volume change, it only causes the change of external dimension of casting, so it is usually expressed by linear shrinkage.
Solid shrinkage is the main cause of internal stress, crack and deformation.
The shrinkage of different alloys is different. Among the commonly used alloys, cast steel has the largest shrinkage. The volume shrinkage of carbon steel is shown in table 2.
Table 2 volume shrinkage of carbon steel
|Types of alloys||Carbon content%||Casting temperature (℃)||Liquid shrinkage%||Solidification shrinkage%||Solid shrinkage%||Total volume shrinkage%|
|Carbon cast steel||0.35||
(2) Factors affecting shrinkage
The factors affecting shrinkage include chemical composition, pouring temperature, casting structure and mold conditions.
① Chemical composition
With the increase of carbon content, the solidification shrinkage of carbon steel increases, while the solid shrinkage decreases slightly. In gray cast iron, carbon is the element to form graphite, silicon is the element to promote graphitization, so the more carbon and silicon content, the smaller the shrinkage. Sulfur can prevent the graphite from breaking out and increase the shrinkage of castings. However, appropriate manganese content can combine with sulfur to synthesize MNS, which counteracts the effect of sulfur on graphitization and reduces the shrinkage. If the manganese content is too high, the shrinkage of cast iron will increase.
② The higher the pouring temperature, the higher the superheat and the higher the liquid shrinkage.
③ Casting structure and mold condition the alloy does not shrink freely in the mold, but restrains.
The resistance comes from the following two aspects
1) Because of the different cooling rate of different parts of casting, the resistance to shrinkage is caused by mutual restriction.
2) Mechanical resistance of mold and core to shrinkage. Obviously, the actual linear shrinkage of the casting is smaller than the free linear shrinkage of the alloy. Therefore, in the design of the model, the appropriate shrinkage must be selected according to the alloy variety, the specific shape, size, molding process and other factors of the casting.
2. Formation and prevention of shrinkage cavity
(1) In the solidification process of liquid metal in the mold, due to liquid shrinkage and solidification shrinkage, volume reduction, if the shrinkage can not be made up, holes will be formed in the last solidification part of the casting, which is called shrinkage cavity.
① Shrinkage cavity
Shrinkage cavity is usually hidden in the upper part of the casting or the last solidification part, and sometimes can be exposed by machining. In some cases, the shrinkage cavity is produced on the upper surface of the casting, showing obvious pits. This kind of shrinkage cavity is also called “open shrinkage cavity”. The shape characteristics of shrinkage cavity are: irregular shape, but mostly close to the inverted cone, its inner surface is not smooth. The forming process of shrinkage cavity is shown in Fig. 3-1. After the liquid metal fills the mold (Fig. 3-1, a), due to the heat absorption of the mold, the metal near the surface of the mold cavity will soon drop to the solidification temperature and solidify into a shell (Fig. 3-1, b).
Figure 3-1 schematic diagram of shrinkage cavity formation process
As the temperature continues to drop, the solidification layer thickens, and the remaining liquid inside the casting shrinks due to liquid shrinkage and solidification shrinkage of supplementary solidification layer, the volume shrinks, and the liquid level drops, resulting in voids in the casting (Fig. 3-1, c). The temperature continues to drop, the shell continues to thicken, and the liquid level continues to drop. When the internal solidification is complete, the shrinkage cavity is formed in the upper part of the casting (Fig. 3-1, d). The casting with shrinkage cavity is cooled to room temperature from the end temperature of solidification, and the external dimension is slightly reduced due to solid shrinkage (Fig. 3-1, e).
② Shrinkage porosity
Shrinkage porosity is essentially to disperse the concentrated shrinkage pores into a large number of small shrinkage pores. For the same shrinkage volume, the distribution area of shrinkage porosity is much larger than that of shrinkage cavity. The reason of shrinkage porosity is that the liquid shrinkage and solidification shrinkage of the alloy can not make up for each other, but the specific factor has its particularity compared with the concentrated shrinkage. The formation of shrinkage porosity can be illustrated by the cylindrical casting shown in Figure 3-2. The casting begins to solidify from the outer layer, but the front edge of solidification is uneven (Fig. 3-2a). Because the casting has similar heat dissipation conditions in the circumferential direction, the front edge of solidification almost reaches the center at the same time in the late solidification stage, forming a simultaneous solidification zone. In this region, the remaining liquid is divided into many small liquid regions by the uneven solidification front (Fig. 3-2b). Finally, when these large amount of small liquid regions solidify and contract, shrinkage porosity is formed due to lack of feeding (Fig. 3-2c).
