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Causes of casting cracks and six kinds of common defects and their prevention

There are two types of cracks in casting, hot crack and cold crack.

Hot cracking

Hot cracking is a kind of hot cracking, which is characterized by crooked crack shape, irregular fracture surface, wide surface and narrower inside. The mechanism is as follows: the molten steel begins to condense after it is injected into the mold cavity. When the crystal framework has been formed and linear shrinkage begins, the shrinkage is blocked because the molten steel is not solidified into a solid state at this time, and the stress or plastic deformation will occur in the casting. When they exceed the material strength limit at this high temperature, the casting will crack.

Morphology and characteristics of hot cracks

Hot crack is a kind of crack which is caused by the blocked solid shrinkage of the casting when the strength and plasticity of the casting are very low at the end of solidification or shortly after solidification. Hot crack is one of the common casting defects in steel castings, malleable iron castings and some light alloy castings. The hot cracks initiate and propagate along the grain boundary, and their shape is uneven, tortuous and irregular. The surface of the crack is oxidation color, no metallic luster. The crack surface of steel casting is nearly black, while that of aluminum alloy is dark gray. The external crack can be seen by naked eye, and can be distinguished from cold crack according to the shape and fracture characteristics.
Hot crack can be divided into outer crack and inner crack. The hot crack that can be seen on the surface of casting is called external crack. The external cracks often occur at the corner of casting, the sharp change of section thickness, the slow solidification and the stress concentration. It is characterized by wide surface, narrow interior and tearing shape. Sometimes the fracture will run through the whole casting section. Another characteristic of hot crack is that the crack distributes along the grain boundary. The internal crack usually occurs in the final solidification part of the casting. The shape of the crack is very irregular, and the cross section is often accompanied by dendrites. Generally, 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 fundamental reason is the solidification mode and the thermal stress and shrinkage stress during solidification.
After the liquid metal is poured into the mold, the heat loss is mainly through the mold wall, so the solidification always starts from the surface of the casting. When a large number of dendrites appear at the later stage of solidification and form a complete skeleton, solid shrinkage begins to occur. However, there is a layer of liquid metal film (liquid film) between the dendrites. If the shrinkage of the casting is not hindered, the dendrite skeleton can contract freely without force. When the shrinkage of dendrite skeleton is hindered by sand mold or sand core, tensile stress will be produced if it can not shrink freely. When the tensile stress exceeds the strength limit of the material, the dendrites will crack. If the pulling speed of dendrite skeleton is very slow, and there is enough liquid metal around the pulling part to flow into the crack in time, the casting will not produce hot cracks. On the contrary, if the crack is not replenished by liquid metal, hot cracks will appear in the casting.
It can be seen that the alloy with wide solidification temperature range and mushy or sponge network solidification mode is most prone to hot cracking. With the narrowing of the solidification temperature range, the hot cracking tendency of the alloy decreases, and the alloy with eutectic composition solidified at constant temperature is the least likely to form hot cracking. Hot cracking occurs in the solidification period of castings, but it does not necessarily mean that hot cracking occurs during solidification. It mainly depends on the thermal stress and shrinkage stress during solidification. The thermal stress in the solid grain skeleton in the solidification region of the casting is easy to cause hot cracking or subcutaneous hot cracking; The shrinkage stress caused by external factors is the main condition of hot cracking. When the casting shell is in solidification state, its linear shrinkage is hindered by external factors such as sand core, molding sand, surface friction between casting surface and sand mold, there will be shrinkage stress (tensile stress) in the shell. The thin shell formed by the hot spot, especially the sharp corner at the hot spot, becomes the place where the shrinkage stress is concentrated, and the casting is most likely to produce hot cracking in these places.

  • The causes of hot cracks are reflected in the process and casting structure, including uneven wall thickness and small internal angle;
  • There are too many bifurcations in the lap joint, which hinder the normal shrinkage of the casting;
  • For example, the pouring riser is close to the box belt or the sand strength between the pouring riser is very high, which limits the free shrinkage of the casting;
  • The riser is too small or too large;
  • The linear shrinkage of the alloy is too large;
  • The forming elements of medium and low melting point phase in the alloy exceed the standard, and the contents of sulfur and phosphorus in cast steel and cast iron are high;
  • It is too early to open the box and the cooling is too fast.

How to prevent hot crack

01. Improve casting structure
The wall thickness should be uniform, and the corner should be rounded to reduce the stress concentration. The spokes of wheel castings can be bent if necessary.
02. Improve the melting quality of alloy materials
Refining and degassing processes are used to remove the oxide inclusions and gases in the liquid metal. Control the content of harmful impurities and adopt reasonable melting process to prevent cold cracks.
03. Adopt correct casting process measures
It is not only helpful to prevent hot crack, but also helpful to prevent cold crack to make casting solidify at the same time. The position and size of the riser should be set reasonably to make the cooling rate of each part of the casting as uniform as possible and reduce the tendency of cold cracks.
Determining the residence time of castings in sand mold correctly sand mold is a good heat preservation container, which can further homogenize the temperature of thicker and thinner parts of castings, reduce the temperature difference between them, reduce the thermal stress, and reduce the tendency of cold cracks. Prolong the residence time of the casting in the mold, so as to avoid large internal stress and cold crack in the casting caused by premature unpacking.
It is an effective measure to prevent cold cracking of castings due to shrinkage stress by increasing the concession of sand mold and sand core, removing the pressing box iron as soon as possible after solidification and loosening the fastening device of sand box. The sand mold and sand core of large castings can be removed in advance after pouring, so as to reduce their shrinkage resistance to castings and promote the uniform cooling of all parts of castings. In the process of sand dropping, cleaning and handling, collision and extrusion should be avoided to prevent cold cracks.
04. Aging heat treatment
The casting with large casting stress should be treated with aging heat treatment in time to avoid cold cracks caused by excessive residual stress. If necessary, an aging heat treatment should be carried out after cutting the riser or welding.

Cold cracking

The cold crack is caused by the local casting stress greater than the ultimate strength of the alloy when the casting is cooled to the elastic state after solidification. The cold crack always occurs in the place where the tensile stress is concentrated during cooling. Cold crack is different from hot crack. Cold crack usually propagates through the whole section, and its shape is straight or broken line with uniform width. The fracture surface of cold crack is pure metallic luster or mild oxidation color, and the crack trend is smooth, rather than along the grain boundary. This is significantly different from hot cracking. Cold cracks can be seen by naked eyes, and can be distinguished from hot cracks according to their macroscopic morphology and microscopic characteristics of transgranular growth.
When the casting stress in the casting is greater than the strength limit of the metal, the casting will produce cold cracks. Therefore, all the factors that increase the stress and decrease the strength of the metal may cause cold cracks in the casting.

Main causes of cold cracks

01. Casting structure
The uneven wall thickness of castings causes casting stress and sometimes cold crack defects. Due to the obstruction of rigid structure, the casting is easy to produce thermal stress, which makes the casting produce cold cracks. For example, a box casting with uniform wall thickness of “thin wall and large core” will produce temporary shrinkage stress due to the obstruction of sand core. When it exceeds the tensile strength of alloy material, the casting will produce cold cracking.
02. The design of riser system is unreasonable
For the castings with uneven wall thickness, if the inner gate is set in the wall thickness part of the casting, the cooling speed of the thick wall part of the casting will be slower, which will lead to or aggravate the difference of the cooling speed of each part of the casting, increase the thermal stress of the casting, and promote the casting to produce cold cracks. Improper setting of pouring and riser directly hinders the shrinkage of castings and causes cold cracks in castings. Because the gate is thinner than the casting, the gate solidifies first. When the casting shrinks inward and is hindered by the gate, the tensile stress will be produced, and it is easy to produce cold cracks on the wall between the two gates. Secondly, the high-temperature strength or dry strength of molding sand or core sand is too high, and the high-temperature yield is poor, which hinders the shrinkage of the casting and produces great tensile stress, resulting in cold cracks in the casting.
03. Unqualified chemical composition of alloy material
The high content of carbon and other alloy elements in steel makes the casting prone to cold cracks. Ductile alloy materials are not easy to produce cold cracks, brittle alloy materials are easy to produce cold cracks. Phosphorus is a harmful element in steel ω( P) When the ratio is more than 0.05%, the cold brittleness of the steel increases and cold cracks are easy to occur. In gray cast iron, the existence of excessive anti graphitization elements will also increase the shrinkage of castings, resulting in cold cracks.
04. Control the unpacking time
If the casting is opened too early and the temperature of sand dropping is too high, the casting will crack when it is impacted and squeezed during sand cleaning.

