In the field of modern manufacturing, heat treatment is a key process for enhancing the strength, hardness, and other properties of metallic materials.

However, during heat treatment, temperature changes and phase transformation reactions generate internal stresses in the material.

These internal stresses can easily cause deformation or even cracking, severely affecting the dimensional accuracy and assembly performance of components.

Currently, research on heat treatment deformation has largely focused on single factors, lacking a mechanistic analysis of the coupled effects of multiple factors; consequently, the effectiveness of deformation control in actual production remains limited.

In light of this, this paper integrates heat conduction theory with the principles of phase transformation kinetics to conduct a systematic analysis of the mechanisms underlying heat treatment deformation.

By identifying key influencing factors and establishing a multidimensional control system, this study aims to provide theoretical references and practical guidance for addressing deformation challenges in actual production processes.

Analysis of the Mechanism of Heat Treatment Deformation

• Mechanism of Thermal Stress Deformation

Thermal stress refers to the internal stress generated when thermal expansion and contraction are impeded by uneven temperature distribution during the heating or cooling of a material.

During the heating phase of heat treatment, the surface temperature of the part rises rapidly, while the internal temperature remains relatively low; therefore, thermal expansion occurs first at the surface.

However, because internal expansion is constrained, compressive stress develops at the surface, while tensile stress develops internally.

During the cooling phase, the surface cools rapidly and contracts significantly, while the interior cools more slowly and contracts less.

Consequently, the surface experiences tensile stress due to internal resistance, while the interior experiences compressive stress.

The magnitude of thermal stress is closely related to factors such as the temperature gradient, the material’s coefficient of thermal expansion, and the modulus of elasticity.

Specifically, the greater the temperature gradient, the more pronounced the difference in thermal expansion and contraction, and the greater the resulting thermal stress.

The higher the material’s coefficient of thermal expansion, the greater the volume change during temperature fluctuations, and the more stress accumulates.

When thermal stress exceeds the material’s yield strength, the part undergoes plastic deformation; when it exceeds the fracture strength, the part may crack.

For example, during rapid cooling, the temperature difference between the surface and the core of high-carbon tool steel can reach several hundred degrees Celsius, making it highly susceptible to deformation or even cracking due to excessive thermal stress.

• Mechanism of Microstructural Stress Deformation

Microstructural stress refers to the internal stress generated during heat treatment when a material undergoes a phase transformation.

This stress arises from volume changes before and after the transformation due to differences in the specific volumes of the various phases, which are further exacerbated by constraints imposed by the part itself or external factors.

Metallic materials undergo a wide variety of phase transformations, including pearlite transformation, bainite transformation, and martensite transformation, and the specific volume changes associated with each transformation vary.

Among these, the specific volume change during martensite transformation is the most significant.

When austenite transforms into martensite, its specific volume increases by approximately 4% to 5%, which is a key factor causing deformation in high-hardness materials during heat treatment.

During the phase transformation process, if the transformation occurs in a localized area of the component, the volume change in that region is constrained by the surrounding areas that have not yet transformed, thereby generating internal stresses.

For example, in the quenching process, the component’s surface first transforms from austenite to martensite, causing volume expansion.

This expansion is constrained by the internal austenitic structure, resulting in compressive stress on the surface.

As cooling continues, a phase transformation and volume expansion also occur within the part.

At this point, the surface has already formed martensite, which has a higher hardness; therefore, the internal expansion induces tensile stress on the surface.

The direction and magnitude of structural stress depend on the sequence of phase transformations, the specific volume of the transformation products, and the constraints.

When structural stress and thermal stress combine, if the total stress exceeds the material’s load-bearing capacity, deformation will occur.

• Coupling Effect of Thermal Stress and Microstructural Stress

In actual heat treatment processes, thermal stress and microstructural stress do not act independently; rather, they overlap and influence each other, ultimately leading to deformation of the component.

The dominant role of these two stresses varies across different stages of heat treatment.

For example, during the heating stage, thermal stress plays the primary role, while microstructural stress is relatively minor.

In the early stages of cooling, thermal stress remains dominant.

As the temperature continues to drop, phase transformations begin to occur, and structural stress gradually increases.

During the low-temperature stage, structural stress may become the dominant stress.

The coupled action of these two stresses makes the stress state of the component more complex.

For instance, during the intermediate-temperature stage of quenching cooling, thermal stress may cause the component’s surface to experience tensile stress while the interior experiences compressive stress, whereas structural stress may cause the surface to experience compressive stress and the interior to experience tensile stress.

These two stresses offset each other, resulting in a reduction in the total stress.