Fig. 3-2 formation process of shrinkage porosity
Figure 3-3 microscopic shrinkage
When the crystallization interval of the alloy is very large, in addition to the increase of shrinkage porosity caused by the above reasons, micro shrinkage porosity will occur in a larger area. At this time, crystallization takes place simultaneously in a wide solidification area. There are both growing solids and liquid metals (see Fig. 3-3). The primary crystals often grow in dendritic shape, so that the liquid is divided into many small liquid areas. If the feeding condition is poor, the casting will also produce shrinkage porosity, which is even smaller and needs to be observed under the microscope. It is difficult to avoid micro shrinkage in castings. Generally, castings are not regarded as defects. However, if high air tightness is required to prevent leakage under pressure, or physical and chemical properties are considered, measures should be taken to prevent or reduce micro shrinkage.
It can be seen from the formation of shrinkage cavity and porosity:
- ① The larger the liquid shrinkage and solidification shrinkage of the alloy (such as cast steel, white cast iron, aluminum bronze, etc.), the larger the shrinkage volume, the easier the casting to form shrinkage cavity.
- ② The higher the pouring temperature is, the greater the liquid shrinkage is (usually the volume shrinkage increases by about 1.6% for every 100 ℃), and the easier the shrinkage cavity is.
- ③ The alloy with large crystallization interval is easy to form shrinkage porosity, while the alloy with pure metal or eutectic composition is easy to form concentrated shrinkage porosity due to small tendency of shrinkage porosity.
Shrinkage porosity, especially microscopic shrinkage porosity, is widely distributed, which is difficult to feed and find. Concentrated shrinkage is easy to check and repair, and also easy to take process measures to prevent. Therefore, the alloy with near eutectic composition or small crystallization interval is often used in casting production.
Any form of shrinkage cavity can significantly reduce the mechanical properties of the casting. Shrinkage porosity can also affect the air tightness, physical and chemical properties of the casting. Therefore, shrinkage cavity and porosity are the major defects of castings, which must be prevented by taking appropriate technological measures according to the technical requirements of castings.
(2) As mentioned above, shrinkage is the physical nature of the alloy. Under normal pouring temperature, the shrinkage volume of the alloy with certain composition can not be changed, but it does not mean that the shrinkage of the casting can not be avoided. It has been proved by practice that even though the shrinkage of the alloy is very large, as long as the solidification of the steel is controlled reasonably and the sequential solidification is realized, the dense casting without shrinkage cavity can be obtained. The so-called sequential solidification means that at the hot spot where shrinkage cavity may appear (that is, the thickest part with the largest diameter of the inscribed circle), a series of technological measures such as adding riser or chill are adopted to make the part far away from the riser solidify first, then the part near the riser solidifies, and finally the riser itself solidifies. According to this cooling sequence, the solidification shrinkage of each part of the casting can be fed by liquid metal, and the shrinkage cavity is transferred to the riser. The riser is the redundant part of the casting, which should be cut off when the casting is cleaned. Fig. 3-4 is the schematic diagram of riser feeding.
A) in the figure shows that shrinkage cavity occurs in the upper thick wall of the casting when the riser is not installed.
B) in the figure shows that the casting solidifies in sequence and the shrinkage cavity is transferred to the riser after the riser is added.
Bottom flange→tube→Upper flange→Riser
There are many kinds of risers. The common riser shown in Figure 3-4 is exposed on the upper box, which is fed by the static pressure of metal. This kind of riser is convenient in modeling, flexible in operation, and easy to supplement hot metal during pouring, so it is widely used. However, the feeding efficiency is poor, the metal consumption is high, and the use of some parts of the casting is limited. In order to make up for the shortage of common risers, various forms of concealed risers are often used in batch and mass production. The dark riser has the advantages of slow heat dissipation, high feeding efficiency and easy feeding to the side or lower part of the casting. In order to further improve feeding efficiency, measures can be taken to increase the internal temperature or pressure of riser, such as heating riser and atmospheric pressure riser.
In order to control the solidification of the casting, the chill can also be placed at the hot spot of the casting. The function of chill is to increase the cooling rate of the thick part of the casting to prevent shrinkage, but the chill itself does not play the role of feeding. Chill is usually made of cast iron or steel.
