There are many kinds of casting defects, and the causes of defects are also very complex. It is not only related to the casting process, but also related to a series of factors, such as the properties of casting alloy, the melting of alloy, the properties of molding materials and so on. Therefore, when analyzing the causes of casting defects, we should start from the specific situation, make a comprehensive analysis according to the characteristics, location, process and molding sand of the defects, and then take corresponding technical measures to prevent and eliminate the defects.
1、 Can't pour
1. The characteristic casting is partially incomplete, often in the thin-wall part, the farthest part from the sprue or the upper part of the casting. The incomplete edges and corners are smooth, bright and non stick sand.
2. Cause
(1) Low pouring temperature, too slow pouring speed or intermittent pouring;
(2) The cross-sectional area of transverse sprue and internal sprue is small;
(3) The content of carbon and silicon in molten iron is too low;
(4) Too much moisture and pulverized coal in molding sand, large gas generation, or too high mud content, poor air permeability;
(5) Insufficient height of upper sand mold and insufficient hot metal pressure;
3. Prevention method
(1) Increase the pouring temperature and speed up the pouring speed to prevent intermittent pouring;
(2) Increase the cross-sectional area of transverse sprue and inner sprue;
(3) Adjust the ingredients after the furnace to appropriately increase the content of carbon and silicon;
(4) Strengthen the exhaust in the mold to reduce the amount of pulverized coal and organic matter in the molding sand;
(5) Increase the height of the upper sand box;
2、 Not fully poured
1. Features: the upper part of the casting is incomplete, the level of molten iron in the sprue is flat with the level of molten iron in the casting, and the edge is slightly circular.
2. Cause
(1) Insufficient iron water in ladle;
(2) The runner is narrow and the pouring speed is too fast. When the molten iron overflows from the gate cup, the operator mistakenly thinks that the mold is full and stops pouring too early.
3. Prevention method
(1) Correctly estimate the amount of iron in the ladle;
(2) For the mold with narrow sprue, appropriately slow down the pouring speed to ensure that the mold is full.
3、 Damage
1. Features: the casting is damaged and broken.
2. Cause
(1) The sand falling of castings is too violent, or the castings are damaged by collision during handling;
(2) When the drum is cleaned, the casting is improperly loaded, and the weak part of the casting is broken when rolling;
(3) The section size of riser and riser neck is too large; the riser neck does not make a knock section (groove), or the method of knocking out the pouring riser is incorrect, resulting in damage and meat shortage of the casting body.
3. Prevention method
(1) During sand falling cleaning and handling of castings, pay attention to avoid various forms of excessive collision and vibration, and avoid unreasonable throwing;
(2) The drum shall be cleaned in strict accordance with the process regulations and requirements;
(3) Modify the size of riser and riser neck, make the section of riser neck, and correctly grasp the direction of pouring riser.
4、 Sticky sand and rough surface
1. Features: sand sticking is a kind of casting surface defect, which is manifested in the adhesion of sand particles on the casting surface that are difficult to remove; If the casting has uneven and unsmooth surface after removing sand particles, it is called rough surface.
2. Cause
(1) The sand is too coarse and the compactness of sand mold is not enough;
(2) The water content in the molding sand is too high, so that the molding sand is not easy to compact;
(3) Pouring speed is too fast, pressure is too high and temperature is too high;
(4) Too little pulverized coal in molding sand;
(5) The drying temperature of the formwork is too high, resulting in the drying of the molding sand on the surface; or the drying temperature of the formwork is too low, and the molding sand adheres to the formwork.
3. Prevention method
(1) Under the condition of sufficient air permeability, use finer raw sand and appropriately improve the compactness of molding sand;
(2) Ensure stable effective pulverized coal content in molding sand;
(3) Strictly control sand moisture;
(4) Improve the pouring system, improve the pouring operation and reduce the pouring temperature;
(5) Control the baking temperature of the formwork, which is generally equal to or slightly higher than the molding sand temperature.
