SAND CASTING EQUIPMENT.

Defect 1: unable to water. 
Features: the appearance of the casting is incomplete, the edges are smooth, and most of them are found in thin-walled parts. 
Cause: 
1.1 the molten iron is oxidized seriously, the content of carbon and silicon is low, and the content of sulfur is high; 1.2 the pouring temperature is low, the pouring speed is slow or intermittent pouring. 
Prevention methods: 
1.11 check whether the air volume is too large; 1.12 add relay coke to adjust the height of the bottom coke; 1.13 increase the pouring temperature and speed, and do not cut off the flow during pouring. 
Defect 2: shrinkage, porosity. 
Features: the surface of the hole is rough and uneven, with dendritic crystals, the pores are concentrated, and the fine ones are loose, which are more common in hot joints. 
Cause: 
2.1 carbon and silicon content is too low, shrinkage is large, riser feeding is insufficient; 2.2 pouring temperature is too high, shrinkage is large; 2.3 riser neck is too long, cross section is too small; 2.4 pouring temperature is too low, molten iron fluidity is poor, which affects feeding; 2.5 improper inoculation, solidification is lath white structure, it is not easy to make up. 
Prevention methods: 
2.11 to control the chemical composition of molten iron to prevent the low content of carbon and silicon; 2.12 to strictly control the pouring temperature; 2.13 to design the riser reasonably and spread cold iron if necessary to ensure sequential solidification; 2.14 to increase the amount of bismuth appropriately. 
Defect 3: hot crack, cold crack. 
Features: hot cracking is high temperature fracture along the grain boundary, tortuous shape, oxidizing color, internal hot cracks often coexist with shrinkage; cold cracking occurs at lower temperature, transgranular fracture, straight shape, metallic luster or slight oxidation color on the surface. 
Cause: 
3.1 the shrinkage is blocked during solidification; 3.2 the carbon content in molten iron is too low, the sulfur content is too high, and the pouring temperature is too high; 3.3 the gas content in molten iron is high, and 3.4 complex parts are packed too early. 
Prevention methods: 
3.11 improved type, core concession; 3.12 carbon content should not be less than 2.3%; 3.13 control sulfur content; 3.14 cupola should be fully baked, air volume should not be too large; 3.15 avoid excessive pouring temperature and improve cooling rate to refine grains; 3.16 control box temperature. 
Defect 4: loose branches and pinholes. 
Features: the fracture surface of the casting has acicular dendritic looseness, extending to the inside, showing a black-gray oxidized color, which is more common in the subcutaneous, slow cooling parts and the junction of the two walls. 
Cause: 
4.1 excessive addition of aluminum before furnace; 4.2 misuse of waste parts containing aluminum after furnace; 4.3 excessive moisture content of molding sand; 4.4 insufficient shrinkage. 
Prevention methods: 
4.11 to control the amount of aluminum, thick castings can not add aluminum; 4.12 to use qualified charge; 4.13 to reduce the moisture of molding sand to prevent pinholes caused by reactive hydrogen; 4.14 to increase the pouring temperature and strengthen feeding.
Defect 5: grey spot, gray mouth. 
Features: there are small black spots on the casting fracture, or the fracture is grayish black. 
There is flake graphite in metallographic observation. 
Cause: 
5.1 the content of carbon and silicon in molten iron is too high; 5.2 the content of bismuth is insufficient, or too much burning loss of bismuth is caused by adding bismuth into the ladle too early; 5.3 larger castings, the gate is too concentrated. 
Prevention methods: 
5.11 adjust the amount of carbon and silicon according to the wall thickness of the casting; 5.12 strictly control the process of adding bismuth; 5.13 disperse the inner sprue to prevent ash spots at the gate due to overheating. 
Defect 6: anti-white mouth. 
Features: the gray mouth appears on the edge of the casting, while the white mouth appears in the central part of the casting. 
Cause: 
6.1 the content of carbon and silicon in molten iron is too high, which leads to the high content of hydrogen, and the segregation of carbide-forming elements such as 6.2 chromium. 
Prevention methods: 
6.11 control the amount of carbon and silicon within the prescribed range, and pay attention to the oven and flood ladle; 6.12 carefully select scrap and control the size of scrap. 
Defect 7: insufficient annealing (hard product). 
Features: mechanical properties are inconsistent with requirements: low elongation after breaking, hard and brittle. 
Excess pearlite and cementite in metallographic structure. 
Bright white spots appear on the fracture surface of black heart malleable cast iron. 
Cause: 
7.1 low silicon or high sulfur in molten iron, improper ratio of manganese to sulfur; 7.2 high chromium in molten iron or excessive oxygen, hydrogen and nitrogen; 7.3 low annealing temperature and insufficient time in the first stage; short annealing time or high holding temperature in the second stage; 7.4 decarburization annealing temperature is too low, or improper annealing atmosphere control; 7.5 annealing furnace temperature difference is large; 7.6 thermometer is not accurate, can not guide production. 
7.7 the excessive accumulation of pulverized coal on the annealing box affects the heat transfer. 
Prevention methods: 
7.11 adjust and control the chemical composition of molten iron; 7.12 when the composition is found to be inappropriate, the annealing temperature of the first stage should be increased and the annealing time should be prolonged; 7.13 the annealing furnace should be improved and the temperature difference should be reduced. 
