Stomata (bubble, pupil, air pocket)
feature
Pores are holes that are present on the surface or inside of a casting. They are round, elliptical or irregular. Sometimes a plurality of pores form an air mass, and the skin is generally pear-shaped. The shape of the pupil is irregular and the surface is rough. The air pocket is a concave surface of the casting and the surface is smooth. The visual inspection of the open hole reveals that the subcutaneous stomata can only be found after mechanical processing.
Cause of formation
1. The mold preheating temperature is too low, and the liquid metal cools too quickly when passing through the pouring system.
2. The mold exhaust design is poor, and the gas cannot be discharged smoothly.
3, the paint is not good, its own exhaustibility is not good, and even its own volatilization or decomposition of gas.
4. There are holes and pits on the surface of the mold cavity. After the liquid metal is injected, the gas in the hole and the pit rapidly expands and compresses the liquid metal to form the pupil.
5. The surface of the mold cavity is rusted and not cleaned.
6. Raw materials (sand core) are improperly stored and are not preheated before use.
7, poor deoxidizer, or insufficient amount or improper operation.
Prevention method
1. The mold should be fully preheated. The particle size of the coating (graphite) should not be too fine and the gas permeability should be good.
2. Cast using a tilt casting method.
3. Raw materials should be stored in a ventilated and dry place, and should be preheated when used.
4. Select a deoxidizer (magnesium) with good deoxidation effect.
5. The pouring temperature should not be too high.
Shrinkage hole
feature
The shrinkage hole is a surface roughened hole existing on the surface or inside of the casting, and the slight shrinkage hole is a plurality of small shrinkage holes which are dispersed, that is, the shrinkage, shrinkage or shrinkage of the crystal grains. It often occurs near the casting sprue, the root of the riser, the thick part, the thick and thin transition of the wall and the thick part with a large plane.
Cause of formation
1. The mold working temperature control does not meet the directional solidification requirements.
2, the paint selection is not appropriate, the thickness of the paint layer in different parts is not well controlled.
3. The position of the casting in the mold is not properly designed.
4. The design of the riser failed to achieve full complementation.
5. The pouring temperature is too low or too high.
Prevention method
1. Increase the temperature of the abrasive.
2. Adjust the thickness of the coating layer, the coating should be evenly sprayed, and the coating should not form part of the coating when it is applied.
3. Locally heat the mold or partially heat it with a heat insulating material.
4, the hot section is set with copper blocks, and the part is chilled.
5. Design the heat sink on the mold, or accelerate the cooling rate in the local area by water, etc., or spray water and spray outside the mold.
6. Use a detachable chiller block and place it in the cavity in turn to avoid insufficient cooling of the chill block itself during continuous production.
7. Design a pressurizing device on the mold riser.
8. The design of the gating system should be accurate and the appropriate pouring temperature should be selected.
Slag hole (solvent slag or metal oxide slag)
feature
The slag hole is a bright hole or a dark hole on the casting. All or part of the hole is filled with slag, the shape is irregular, and the small-point flux slag is not easy to find. After the slag is removed, the hole is smooth, generally distributed in the pouring. In the lower part of the position, near the inner runner or at the corner of the casting, the oxide slag is mostly distributed in the form of a mesh on the surface of the casting near the inner runner, sometimes in the form of flakes, or irregular clouds with wrinkles, or a sheet-like interlayer. Or the inside of the casting is in the form of a floc. When breaking, it often breaks from the interlayer, and the oxide is among them, which is one of the roots of the crack formation of the casting.
Cause of formation
The slag hole is mainly caused by the alloy smelting process and the casting process (including the incorrect design of the casting system), the mold itself does not cause slag holes, and the metal mold is one of the effective methods to avoid slag holes.
Prevention method
1. The gating system is set up correctly or a cast fiber filter is used.
2. Adopt inclined pouring method.
3. Choose flux and strictly control quality.
Crack (hot crack, cold crack)
feature
The appearance of the crack is a straight or irregular curve. The surface of the hot crack fracture is strongly oxidized to dark gray or black, no metallic luster, and the surface of the cold crack fracture is clean and has a metallic luster. Generally, the outer crack of the casting can be seen directly, and the internal crack can be seen by other methods. Cracks are often associated with defects such as shrinkage and slag inclusion, which occur mostly in the inner side of the sharp corner of the casting, at the junction of the thick section, and the hot joint where the riser is connected to the casting.