Figure 3-4 shows the feeding method of valve body castings.
Figure 3-4 two casting methods of valve body casting
The left side of the figure shows the possible shrinkage cavity at the hot spot without riser; the right side of the figure shows that after adding riser and chill, the casting realizes sequential solidification and prevents shrinkage cavity. However, the riser wastes metal, consumes man hours, increases the cost of the casting, and the internal stress of the casting increases, so it is easy to produce deformation and cracks. Therefore, it is mainly used for alloys with large solidification shrinkage and small crystallization interval, such as cast steel, high grade gray cast iron, nodular cast iron, malleable cast iron, brass, etc.
Internal stress, deformation and crack in casting
During the continuous cooling process after solidification, if the solid shrinkage of the casting is obstructed, internal stress will be produced in the casting, which is called internal stress in casting. This kind of internal stress is temporarily stored in the cooling process, and sometimes remains until room temperature, which is called residual internal stress. The internal stress of casting is the main cause of deformation, cold crack and hot crack. According to the causes, it can be divided into thermal stress and shrinkage stress.
1. Thermal stress
The thermal stress is caused by the uneven wall thickness and different cooling rate of each part of the casting, resulting in the inconsistent shrinkage of each part of the casting in the same period.
In order to analyze the formation of thermal stress, it is necessary to understand the change of metal state from high temperature to room temperature, that is, to distinguish plastic state and elastic state.
The metal is in a plastic state from the end of solidification to the recrystallization temperature (620-650 ℃ for steel and castings). At this time, the elongation is very high and the plasticity is good. Under the small external force, the plastic deformation (i.e. permanent deformation) occurs, and the internal stress eliminates itself.
The metal below recrystallization temperature is in elastic state. At this time, under the action of external force, the metal is elastic deformation, and the stress continues to exist after deformation.
2. Shrinkage stress
It is the internal stress caused by the mechanical obstruction of shrinkage, so it is also called mechanical stress. There are many reasons for the formation of mechanical obstacles, such as too high high temperature strength of molding sand or core, poor yield, too little sand consumption of sand box belt or core bone, etc.
The shrinkage stress usually causes tensile or shear stress in the casting, which is temporary. After the casting is boxed or the core is taken out, the stress will disappear. However, the shrinkage stress can act together with the thermal stress in the mold, which enhances the tensile stress and causes the casting to produce hot crack at high temperature or cold crack at low temperature.
Casting process example of valve body
Take the valve body as an example to briefly introduce the casting process design.
Figures 3-5 and 3-6 show two processes of a valve body.
1. Selection of pouring position and parting surface
According to the selection principle of pouring position, the three flange and sealing surface of valve body are important parts. Due to the structural characteristics of the valve body, in order to ensure the convenience of demoulding and quality, the center line of the horizontal direction of the three flange is used as the parting surface. The pouring position is designed according to the flange position on both sides of the Haff face. In this way, the minimum number of mud cores can be ensured, and it is also convenient for core setting and inspection (see Fig. 3-5).
2. Process parameters
According to the main dimensions of the casting, refer to the relevant manual, and determine the machining allowance according to the size of the product.
3. Riser design
Riser design is the key to foundry technology. It is the key to ensure the internal quality of castings. To ensure that the riser solidifies later than the casting, so that the liquid metal in the riser can fully supply the volume shrinkage of the casting during solidification. Firstly, the distribution of hot spot is analyzed. Riser should be set at the thick part of the casting. For the valve body, the three flange root and the sealing ring are the hot spot area. Therefore, risers should be set in these parts.
As shown in the figure below, both valve bodies are considered in this way. But there are some differences. In Figure 3-6, a blind riser is used,
And the gate through the riser is conducive to the riser metal temperature higher than the casting. The open top riser is used in Figure 3-5.
Although the riser forms are different, the results are consistent. It can ensure the feeding effect of the riser, and the specific calculation method of the riser is omitted.
Repair welding of nuclear power valve castings
Because of the particularity of the working condition, the internal quality requirements of the castings are very strict. Generally, RT detection is required to ensure the internal quality of the castings. However, there are always some defects in the castings, some of which are allowed to exist, while others are not allowed. For the defects that are not allowed, if the quantity and size meet the welding and repair specification of castings, the internal quality of the castings can be ensured To be repaired by welding. As far as casting itself is concerned, the casting defects in a certain range can be eliminated by digging. For the casting of nuclear power valve, the control of repair welding is more strict. The following are the control requirements for specific repair welding:
1. record the defect truthfully
For defects of castings, defect records shall be made. For important parts (general body, cover and door), each casting requiring repair shall be provided with a sketch and the type, position of defects and the size of the pit left after excavation. Write down the specific operation parameters during welding repair. In the future, the user can track these parts in the future.