7.14 improve the heat preservation capacity of the annealing furnace to avoid re-adding fuel during the second stage annealing, while ensuring that the second stage annealing can have a slow enough cooling rate 7.15 pay attention to the pulverized coal particle size and coal addition, so as to prevent the accumulation of pulverized coal in the furnace; 7.16 regular maintenance of the thermometer; 7.17 thick castings should be installed in the high temperature part of the furnace. 
Defect 8: deformation. 
Features: casting warped, not round, size, shape and position is not correct. 
Cause: 
8.1 improper packing of castings, compression and extrusion, or lack of support for long parts; 8.2 inaccurate billet scale, not taking into account the graphitization expansion of castings in the annealing process; 8.3 the annealing temperature in the first stage is too high. 
Prevention methods: 
8.11 reasonable packing, long and large castings should be placed on the partition, pay attention to the spacing between the diaphragms, and add fillers or struts if necessary; 8.12 annealing temperature should not be too high; 8.13 deformed castings can be reshaped and corrected.
Defect 9: severe oxidation of the surface. 
Features: there is a purple-black oxidation layer on the surface, and white malleable cast iron is even prone to peeling and peeling. 
Cause: 
9.1 the seal of the annealing box is not strict, or the mud seal falls off; 9.2 the annealing temperature of the first stage is too high and the time is too long; 9.3 furnace gas oxidizing is too strong; 9.4 the pulverized coal nozzle is not directly facing the fire channel, but is directly sprayed into the annealing box, which leads to the oxidation of the castings. 
Prevention methods: 
9.11 attach importance to the mud sealing process, sealing mud can not be too thin, otherwise easy to crack by heat; 9.12 control the temperature of the first stage should not be too high; 9.13 the excess air of pulverized coal furnace can not be large; 9.14 when necessary, add yellow sand to the oxidation filler of white heart malleable cast iron; 9.15, when placing the annealing box, make the nozzle face the fire path; 9.16 adopt high chromium steel annealing box. 
Defect 10: overheating. 
Features: the surface of the casting is rough, the edge is melted, the grain is coarse, and the graphite is thick in the shape of chicken claw. 
It is characterized by white bright pearlite zone, high hardness, affecting cutting, and casting bonding in serious cases. 
Cause: 
10.1 the annealing temperature of the first stage is too high and the time is too long; 10.2 the temperature difference of the annealing furnace is too high, so that the furnace temperature in the local area is too high; 10.3 the thermometer fails and the indicated temperature is on the low side. 
Prevention methods: 
10.11 the first stage annealing temperature should not exceed 980 ℃; 10.12 improve the annealing furnace to uniform furnace temperature; 10.13 check the thermometer regularly. 
Defect 11: poor morphological distribution of graphite. 
Features: graphite is loose, bifurcated, even chicken claw-like, or dendritic graphite, or graphite distributed in strings. 
Cause: 
11.1 the silicon content of molten iron is too high, 11.2 carbon and silicon content and wall thickness are mismatched, there is D-type graphite in the billet, 11.3 low melting point phosphorus at grain boundary, more sulfur eutectic, 11.4 high annealing temperature in the first stage, 11.5 improper inoculation. 
Prevention methods: 
The mass fraction of 11.11 silicon should not exceed 2.1%, the content of carbon and silicon should be controlled correctly by 11.12, the content of phosphorus and sulfur should be controlled by 11.13, the annealing temperature of the first stage of 11.14 should be strictly controlled, and the mass fraction of boron inoculation should not exceed 0.004%, and the mass fraction of aluminum should not exceed 0.02%. 
Defect 12: the epidermis is too thick. 
Features: observe the fracture, the depth of the white circle on the edge exceeds that of 1.5mm. 
Affect the cutting process. 
Cause: 
12.1 the annealing temperature in the first stage is on the high side, the mud seal of 12.2 annealing box is not strict, and the oxidation of 12.3 furnace gas is strong. 
Prevention methods: 
12.11 Control the first stage annealing temperature, generally, the epidermis is not large when the temperature is below 950 ℃; 12.12 seal the box carefully; 12.13 do not pack the wet castings; 12.14 control the air supply of the annealing furnace. 
Defect 13: temper brittleness (white brittleness). 
Features: impact toughness and elongation after fracture are very low, the fracture surface is white, fracture along the grain boundary. 
Under a single optical microscope, there is no abnormality, and it is still ferrite flocculent graphite. 
Cause: 
13.1 the discharge temperature is too low, so the cooling is too slow at 550-400 ℃, and carbides, phosphides and nitrides are precipitated at the ferrite grain boundary. 13.2 cast iron with high phosphorus and high silicon is more prone to temper brittleness. 
Prevention methods: 
13.11 should be quickly cooled at 650-600 ℃; 13.12 should control the amount of phosphorus, silicon and nitrogen in molten iron; adding rare earth ferrosilicon to 13.13 molten iron is beneficial to prevent temper brittleness to a certain extent; 13.14 when temper brittleness has occurred, it can be reheated to 650-700 ℃ and then cooled quickly, and the brittleness can disappear.