During the low-temperature stage of cooling, thermal stress and microstructural stress may superimpose in the same direction, causing the total stress to increase sharply; at this point, the part is most susceptible to deformation.

Additionally, numerous factors such as the part’s shape, dimensions, and heat treatment process parameters can influence the superposition of these two stresses, thereby affecting the final degree of deformation.

Major Factors Affecting Heat Treatment Deformation

• Material Factors

The chemical composition and initial microstructure of the material have a critical impact on heat treatment deformation:

1) In terms of chemical composition, carbon content is a key factor affecting heat treatment deformation.

The higher the carbon content, the greater the specific volume change during martensitic transformation, the higher the microstructural stress, and the more pronounced the tendency for deformation.

The type and content of alloying elements also affect phase transformation temperature, phase transformation rate, and thermal expansion coefficient.

For example, while elements such as chromium and molybdenum can improve the material’s hardenability, they may also increase microstructural stress.

Regarding the initial microstructure, if the material exhibits significant microstructural non-uniformity before heat treatment—such as segregation, banding, or network carbides—this can lead to uneven temperature distribution and inconsistent phase transformations during heating and cooling.

Consequently, stress concentration is exacerbated, increasing the risk of deformation.

For example, cast steel components with coarse grains and segregation in their initial microstructure are highly prone to severe deformation if subjected to direct quenching.

Additionally, the material’s mechanical properties, such as hardness and strength, also influence its resistance to deformation.

The higher the hardness and strength, the poorer the material’s ductility, making it more prone to elastic deformation or cracking under stress; conversely, materials with good ductility can release some stress through plastic deformation, thereby reducing the final amount of deformation.

• Process Factors

1. Heating Process

Heating parameters such as heating temperature, heating rate, and holding time directly affect the generation and accumulation of thermal stress.

If the heating temperature is too high, the grain size of the material will increase, its mechanical properties will deteriorate, and the extent of thermal expansion and contraction will increase, thereby leading to higher thermal stress.

If the heating rate is too fast, the temperature difference between the surface and the interior of the part will widen rapidly, causing an instantaneous rise in thermal stress and potentially triggering premature plastic deformation.

For example, when heating high-speed steel cutting tools, a heating rate exceeding 10 °C/min can result in excessive thermal stress, causing deformation of the cutting edge.

Conversely, an excessively long holding time may lead to problems such as coarse grain structure and oxidation-decarburization, which similarly affect the deformation of the cutting tool.

2. Cooling Process

The cooling process plays a central role in heat treatment deformation.

Factors such as cooling rate, cooling medium, and cooling method all have a direct impact on the distribution of thermal and microstructural stresses:

1) Regarding cooling rate, when cooling is too rapid, thermal stress increases sharply, while the accelerated martensitic transformation rate leads to increased microstructural stress; the superposition of these two stresses can easily trigger deformation.

If the cooling rate is too slow, the desired microstructure and properties cannot be achieved, and the phase transformation process may be prolonged with slow stress release.

Although the amount of deformation is relatively small in this case, it still requires control.

2) Different cooling media possess varying cooling capacities, which in turn affect deformation differently.

For example, water has a strong cooling capacity and is suitable for materials requiring rapid quenching, but it carries a relatively higher risk of deformation; oil has a weaker cooling capacity, and while the risk of deformation is relatively lower, it lacks sufficient quenching depth; aqueous polymer solutions allow for adjustment of cooling capacity by varying concentration, balancing the requirements for hardness and deformation control.

3) Regarding cooling methods, uniform cooling can reduce temperature distribution irregularities and lower thermal stress.

Specific methods include circulation cooling and spray cooling.

If cooling is uneven—such as when a part comes into contact with the cooling medium too rapidly in certain areas—it can lead to localized stress concentration and cause irregular deformation.

3. Heat Treatment Methods

Different heat treatment methods result in different deformation patterns.

The quenching process involves rapid cooling and intense phase transformations, making it the heat treatment method that causes the most severe deformation.

The tempering process is performed at relatively low temperatures, and its primary function is to relieve quenching stresses and reduce deformation.

However, if the tempering temperature is too high or the holding time is insufficient, residual stresses may not be fully released, potentially leading to secondary deformation;

Normalizing and annealing processes involve slower cooling rates and more gradual phase transformations, resulting in relatively minor deformation;

However, they require strict control over the uniformity of heating and cooling; composite heat treatment processes, such as “quenching + tempering + aging,” can effectively reduce total deformation through multi-stage stress relief and microstructural stabilization.

