The changes in volume and shape of the workpiece during heat treatment are caused by the volume expansion caused by the specific volume change during the structural transformation in the steel, as well as the plastic deformation caused by heat treatment stress. Therefore, the greater the heat treatment stress and the more uneven the phase transformation, the greater the deformation, and vice versa. To reduce deformation, efforts must be made to reduce quenching stress and improve the yield strength of the steel.

The influence of chemical composition on heat treatment deformation

The chemical composition of steel affects the heat treatment deformation of workpieces by influencing the yield strength, Ms point, hardenability, specific volume of microstructure, and residual austenite content of the steel.

The carbon content of steel directly affects the specific volume of various microstructures obtained after heat treatment (the relationship between specific volume of different microstructures at room temperature and carbon content - abbreviated, the relationship between carbon content of carbon steel and Ms point and residual austenite - abbreviated). As the carbon content of steel increases, the volume of martensite increases and the yield strength increases. The increase in hardenability and martensite specific volume increases the stress and heat treatment deformation of the quenched microstructure. The increase in residual austenite content and yield strength reduces the specific volume change, leading to a decrease in tissue stress and a reduction in heat treatment deformation. The influence of carbon content on the deformation of workpieces during heat treatment is the result of the combined effect of the aforementioned contradictory factors.

The influence of carbon content on the volume change during quenching(sample size: ￠25 * 100)

The quenching deformation trend of the 08 steel sample is to shorten the length, increase the diameter in the middle of the sample, and reduce the diameter at the end, forming a waist drum shape. This is because although the Ms point of low-carbon steel is high, when martensitic transformation occurs, the yield strength of the steel is low, the plasticity is good, and it is easy to deform. However, due to the small volume of martensite, the tissue stress is not large, and it will not cause large plastic deformation. On the contrary, the deformation caused by thermal stress is relatively large, ultimately manifested as thermal stress deformation.

The microstructure stress became the dominant factor causing deformation in the 55 steel specimen, resulting in a reduction in the middle diameter, an increase in the end diameter, and an increase in length.

When the mass fraction of carbon further increases to above 0.8%, due to the decrease in Ms point and the increase in residual austenite content, its deformation becomes a thermal stress type deformation with shortened length and increased diameter. And due to the increase in yield strength of high carbon steel, its deformation is smaller than that of medium carbon steel. For carbon steel, in most cases, the deformation of T7A steel is the smallest. When the mass fraction of carbon is greater than 0.7%, it tends to shrink; But when the mass fraction of carbon is less than 0.7%, both the inner and outer diameters tend to expand.

Generally speaking, in the case of complete quenching, due to the higher Ms point of carbon steel compared to alloy steel, its martensitic transformation begins at higher temperatures. Due to the good plasticity of steel at higher temperatures and the relatively low yield strength of carbon steel itself, carbon steel parts with inner holes (or cavities) tend to deform more and the inner holes (or cavities) tend to swell. Due to its high strength, low Ms point, and high residual austenite content, alloy steel has relatively small quenching deformation, mainly manifested as thermal stress deformation, and its internal holes (or cavities) tend to shrink. Therefore, when quenched under the same conditions as medium carbon steel, high carbon steel and high alloy steel workpieces often experience mainly inner hole shrinkage.

The influence of alloying elements on the heat treatment deformation of workpieces is mainly reflected in their influence on the Ms point and hardenability of the steel. Most alloying elements, such as manganese, chromium, silicon, nickel, molybdenum, boron, etc., lower the Ms point of the steel, increase the amount of residual austenite, reduce the specific volume change and microstructure stress during quenching, and thus reduce the quenching deformation of the workpiece. The alloying elements significantly improve the hardenability of steel, thereby increasing the volume deformation and structural stress of the steel, leading to an increase in the tendency of workpiece heat treatment deformation. In addition, due to the improved hardenability of steel by alloying elements, the critical quenching cooling rate is reduced. In actual production, a mild quenching medium can be used for quenching, thereby reducing thermal stress and reducing the heat treatment deformation of the workpiece. Silicon has little effect on the Ms point and only has a reducing effect on the deformation of the sample. Tungsten and vanadium have little effect on the hardenability and Ms point, and have little effect on the deformation of the workpiece during heat treatment. Therefore, the so-called micro deformed steel in industry contains a large amount of alloy elements such as silicon, tungsten, and vanadium.