5、 Sand hole
1. Features: holes filled with molding sand inside or on the surface of the casting.
2. Cause
(1) Insufficient surface strength of molding sand;
(2) There is no fillet on the pattern or the draft angle is small, which leads to the failure to repair or close the box after the hook sand and mold are damaged;
(3) The sand mold is placed for too long before pouring, and the surface strength decreases after air drying;
(4) The mold is damaged during box closing or handling;
(5) When closing the box, the floating sand in the mold was not cleaned, the gate cup was not covered well after closing the box, and the broken sand fell into the mold.
3. Prevention method
(1) Increase the content of clay in molding sand, add new sand in time, and improve the surface strength of molding sand;
(2) The pattern finish shall be high, and the draft angle and casting fillet shall be made reasonably. The damaged mold shall be repaired before closing the box;
(3) Shorten the placement time of sand mold before pouring;
(4) Be careful when closing the box or handling the mold to avoid damage or falling into the sand mold cavity;
(5) Before closing the box, remove the floating sand in the mold and cover the gate.
6、 Jointing and sand expansion
1. Features: the drape joint often appears at the parting surface of the casting. It is a thin sheet metal protrusion with uneven thickness perpendicular to the casting surface. Sand bulging is the local swelling of the inner and outer surfaces of castings to form irregular nodular metal protrusions.
2. Cause
(1) Insufficient or uneven compactness;
(2) The strength of surface sand is not enough or the moisture of molding sand is too high;
(3) The liquid metal head is too large and the pouring speed is too fast.
3. Prevention method
(1) Improve mold compactness and avoid local looseness;
(2) Adjust sand mixing process, control moisture and improve molding sand strength;
(3) Reduce the head of liquid metal and pouring speed.
7、 Lifting box
1. Features: the casting has a large area of drape joint at the parting surface, which changes the overall dimension of the casting mold. The box lifting is too large, resulting in fire running - the molten iron overflows from the parting surface, and the defect of insufficient pouring is caused in serious cases.
2. Cause
(1) The sand box is not fastened, the quality of the pressing iron is not enough, or the pressing iron is removed too early;
(2) Pouring is too fast and the impact force is too large;
(3) The template is curved.
3. Prevention method
(1) Increase the weight of the pressed iron, and remove the pressed iron after the molten iron solidifies;
(2) Reduce the ladle position and pouring speed;
(3) Revise the template.
8、 Desanding
1. Features: massive metal protrusions appear on the surface of the casting, and their shape is very similar to the falling sand block. In other parts of the casting, sand holes or defects often appear.
2. Cause
(1) There are deep and small grooves on the pattern, which is the same as the structural characteristics or the draft angle is small, and the sand mold will be damaged or cracked during mold lifting;
(2) Uneven compactness and insufficient local strength of mold;
(3) When the box is closed and the mold is transported, the sand block in part of the mold is accidentally dropped.
3. Prevention method
(1) The draft angle of the pattern shall be appropriate and the surface shall be smooth;
(2) High and uniform mold compactness;
(3) Be careful in case closing and handling.
9、 Wrong type (wrong box)
1. Features: one part of the casting is staggered with the other part at the joint of the parting surface, resulting in relative displacement, so that the shape of the casting is inconsistent with the drawing.
2. Cause
(1) The pattern is poorly made, the upper and lower molds are not aligned or the pattern is deformed;
(2) Inaccurate positioning of sand box or formwork, or loose positioning pin;
(3) Wear of parts on the extrusion molding machine, such as the wear of the lower lining plate of the positive pressing plate and the bearing of the reverse pressing plate;
(4) The casing used during pouring is deformed, and the upper and lower molds are displaced due to careless handling and enclosure.
3. Prevention methods: strengthen the inspection and repair of formwork;
(1) Regularly check the locating pins and pin holes of sand box and formwork and install them reasonably;
(2) Check the relevant parts of the extrusion molding machine, adjust them in time, and replace those with large wear;
(3) Regularly reshape the casing. Be careful when handling the mold after taking off the casing. The sand mold poured on the surface should be lined up in a row.
10、 Gray mouth and pitting
1. Features the fracture of the casting is gray black or small black spots, with more central parts and less edges. Flake graphite can be seen by metallographic observation.
2. Cause
(1) The chemical composition of molten iron does not meet the requirements, and the content of carbon and silicon is too high;
(2) Bismuth inoculated in front of the furnace is added into the ladle too early or too late, or the amount of bismuth is insufficient.
3. Prevention method
(1) Select the chemical composition correctly and mix reasonably to make the amount of carbon and silicon in molten iron within the specified range;
(2) Increase the amount of bismuth and strictly control the pre furnace inoculation process.
11、 Crack (hot crack, cold crack)
1. Features there are penetrating or non penetrating cracks on the outside or inside of the casting. During hot cracking, there is a dark or black oxidized surface, and the fracture shape is tortuous. Cold crack is a relatively clean brittle crack with flat fracture and metallic luster or slight oxidation color.
2. Cause
(1) The content of carbon and silicon in molten iron is too low and the content of sulfur is too high;
(2) Pouring temperature is too high;
(3) The riser neck is too large or too short, resulting in serious local overheating, or the heavy port is too small, resulting in poor feeding;
(4) During cleaning and transportation of castings, the impact is too large.
3. Prevention method
(1) Control the chemical composition of molten iron within the specified range;
(2) Reduce pouring temperature;
(3) Rational design of riser system;
(4) Avoid excessive impact during cleaning and transportation of castings.
12、 Stomata
1. Features: the pore wall of the pore is smooth and bright, with round, pear and needle shapes. The size of the pore varies from large to small, which is produced on the surface or inside of the casting. Pores in castings can only be found after crushing or machining.
2. Cause
(1) The small furnace charge is wet, seriously rusted or greasy, resulting in too much gas content and serious oxidation of molten iron;
(2) Tap hole, tap trough, furnace lining and ladle lining are not flooded;
(3) The pouring temperature is low, so that the gas has no time to float and escape;
(4) The aluminum content in the charge is high, which is easy to form hydrogen pores;
(5) Poor air permeability of sand mold, high moisture content of molding sand and more pulverized coal or organic matter make it difficult to discharge a large amount of gas during pouring.
3. Prevention method
(1) The charge shall be properly managed and the surface shall be clean;
(2) Hearth, front furnace, taphole, taphole and ladle must be dried;
(3) Increase pouring temperature;
(4) Scrap steel with excessive aluminum content shall not be used;
(5) Properly reduce the moisture of molding sand, control the amount of pulverized coal, and plug vent holes.
13、 Shrinkage and looseness
1. Features: dispersed and small shrinkage cavities, those with dendrite crystallization are called shrinkage porosity, and those smaller than shrinkage porosity are called porosity. It often occurs in the hot world.
2. Cause
(1) The content of carbon and silicon in molten iron is too low and the shrinkage is large;
(2) The pouring speed is too fast and the pouring temperature is too high, resulting in large liquid shrinkage;
(3) Improper design of gating system and riser can not realize sequential solidification;
(4) The riser is too small and the feeding is insufficient.
3. Prevention method
(1) Control the chemical composition of molten iron within the specified range;
(2) Reduce pouring speed and pouring temperature;
(3) Improve the pouring and riser system and use sequential solidification;
(4) Increase the riser volume to ensure full feeding.
14、 Anti white mouth
1. Features: white mouth structure appears inside the fracture of the casting, and gray mouth appears at the edge.
2. Cause
(1) The hydrogen content of molten iron with high carbon and silicon content is too high;
(2) Excessive chromium and other white mouth forming elements brought into the charge;
(3) Serious element segregation;
3. Prevention method
(1) Control chemical composition, carbon and silicon content should not be too high;
(2) The furnace lining and ladle lining shall be dried; the moisture content of molding sand shall not be too high;
(3) Strengthen charge management and reduce white elements.
