Casting is a kind of metal thermal processing technology mastered by human beings earlier, with a history of about 6,000 years. China has entered the heyday of bronze castings between 1700 BC and 1000 BC, and the craftsmanship has reached a very high level.
Definition and classification of casting
Definition of casting: It is a molding method of casting liquid metal into a casting cavity suitable for the shape of the part, and after it is cooled and solidified, a metal part blank with a certain shape, size and performance is obtained.
Common casting methods include sand casting and precision casting, and the detailed classification methods are shown in the table below.
Sand Casting: Sand Casting – A casting method in which castings are produced in sand molds. Steel, iron and most non-ferrous alloy castings can be obtained by sand casting. Because the molding materials used in sand casting are cheap and easy to obtain, the casting mold is easy to manufacture, and it can adapt to the single-piece production, batch production and mass production of castings. It has been the basic process in casting production for a long time.
Precision casting: Precision casting is a general term for precise casting processes obtained by precise molding methods. Its products are precise, complex, and close to the final shape of the parts, and can be used directly without or with little processing. It is an advanced technology near net shape.
Commonly used casting methods and their advantages and disadvantages
1. Ordinary sand casting
The basic raw materials for making sand molds are foundry sand and sand binder. The most commonly used foundry sand is siliceous sand. When the high-temperature performance of siliceous sand cannot meet the requirements of use, special sand such as zircon sand, chromite sand, and corundum sand are used. The most widely used molding sand binder is clay, and various drying oils or semi-drying oils, water-soluble silicates or phosphates and various synthetic resins can also be used as molding sand binders.
The outer sand molds used in sand casting are divided into three types: clay wet sand molds, clay dry sand molds and chemically hardened sand molds according to the binder used in the sand and the way to build strength.
Sand casting is the most popular and simplest type of casting that has been used for centuries. Sand casting is the process of making large parts such as gray iron, ductile iron, stainless steel and other types of steel. The main steps include painting, mold, core making, molding, melting and pouring, cleaning and so on.
Selection of Process Parameters
Machining allowance: The so-called machining allowance is the surface that needs to be cut on the casting. A certain amount of machining allowance should be reserved in advance. Its size depends on the type of casting alloy, molding method, casting size and the position of the processing surface in the mold. location and many other factors.
Draft slope: In order to make the pattern easy to take out from the mold, the slope added to the vertical wall perpendicular to the parting surface is called the draft slope.
Casting rounded corners: In order to prevent stress and cracks at the joints and corners of the castings, and to prevent damage to the sharp corners of the mold and sand holes, when designing castings, the joints and corners of the casting walls should be designed as rounded corners.
Core head: In order to ensure the positioning, fixation and exhaust of the core in the mold, a core head must be designed for both the pattern and the core.
Shrinkage allowance: Due to the cooling shrinkage of the casting after pouring, this part of the shrinkage size should be added when making the shape.
∙ Clay resources are abundant and cheap. Most of the used clay wet sand can be recycled and reused after proper sand treatment;
∙ The cycle of manufacturing mold is short and the work efficiency is high;
∙ The mixed molding sand can be used for a long time;
∙ Wide adaptability. Small pieces, large pieces, simple pieces, complex pieces, single pieces and large batches can be used;
Disadvantages and limitations:
∙ Since each sand mold can only be poured once, the mold is damaged after the casting is obtained and must be reshaped, so the production efficiency of sand casting is low;
∙ The rigidity of the mold is not high, and the dimensional accuracy of the casting is poor;
∙ Castings are prone to defects such as sand washing, sand inclusions, and pores.
2. Investment casting
Investment casting is also called “lost wax casting” when wax is used to make the pattern. Investment casting usually refers to making patterns from fusible materials, covering the surface of the patterns with several layers of refractory materials to make shells, and then melting the patterns and discharging them to obtain molds without parting surfaces. After high-temperature roasting Ready-to-sand casting casting solution. Since patterns are widely made of waxy materials, investment casting is often referred to as “lost wax casting”.
The types of alloys that can be produced by investment casting include carbon steel, alloy steel, heat-resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and ductile iron.