Common defects of six kinds of castings

01. Air hole (bubble, choke hole, air pocket)
Features: air hole is the hole existing on the surface or inside of the casting, which is round, oval or irregular. Sometimes multiple air holes form an air mass, and the skin is generally pear shaped. The shape of choke hole is irregular, and the surface is rough. The air pocket is a concave part in the surface of casting, and the surface is smooth. It can be found by the appearance inspection of the open hole, and it can only be found by the mechanical processing of the subcutaneous pores.
20210530004912 14573 - Causes of casting cracks and six kinds of common defects and their prevention
The causes are as follows:

  • 1. The preheating temperature of the mould is too low, and the liquid metal cools too fast when passing through the gating system.
  • 2. The mold exhaust design is poor, and the gas cannot be discharged smoothly.
  • 3. The coating is not good, and its exhaust is not good, even it volatilizes or decomposes gas.
  • 4. There are holes and pits on the surface of the mold cavity. After the liquid metal is injected, the gas in the holes and pits expands rapidly and compresses the liquid metal to form choking holes.
  • 5. The surface of mold cavity is rusted and not cleaned up.
  • 6. The raw material (sand core) is not properly stored and preheated before use.
  • 7. Poor deoxidizer, insufficient dosage or improper operation.

Prevention methods:

  • 1. The mold should be fully preheated, the particle size of the coating (graphite) should not be too fine, and the air permeability should be good.
  • 2. Use inclined pouring method.
  • 3. Raw materials should be stored in a ventilated and dry place and preheated during use.
  • 4. Choose the deoxidizer (magnesium) with better deoxidation effect.
  • 5. Pouring temperature should not be too high.

02. Shrinkage (porosity)
Features: shrinkage cavity is a kind of rough surface hole existing on the surface or inside of casting. Slight shrinkage cavity is many scattered small shrinkage cavities, that is shrinkage porosity, and the grains at the shrinkage cavity or shrinkage porosity are coarse. It often occurs near the ingate, riser root, thick and large parts, wall thickness transition and thick and thin parts with large plane.
The causes are as follows:

  • 1. The mold working temperature control does not meet the requirements of directional solidification.
  • 2. Improper selection of coating and poor control of coating thickness at different parts.
  • 3. Improper design of the position of the casting in the mold.
  • 4. The design of pouring and riser failed to play the role of full feeding.
  • 5. The pouring temperature is too low or too high.

Prevention and control methods:

  • 1. Increase the temperature of abrasive tools.
  • 2. Adjust the thickness of the coating layer, spray the coating evenly, and do not form local coating accumulation when the coating falls off and is repainted.
  • 3. Local heating of the mold or local insulation with insulation material.
  • 4. Insert copper block at the hot spot to chill the local part.
  • 5. design heat sink on the die, or accelerate the local cooling rate through water, or spray water outside the mold, spray.
  • 6. The demountable chilling block is placed in the mold cavity in turn to avoid insufficient cooling of the chilling block itself in continuous production.
  • 7. The pressure device is designed on the mold riser.
  • 8. The gating system should be designed accurately and the suitable pouring temperature should be selected.

03. Slag hole (slag inclusion of flux or metal oxide)
Features: the slag hole is the open hole or the dark hole on the casting. The hole is completely or partially filled by the slag. The shape is irregular, and it is difficult to find the small spot slag inclusion. After the slag is removed, the smooth hole appears, which is generally distributed in the lower part of the pouring position, near the ingate or the dead corner of the casting. The oxide slag inclusion is mostly distributed on the surface of the casting near the ingate in a network, sometimes in a thin sheet shape, In the process of fracture, it often breaks from the interlayer, in which oxide is one of the causes of cracks.
The causes are as follows:
Slag hole is mainly caused by alloy melting process and pouring process (including incorrect design of pouring system). The mold itself will not cause slag hole, and metal mold is one of the effective methods to avoid slag hole.
Prevention and control methods:

  • 1. The gating system is set correctly or the cast fiber filter is used.
  • 2. Inclined pouring method is adopted.
  • 3. Select flux and strictly control quality.

04. Crack (hot crack, cold crack)
Features: the appearance of crack is straight or irregular curve, the surface of hot crack fracture is strongly oxidized to dark gray or black, without metallic luster, while the surface of cold crack fracture is clean and metallic luster. Generally, the external cracks of castings can be seen directly, while the internal cracks can only be seen by other methods. Cracks are often associated with shrinkage porosity, slag inclusion and other defects, which often occur in the inner side of the casting sharp corner, the junction of thick and thin sections, and the hot spot area where the riser is connected with the casting.
The causes are as follows:
Metal mold casting is easy to produce crack defects, because the metal mold itself has no concession, the cooling speed is fast, it is easy to cause the internal stress of the casting to increase, the mold opening is too early or too late, the pouring angle is too small or too large, the coating layer is too thin and so on, which are easy to cause cracks in the mold cavity itself.
Prevention and control methods:

  • 1. Attention should be paid to the structural and technological properties of the castings, so that the parts with uneven wall thickness can be evenly transited, and appropriate fillet size should be adopted.
  • 2. Adjust the coating thickness to make each part of the casting reach the required cooling rate as far as possible to avoid forming too large internal stress.
  • 3. Pay attention to the working temperature of the metal mold, adjust the inclination of the mold, timely core pulling and cracking, and take out the casting for slow cooling.

05. Cold insulation (poor fusion)
Features: cold shut is a kind of through seam or surface crack with round edge, which is separated by oxide skin in the middle and is not completely integrated. When cold shut is serious, it becomes “under cast”. Cold shut often appears on the top wall of casting, thin horizontal or vertical plane, thick thin wall connection or thin auxiliary plate.
The causes are as follows

  • 1. The exhaust design of metal mold is unreasonable.
  • 2. The working temperature is too low.
  • 3. Poor coating quality (artificial, material).
  • 4. Improper position of runner.
  • 5. The pouring speed is too slow.

Prevention and control methods:

  • 1. Design runner and exhaust system correctly.
  • 2. For large area thin-walled castings, the coating should not be too thin, and properly thickening the coating layer is conducive to forming.
  • 3. Properly increase the working temperature of the die.
  • 4. Inclined pouring method is adopted.
  • 5. Pouring with mechanical vibration metal mold.

06. Sand hole (sand hole)
Features: relatively regular holes are formed on the surface or inside of the casting, the shape of which is consistent with the shape of sand particles. The sand particles embedded on the surface of the casting can be seen when the casting is just out of the mold, from which the sand particles can be taken out. When multiple sand holes exist at the same time, the surface of the casting is orange peel.
The causes are as follows
The sand particles falling from the surface of the sand core are wrapped by liquid copper and form holes on the surface of the casting.

  • 1. The surface strength of sand core is not good, burnt or not completely solidified.
  • 2. The size of the sand core is not consistent with the external mold, and the sand core is crushed when the mold is closed.
  • 3. The mould is dipped in graphite water polluted by sand.
  • 4. The sand dropped from the friction between the ladle and the sand core at the runner will rush into the mold cavity with copper water.

Prevention and control methods:

  • 1. The sand core should be produced strictly according to the process and the quality should be checked.
  • 2. The size of sand core is consistent with that of outer mold.
  • 3. The ink should be cleaned up in time.
  • 4. Avoid friction between ladle and sand core.
  • 5. Clean the sand in the mold cavity when setting the sand core.

Source: Network Arrangement – China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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