4. Hole defects
1) Air holes (concentrated air holes)
(1) Features: There are single or several large smooth holes in individual parts of the casting. The eyelets are sometimes oxidized in color and are usually discovered after processing.
(2) Causes of formation: the gas generated by the mold shell penetrates the molten metal; or the gas permeability of the mold shell is too poor, making it difficult to discharge the gas in the cavity, invading the molten metal or being involved in the gas during casting and failing to discharge the molten metal. stomata.
(3) Prevention reasons:
a. When dewaxing, the mold material should be removed, and the residual mold material should be as little as possible
b. The shell roasting should be sufficient and the temperature should be high enough
c. For complex thin-walled castings, to improve the air permeability of the shell, vent holes can be provided at the highest point if possible
d. Reasonably set the pouring system to prevent the pouring from being involved in gas, and facilitate the discharge of the gas in the cavity
e. Appropriately increase the pouring temperature, reduce the pouring distance and speed as much as possible, so that the molten metal can be filled smoothly, prevent gas from being involved, and enable the gas in the cavity and liquid metal to be discharged smoothly.
2) Diffuse stomata
(1) Features: There are small, scattered, or dense holes on the casting, sometimes on the entire section
(2) The reason for the formation; is the precipitation pores formed in the casting by the precipitation of the gas contained in the molten metal.
(3) Preventive measures:
a. Use cleaned charge
b. Reasonable ingredients
c. Covering agent should be added to the metal liquid surface during the smelting process to shorten the smelting time as much as possible
d. Deoxidation and degassing of molten metal should be sufficient
e. The molten metal should be properly placed before pouring to facilitate the escape of gas
f. Prevent secondary oxidation of molten metal during the pouring process
3) Shrinkage cavity, shrinkage porosity
(1) Features: There are large and irregular holes with rough walls inside the casting, which are called shrinkage holes.
(2) Causes of formation: When a certain part of the casting shrinks and solidifies in the liquid state of the alloy, the liquid metal cannot be fed, and shrinkage holes will be generated there.
(3) Preventive measures:
a. The casting structure should conform to the casting principles
b. Set the gating system reasonably
c. Reasonable grouping
d. The casting temperature of the shell and molten metal should be appropriate
e. Fill it up when pouring, or add insulation
4) retraction
(1) Features: There are many small, scattered, and irregularly shaped holes inside the casting with rough walls, which are called shrinkage porosity.
(2) Reason for formation: unreasonable design of gating system
(3) Preventive measures; reasonable setting of gating system
5. Cracks and deformation
1) Hot cracking, cold cracking
Hot crack
(1) Features; discontinuous, distorted, irregularly oriented intergranular cracks occur on the surface or inside of the casting, and the fracture is strongly oxidized and discolored, which is called hot cracking
(2) Causes of formation; hot cracks are formed within a certain temperature range, generally above the solidus temperature of the alloy. The reason is that the solidification shrinkage part is blocked and the design of the gating system is unreasonable.
(3) Preventive measures; the casting structure should conform to the casting principle, and the content of P and S in the steel should be strictly controlled; the pouring should be properly designed
Cold crack
(1) Features; there are continuous and linear cracks on the casting that pass through the crystal, and the fracture is clean and metallic, which is called cold cracking.
(2) Causes of formation; cold cracking is caused by the casting stress (thermal stress, transformation stress | shrinkage stress) at the casting site exceeding the strength limit of its combined multi-material.
(3) Preventive measures: rationally design the gating system
2) Brittle fracture and deformation of castings
Brittle fracture is related to the amount of aluminum (Al), B, and S in the steel, which should be strictly controlled
The deformation is mainly investment deformation
6 .Other defects
1) Trachoma
(1) Features; there are holes filled with sand on the surface or inside of the casting, called trachoma
(2) Causes of formation; sand may be brought into the cavity due to inadvertent operation; or due to poor inner surface quality of the shell, such as local flaky peeling; or there is a gap during the welding of the module, and the paint penetrates the gap, and this is the case during pouring. The paint was washed down and fell into the molten metal, causing sand holes
(3) Preventive measures:
a. The gate rod should be clean and free of impurities such as sand particles
b. There should be no gaps or grooves in the welding of the module
c. When dewaxing, the edge of the shell pouring cup should be smoothed and the loose sand should be removed to prevent the loose sand from falling into the cavity
d. It is best to use a flanged pouring cup
2) Slag hole
Features; cavities caused by slag on or in the casting
Formation reasons: slag holes are caused by factors such as the slag in the molten metal not being removed cleanly
Preventive measures: use clean charge; pay attention to slag making and slagging
3) Cold isolation and pouring
Cold insulation
(1) Features; there are incompletely fused gaps on the casting, and the edges where they meet are rounded
(2) The reason for the formation: when the metal liquid is filled, when the two metal liquid streams meet, the temperature is too low, so that they cannot be fused.
(3) Preventive measures: appropriately increase the pouring temperature of the molten metal and the temperature of the shell, increase the metal hydraulic head to prevent the interruption of flow; improve the design of the pouring system, and increase the cross-sectional area of the pouring system, etc.
Insufficient pouring (undercasting)
(1) Features: The casting is partially filled, causing the casting to be “lack meat”, and its end is arc-shaped. Castings that cannot be poured are scraps
(2) Causes of formation: The temperature of the molten metal or the shell temperature is too low during pouring; the pouring speed is too slow; the flow interruption phenomenon occurs during pouring; the height of the sprue is not enough, the hydrostatic head of the metal liquid is too small; the wall thickness of the casting is too thin, The molten metal flow is too long; the air permeability of the mold shell is poor, and the mold shell emits gas during pouring. These gases and the original gas in the cavity cannot be discharged in time, and a large gas back pressure is formed at the final filling of the molten metal, which prevents the molten metal from filling.
(3) Preventive measures:
a. Appropriately increase the molten metal pouring temperature and the shell temperature
b. The pouring speed should not be too slow, and the liquid flow should not be interrupted during the pouring process
c. Correctly set the gating system to ensure sufficient head height and shorten the molten metal flow as much as possible
d. For castings that are too thin, the wall thickness can be appropriately increased if permitted. For thin-walled parts that cannot be thickened, centrifugal casting, low-pressure casting, or vacuum suction casting can be used
e. The shell roasting should be sufficient to ensure good air permeability and less gas generation
f. If necessary, vent holes can be provided at the highest part of the casting
4) Run fire
The main factors of investment casting defects are easy investment mold quality, shell quality and metal night quality, etc.
(1) Features: There is excess metal with irregular shapes on the surface or inner cavity of the casting
(2) Causes of formation: The outer fire is caused by the insufficient strength of the shell, and the shell is cracked during pouring, causing the outflow of metal liquid. The inner fire is caused by the formation of inner holes and grooves. Partially not coated with paint, or paint strips Air, or not sprinkled with sand so that there are pores in the shell. When the molten metal is poured, the liquid enters the pores or penetrates the defective shell, causing internal fire
(3) Preventive measures:
a. Reasonable design of gating system
b. For parts with small holes and grooves, use secondary surface sand; if necessary, use paint and fine sand with lower viscosity. A careful operation to ensure that the paint is uniform and dense without pores
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