- High dimensional accuracy. Generally up to CT4-6 (CT10~13 for sand casting, CT5~7 for die casting);
- Can improve the utilization rate of metal materials. Investment casting can significantly reduce the amount of processing on the forming surface and mating surface of the product, saving processing time and cutting tool material consumption;
- It can maximize the similarity between blanks and parts, and bring great convenience to the structural design of parts. Castings with complex shapes Investment casting can cast castings with very complex shapes, castings with a wall thickness of 0.5mm and a weight as small as 1g, and combined and integral castings;
- Not limited by alloy material. The investment casting method can cast carbon steel, alloy steel, ductile iron, copper alloy and aluminum alloy castings, and can also cast castings of superalloys, magnesium alloys, titanium alloys and precious metals. For alloy materials that are difficult to forge, weld and cut, it is especially suitable for casting by precision casting;
- High production flexibility and adaptability Investment casting is suitable for both large batch production and small batch production or even single piece production.
Disadvantages and limitations:
The size of the casting should not be too large, the process is complicated, and the cooling speed of the casting is slow. Among all blank forming methods, investment casting has the most complicated process and high casting cost, but if the product is selected properly and the parts are designed reasonably, the high casting cost can be compensated by reducing cutting, assembly and saving metal materials. Then investment casting has good economy.
3. Die casting
The principle of die-casting process is to use high pressure to press molten metal into a precision metal mold cavity at high speed, and the molten metal is cooled and solidified under pressure to form a casting.
Cold and hot chamber die casting are two basic methods of die casting process. In cold chamber die casting, the molten metal is poured into the pressure chamber by manual or automatic pouring device, and then the injection punch advances to hydraulically inject the metal into the cavity. In the hot chamber die-casting process, the pressure chamber is perpendicular to the crucible, and the molten metal flows into the pressure chamber automatically through the feed port on the pressure chamber. The injection punch moves downward, pushing the molten metal through the gooseneck into the cavity. After the molten metal solidifies, the die-casting mold is opened, and the casting is taken out to complete a die-casting cycle.
∙ Good product quality. The dimensional accuracy of castings is high, generally equivalent to grade 6~7, or even grade 4; the surface finish is good, generally equivalent to grade 5~8; the strength and hardness are high, and the strength is generally 25~30% higher than that of sand casting, but the extension The rate is reduced by about 70%; the size is stable and the interchangeability is good; it can die-cast thin-walled and complex castings;
∙ High production efficiency. The machine productivity is high, for example, the domestic JⅢ3 horizontal cold-air die-casting machine can die-cast 600-700 times on average in eight hours, and the small-scale hot chamber die-casting machine can die-cast 3000-7000 times on average every eight hours; Die-casting bell alloy, life can reach hundreds of thousands of times, even millions of times; easy to realize mechanization and automation;
∙ Excellent economic effect. Due to the precise size of the die-casting parts, the surface is smooth and clean. Generally, it is used directly without mechanical processing, or the processing amount is very small, so it not only improves the metal utilization rate, but also reduces a large amount of processing equipment and man-hours; the casting price is easy; combined die-casting can be used with other metal or non-metal materials . It not only saves assembly man-hours but also saves metal.
Disadvantages and limitations:
∙ Due to the high speed of filling the cavity with liquid metal during die casting, the flow state is unstable. Therefore, if the general die casting method is adopted, the casting is prone to produce pores and cannot be heat treated;
∙ Die casting is more difficult for castings with complex concaves;
∙ For high melting point alloys (such as copper, ferrous metals), the life of die-casting models is relatively low;
∙ It is not suitable for small batch production, the main reason is that the manufacturing cost of die casting mold is high, the production efficiency of die casting machine is high, and small batch production is uneconomical.
4. Metal mold casting
Also known as die casting, it is a casting method in which liquid metal is poured into a metal mold to obtain a casting. The casting mold is made of metal and can be used repeatedly (hundreds to thousands of times), also called permanent casting.
Generally, metal molds are made of cast iron and cast steel. The inner cavity of the casting can be either a metal core or a sand core. There are many types of metal structures, such as horizontal type, heavy vertical type and compound type. Among them, the vertical parting is convenient to open the gate and take out the casting; the horizontal parting is mostly used to produce thin-walled wheel-shaped castings; It is a fixed horizontal bottom plate, mainly used in the casting of more complex castings.
Process characteristics of metal mold casting: metal mold has fast heat conduction speed and non-concession, which makes castings prone to defects such as insufficient pouring, cold shut, cracks and white holes. In addition, the metal type is repeatedly washed by hot metal liquid, which will reduce the service life. For this reason, the following auxiliary process measures should be adopted.
Preheating the metal mold: Preheating the metal mold before pouring can slow down the cooling capacity of the mold, which is beneficial to the filling of the molten metal and the graphitization process of cast iron. For the production of cast iron parts, the metal mold is preheated to 250-350 °C; for the production of non-ferrous metal parts, the temperature is preheated to 100-250 °C.
Brushing paint: In order to protect the metal mold and facilitate exhaust, usually a refractory paint layer is sprayed on the surface of the metal mold to prevent the metal mold from being directly eroded and heated by the metal liquid. Because adjusting the thickness of the coating layer can change the cooling rate of each part of the casting and facilitate the discharge of gas in the metal mold. For casting different alloys, different coatings should be sprayed. For example, cast aluminum alloy parts should be sprayed with coatings made of zinc oxide powder, talc powder and water glass; gray iron parts should be coated with graphite powder, talcum powder, refractory clay powder, peach gum and water.
Pouring: Metal molds have strong thermal conductivity, so when metal molds are used, the pouring temperature of the alloy should be 20-30°C higher than that of sand molds. Generally, the temperature of aluminum alloy is 680℃～740℃; the temperature of cast iron is 1300℃～1370℃; the temperature of tin bronze is 1100～1150℃. Take the upper limit for thin-walled parts and the lower limit for thick-walled parts. The wall thickness of cast iron parts should not be less than 15mm to prevent white structure.
Opening: The later the opening, the greater the shrinkage of the casting in the metal mold, it is difficult to take out and use, and the casting is prone to large internal stress and cracks. Usually, the casting temperature of iron castings is 700-950°C, and the mold opening time is 10-60 seconds after pouring.
Compared with sand casting, metal casting has the following advantages:
∙ Good reusability, can “multiple castings in one mold”, saving molding materials and molding man-hours.
∙ Due to the strong cooling ability of the metal mold on the casting, the structure of the casting is dense and the mechanical performance is high.
∙ The dimensional accuracy of the casting is high, the tolerance class is IT12~IT14; the surface roughness is low, Ra is 6.3m.
∙ Metal mold casting does not use sand or uses less sand, which improves working conditions.
Disadvantages and limitations:
The manufacturing cost of metal type is high, the cycle is long, and the process requirements are strict. It is not suitable for the production of single small batch castings. It is mainly suitable for the mass production of non-ferrous alloy castings, such as aluminum pistons for aircraft, automobiles, internal combustion engines, motorcycles, etc. , cylinder block, cylinder head, oil pump housing and copper alloy bearing bush, bushing, etc. For ferrous alloy castings, it is limited to medium and small castings with relatively simple shapes.
5. Low pressure casting
Low-pressure casting refers to the method of filling the mold with liquid metal under a relatively low pressure (0.02-0.06MPa) and crystallizing under pressure to form a casting.
Pour the smelted molten metal into the insulated crucible, install the sealing cover, the liquid metal riser pipe connects the molten metal with the mold, lock the mold, and slowly introduce dry compressed air into the crucible furnace, and the molten metal is subjected to gas pressure The cavity is filled from bottom to top along the riser and gating system, and crystallized under pressure. After the casting is formed, the pressure in the crucible is removed, and the metal liquid in the riser drops back to the metal liquid level in the crucible. Open the mold and take out the casting.
∙ The rising speed and crystallization pressure of molten metal can be adjusted during pouring, so it can be applied to various casting molds (such as metal molds, sand molds, etc.), casting various alloys and castings of various sizes;
∙ Bottom injection type filling is adopted, the liquid metal filling is stable, without splashing, which can avoid the gas involved and the erosion of the mold wall and core, and the casting has fewer defects such as pores and slag inclusions, which improves the qualified rate of the casting;
∙ The casting crystallizes under pressure, the casting structure is dense, the outline is clear, the surface is smooth, and the mechanical properties are high, which is especially beneficial for the casting of large and thin-walled parts;
∙ The feeding riser is omitted, and the metal utilization rate is increased to 90% to 98%;
∙ Low labor intensity, good working conditions, simple equipment, easy to realize mechanization and automation.