The influence of original organization and stress state on heat treatment deformation

The original structure of the workpiece before quenching, such as the morphology, size, quantity, and distribution of carbides, segregation of alloy elements, and fiber direction formed by forging and rolling, all have a certain impact on the heat treatment deformation of the workpiece. Spherical pearlite has a larger volume and higher strength than flake pearlite, so the quenching deformation of the workpiece after pre spheroidization treatment is relatively small. For some high carbon alloy tool steels, such as 9Mn2V, CrWMn, and GCr15 steel, the spheroidization level has a significant impact on the correction of heat treatment deformation cracking and quenching deformation, and it is usually advisable to use a 2.5-5 level spheroidized structure. Quenching and tempering not only reduces the absolute value of deformation of the workpiece, but also makes the quenching deformation of the workpiece more regular, which is beneficial for controlling deformation.

The distribution of strip carbides has a significant impact on the heat treatment deformation of workpieces. After quenching, the workpiece expands parallel to the direction of the carbide strip, and shrinks in the direction perpendicular to the carbide strip. The coarser the carbide particles, the greater the expansion in the direction of the strip. For martensitic steels such as Cr12 type steel and high-speed steel, the morphology and distribution of carbides have a particularly significant impact on quenching deformation. Due to the small coefficient of thermal expansion of carbides, which is about 70% of the matrix, during heating, the carbides with smaller expansion along the strip direction inhibit the elongation of the matrix, while during cooling, the carbides with smaller shrinkage hinder the shrinkage of the matrix. Due to the slow heating temperature of austenitization, the inhibitory effect of carbides on basic expansion is weak. Therefore, the directional effect of carbides distributed in strips on the quenching and heating deformation of workpieces is relatively small. However, during quenching and cooling, due to the fast cooling rate, the inhibitory effect of carbides on matrix shrinkage increases, resulting in a significant elongation along the direction of carbide strips after quenching.

Materials that have been rolled and forged exhibit different heat treatment deformation behaviors along different fiber directions. The difference in size variation along the longitudinal and transverse directions is relatively small for normalized specimens with unclear fiber orientation. When there is a clear banded structure in the annealed sample, the size changes along the fiber direction and perpendicular to the fiber direction are significantly different. When the forging ratio is large and the fiber direction is obvious, the size change rate of the longitudinal specimen along the fiber direction is greater than that of the transverse specimen perpendicular to the fiber direction.

When there are network carbides in hypereutectoid steel, a large amount of carbon and alloy elements are enriched near the network carbides. In areas far from the network carbides, the carbon and alloy elements are lower, resulting in increased quenching microstructure stress, increased quenching deformation, and even cracking. Therefore, the network carbides in hypereutectoid steel must be eliminated through appropriate pre heat treatment.

In addition, the macroscopic segregation of steel ingots often results in square segregation on the cross-section of the steel material, which often leads to uneven quenching deformation of disc-shaped parts. In short, the more uniform the original structure of the workpiece, the smaller the heat treatment deformation, the more regular the deformation, and the easier it is to control.

The stress state of the workpiece itself before quenching has a significant impact on deformation. Especially for workpieces with complex shapes that have undergone high feed rate cutting, if the residual stress is not eliminated, it has a significant impact on quenching deformation.

The influence of workpiece geometry on heat treatment deformation

Workpieces with complex geometric shapes and asymmetric cross-sectional shapes, such as shafts with keyways, keyway cutters, tower shaped workpieces, etc., experience uneven cooling when quenched and cooled, with one side dissipating heat quickly and the other side dissipating heat slowly. If the deformation caused by uneven cooling above Ms is dominant, the side with faster cooling is concave. If the deformation caused by uneven cooling below Ms is dominant, the side with faster cooling is convex. Increasing the isothermal time, increasing the bainite transformation variable, making residual austenite more stable, reducing the amount of martensite transformation in air cooling, can significantly reduce the deformation of the workpiece.