• Part Geometry and Tooling Factors

The geometric shape and dimensions of a part are key external factors influencing heat treatment deformation.

Parts with complex geometries—such as those featuring holes, grooves, and steps—are highly prone to uneven temperature distribution during heating and cooling, resulting in significant stress concentrations and more complex deformation patterns.

In contrast, slender shafts and thin-walled parts have relatively low stiffness and are susceptible to deformation such as bending and warping under stress.

Uneven wall thickness in parts can also lead to variations in cooling rates, with thicker sections cooling more slowly and thinner sections cooling more rapidly; the resulting stress differences can induce deformation.

For example, the crankpin and main journal of an engine crankshaft have significantly different wall thicknesses, making them prone to twisting deformation during quenching due to uneven stress distribution.

The design and usage of fixtures and jigs also influence deformation.

If fixtures impose improper constraints on the part, they restrict normal thermal expansion and contraction as well as volume changes due to phase transformations, thereby exacerbating stress concentration.

Conversely, proper fixture design can guide the part to deform uniformly, reducing stress concentration.

For example, during the heating and cooling of the part, using elastic fixtures to provide some freedom of movement can release some of the stress; employing positioning fixtures can ensure that the part maintains a stable orientation during heat treatment, preventing deformation caused by its own weight or external forces.

Measures to Control Heat Treatment Distortion

• Appropriate Selection of Materials and Pre-treatment

The appropriate selection of materials is the foundation for controlling heat treatment distortion.

Based on the service requirements of the part, selecting materials with moderate carbon content and an appropriate balance of alloying elements can prevent excessive distortion caused by the material’s inherent phase transformation characteristics.

For precision parts, alloy steels with good hardenability and low deformation tendency, such as Cr12MoV and W18Cr4V, can be selected.

Prior to heat treatment, the material must undergo preliminary heat treatment to improve its initial microstructure.

Common preliminary treatment processes include annealing and normalizing.

Annealing eliminates casting and forging stresses, refines grain size, homogenizes the microstructure, and reduces segregation; normalizing not only enhances the material’s hardness and strength and improves machinability but also provides a uniform microstructural foundation for subsequent heat treatment.

For materials with defects such as surface cracks or inclusions, non-destructive testing and repairs must be performed in advance to prevent deformation from being exacerbated by defect propagation during heat treatment.

• Improving Heat Treatment Process Parameters

1. Optimizing the Heating Process

Using a slow heating method can reduce temperature gradients on the surface and inside the part, thereby minimizing thermal stress.

Manufacturers apply staged heating processes to large, complex parts. They first hold the part at a lower temperature for a period of time.

This allows the internal temperature to rise gradually. They then heat the part to the quenching temperature.

Engineers divide the heating process of high-carbon, high-chromium steel into three stages. They implement a preheating stage, a secondary preheating stage, and a final heating stage.

This approach effectively minimizes the accumulation of thermal stress.

Process engineers strictly control heating temperature and holding time. They prevent grain growth caused by excessively high temperatures or prolonged holding times.

Engineers set appropriate heating parameters based on material properties and part dimensions. When quenching medium-carbon structural steel, they set the temperature at Ac3+(30–50)°C.

They calculate the holding time based on the effective thickness of the part, typically 1.5 to 2.5 minutes per millimeter.

Manufacturers use controlled-atmosphere heating or vacuum heating. These methods reduce oxidation and decarburization of the material. They also prevent uneven stress distribution caused by changes in surface properties.

2. Optimizing the Cooling Process

Selecting the appropriate cooling medium and method allows for a balance between hardness requirements and distortion control.

Manufacturers use step quenching or isothermal quenching processes for parts with strict distortion requirements. In graded quenching, operators heat the part to the quenching temperature.

They then rapidly immerse it in a salt or oil bath held at a temperature slightly above the martensite transformation point (Ms point) for a period of time. This step equalizes the temperatures of the part’s surface and core.

Operators then air-cool the part to room temperature. This cooling reduces thermal and microstructural stresses.

Isothermal quenching, on the other hand, involves placing the part in an isothermal medium below the Ms point and holding it there until the phase transformation is complete, resulting in a lower bainite microstructure.

This process significantly reduces part deformation compared to conventional quenching.

Controlling the cooling rate prevents excessive cooling.

Precise control of the cooling rate can be achieved by adjusting the concentration and temperature of the cooling medium, or by employing methods such as spray cooling or air cooling.

For example, when using a polymer aqueous solution as the cooling medium, increasing its concentration reduces the cooling rate and minimizes the risk of deformation.