Disadvantages and limitations:
The life of the riser is short, and the molten metal is easy to oxidize and generate slag inclusions during the heat preservation process. It is mainly used to cast some aluminum alloy and magnesium alloy castings with high quality requirements, such as thin-walled parts such as cylinder block, cylinder head, crankcase and aluminum piston of high-speed internal combustion engine.
6. Centrifugal casting
Centrifugal casting is a casting method in which molten metal is poured into a rotating mold, and the mold is filled and solidified under the action of centrifugal force.
Classification of centrifugal casting
According to the position of the mold rotation axis in space, common centrifugal casting can be divided into two types:
Horizontal centrifugal casting: Centrifugal casting when the axis of rotation of the mold is horizontal or the angle with the horizontal line is small (<4°).
Vertical centrifugal casting: Centrifugal casting when the rotation axis of the mold is in a vertical state is called vertical centrifugal casting.
Centrifugal casting in which the axis of rotation of the mold forms a large angle with the horizontal and vertical lines is called inclined-axis centrifugal casting, but it is rarely used.
∙ The core, gating system and riser can be omitted when centrifugal casting is used to produce hollow rotating body castings;
∙ Due to the centrifugal force generated by the liquid metal during rotation, the metal with high density is pushed to the outer wall, while the gas and slag with low density move to the free surface, forming a directional solidification from the outside to the inside, so the feeding conditions are good, The casting structure is dense and the mechanical properties are good;
∙ It is easy to cast “bimetallic” bushings and bearing bushes, such as casting a thin layer of copper bushing in the steel bushing, which can save expensive copper materials;
∙ Good filling ability;
∙ Elimination and reduction of consumption in gating systems and risers.
Disadvantages and limitations:
∙ The free surface inside the casting is rough, with large dimensional error and poor quality;
∙ Not suitable for alloys with large density segregation (such as lead bronze) and alloys such as aluminum and magnesium.
Casting defects and their control methods
There are many kinds of casting defects, and the causes of defects are also very complicated. It is not only related to the casting process, but also related to a series of factors such as the properties of the casting alloy, the melting of the alloy, and the performance of the molding material. Therefore, when analyzing the causes of casting defects, it is necessary to proceed from the specific situation and conduct a comprehensive analysis based on the characteristics, location, process and molding sand used of the defects, and then take corresponding technical measures to prevent and eliminate defects.
1. Not enough to pour
There are partial defects in the casting, which often appear in the thin-walled part, the part farthest from the runner or the upper part of the casting. Incomplete corners are smooth and bright without sticking to sand.
∙ Pouring temperature is low, pouring speed is too slow or intermittent pouring;
∙ The cross-sectional area of runners and inner runners is small;
∙ The content of carbon and silicon in molten iron is too low;
∙ Excessive moisture and coal powder content in the molding sand, large gas generation, or too high mud content, poor air permeability;
∙ The height of upper sand mold is not enough, and the pressure of molten iron is not enough.
∙ Increase pouring temperature, accelerate pouring speed, and prevent intermittent pouring;
∙ Increase the cross-sectional area of runner and inner runner;
∙ Adjust the ingredients after the furnace to properly increase the content of carbon and silicon;
∙ Strengthen the exhaust in the mold, reduce the amount of coal powder and organic matter added in the molding sand;
∙ Increase the height of the upper flask.
The upper part of the casting is incomplete, the level of the molten iron in the sprue is equal to that of the casting, and the edges are slightly rounded.
∙ The amount of molten iron in the ladle is not enough;
∙ The sprue is narrow and the pouring speed is too fast. When the molten iron overflows from the sprue cup, the operator mistakenly thinks that the mold is full and stops pouring prematurely.
∙ Correctly estimate the amount of molten iron in the ladle;
∙ For the mold with narrow sprue, slow down the pouring speed appropriately to ensure the mold is full.
The casting is damaged and broken.
∙ The sand falling out of the casting is too violent, or the casting is damaged by impact during handling;
∙ When the drum is cleaned, the casting is not loaded properly, and the weak part of the casting is broken when it is tumbled;
∙ Riser and riser neck section size is too large; riser neck does not have a knockout section (groove). Or the method of knocking out the riser is not correct, which will damage the casting body and lack flesh.
∙ When cleaning and transporting the castings, pay attention to avoid various forms of excessive impact and vibration, and avoid unreasonable throwing away;
∙ Operate strictly in accordance with the process regulations and requirements when cleaning the drum;
∙ Modify the size of the riser and riser neck, make a knock-out section of the riser neck, and correctly grasp the direction of the pouring riser.
