Thermal Stress and Its Classification

Thermal stresses can be broadly categorized into two types: thermal stresses and microstructural stresses. The distortion that occurs during heat treatment of a workpiece is the result of the combined effects of these two types of stresses. The state in which thermal stresses exist within a workpiece and their associated effects vary depending on the specific circumstances. Internal stresses caused by uneven heating or cooling are referred to as thermal stresses; internal stresses arising from the non-uniformity of phase transformations are called microstructural stresses. Additionally, internal stresses induced by the non-uniformity of microstructural transformations within the workpiece itself are known as residual stresses. The final stress state and magnitude of a workpiece after heat treatment—determined by the sum of thermal stresses, microstructural stresses, and residual stresses—are collectively referred to as residual stresses.

Thermal stresses can be broadly categorized into two types: thermal stresses and structural stresses. The distortion that occurs during heat treatment of a workpiece is the result of the combined effects of these two types of stresses. The state in which thermal stresses exist within a workpiece and their respective effects vary depending on the situation. Internal stresses caused by uneven heating or cooling are referred to as thermal stresses; internal stresses arising from the non-uniformity of phase transformations are called structural stresses. Additionally, internal stresses induced by the non-uniformity of microstructural transformations within the workpiece itself are known as residual stresses. The final stress state and magnitude of a workpiece after heat treatment—determined by the sum of thermal stresses, structural stresses, and residual stresses—are collectively referred to as residual stresses.

The distortion and cracking that occur during the heat treatment of workpieces are the result of the combined effects of these internal stresses. At the same time, under the influence of thermal treatment stresses, certain parts of the workpiece may be subjected to tensile stress while other parts are under compressive stress; in some cases, the distribution of stress states within the workpiece can become extremely complex. In such situations, it is essential to analyze the specific circumstances on a case-by-case basis.

1. Thermal stress

Thermal stress is the internal stress that arises during heat treatment due to uneven thermal expansion and contraction between the surface and the core of a workpiece, or between thin and thick sections, caused by differences in heating or cooling rates. Generally, the faster the heating or cooling rate, the greater the thermal stress produced.

2. Residual stress

The internal stress arising from the time-inconsistent volumetric changes caused by phase transformations is called structural stress, also known as transformation stress. Generally, the greater the volumetric change before and after a structural transformation, and the larger the time difference between the transformation of various regions, the greater the resulting structural stress will be.

3. Additional stress

During the heat treatment process, in addition to thermal stresses and microstructural stresses, internal stresses—known as residual stresses—can also arise due to the non-uniformity of microstructure between the surface and the core of the workpiece, as well as inconsistencies in elastic deformation within the workpiece. For example, carburization or decarburization on the workpiece surface, surface hardening or local hardening, and other factors that lead to microstructural non-uniformity between the surface and the core can all induce residual stresses in the vicinity of the heat-treated area.

( 1 Additional stresses generated during surface hardening or local hardening.   

During local quenching or surface quenching (such as induction quenching, flame quenching, and laser quenching), a martensitic structure forms only in the quenched areas, while the unquenched regions retain their original microstructure, resulting in a difference in specific volume throughout the entire workpiece. At this point, the expansion caused by the increased specific volume due to the martensite layer on the workpiece surface is constrained by the core region, leading to compressive stress on the surface and tensile stress in the core.

( 2 ) Carburizing and quenching Additional stress formed at the time   

During the quenching of carburized workpieces, since the surface layer has a higher carbon content while the core has a lower carbon content (the original carbon content of the steel), the phase transformation temperatures at the surface and in the core differ—specifically, the transformation temperature at the surface is... Ms. The transformation temperatures differ between the surface and the core (the surface has a lower transformation temperature than the core). Consequently, the internal structure undergoes transformation and expands first. At this stage, the surface microstructure remains austenitic and is still in a plastic state. Initially, the surface is subjected to tensile stress, while the core experiences compressive stress. Due to the excellent plasticity of the surface layer, it readily undergoes plastic deformation under tensile stress, leading to stress relaxation—i.e., the stress value at the surface decreases somewhat. Later, as the high-carbon surface layer also undergoes martensitic transformation and expands, the stress distribution between the surface and the core reverses: the surface now experiences compressive stress, while the core experiences tensile stress.

4. Residual stress

During heat treatment, as long as a phase transformation is involved, both thermal stresses and microstructural stresses will arise simultaneously. The final stress state of the workpiece depends on the sum of thermal stresses, microstructural stresses, and any additional stresses. The internal stresses that remain after heat treatment are referred to as residual stresses. These can be categorized into residual tensile stresses (indicated by “+” indicates) and residual compressive stress (in “ – ” Indicates).