The influence of process parameters on heat treatment deformation

Whether it is conventional heat treatment or special heat treatment, heat treatment deformation may occur. When analyzing the influence of heat treatment process parameters on heat treatment deformation, the most important thing is to analyze the effects of heating and cooling processes. The main parameters of the heating process are the uniformity of heating, heating temperature, and heating speed. The main parameters of the cooling process are the uniformity of cooling and the cooling speed. The impact of uneven cooling on quenching deformation is the same as that caused by asymmetric cross-sectional shape of the workpiece. This section mainly discusses the influence of other process parameters.

Deformation caused by uneven heating - excessive heating speed, uneven temperature in the heating environment, and improper heating operation can all cause uneven heating of the workpiece. The uneven heating has a significant impact on the deformation of slender or thin workpieces. The term non-uniform heating here does not refer to the inevitable temperature difference between the surface and center of the workpiece during the heating process, but specifically refers to the temperature gradient in various parts of the workpiece due to various reasons. In order to reduce deformation caused by uneven heating, for high alloy steel workpieces with complex shapes or poor thermal conductivity, slow heating or preheating should be used. However, it should be pointed out that although rapid heating can lead to an increase in deformation of long axis workpieces and thin plate shaped parts. However, for workpieces with mainly volumetric deformation, rapid heating often plays a role in reducing deformation. This is because when only the working part of the workpiece requires quenching and strengthening, rapid heating can keep the center of the workpiece in a low temperature and high strength state, and the working part can reach the quenching temperature. This high-strength core can prevent significant deformation of the workpiece after quenching and cooling. In addition, fast heating can use higher heating temperatures and shorter heating and insulation time can reduce the deformation caused by the weight of the workpiece during prolonged stays at high temperatures. Rapid heating only reaches the phase transition temperature in the surface and local areas of the workpiece, which correspondingly reduces the volume change effect after quenching, which is also beneficial for reducing quenching deformation.

The influence of heating temperature on deformation - Quenching heating temperature affects quenching deformation by changing the temperature difference during quenching cooling, changing the hardenability, Ms point, and the amount of residual austenite. Raising the quenching heating temperature increases the residual austenite content, lowers the Ms point, reduces the deformation caused by structural stress, and tends to shrink the hole cavity of sleeve type workpieces. However, on the other hand, the increase in quenching heating temperature enhances the hardenability, increases the temperature difference during quenching cooling, increases thermal stress, and has a tendency to cause internal hole expansion. Practice has shown that for low-carbon steel workpieces, if the inner hole shrinks after quenching at normal heating temperature, increasing the quenching heating temperature will result in greater shrinkage. In order to reduce shrinkage, the quenching heating temperature should be lowered; For workpieces made of medium carbon alloy steel, if the inner hole expands after quenching at normal heating temperature, increasing the quenching heating temperature will cause greater expansion. In order to reduce the expansion of the hole cavity, it is also necessary to lower the quenching heating temperature. For Cr12 type high alloy mold steel, increasing the quenching heating temperature leads to an increase in residual austenite content and a tendency to reduce the pore size.

The effect of quenching cooling speed on deformation - Generally speaking, the more intense quenching cooling, the greater the temperature difference between the inside and outside of the workpiece and different parts (parts with different cross-sectional sizes), the greater the internal stress generated, leading to an increase in heat treatment deformation. The deformation of hot die steel specimens (150 length * 100 width * 50 height) after quenching and tempering at different cooling rates. The cooling speed of the three media is the fastest with oil cooling, followed by hot bath cooling, and the slowest with air cooling. After being quenched at three different cooling rates, the length and width of the workpiece tend to shrink, with little difference in deformation amount; However, the deformation caused by air cooling and hot bath quenching with slow cooling speed in the thickness direction is much smaller, with a deformation expansion of 0.05%, while oil quenching undergoes shrinkage deformation, with a maximum deformation of about 0.28%. However, when the change in cooling rate causes a change in the phase transformation of the workpiece, an increase in cooling rate does not necessarily lead to an increase in deformation, sometimes it can actually reduce deformation. For example, when low-carbon alloy steel undergoes shrinkage due to the presence of a large amount of ferrite in the center after quenching, increasing the quenching cooling rate to obtain more bainite in the center can effectively reduce shrinkage deformation. On the contrary, if the workpiece swells due to the martensite obtained in the center after quenching, reducing the cooling rate to reduce the relative amount of martensite in the center can also reduce the swelling. The effect of quenching cooling rate on quenching deformation is a complex problem, but the principle is to minimize the quenching cooling rate while ensuring the required microstructure and properties.