4. Sticky sand and surface roughness
Sand sticking is a surface defect of castings, which is characterized by the adhesion of sand particles that are difficult to remove on the surface of castings; if a casting has an uneven and non-smooth surface after removing sand particles, it is called rough surface.
∙ The sand grains are too coarse, and the compactness of the sand mold is not enough;
∙ Moisture in the molding sand is too high, making the molding sand difficult to compact;
∙ The pouring speed is too fast, the pressure is too high, and the temperature is too high;
∙ There is too little coal powder in the molding sand;
∙ If the template baking temperature is too high, the molding sand on the surface will dry out; or if the template baking temperature is too low, the molding sand will adhere to the template.
∙ When the air permeability is sufficient, use finer raw sand and properly increase the compactness of the molding sand;
∙ Guarantee a stable effective coal powder content in the molding sand;
∙ Strictly control sand moisture;
∙ Improve pouring system, improve pouring operation, reduce pouring temperature;
∙ Control template baking temperature, generally equal to or slightly higher than molding sand temperature.
Holes filled with molding sand in or on the surface of the casting.
∙ The surface strength of the molding sand is not enough;
∙ There is no rounded corner on the shape or the draft angle is small, which leads to sand hooking, and the mold is not repaired after damage or the box is closed without repair;
∙ If the sand mold is placed for too long before pouring, the surface strength will decrease after air drying;
∙ The mold is damaged during packing or handling;
∙ The floating sand in the mold was not cleaned when the box was closed, and the sprue cup was not covered properly after the box was closed, and the broken sand fell into the mold.
∙ Increase the clay content in the molding sand, add new sand in time, and improve the surface strength of the molding sand;
∙ The pattern finish should be high, and the draft angle and casting fillet should be made reasonably. Damaged molds should be repaired before repacking;
∙ Shorten the placement time of the sand mold before pouring;
∙ Be careful when closing or handling the mold to avoid damage or sand grains falling into the sand cavity;
∙ Clear the floating sand in the mold before closing the box, and cover the gate.
6. Draped joints and expanding sand
The drape often appears at the parting surface of the casting, which is a thin sheet metal protrusion with uneven thickness that is perpendicular to the surface of the casting. Sand swelling is the local expansion of the inner and outer surfaces of the casting, forming irregular nodular metal protrusions.
∙ Insufficient or uneven firmness;
∙ The strength of the surface sand is not enough, or the moisture content of the molding sand is too high;
∙ The liquid metal head is too large and the pouring speed is too fast.
∙ Improve the compactness of the mold and avoid local looseness;
∙ Adjust the sand mixing process, control moisture, and improve the strength of the molding sand;
∙ Reduce the pressure head of liquid metal and reduce the pouring speed.
7. Carrying boxes
The casting has a large area of seams at the parting surface, which changes the shape and size of the mold. If the lifting box is too large, it will cause a fire run-the molten iron overflows from the parting surface, and in severe cases, it will cause insufficient pouring defects.
∙ The sand box is not fastened, the quality of the weight is not enough or the weight is removed too early;
∙ The pouring is too fast and the impact force is too large;
∙ Curved formwork.
∙ Increase the weight of pressed iron, remove the pressed iron after the special molten iron solidifies;
∙ Lower the ladle position and lower the pouring speed;
∙ Correct templates.
8. Sand drop
Lumpy metal protrusions that appear on the surface of a casting, resembling fallen sand clods in appearance. In other parts of the casting, sand holes or defects often appear.
∙ There are deep and small grooves on the pattern, which are the same as the structural features or the draft angle is small, and the sand mold will be damaged or cracked when the mold is pulled out;
∙ Uneven compactness and insufficient local strength of the mold;
∙ Accidentally drop some sand blocks of the mold when closing the box and transporting the mold.
∙ The draft angle of the pattern should be suitable and the surface should be smooth;
∙ High and uniform mold compactness;
∙ Be careful when packing and transporting.
9. Wrong type (wrong box)
One part of the casting is staggered from the other part at the joint of the parting surface, and relative displacement occurs, so that the shape of the casting does not match the drawing.