Cause of formation
Metal mold casting is prone to crack defects, because the mold itself has no retractability, the cooling speed is fast, and it is easy to cause the internal stress of the casting to increase. The opening type is too early or too late, the pouring angle is too small or too large, and the coating layer is too thin. It is easy to cause cracking of the casting, and it is easy to cause crack when the mold cavity itself has cracks.
Prevention method
1. Attention should be paid to the structural and structural properties of the castings, so that the parts with uneven wall thickness of the castings are evenly transitioned, and the appropriate rounded corners are used.
2, adjust the thickness of the coating, as far as possible to achieve the required cooling rate of each part of the casting, to avoid the formation of too much internal stress.
3, should pay attention to the working temperature of the metal mold, adjust the mold slope, and timely core cracking, take out the casting slowly.
Cold separation (poor fusion)
feature
The cold partition is a kind of slit or surface with a rounded edge. The middle is separated by the scale, which is not completely integrated. When the cold partition is severe, it becomes “under-cast”. Cold insulation often occurs on the top wall of the casting, a thin horizontal or vertical surface, a thick wall joint or a thin support plate.
Cause of formation
1. Metal mold exhaust design is unreasonable.
2. The working temperature is too low.
3. The quality of the paint is not good (man-made, material).
4. The location of the sprue is improper.
5. The pouring speed is too slow.
Prevention method
1. Design the runner and exhaust system correctly.
2, large-area thin-walled castings, coatings should not be too thin, appropriate thickening of the coating layer is conducive to molding.
3. Appropriately increase the working temperature of the mold.
4, using the oblique casting method.
5. Casting with mechanical vibration metal mold.
Sand hole (sand hole)
feature
A relatively regular hole is formed on the surface or inside of the casting, and its shape is consistent with the shape of the sand. When the mold is just released, sand grains embedded on the surface of the casting can be seen, and the sand can be extracted from the sand. When a plurality of sand eyes are simultaneously present, the surface of the casting is orange peel.
Cause of formation
The sand falling from the surface of the core is surrounded by the copper liquid and forms a hole with the surface of the casting.
1. The surface strength of the sand core is not good, burnt or not fully cured.
2. The size of the sand core does not match the outer mold, and the sand core is crushed when the mold is closed.
3. The mold smashed the graphite water contaminated with sand.
4. The sand that is rubbed off by the ladle and the sand core at the sprue is flushed into the cavity with the copper water.
Prevention method
1. The sand core is manufactured strictly according to the process and the quality is checked.
2. The size of the sand core matches the size of the outer mold.
3. The ink should be cleaned up in time.
4. Avoid rubbing the ladle with the sand core.
5. When blowing the sand core, blow the sand in the mold cavity.
Casting cracks are mainly divided into two categories, hot cracking and cold cracking!
Hot crack
The hot crack is the bending shape of the crack, the fracture is very irregular and the broken wire is connected, and the surface is wider, the narrower the inside, the hot cracking mechanism is: the molten steel begins to condense after injection into the cavity, when the crystal skeleton has formed and started After the wire is shrunk, since the internal molten steel is not solidified into solid state and the shrinkage is hindered, stress or plastic deformation occurs in the casting, and when they exceed the material strength limit at this high temperature, the casting will crack.
Hot crack morphology and characteristics
The hot crack is a crack caused by the solid shrinkage of the casting under the condition that the casting is in a state of low strength and plasticity at the end of solidification or shortly after solidification. Thermal cracking is one of the casting defects commonly found in the production of steel castings, malleable iron castings and certain light alloy castings. The hot cracks sprout at the grain boundaries and expand along the grain boundaries, and their shapes are uneven in thickness and irregular in twists and turns. The cracked surface is oxidized and has no metallic luster. The crack surface of the cast steel is approximately black, while the aluminum alloy is dark gray. The outer crack is visible to the naked eye and can be distinguished from the cold crack according to the shape and fracture characteristics.
