What is quenching crack

What is quenching crack?

During quenching, the internal stress caused by volume effect in steel parts will produce different types of quenching cracks if the internal stress cannot be relaxed due to insufficient plasticity.

Quenching cracks are cracks that occur during quenching or during room temperature storage after quenching. The latter is also called aging crack. There are many reasons for quenching crack. When analyzing quenching crack, it should be distinguished according to crack characteristics.

Main factors causing quenching cracks

The main factors causing quenching cracks are: improper control of quenching heating mode and heating speed, uneven heating, too high quenching temperature, improper cooling mode during quenching, too fast cooling speed and uneven cooling; Improper selection of cooling medium; Before quenching, the workpiece is not subject to preliminary heat treatment or improper treatment; Untimely tempering; High stress concentration caused by material defects or workpiece surface defects, etc.

Type of quenching crack

The cracks of the parts mainly occur in the later stage of quenching and cooling, that is, after the martensitic transformation is basically completed or completely cooled, the brittle failure is caused by the tensile stress in the parts exceeding the fracture strength of the steel. Cracks are usually perpendicular to the direction of maximum tensile deformation, so different forms of cracks in parts mainly depend on the stress distribution.
Common types of quenching cracks: longitudinal (axial) cracks mainly occur when the tangential tensile stress exceeds the fracture strength of the material; When the large axial tensile stress formed on the inner surface of the part exceeds the breaking strength of the material, a transverse crack is formed; The network crack is formed under the action of surface two-dimensional tensile stress; The peel crack occurs in a very thin hardened layer. This crack may occur when the stress changes sharply and excessive tensile stress acts in the radial direction.

Longitudinal crack

The crack occurs at the maximum tensile stress near the surface layer of the part, and the crack has a certain depth to the center. The crack trend is generally parallel to the axial direction, but the trend can also be changed when the part has stress concentration or internal structural defects.

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After the workpiece is completely quenched, it is easy to produce longitudinal cracks, which is related to the large tangential tensile stress on the surface of the quenched workpiece, and the tendency to form longitudinal cracks increases with the increase of carbon content of steel. Due to the small specific volume of martensite, strong thermal stress and large residual compressive stress on the surface, low carbon steel is not easy to quench and crack. With the increase of carbon content, the compressive stress on the surface decreases, the effect of structural stress increases, and the peak tensile stress moves to the surface layer. Therefore, high carbon steel is easy to form longitudinal quench and crack under overheating.
The size of parts directly affects the magnitude and distribution of residual stress, and its quenching cracking tendency is also different. Longitudinal cracks can also be easily formed by quenching in the range of dangerous section dimensions. In addition, the block depression of raw materials of steel often causes longitudinal cracks. Because most steel parts are rolled, non-metallic inclusions and carbides in the steel are distributed along the deformation direction, resulting in the anisotropy of the steel. If the tool steel has banded structure, its transverse fracture strength after quenching is 30% less than that in the longitudinal direction? In addition to 50%, if there are factors such as non gold chip inclusions in the steel that lead to stress concentration, longitudinal cracks are easy to form even when the tangential stress is smaller than the axial stress. Therefore, strictly controlling the level of non-metallic inclusions and sugar inhibitor in steel is an important factor to prevent quenching cracks.

Transverse crack or arc crack

As shown in Figure 2, cracks often occur at the sharp corners of the workpiece. The tensile stress peak is easy to occur in the transition zone of non quenched high carbon steel or carburized parts. Such cracks often originate in the surface layer or inside the workpiece at a certain depth. When there are soft spots on quenched steel parts, it is also easy to form small arc cracks.

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Figure.2

The internal stress distribution characteristics of transverse crack and arc crack are: surface compressive stress. After leaving the surface for a certain distance, the compressive stress becomes a large tensile stress. The crack occurs in the bee value area of tensile stress, and then spreads to the part surface when the internal stress is redistributed or the brittleness of steel further increases.
Transverse cracks often occur on large shaft parts, such as rolls, steam turbine rotors or other shaft parts. The crack is characterized by fracture from inside to outside perpendicular to the axis, which is often formed without quenching, which is caused by thermal stress. Large forgings often have metallurgical defects such as pores, inclusions, forging cracks and white spots. These defects, as the starting point of fracture, fracture under axial tensile stress. Arc cracks are caused by thermal stress. They are usually distributed in an arc at the part with sudden change of shape. It is mainly generated inside the workpiece or near sharp edges, grooves and holes, and is distributed in an arc. When the diameter or thickness is 80? When high carbon steel parts above 100mm are not quenched thoroughly, the surface presents compressive stress and the core presents tensile stress. The maximum tensile stress occurs in the transition zone from hardened layer to non hardened layer, and arc cracks occur in these areas. In addition, the cooling speed at sharp edges and corners is fast and fully quenched. When transitioning to gentle parts, that is, to unhardened areas, there is a maximum tensile stress area, so it is easy to produce arc cracks. The cooling speed near the pin hole, groove or center hole of the workpiece is slow, the corresponding hardening layer is thin, and the tensile stress near the hardening transition zone is also easy to cause arc cracks.