2. implement the application system for welding repair
If the casting of nuclear power valve exceeds the standard, the casting unit must apply for it before welding repair. After the approval of design and process department, the specific implementation plan shall be formulated and submitted to the chief engineer for approval before welding repair can be carried out. The important event of welding repair of castings has been checked at various levels.
3. operation specification for serious welding repair
During the welding process of casting, welding must be carried out by welders with nuclear power welding repair qualification. During the welding repair process, welding must be carried out strictly according to the process requirements and operation specifications, and the welding rod drying and heat preservation during the use must be done to maintain a certain layer temperature.
4. Do well post welding heat treatment
After the repair welding of castings, the change from liquid to solid state occurred in the repair welding area. The edge area of the repair welding was subjected to the heating and cooling process to produce stress, and the local structure of the welding repair also changed. Therefore, the casting must be heat treated after repair welding to make the structure and performance of the casting consistent and the stress eliminated. But the heat treatment times of castings can not be too many, so we use the raw RT and repair welding, and the heat treatment of castings is not more than 2 times.
Analysis of casting defects
There will be defects in any casting. The existence of these defects will bring great hidden danger to the internal quality of the casting. In the production process, the welding repair to eliminate these defects will also bring great burden to the production process. In particular, as a thin shell casting under pressure and temperature, the internal structure compactness of valve is very important. Therefore, the internal defects of castings become the decisive factor affecting the quality of castings.
The main internal defects of valve castings are porosity, slag inclusion, shrinkage porosity and cracks.
(1) Blowhole: the blowhole is produced by gas, and the surface of the hole is smooth. It is produced in the interior or near the surface of the casting, and the shape is mostly round or oblong.
The main sources of gas forming pores are as follows
① Nitrogen and hydrogen dissolved in the metal are contained in the metal during the solidification process of the casting, forming closed round or oval pores with metallic luster on the inner wall.
② The moisture or volatile matter in the molding material will become gas due to heating, forming dark brown pores on the inner wall.
③ In the process of metal pouring, the air is drawn in and pores are formed due to the unsteady flow.
Prevention methods of air hole defects:
- ① In smelting, rusty metal materials should be used or not used as much as possible, and tools and ladle should be baked and dried.
- ② The molten steel should be discharged at high temperature and poured at low temperature. The molten steel should be properly sedated to facilitate the gas floating.
- ③ The process design of pouring and riser should increase the head of molten steel, avoid gas involvement, and set up artificial gas path to exhaust reasonably.
- ④ The molding materials should control the moisture content and gas generation, increase the air permeability, and the sand mold and sand core should be baked and dried as far as possible.
(2) Shrinkage cavity (porosity): it is a continuous or discontinuous round or irregular cavity (cavity) produced in the casting (especially in the hot spot). The inner surface is rough, the color is dark, the metal grains are coarse, and most of them are dendritic crystals, which gather in one or more places. Leakage is easy to occur during the hydraulic test.
Causes of shrinkage cavity (porosity): volume shrinkage occurs when the metal solidifies from liquid to solid. At this time, if not enough molten steel is supplied, shrinkage cavity will inevitably occur. The shrinkage of steel castings is basically caused by improper control of sequential solidification process, which may be caused by incorrect riser setting, too high pouring temperature of molten steel and large metal shrinkage.
Methods to prevent shrinkage cavity (porosity)
- ① The gating system of castings should be designed scientifically to realize the sequential solidification of molten steel. The first solidification part should be supplemented with molten steel.
- ② Correct and reasonable setting of risers, subsidies, internal and external chill, to ensure the sequence of solidification.
- ③ During the pouring of molten steel, the last top pouring from the riser is beneficial to ensure the temperature of molten steel and feeding, and reduce the occurrence of shrinkage cavity.
- ④ In the aspect of pouring sequence, high pouring speed is better than low pouring speed.
- ⑤ The pouring temperature should not be too high. The molten steel is discharged from the furnace at high temperature and poured after sedation, which is conducive to reducing shrinkage cavity.
(3) Sand inclusion (slag): sand inclusion (slag), commonly known as sand hole, is a kind of incoherent round or irregular hole in the casting, which is mixed with molding sand or steel slag. The size of the hole is irregular, and it gathers in one or more places, often in the upper part of the mold.
The reason of sand inclusion (slag): slag inclusion is caused by the discrete steel slag entering the casting with the molten steel in the process of smelting or pouring. Sand inclusion is caused by the insufficient compactness of the mold cavity during molding. When the molten steel is poured into the mold cavity, the sand is flushed by the molten steel into the casting. In addition, improper operation during mold repair and box closing and sand dropping are also the causes of sand inclusion.
Methods to prevent the occurrence of sand (slag):
- ① During molten steel smelting, exhaust gas and discharge slag thoroughly as far as possible. After the molten steel comes out of the furnace, calm down in the ladle, which is conducive to the floating of slag.
- ② The pouring ladle of molten steel should not be turned over as far as possible, but teapot ladle or bottom ladle should be used to avoid the slag in the upper part of molten steel entering the casting cavity along with the molten steel.
- ③ Castor slag measures should be taken during molten steel pouring to minimize the slag entering the cavity with molten steel.
- ④ In order to reduce the possibility of sand inclusion, it is necessary to ensure the compactness of sand mold during molding, pay attention not to drop sand during mold repair, and blow the cavity clean before closing the box.
(4) Cracks: most of the cracks in the casting are hot cracks with irregular shape, penetrating or non penetrating, continuous or intermittent, and the metal at the crack is dark or has surface oxidation.
There are two reasons for cracks: high temperature stress and liquid film deformation.
High temperature stress is the stress caused by the blockage of shrinkage deformation of molten steel at high temperature. When the stress exceeds the strength or plastic deformation limit of metal at this temperature, cracks will appear. Liquid film deformation is the liquid film formed between grains during solidification and crystallization of molten steel. With the solidification and crystallization, the liquid film deforms. When the deformation amount and deformation speed exceed a certain limit, cracks appear. The temperature range of hot crack is about 1200 ~ 1450 ℃.
The influencing factors of cracks are as follows
- ① S and P elements in steel are harmful factors for cracks. Their eutectic with iron reduces the strength and plasticity of cast steel at high temperature, resulting in cracks.
- ② Slag inclusion and segregation in the steel increase the stress concentration and hence the hot cracking tendency.
- ③ The larger the linear shrinkage coefficient of steel grade is, the greater the tendency of hot cracking is.
- ④ The higher the thermal conductivity of the steel, the greater the surface tension, the better the high temperature mechanical properties, and the smaller the tendency of hot cracking.
- ⑤ If the fillet is too small, the wall thickness difference is too large, and the stress concentration is serious, cracks will occur.
- ⑥ If the compactness of sand mold is too high and the core is poor, the shrinkage of casting will be hindered and the tendency of crack will be increased.
- ⑦ Other factors such as improper arrangement of risers, too fast cooling speed of castings, excessive stress caused by cutting 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 cracks.
Based on the above analysis of the causes of casting defects, find out the existing problems, and take corresponding improvement measures, we can find the method to solve the casting defects, which is conducive to the improvement of casting quality.
Quality control of castings
What is casting quality
In casting production, the word “quality” has two meanings. The first is the quality of the casting itself, that is, the extent to which the casting meets the user’s requirements in the process of use; the second is the engineering quality, that is, the extent to which the production process of the casting can guarantee the product quality. The two contents are different, but they are related. For example, the foundry enterprises with backward management technology and poor engineering quality can produce castings with good quality through meticulous work and sparing no cost. For example, the foundry enterprises with good facilities and management do not need to strive for refinement when they produce castings with low requirements.
1. Quality of castings
To evaluate the quality of castings, the first is to see whether it can meet the use conditions and the designer’s requirements for its structure and function. The second is economy. It is required to meet the quality requirements of products with the lowest production cost. It is necessary to analyze all aspects of production from the perspective of value engineering, so as to maintain the competitiveness of products for casting enterprises.
2. Project quality
Engineering quality refers to the guarantee degree of the management level and technical quality of the enterprise to the product quality requirements. Its main factors are the quality of employees, the level of technology, the control and management of raw materials for production, the status of equipment, the quality of process equipment and the quality assurance system of the enterprise.
Requirements for Castings
1. Requirements for casting material
The composition and mechanical properties of the material should meet the design standards and the selected material brand.
2. Dimensional tolerance, weight tolerance and surface roughness
Dimensional tolerance, quality tolerance and surface roughness are all important indexes to reflect the quality of castings, but the requirements for all kinds of castings are not the same.
(1) Dimensional tolerance of castings
It should conform to the Chinese standard GB / t6414 “casting dimensional tolerance and machining allowance”, which is divided into 16 grades. According to the casting material and the selected production mode, the casting tolerance grade CT (casting dimensional tolerance grade) is determined, and the specific tolerance value is found according to different nominal dimensions. General clay sand is grade 14 and self hardening sand is grade 11-12.
(2) Weight tolerance MT of castings
According to GB / t11351 “weight tolerance of castings”, 16 grades are also specified. The tolerance grade CT is determined as above, and the tolerance value is found according to different weights. The general confirmation methods are as follows:
- 1) In mass production, both the supplier and the demander randomly select 10 pieces and take the average value;
- 2) The actual weight of small batches and single castings shall be confirmed by both parties;
- 3) The actual weight of the standard sample is taken as the nominal weight.
(3) Surface roughness m of casting
So far, there is no classification standard of casting roughness in China and no international standard. At present, the parameters for evaluating the surface roughness in most countries are the arithmetic mean deviation (RA) of the surface profile of castings, and the roughness measured in micrometers (UM) requires special instruments. At present, standard samples are used for comparison evaluation in various industrial countries.
3. Casting quality
(1) Appearance quality of castings
Generally, the designer of the casting shall specify the defects according to the service conditions of the casting, or determine the allowable defects on the casting through negotiation between the supplier and the demander. The defects are generally inspected by the inspector with visual inspection. It is generally in accordance with msssp-55 “quality system of steel castings for valves, flanges, fittings and other piping components – visual inspection method for irregular surfaces”.
If the surface quality requirements are high, it can be proposed that the castings should pass the penetrant inspection or magnetic particle inspection. These two kinds of nondestructive testing should be accepted according to the provisions of Appendix B of ASTM B16.34.
(2) Internal quality of castings
In general, it is not required to detect the internal defects of castings, only for special products. There are generally two ways to inspect the internal quality of castings: one is casting anatomy, the other is non-destructive testing.
Nondestructive testing mainly includes penetrant testing (PT), magnetic particle testing (MT), ultrasonic testing (UT) and radiographic testing (RT).
3、 Investigation on quality assurance system of foundry
The buyer of the casting not only pays attention to the quality of the casting itself, but also cares about the engineering quality of the foundry.
For the purchase of valves, the casting accounts for about 90% of the weight of the whole valve by weight, which shows the importance of casting. The quality reliability and economy of products are closely related to the engineering quality of the manufacturer. Therefore, when purchasing the valve, the actual capacity of the manufacturer of the valve, that is, whether the casting is produced or purchased, can also say whether the engineering quality can meet the requirements of the goods you purchased, and carry out quality assurance inspection.
On the one hand, there are three aspects in the quality assurance investigation:
1. The quality assurance system of the production unit includes:
- Quality management organization;
- Quality responsibility system;
- Quality information network;
- Standardization organization;
- Measurement work;
- Quality education for employees;
- Product technical file management, technical data and other requirements to achieve traceability.
2. The basic facilities of the production unit include:
- Production equipment (degree of mechanization of the enterprise);
- Process equipment (such as sand box finishing car, positioning pin and other auxiliary equipment);
- Monitoring in the production process;
- Product inspection means.
3. The personnel quality of the production unit includes:
- Whether the production workers have received technical training before going on duty;
- Whether the operators of key processes have received necessary training and qualification certification;
- The civilized production habits of employees.
Difference between casting and forging process of valve
Casting valve is the valve made by casting. Generally, the pressure rating of casting valve is relatively low (such as PN16, PN25, PN40, but there are also high pressure valves, which can reach 1500Lb, 2500lb), and most of the diameter are above DN50. Forged valve is forged out, generally used in high-grade pipeline, the diameter is relatively small, generally below DN50.
1. Casting: it is the process of melting the metal into a liquid that meets certain requirements and pouring it into the mold. After cooling, solidification and cleaning, the casting (part or blank) with predetermined shape, size and performance can be obtained. The basic technology of modern machinery manufacturing industry.