Thermal cracks can be further divided into outer cracks and inner cracks. The hot cracks that can be seen on the surface of the casting are called external cracks. External cracks often occur at the corners of the casting, at a sharp change in the thickness of the section, or in a partially suspected slow place where stress concentration is likely to occur. It is characterized in that the surface is narrow inside and is tear-like. Sometimes the fracture will run through the entire casting section. Another feature of thermal cracking is that cracks are distributed along grain boundaries. Internal cracks generally occur in the final solidified part of the casting. The crack shape is very irregular, and the section is often accompanied by dendrites. Under normal circumstances, the internal crack does not extend to the surface of the casting.
Causes of hot crack formation
There are many theoretical and practical reasons for the formation of hot cracks, but the root cause is the solidification mode of the casting and the thermal stress and shrinkage stress of the casting during solidification.
After the liquid metal is poured into the mold, the heat is dissipated mainly through the wall, so the solidification always starts from the surface of the casting. When a large amount of dendrites appear in the late stage of solidification and are lapped into a complete skeleton, solid shrinkage begins to occur. However, at this time, there is a liquid metal film (liquid film) which has not been solidified between the dendrites. If the casting shrinks without any hindrance, the dendritic skeleton can be freely contracted and is not affected by the force. When the shrinkage of the dendritic skeleton is hindered by the sand type or the sand core or the like, the tensile stress is generated when the shrinkage is not freely contracted. When the tensile stress exceeds the limit of its material strength, cracks will occur between the dendrites. If the dendritic skeleton is pulled slowly, and there is enough metal liquid around the pulled portion to flow into the crack and replenish in time, the casting will not generate hot cracks. Conversely, if the crack is not replenished with molten metal, hot cracks will occur in the casting.
From this, it can be seen that an alloy having a wide solidification temperature range and a paste-like or sponge network-like solidification method is most likely to be thermally cracked. As the solidification temperature range is narrowed, the thermal cracking tendency of the alloy becomes smaller, and the alloy of the isothermally solidified eutectic composition is least likely to form a thermal crack. Thermal cracking is formed during the solidification of the casting, but does not mean that thermal cracking will occur when the casting solidifies. It mainly depends on the thermal stress and shrinkage stress during the solidification of the casting. The thermal stress in the solid phase grain skeleton of the solidification zone of the casting is easy to cause thermal cracking or subcutaneous thermal cracking of the casting; the shrinkage stress caused by the external obstruction factor is the main condition for the thermal cracking of the casting. In the solidified casting shell, the line shrinkage is hindered by external factors such as the sand core, the molding sand, the surface of the casting and the surface friction of the sand surface. There is shrinkage stress (tensile stress) in the outer casing, and the hot section of the casting, especially the hot section. The thinner shell formed by the corners becomes a place where the shrinkage stress concentrates, and the castings are most likely to be thermally cracked in these places.
The causes of hot cracks are reflected in the process and the structure of the castings: the wall thickness of the casting is not uniform, the inner angle is too small; the lap joint is too much, the outer frame of the casting, the ribs and the like hinder the normal shrinkage of the casting; the riser system hinders The casting shrinks normally. For example, the strength of the sand between the riser near the tank or the riser is high, which limits the free shrinkage of the casting; the riser is too small or too large; the shrinkage of the alloy wire is too large; the low melting point of the alloy forms The element exceeds the standard, and the content of sulfur and phosphorus in cast steel cast iron is high; the castings are out of the box too early and the cooling is too fast.
How to prevent hot cracks from occurring
1. Improve the structure of castings
The wall thickness should be uniform, and the corners should be rounded to reduce stress concentration. The spokes of the wheel castings can be curved as necessary.
2. Improve the melting quality of alloy materials
The refining and degassing processes are used to remove oxidizing impurities and gases in the molten metal. Control the content of harmful impurities and use a reasonable smelting process to prevent cold cracks.