Inner hole longitudinal crack

When the hardenability of the steel is large enough, the internal stress on the inner hole surface is mainly tissue stress, and the tangential tensile stress is large. It is easy to form longitudinal cracks on the inner hole wall, which are radial from the end face, as shown in Fig. 3.

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Fig.3

Quenching crack caused by great thickness difference of section

During cooling, the time difference of martensitic transformation in parts with great thickness difference is very large, forming a large structural stress, resulting in cracks, as shown in Fig. 4:

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Fig.4

Crack caused by stress concentration

When there are sharp corners and notches on the steel parts, it is easy to cause stress concentration and crack during quenching, especially under the joint action of stress concentration and sharp change of section size, the risk of quenching crack is greater. As shown in Figure 5, the 3mm thick flange root is easy to crack.

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Figure.5

Reticular crack

This crack has arbitrary directionality and is independent of the shape of the workpiece, as shown in Fig. 6. The depth of network crack is generally in the range of 0.01-0.15mm, which is a kind of surface crack. Network crack is easy to form after decarburization and quenching on the surface of high carbon tool steel and alloy tool steel.
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Fig.6

The main feature of this kind of crack is that the arbitrary direction of the crack has nothing to do with the shape of the part. Many cracks are connected to form a network and widely distributed. When the crack depth is large, if it reaches more than 1mm, the network characteristics disappear and become cracks with arbitrary orientation or longitudinal distribution. The network crack is related to the state of two-dimensional tensile stress on the surface.
High carbon or carburized steel parts with decarburization layer on the surface are easy to form network cracks during quenching. This is because the carbon content of martensite in the surface layer is lower than that in the inner layer, and the specific volume is small. During quenching, the surface layer of carbon is subjected to tensile stress. When the dephosphorization layer is not completely removed during machining, the parts will also form network cracks during high-frequency or flame surface quenching. In order to avoid such cracks, the surface quality of the parts shall be strictly controlled, and oxidation and joint shall be prevented as far as possible during heat treatment. In addition, after the forging die is used for a certain time, the strip or network thermal fatigue cracks in the cavity and the cracks of quenched parts in the grinding process belong to this form.
The peeling crack occurs in a very narrow area of the surface layer, which acts on the compressive stress in the axial and tangential directions, and is in the state of tensile stress in the radial direction. The crack is parallel to the surface of the part. The peeling of the hardened layer after surface quenching and carburized parts cooling belong to this kind of crack. Its generation is related to the uneven structure in the hardened layer. For example, after the alloy carburized steel is cooled at a certain speed, the structure in the carburized layer is: the outer layer is very fine pearlite + carbide, the secondary layer is martensite + residual austenite, and the inner layer is fine pearlite or very fine pearlite. As the formation specific volume of the sublayer martensite is the largest, the volume expansion results in the compressive stress acting on the surface in the axial and tangential directions, the tensile stress in the radial direction, the stress mutation to the inside, and the transition to the compressive stress state. The peel crack occurs in the extremely thin area where the stress transits rapidly. In general, the crack is latent in the interior parallel to the surface, and in serious cases, it will cause surface spalling. If the cooling rate of carburized parts is accelerated or reduced, uniform martensite or very fine pearlite structure can be obtained in the carburized layer, which can prevent the occurrence of such cracks. In addition, during high-frequency or flame surface quenching, such surface cracks are easy to form due to surface overheating and uneven structure along the hardened layer.