2. The cost of casting blank is low, and it is more economical for the parts with complex shape, especially for the parts with complex cavity; at the same time, it has wide adaptability and better comprehensive mechanical properties.
3. However, there are many materials (such as metal, wood, fuel, molding materials, etc.) and equipment (such as metallurgical furnace, sand mixer, molding machine, core making machine, sand dropping machine, shot blasting machine, cast iron plate, etc.) needed for foundry production, which will produce dust, harmful gas and noise and pollute the environment.
4. Casting is a kind of metal hot working technology mastered by human beings earlier, which has a history of about 6000 years. In 3200 BC, copper frog castings appeared in Mesopotamia. From the 13th century BC to the 10th century BC, China has entered the heyday of bronze casting, and the technology has reached a fairly high level. For example, the simuWe square tripod weighing 875 kg in the Shang Dynasty, the zunpan of marquis Zeng Yi in the Warring States period and the transparent mirror of the Western Han Dynasty are representative products of ancient casting. The early casting was greatly influenced by pottery. Most of the castings were tools or utensils for agricultural production, religion, life and other aspects, with strong artistic color. In 513 B.C., China produced the earliest recorded iron casting in the world, the Jin Ding (about 270 kg). Around the 8th century, iron castings were produced in Europe. In the new era of industrial revolution, the casting industry entered the post-18th century. In the 20th century, the development of casting is very fast. Ductile iron, malleable iron, ultra-low carbon stainless steel, aluminum copper, aluminum silicon, aluminum magnesium alloy, titanium base, nickel base alloy and other casting metal materials have been developed, and a new process of inoculation treatment for gray cast iron has been invented. After 1950s, new technologies such as wet sand high pressure molding, chemical hardening sand molding, core making, negative pressure molding and other special casting, shot blasting and cleaning appeared.
5. There are many kinds of casting, which can be divided into:
- ① Ordinary sand mold casting, including wet sand mold, dry sand mold and chemical hardening sand mold.
- ② According to the molding materials, the special casting can be divided into the special casting with natural mineral sand and stone as the main molding materials (such as investment casting, clay mold casting, shell mold casting in foundry, negative pressure casting, full mold casting, ceramic mold casting, etc.) and the special casting with metal as the main molding materials (such as metal mold casting, pressure casting, continuous casting, low pressure casting, centrifugal casting, etc.).
6. Casting process usually includes:
- ① preparation of casting mold (making liquid metal a container for solid casting), casting mold can be divided into sand mold, metal mold, ceramic mold, clay mold, graphite mold, etc. according to the materials used, casting mold can be divided into disposable mold, semi permanent mold and permanent mold according to the times of use, and the advantages and disadvantages of casting mold preparation are the main factors affecting the casting quality;
- ② melting and pouring of casting metal, casting process Casting metal (casting alloy) mainly includes cast iron, cast steel and cast non-ferrous alloy;
- ③ casting treatment and inspection, including removing foreign matters on the core and casting surface, removing casting riser, scraping burr and seam and other protrusions, as well as heat treatment, shaping, anti rust treatment and rough processing.
1. Forging: is the use of forging machinery to exert pressure on metal billets to produce plastic deformation, in order to obtain a certain mechanical properties, a certain shape and size of forgings processing method.
2. One of the two major components of forging. The casting porosity and welding holes of metal can be eliminated by forging, and the mechanical properties of forgings are generally better than those of castings of the same material. For the important parts with high load and severe working conditions in machinery, forgings are mostly used except for the simple shape of rolled plates, profiles or weldments.
3. Forging can be divided into:
- ① open forging (free forging). There are mainly two kinds of forging, manual forging and mechanical forging, which are used to deform the metal between the upper and lower anvil blocks by impact force or pressure.
- ② Closed mode forging. The forging is made of metal billet in a certain shape forging die, which can be divided into die forging, cold heading, rotary forging, extrusion and so on. According to deformation temperature, forging can be divided into hot forging (processing temperature is higher than recrystallization temperature of blank metal), warm forging (lower than recrystallization temperature) and cold forging (normal temperature).
4. The main forging materials are carbon steel and alloy steel, followed by aluminum, magnesium, titanium, copper and their alloys. The original state of materials includes bar, ingot, metal powder and liquid metal. The ratio of the cross-sectional area of the metal before deformation to that of the die after deformation is called forging ratio. The correct choice of forging ratio has a great relationship with the improvement of product quality and the reduction of cost.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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