3, using the correct casting process measures
Simultaneous solidification of the casting not only helps prevent hot cracking, but also helps prevent cold cracking. Reasonably set the position and size of the pouring riser so that the cooling speed of each part of the casting is as uniform as possible, reducing the tendency of cold cracking.
Correctly determining the residence time of the casting in the sand mold The sand type is a good thermal insulation container, which can further homogenize the temperature of the thicker and thinner parts of the casting, reduce the temperature difference between them, reduce the thermal stress and reduce the tendency of cold cracking. Extend the residence time of the casting in the mold to avoid premature opening of the internal stress in the casting, and cold cracking.
Increasing the sand type and the core of the concealed casting. After solidification, the pressure box iron is removed early, and the sand box fastening device is loosened. This is an effective measure to prevent cold cracking of the casting due to shrinkage stress. For the sand mold and core of large castings, some sand and core sand can be excavated in advance after casting to reduce their shrinkage resistance to the casting and promote uniform cooling of all parts of the casting. During the process of falling sand, cleaning and handling, the castings should avoid collision and extrusion to prevent cold cracks in the castings.
4, aging heat treatment
Castings with high casting stress should be subjected to aging heat treatment in time to avoid excessive residual stress and cold cracking of the casting. If necessary, the casting is subjected to an aging heat treatment after cutting the riser or welding.
Cold crack
Cold cracking is the cracking caused by the local casting stress being greater than the ultimate strength of the alloy when the casting is solidified and then cooled to an elastic state. Cold cracks always occur in the part subjected to tensile stress during cooling, especially where the tensile stress is concentrated. Cold cracks are different from hot cracks. Cold cracks tend to penetrate through the entire section, and the shape is uniform or slender in shape. The shape of the fracture surface of the cold crack is metallic luster or mild oxidation, and the crack direction is smooth. It does not occur along the grain boundary. This is significantly different from hot cracking. The cold crack test is visible to the naked eye and can be distinguished from the hot crack according to its macroscopic morphology and microscopic characteristics of transgranular expansion.
When the casting stress in the casting is greater than the strength limit of the metal, the casting will produce cold cracks. Therefore, any factors that increase the stress of the casting and reduce the strength of the metal may cause cold cracking in the casting.
The main causes of cold cracks are as follows:
1. Casting structure
The wall thickness of the casting is not uniform, which causes casting stress to be generated in the casting, and sometimes causes cold cracking defects. Castings of rigid structures are prone to thermal stress due to structural obstruction, resulting in cold cracking of the casting. For example, a "thin-walled large core" box-shaped casting with uniform wall thickness produces temporary shrinkage stress due to the obstruction of the core, and when the tensile strength of the alloy material is exceeded, the casting is cold-cracked.
2, the design of the riser system is unreasonable
For castings with uneven wall thickness, if the ingate is placed in the wall thickness portion of the casting, the cooling rate of the thick portion of the casting will be slower, causing or aggravating the difference in cooling speed of each part of the casting and increasing the thermal stress of the casting. , causing cold cracks in the casting. Improper setting of the riser directly hinders the shrinkage of the casting and also causes cold cracking in the casting. Since the gate is thinner than the casting, the gate first solidifies, and when the casting is inwardly contracted by the gate, tensile stress is generated, and it is usually easy to generate cold cracks on the wall between the two gates. Secondly, the high-temperature strength or dry strength of the molding sand or the core sand is too high, and the high-temperature repellent property is poor, so that the shrinkage of the casting is hindered, and a large tensile stress is generated, resulting in cold cracking of the casting.
3. The chemical composition of the alloy material is unqualified.
The high carbon content and other alloying elements in the steel make the castings susceptible to cold cracking. The tough alloy material is not easy to produce cold cracks, and the brittle alloy material is prone to cold cracks. Phosphorus is a harmful element in steel. When ω(P)>0.05%, the cold brittleness of steel increases, and cold cracks are likely to occur. In gray cast iron, the presence of an excessive amount of anti-graphitizing elements also causes an increase in the amount of shrinkage of the casting, resulting in cold cracking of the casting.
4, control the time of opening the box
Castings are prematurely opened, and the falling sand temperature is too high. Collision and extrusion during sand cleaning can cause cracking of the casting.