For complex parts, localized cooling can be employed to slow the cooling rate in areas prone to deformation while accelerating it in areas requiring high hardness.

3. Selecting the Appropriate Heat Treatment Method

It is crucial to select the appropriate heat treatment method based on the performance requirements and deformation sensitivity of the part.

For parts with high precision requirements and high deformation sensitivity, a combined process of “quenching + high-temperature tempering + low-temperature tempering” can be used.

This involves first eliminating most of the quenching stresses through high-temperature tempering, followed by low-temperature tempering to stabilize the microstructure and dimensions.

For parts subjected to impact loads, quenching and tempering can be performed to minimize deformation while ensuring both strength and toughness.

Additionally, surface heat treatment processes such as induction hardening and carburizing can impart high surface hardness to the parts while maintaining good plasticity and toughness in the core, thereby reducing overall deformation.

• Optimizing Part Structure and Fixture Design

Designers consider all heat treatment process requirements during the part design phase.

Engineers prioritize symmetrical designs in structural optimization.
They avoid conditions that easily cause stress concentration, including uneven wall thickness, sharp corners, and right angles.

Designers convert sharp corners into rounded corners or chamfers to reduce stress concentration.

Engineers enhance the deformation resistance of low-rigidity parts such as slender shafts and thin plates. They achieve this by adding ribs or using composite structures.

Designers arrange holes, slots, and other features rationally.
They prevent uneven temperature distribution caused by structural asymmetry.

Properly designed fixtures and jigs can provide uniform restraint and support for parts.

For circular parts, expansion-type fixtures can be used to ensure precise centering during heat treatment and reduce radial deformation;

Slender shaft-type parts can use multi-point support fixtures to prevent bending deformation caused by self-weight.

Specialized fixtures ensure uniform force distribution on complex-shaped parts during heat treatment.
This design limits harmful deformation in the part.

Engineers select fixture materials with a coefficient of thermal expansion similar to that of the part. This selection minimizes stresses between the fixture and the part caused by temperature changes.

• Application of Pre-deformation and Correction Treatments

Pre-deformation treatment involves applying a counter-deformation to a part prior to heat treatment, based on the part’s inherent deformation characteristics.

The goal is to ensure that the deformation resulting from heat treatment offsets the pre-deformation, thereby meeting dimensional accuracy requirements.

Determining the magnitude of pre-deformation requires extensive experimental data, as well as precise calculations of the counter-deformation’s magnitude and direction, taking into account the part’s material, structure, and heat treatment process.

Slender shafts prone to bending after quenching receive a reverse bending force applied with a press before heat treatment.

This action induces a controlled degree of pre-deformation. The pre-deformation compensates for the bending deformation that occurs after quenching.

Slightly deformed parts after heat treatment undergo corrective treatment. The corrective treatment restores dimensional accuracy.

Common corrective methods include mechanical correction and heat treatment.

Mechanical correction applies external force using equipment such as presses and straightening machines to induce reverse plastic deformation in the part, thereby eliminating the original deformation.

Heat treatment correction, on the other hand, utilizes the material’s elastic recovery properties by first heating the part to a certain temperature and applying constraints, then allowing the part to return to its original state during the cooling process.

For gears that have deformed after quenching, the tooth-top pressure correction method can be employed.

This involves heating the gear to 200–300 °C, applying pressure to the tooth tops, holding for a certain period, and then cooling it to achieve the desired correction of deformation.

Conclusion

This paper presents a systematic study of the mechanisms of heat treatment deformation and control measures.

The essence of heat treatment deformation lies in the uneven distribution of thermal stresses and microstructural stresses, as well as their combined effects.

Thermal stresses arise from thermal expansion and contraction caused by uneven temperature distribution and constrained movement, while microstructural stresses result from the restriction of volume changes during phase transformations.

The dominant role of these two types of stress varies across different stages of heat treatment, and together they influence the degree of part deformation.

Material properties, process parameters, part geometry, and fixture design are the key factors affecting heat treatment deformation.

The chemical composition and initial microstructure of the material determine its deformation tendency.

Heating, cooling processes, and heat treatment methods directly influence the generation and accumulation of stress;

While the rationality of part geometry and the scientific design of fixtures affect the distribution and release of stress.

Appropriate selection of materials and pretreatments provides the foundation for controlling heat treatment deformation.

Refining heat treatment process parameters reduces the generation and accumulation of internal stress. Optimizing part geometry and fixture design improves stress distribution and release during processing.

Employing pre-deformation and corrective treatments compensates for expected and existing deformation.

These measures effectively control heat treatment deformation and improve the dimensional accuracy of parts.