∙ The pattern is poorly made, the upper and lower molds are not aligned or the pattern is deformed;
∙ Inaccurate positioning of the flask or template, or loose positioning pins;
∙ The parts on the extrusion molding machine are worn, such as the wear of the lower liner of the positive pressure plate and the bearing of the reverse pressure plate;
∙ The box used for pouring is deformed, and the upper and lower molds are displaced due to carelessness in handling and enclosing the box.
∙ Strengthen the inspection and repair of formwork;
∙ Frequently check the positioning pins and pin holes of the sand box and formwork, and install them reasonably;
∙ Check the relevant parts of the extrusion molding machine, adjust them in time, and replace the worn ones;
∙ Regularly reshape the casing. Care must be taken when handling molds that have been removed from the box. Sand molds poured on the surface should be surrounded by a row of sand molds.
10. Gray mouth and pitting
The fracture of the casting is gray-black or black dots appear, with more central parts and fewer edges, and flake graphite can be seen in metallographic observation.
∙ The chemical composition of molten iron does not meet the requirements, and the content of carbon and silicon is too high;
∙ The bismuth inoculated in front of the furnace is added to the ladle too early or too late, or the amount of bismuth is insufficient.
∙ Correctly select the chemical composition and reasonable ingredients, so that the amount of carbon and silicon in the molten iron is within the specified range;
∙ Increase the amount of bismuth added and strictly inoculate the furnace.
11. Cracks (hot cracks, cold cracks)
There are penetrating or non-penetrating cracks on the outside or inside of the casting. The fracture surface with dark or black oxidized surface is tortuous during thermal cracking. Cold cracks are relatively clean brittle cracks with flat fractures and metallic luster or slight oxidized color.
∙ The amount of carbon and silicon in molten iron is too low, and the sulfur content is too high;
∙ Pouring temperature is too high;
∙ If the riser neck is too large or too short, it will cause severe local overheating, or if the feeder is too small, the feeding will not be good;
∙ Castings are subjected to excessive impact during cleaning and transportation.
∙ Control the chemical composition of molten iron within the specified range;
∙ lower pouring temperature;
∙ Rational design of riser system;
∙ Castings should avoid excessive impact during cleaning and transportation.
The pore walls of the pores are smooth and bright, and the shapes are round, pear-shaped and needle-shaped. The sizes of the pores are large and small, and they are generated on the surface or inside of the casting. The pores inside the casting can only be found after breaking or machining.
∙ The small charge is damp, heavily corroded or oily, resulting in too much gas content in the molten iron and serious oxidation;
∙ The tapping holes, tapping troughs, furnace linings and ladle linings are not dry;
∙ The pouring temperature is low, so that the gas has no time to float up and escape;
∙ High aluminum content in the furnace charge is easy to cause hydrogen pores;
∙ The air permeability of the sand mold is not good, the moisture content of the sand is high, and it contains a lot of coal powder or organic matter, which makes a large amount of gas generated during pouring and is not easy to discharge.
∙ The charge should be properly managed and the surface should be clean;
∙ Hearth, forehearth, taphole, tapping trough and ladle must be dried;
∙ Increase pouring temperature;
∙ Do not use steel scrap with high aluminum content;
∙ Appropriately reduce the moisture content of the molding sand, control the amount of pulverized coal, and tie air holes, etc.
13. Shrinkage, porosity
Scattered and small shrinkage cavities, those with dendritic crystals are called shrinkage porosity, and those smaller than shrinkage porosity are called porosity. Often appear in hot world parts.
∙ The content of carbon and silicon in molten iron is too low and the shrinkage is large;
∙ The pouring speed is too fast and the pouring temperature is too high, which makes the liquid shrink greatly;
∙ Improper design of gating system and riser, unable to achieve sequential solidification;
∙ Riser is too small, insufficient feeding.
∙ Control the chemical composition of molten iron within the specified range;
∙ Reduce pouring speed and pouring temperature;
∙ Improved gating and riser system, using sequential solidification;
∙ Increase the riser volume to ensure full feeding.
White mouth tissue appears inside the fracture of the casting, and gray mouth appears on the edge.
∙ Molten iron with high carbon and silicon content contains too much hydrogen;
∙ There are too many white-forming elements such as chromium brought into the charge;
∙ Severe element segregation;
∙ Control chemical composition, carbon and silicon content should not be too high;
∙ The furnace lining and bag lining should be dried; the moisture content of the molding sand should not be too high;
∙ Strengthen charge management to reduce whitening elements.