The influence of aging and cold treatment on heat treatment deformation - For precision parts and measuring tools, in order to maintain accuracy and dimensional stability during long-term use, it is often necessary to undergo cold treatment and tempering to make their structure more stable. Therefore, understanding the deformation laws of tempering process and cold treatment on workpieces during aging is of great significance for improving the heat treatment quality of such workpieces. Cold treatment transforms residual austenite into martensite, leading to volume expansion; Low temperature tempering and aging, on the one hand, promote the precipitation of ∈ - carbides and the decomposition of martensite, causing volume shrinkage, and on the other hand, cause a certain degree of stress relaxation, resulting in shape distortion of the workpiece. The chemical composition of steel, tempering temperature, and aging temperature are the main factors affecting the working deformation during the aging process.

Deformation of Carburized Workpieces - Carburized workpieces are usually made of low-carbon steel and low-carbon alloy steel, with an original structure of ferrite and a small amount of pearlite. According to the service requirements of the workpiece, after carburization, the workpiece needs to be directly quenched, slowly cooled, reheated, quenched, or quenched again. The carburized workpiece undergoes deformation during the slow cooling and carburizing quenching processes after carburization due to the effects of structural and thermal stresses. The size and deformation pattern of the deformation depend on factors such as the chemical composition of the carburized steel, the depth of the carburized layer, the geometric shape and size of the workpiece, and the heat treatment process parameters after carburization and carburization.

Workpieces can be divided into slender parts, flat parts, and cubic parts based on their relative dimensions of length, width, height (thickness). The length of a slender piece is much larger than its cross-sectional size, the length and width of a flat piece are much larger than its height (thickness), and the dimensions in the three directions of a cube are not significantly different. The maximum internal stress during heat treatment is generally generated in the direction of maximum size. If this direction is referred to as the dominant stress direction, workpieces made of low-carbon steel and low-carbon alloy steel generally exhibit shrinkage deformation along the dominant stress direction when ferrite and pearlite are formed in the core after carburization and slow cooling or air cooling, with a shrinkage deformation rate of about 0.08-0.14%. As the content of alloying elements in steel increases and the cross-sectional size of the workpiece decreases, the deformation rate also decreases, and even swelling deformation occurs.

Slender rods with significant differences in cross-sectional thickness and asymmetric shapes are prone to bending deformation after carburization and air cooling. The direction of bending deformation depends on the material. The thin section of low-carbon steel carburized workpieces with fast cooling is often concave on one side. However, for low-carbon alloy steel carburized workpieces with higher alloy elements such as 12CrN3A and 18CrMnTi, the thin section side with fast cooling is often convex.

After carburizing at temperatures of 920-940C, the mass fraction of carbon in the carburized layer of workpieces made of low-carbon steel and low-carbon alloy steel increases to 0.6-1.0%. The high carbon austenite in the carburized layer needs to be undercooled below Ar1 (around 600C) during air cooling or slow cooling before it begins to transform into pearlite. The low-carbon austenite in the center begins to precipitate ferrite at around 900C, and the remaining austenite undergoes eutectoid decomposition and transformation into pearlite below Ar1 temperature. From the undercooling of the carburizing temperature to the Ar1 temperature, the carburized layer of the eutectoid component did not undergo phase transformation, while the high carbon austenite only experienced thermal shrinkage with the decrease of temperature. At the same time, the low carbon austenite in the center expanded due to the increase in the volume ratio of ferrite precipitation, resulting in compressive stress in the center and tensile stress in the carburized layer. Due to cardiac events γ->α During the transformation, the effect of phase change stress reduces its yield strength, leading to compressive deformation at the center. Low carbon alloy steel has higher strength and smaller compressive plastic deformation at the center under the same conditions.

When carburized workpieces with asymmetric shapes are air-cooled, the shrinkage of the austenite line length on the side with fast cooling is greater than that on the side with slow cooling, resulting in bending stress. When the bending stress is greater than the yield strength on the side with slow cooling, the workpiece bends towards the side with fast cooling. For low-carbon alloy steel with high alloying element content, the surface layer after carburization has the composition of high carbon alloy steel. During air cooling, the side with fast cooling undergoes phase transformation, forming a new phase with higher hardness and larger specific volume of structure. On the other side, the new phase formed slowly due to cooling has lower hardness, resulting in opposite bending deformation.

