What is a casting crack

What is a casting crack?

Casting crack is a kind of metal defect with incomplete fracture. The existence of cracks not only destroys the continuity of the casting, but also most of the crack tips are sharp, which is easy to cause stress concentration and promote the fracture of the casting under the action of stress. The causes of crack formation are often complex. Unreasonable design, improper material selection, poor material, improper manufacturing process, improper maintenance and use may lead to cracks.

Classification of cracks in castings

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 and measures of hot cracking of castings

There are many factors affecting the formation of hot crack in castings. When considering the measures to prevent hot crack, it can not be solved from unilateral influencing factors. It is necessary to conduct comprehensive analysis in combination with specific conditions, so as to take corresponding measures.

Structure of casting

The unreasonable structural design of castings is often one of the reasons for hot cracking. Therefore, the following points should be paid attention to when designing castings:

• a. The intersection of two sections shall not be designed to turn at right angles, but shall be provided with rounded corners. The shape and size of fillet shall be determined according to the handover of castings.
• b. Reduce and divide the heat dissipation nodes as far as possible, avoid using cross sections, and stagger the cross sections if conditions permit.
• c. When sections with different thickness must be used on steel parts, the shrinkage of each part of the casting shall not hinder each other. For example, for wheel castings such as handwheel and gear, in addition to making fillet at the corner, if necessary, the spokes can be made into bending shape.

Design of casting process

• a. After the liquid metal enters the mold cavity through the inner gate, the casting near the inner gate cools slowly, forming a weak area on the casting, so it is easy to produce hot cracks. Sometimes, the shrinkage of castings may be hindered by the gate and produce hot cracks. Especially in order to make the liquid metal poured evenly, the danger is greater when the complex and connected gate is applied.
• b. Hot cracks often occur at the junction of sections with uneven wall thickness. For such defects as fire fighting, process bars can be set at these parts with the consent of users. Crack prevention process bars can not only improve the strength of hot crack parts of castings, but also play a role in heat dissipation, so as to slow down the concentration of “hot spots” and alleviate the generation of hot cracks. When setting the process reinforcement, it should not be too thick, which is generally about 1 / 3 of the wall thickness of the corresponding part, otherwise it will also lead to hot crack. At the same time, the shape of the process reinforcement shall be handled well, and the process hole shall be added in the reinforcement if necessary.

Shell making process

Improving the yield of the shell to reduce the shrinkage obstruction is conducive to reducing the tendency of hot cracking, and the factors affecting the yield are: the type and performance of the binder, the refractory, the type of hardener and the shell making process. Therefore, under the condition of meeting the strength, the number of layers of the mold shell should be reduced as far as possible. For castings with special circumstances, the water glass density of the reinforcement layer can be appropriately reduced to 1.32, and ammonia chloride hardening process can be adopted.

Adjustment and refining of alloy composition and melting process

• a. On the premise of not affecting the service performance of the casting, the chemical composition of the alloy can be adjusted appropriately, the solidification temperature range can be reduced, the shrinkage during solidification can be reduced, or the near eutectic composition with better crack resistance can be selected. For example, when casting zg45# the wrench body casting, because this kind of casting is easy to produce hot crack, adjusting its chemical composition is conducive to the prevention of hot crack. The adjustment method is to take the lower limit value of carbon as 0.42-0.44%, the upper limit value of silicon as 0.45%, appropriately increase it to 0.8-0.9%, and control sulfur and phosphorus below 0.04%.
• b. Micro alloying and modification of carbon steel and alloy steel can significantly improve the crack resistance of steel castings. This purpose can be achieved by adding rare earth elements or other elements, and the addition amount is generally less than 0.3%. Elements can be added alone or several elements can be added at the same time. Common elements include vanadium, cerium, calcium, titanium, niobium, etc. This method can refine the grain of cast steel and reduce inclusions, so as to change the primary crystallization process of steel and enhance the crack resistance and mechanical properties of cast steel. If 0.5-0.2% vanadium diluted into 0.2% is added to zg25# carbon steel, the grain can be refined and the coarse widmanstatten structure can be eliminated; Adding 0.05-0.1% cerium to chromium, nickel and copper steel can improve the columnar structure; Adding 0.15% cerium to ZG2Cr13 and zg1cr18n19 cast steel can improve the distribution of sulfide inclusions; Therefore, the crack resistance of cast steel is improved.
• c. Improving the deoxidation process of cast steel can improve the crack resistance of cast steel. The oxidation inclusion at the grain boundary of steel castings is one of the main reasons for the formation of hot cracks. Increasing deoxidation effect, reducing oxide inclusions and changing their distribution can reduce the hot cracking tendency of cast steel. Experiments show that the effect of comprehensive deoxidation is much better than that of single deoxidation. This is because the size of deoxidation products is much larger than that of deoxidation alone, which is conducive to the elimination of deoxidation products from molten steel. Comprehensive deoxidation method: carbon steel and low alloy steel are pre deoxidized with manganese and silicon, and finally deoxidized with aluminum. Stainless steel is deoxidized by diffusion of silicon calcium particles and lime, and then precipitated by massive silicon calcium. The most ideal way is to use multi-component composite Deoxidizer.
• d. Control the crystallization process of cast steel, refine the primary crystal structure and reduce the tendency of hot crack. Suspension casting method is adopted, that is, fine particle metal powder is added through gate or other channels while molten steel is poured to refine the primary crystal structure. For example, when zg35# carbon steel is poured, 2% ferromanganese powder is added; When pouring chromium molybdenum alloy steel, adding molybdenum powder with particle size of 0.1mm can refine grains, improve mechanical properties and reduce hot crack defects of castings.

Pouring conditions

In terms of pouring, the main measures to prevent hot cracking are high-temperature tapping, low-temperature pouring, and the principle of fast pouring speed first and then slow pouring speed. However, considering that reducing the pouring temperature is easy to produce cold shut, insufficient pouring and other defects for thin-walled parts, it is better to produce according to the following methods according to the structural characteristics of castings.

• a. When pouring thin-walled and rod-shaped (wrench, connecting rod and pliers) castings, the pouring temperature of molten steel is controlled at about 1540 ℃, and the temperature of mold shell is greater than 650 ℃, that is, red shell pouring is required.
• b. For castings with wall thickness greater than 20mm, the pouring temperature of molten steel is about 1530 ℃, the temperature of mold shell is below 400 ℃, and even cold shell pouring can be used. This is beneficial to improve the crusting strength of castings in the dangerous period of hot cracking.
• c. When castings with uneven wall thickness are encountered, the pouring temperature of molten steel is 1540 ℃. The temperature of the mold shell is about 400-500 ℃, but the pouring method of fast first and then slow must be strictly implemented. After the casting is poured and the liquid steel solidifies, the mold shell of the wall thickness part shall be removed first and allowed to cool first, so as to achieve the purpose of uniform shrinkage of the casting. The above measures are taken for carbon steel castings prone to thermal cracking. Stainless steel castings can be carried out with reference, but the pouring temperature should be appropriately increased by about 30-60 ℃.

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.

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: China Butt Weld Fittings Manufacturer – Yaang Pipe Industry Co., Limited (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.)

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