Peel crack

The peeling crack occurs in a very narrow area of the surface layer, which acts on the compressive stress in the axial and tangential directions, and is in the state of tensile stress in the radial direction. The crack is parallel to the surface of the part. The peeling of the hardened layer after surface quenching and carburized parts cooling belong to this kind of crack. Its generation is related to the uneven structure in the hardened layer. For example, after the alloy carburized steel is cooled at a certain speed, the structure in the carburized layer is: the outer layer is very fine pearlite + carbide, the secondary layer is martensite + residual austenite, and the inner layer is fine pearlite or very fine pearlite. As the formation specific volume of the sublayer martensite is the largest, the volume expansion results in the compressive stress acting on the surface in the axial and tangential directions, the tensile stress in the radial direction, the stress mutation to the inside, and the transition to the compressive stress state. The peel crack occurs in the extremely thin area where the stress transits rapidly. In general, the crack is latent in the interior parallel to the surface, and in serious cases, it will cause surface spalling. If the cooling rate of carburized parts is accelerated or reduced, uniform martensite or very fine pearlite structure can be obtained in the carburized layer, which can prevent the occurrence of such cracks. In addition, during high-frequency or flame surface quenching, such surface cracks are easy to form due to surface overheating and uneven structure along the hardened layer.

Microcrack

It is caused by microstress. The intergranular cracks after quenching and grinding of high carbon tool steel or carburized workpiece and the cracks caused by untimely tempering of quenched parts are related to the existence and subsequent expansion of microcracks in the steel.
Microcracks must be examined under a microscope. They usually occur at the grain boundary of the original austenite or at the junction of the martensite sheet, and some cracks pass through the martensite sheet. The research shows that microcracks are mostly found in sheet twin martensite, because sheet martensite collides with each other during high-speed growth to produce high stress, while twin martensite itself is brittle and can not produce plastic deformation to relax stress, so microcracks are easy to occur. The coarse austenite grain increases the sensitivity to microcracks. The presence of microcracks in steel will significantly reduce the strength and plasticity of quenched parts, resulting in early failure (fracture) of parts.
To avoid the microcrack of high carbon steel parts, measures such as lower quenching and heating temperature, obtaining fine martensite structure and reducing the carbon content in martensite can be taken. In addition, tempering in time after quenching is an effective method to reduce internal stress. The test shows that after full tempering above 200 ℃, the carbide precipitated at the obvious crack has the effect of “welding” crack, which can significantly reduce the harm of microcrack.

Cracks caused by raw material defects

Slag inclusion in raw materials, network carbide, surface folding in plastic forming process and overheating structure during heating may be called crack sources. It will be exposed or further expanded during quenching. To solve this kind of crack, we should start with controlling the quality of raw materials before quenching.

Characteristics of quenching cracks

In the process of quenching, when the huge stress produced by quenching is greater than the strength of the material itself and exceeds the plastic deformation limit, it will lead to cracks. Quenching cracks often occur shortly after the beginning of martensitic transformation, and the distribution of cracks has no certain law, but they are generally easy to form at the sharp corners and abrupt changes of section of the workpiece.
The quenching cracking observed under the microscope may be intergranular cracking or transgranular cracking; Some are radial, others are single linear or reticular. Quenching cracks caused by too fast cooling in the martensitic transformation zone are often transgranular distribution, and the cracks are straight without branched small cracks around. The quenching cracks caused by too high quenching heating temperature are distributed along the grain, the crack tail is thin, and presents the characteristics of overheating: coarse acicular martensite can be observed in structural steel; Eutectic or angular carbides can be observed in tool steels. High carbon steel workpiece with decarburized surface is easy to form network cracks after quenching. This is because the volume expansion of the surface decarburized layer during quenching and cooling is smaller than that of the non decarburized core, and the surface material is pulled and cracked into a network due to the expansion of the core.

Characteristics of non quenched cracks

The cracks after quenching are not necessarily caused by quenching, but can be distinguished according to the following characteristics:
For the cracks found after quenching, if there is oxidation decarburization on both sides of the crack, it is certain that the crack already exists before quenching. In the process of quenching and cooling, cracks can be formed only when the amount of martensite transformation reaches a certain amount. The corresponding temperature is below 250 ℃. At such a low temperature, even if a crack occurs, decarburization and obvious oxidation will not occur on both sides of the crack. Therefore, the cracks with oxidative decarburization are non quenched cracks.
If the crack already exists before quenching and is not connected with the surface, although such internal crack will not produce oxidation and decarburization, the crack line appears soft and the tail end is round and bald, which is easy to be distinguished from the characteristics of strong and powerful lines and sharp tail end of quenching crack.

How to prevent quenching cracks

The purpose of quenching is to cool austenite without transforming it into pearlite and bainitic limit, so as to obtain martensitic structure. For this purpose, cooling must be carried out at a cooling rate higher than the upper critical cooling rate as mentioned earlier.
On the other hand, in order to reduce quenching deformation and prevent quenching cracks, the cooling speed of each part should be as uniform as possible. In short, quench must be avoided.
If it is handled well at this time, it can obtain martensite and produce small cracks and bending at the same time.
1. Even rapid cooling can achieve good results
The workpiece with simple shape is not easy to crack and bend. Avoiding acute angle, abrupt change of section and adopting symmetrical shape in part design can reduce the occurrence of crack and bending.
2. Cool evenly
Because the edges and corners, thin wall and other parts are easy to cool, they should be cooled slowly, while the concave and thick wall parts accelerate the cooling.
In addition, the mode of quenching into the coolant and the swing mode play an important role in obtaining uniform cooling.
3. Even slow cooling can achieve good results
If the coolant is changed, from water cooling to oil cooling and from oil cooling to air cooling, the temperature difference of each part will be small, and the cracks and bending will be reduced. In this way, if the steel with low critical cooling rate and good hardenability is used for quenching, defects can be prevented.
4. Fast cooling in critical area and slow cooling in dangerous area
During quenching, the transformation from austenite to pearlite (bainite) occurs above 550 ℃, and the contact between the cooling curve and the nose of cot curve has been explained at the same time. Therefore, in this area, it must be cooled at a cooling rate higher than the critical speed.
After that, as we know, since the time of the bainite transformation start line is far to the right, it can be understood that slow cooling is appropriate.
Because the phase transformation stress causing special problems such as crack bending occurs in the structural temperature zone below the MS point, the slow cooling in this region can reduce the temperature difference of each part, which can inhibit and reduce the occurrence of phase transformation stress.
In short, if the critical area is cooled rapidly and the dangerous area is cooled slowly, the ideal quenching without crack, bending and full hardening can be carried out. Intermittent quenching introduced later is one of its methods.
Secondly, isothermal quenching in a hot bath with a certain temperature is a very effective method.

Discussion on quenching crack

1. Shaft, 40Cr, cracks found after forging and quenching. There are signs of oxidation on both sides of the crack. Metallographic examination shows that there is decarburization layer on both sides of the crack, and the ferrite on both sides of the crack presents large columnar grains, and its grain boundary is roughly perpendicular to the crack. Conclusion: the crack is a non quenched crack formed during forging.
When a crack is formed in the forging process, quenching heating causes oxidation decarburization on both sides of the crack. With the decarburization process, the carbon content on both sides of the crack decreases and the ferrite grains begin to nucleate. When the ferrite grains nucleated along both sides of the crack grow into contact with each other, they grow to the matrix far away from both sides of the crack. Due to the decrease of carbon concentration on both sides of the crack during decarburization, it also develops from the opening of the crack to the inside, which provides conditions for the continuous growth of ferrite grains. Therefore, it finally grows into columnar crystals perpendicular to the grain boundary and the crack.
2. Half shaft sleeve seat, 40Cr, cracked after quenching. Metallographic examination shows that there is a full decarburization layer on both sides of the crack, in which the ferrite is coarse columnar grain and perpendicular to the crack. The structure inside the full decarburization layer is lath martensite plus a small amount of Troostite, which is a normal quenched structure. Conclusion: it is not forged in the processing process, so it is a non quenched crack caused by raw materials.
3. Gear milling cutter, high speed steel, cracks appear on the inner hole wall after quenching. Metallographic examination showed that the carbides near the crack were distributed in uneven bands. Conclusion: This is the quenching crack caused by uneven structure.
When there is carbide aggregation in the microstructure of steel, the content of carbon and alloy elements in these places is relatively high, resulting in the reduction of critical temperature. Therefore, even if quenching heating is carried out at normal temperature, the heating temperature is too high for the carbide accumulation. As a result, overheated structure appears in these places, which reduces the strength of the steel. When quenching and cooling, cracking occurs under the action of stress.
The carbide inhomogeneity of high speed steel is one of the important quality indexes of this steel. In order to reduce or prevent the occurrence of such defects, metallurgical plants and user plants are constantly taking measures, such as using factory forging process to homogenize the structure. When the improvement of carbide heterogeneity is limited, lower quenching heating temperature can be used to avoid overheating on the premise of ensuring hardness.
4. W18Cr4V Steel mould was heated in high temperature salt bath and oil cooled, and cracks were found. From the crack characteristics, it is caused by too fast cooling. Due to the large section of the workpiece and the large temperature difference between the inside and outside during cooling, when the surface is transformed into martensite, the interior is still in the state of austenite, which is gradually transformed into martensite in the subsequent cooling process, resulting in the cracking of the surface under great tensile stress due to the expansion of internal volume. Therefore, it can be judged as quenching crack.

Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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@steeljrv.com

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