The quenching deformation law of carburized workpieces can be analyzed using the same method. The quenching temperature of carburized parts is usually 800-820C. During quenching, the high carbon austenite in the carburized layer will undergo significant thermal shrinkage when cooled from the carburizing temperature to the Ms point temperature range. At the same time, the low carbon austenite in the center will transform into ferrite and pearlite, low-carbon bainite or low-carbon martensite. Regardless of the type of tissue it transforms into, the heart undergoes volume expansion due to an increase in tissue specific volume, resulting in significant internal stress in the carburized layer and the heart. Generally speaking, in the absence of quenching, due to the low yield strength of ferrite and pearlite phase transition products in the core, shrinkage deformation occurs in the direction of the dominant stress under the thermal shrinkage compressive stress of the carburized layer. When the phase transformation products in the core are a combination of high-strength low-carbon bainite and low-carbon martensite, the surface high carbon austenite undergoes plastic deformation under the action of core expansion stress, resulting in dominant stress direction and expansion.

With the increase of carbon content and alloy element content in carburized steel, the core hardness of carburized parts increases after quenching, and the tendency of dominant stress direction expansion increases. When the hardness of the core is 28-32HRC, the quenching deformation of the carburized workpiece is very small. As the hardness of the heart increases, the tendency for swelling and deformation increases. It is obvious that factors such as improving the hardenability of carburized parts, which lead to an increase in the hardness of the center of carburized parts, will increase the tendency of carburized parts to swell along the dominant stress direction.

The deformation of nitrided workpieces - nitriding can effectively improve the surface hardness and fatigue resistance of workpieces, and to some extent improve their corrosion resistance. The nitriding temperature is relatively low, about 510-560C. During the nitriding process of steel materials, the base metal does not undergo phase transformation, so the deformation of the nitrided workpiece is relatively small. Nitriding is generally the final process of heat treatment. After nitriding, besides high-precision workpieces, other mechanical processing is generally not carried out. Therefore, nitriding is widely used to treat precision parts that require high hardness and small deformation. However, the nitrided workpiece still undergoes deformation. Due to the infiltration of nitrogen atoms, the specific volume of the nitrided layer increases. Therefore, the most common deformation of the nitrided workpiece is the expansion of the workpiece surface. The expansion of the surface nitrided layer is hindered by the center, and the surface is subjected to compressive stress, while the center is subjected to tensile stress. The magnitude of internal stress is influenced by factors such as the cross-sectional size of the part, the yield strength of the nitrided steel, the nitrogen concentration and depth of the nitrided layer. When the cross-sectional size of the workpiece is small, the cross-sectional shape is asymmetric, and the furnace temperature and nitriding are uneven, the nitrided workpiece will also produce dimensional changes or shape distortions such as bending and warping deformation.

The deformation pattern of shaft parts after nitriding is that the outer diameter expands and the length elongates. The radial expansion usually increases with the increase of the workpiece diameter, but the maximum expansion does not exceed 0.055mm. The length elongation is generally greater than the radial expansion, and its absolute value increases with the length of the shaft, but does not change proportionally with the length of the shaft. The deformation of nitrided sleeve workpieces depends on the wall thickness. When the wall thickness is thin, both the inner and outer diameters tend to expand. As the wall thickness increases, the expansion decreases significantly. When the wall thickness is large enough, the inner diameter tends to shrink.

In general, when the effective cross-sectional size of the workpiece is greater than 50mm, the main deformation mode of nitriding treatment is surface expansion. But as the cross-sectional area of the workpiece decreases, when the ratio of the cross-sectional area of the nitrided layer to the central cross-sectional area is greater than 0.05 but less than 0.7, in addition to surface expansion, deformation caused by internal stress must also be considered. The amount of deformation along the dominant stress direction of the workpiece can be approximately estimated using empirical formulas: Δ L= η （ Ν/Κ)％

Δ L - The increase in the length of the dominant stress direction.

η---- The coefficient depends on the material and the shape of the cross-section of the nitrided workpiece.

Ν------ The cross-sectional area of the nitrided layer.

Κ---- The cross-sectional area of the heart.

Commonly used nitrided steel η value: