Stress relief annealing of steel and metals – temperature, process and application

Stress relief annealing Stress relief is a heat treatment process used to reduce residual stresses in a material after welding, casting, forging, rolling, machining, straightening, or other technological processes. Properly performed stress relief improves the dimensional stability of the component and reduces the risk of deformation during further processing or use.
The process involves heating the material to a temperature lower than that which induces the fundamental structural changes characteristic of other types of heat treatment, heating for an appropriate time, and then controlled cooling. The exact parameters depend on the material type, its condition, the component dimensions, the previous manufacturing technology, and the level and distribution of residual stresses.
Stress relief annealing is used for both carbon and alloy steels, as well as – with appropriate parameters – cast iron, aluminum alloys, copper, titanium, and other metallic materials. It is particularly important for welded structures, large forgings and castings, and precision-machined components, where even minor dimensional changes can have operational implications.

What is stress relief annealing?

Residual stresses can remain in a material despite the absence of external forces. They primarily arise when different areas of a component undergo different deformations, heat or cool at different rates, or undergo different structural changes.
Typical processes that cause residual stresses include:

• casting and uneven solidification of the material,
• welding and the associated local heating and shrinkage of the weld,
• rolling, forging, stamping, bending and straightening,
• drawing and other cold forming processes,
• machining, especially with large material removal,
• grinding and intensive surface treatment,
• hardening and other heat treatment processes,
• uneven heating or cooling of elements with different cross-sections.

During stress relief annealing, increasing temperature increases the potential for stress relaxation processes. The material remains at a given temperature for a specified period of time, allowing stresses to be significantly reduced without completely altering its microstructure.
This doesn't mean, however, that annealing always completely removes stresses. The degree of stress relief depends on factors such as temperature, soaking time, alloy type, material condition, and previous processing history.

Why is stress relief annealing performed?

The primary goal of the process is to increase the component's stability during further production and subsequent use. Residual stresses can lead to changes in the component's shape after milling, turning, drilling, or cutting the material. In extreme cases, they can also promote crack formation.
Stress relief annealing is used, among other things, to:

• reducing residual stresses after welding,
• limiting deformation during further machining,
• increasing the dimensional stability of precision parts,
• reducing stresses generated during forging, rolling or straightening,
• stabilization of large castings and forgings,
• preparation of the semi-finished product for subsequent technological operations,
• reducing the risk of uncontrolled changes in the shape of the finished element.

Stress relief annealing - process temperature

Temperature is one of the most important stress relief parameters. There is no single universal temperature suitable for all materials. The range must be selected based on the specific grade, its microstructure, delivery condition, and required post-treatment properties.
The following values ​​are indicative and should not replace the parameters specified for a specific type of material, standard, technological documentation or manufacturer's recommendations.

In industrial practice, the stress relief temperature must be considered in conjunction with the soak time. Increasing the temperature typically accelerates the stress relaxation process, but excessive temperatures can cause undesirable changes to the material's microstructure and properties.

Soaking time during annealing

The heating time depends on the material type, process temperature, component dimensions, cross-sectional thickness, and batch weight. The timing of the heating time measurement is also crucial – the component must reach the appropriate temperature in the required cross-sectional area.
Small and thin-walled components may require a relatively short heating time, while massive forgings, castings or welded structures require a much longer time for temperature equalization and effective stress relaxation.
For this reason, simple rules such as "one hour for a given thickness" should be considered only as technological guidelines. In the production of critical components, the parameters should be derived from appropriate heat treatment technology.

Heating the element

Controlled heating is particularly important for large components, complex shapes, and significant variations in cross-sectional thickness. Heating too rapidly can cause significant temperature differences between the surface and the core, thus introducing additional thermal stresses.
The larger the component and the more varied its cross-section, the more important uniform heat input becomes. For massive forgings, castings, and welded structures, the heating process should be part of the developed process technology.

Cooling after stress relief annealing

Cooling control is just as important as the heating and soaking process itself. If the stress-relieved component is cooled too quickly or unevenly, temperature differences between individual areas can re-introduce stress.
Depending on the material and dimensions of the component, cooling with the furnace to a specific temperature, controlled furnace cooling, or further cooling in still air are used. The cooling method should take into account both the required degree of stress reduction and the potential for structural changes.

Stages of stress relief annealing

1. Preparing the element – determining the type of material, its condition, dimensions and the cause of stresses.
2. Controlled heating – bringing the element to the required temperature while limiting excessive thermal gradients.
3. Temperature equalization – obtaining the appropriate temperature in the required cross-sectional area of ​​the element.
4. Soaking – maintaining the material at a given temperature for a time enabling appropriate stress reduction.
5. Controlled cooling – lowering the temperature in a way that limits the possibility of renewed stress formation.

The effect of stress relief annealing on material properties

Properly performed stress relief annealing should primarily reduce residual stresses while minimizing the impact on the material's basic properties. However, this does not mean that the mechanical properties always remain completely unchanged.

Hardness

In many steels, properly selected stress relief causes only a slight change in hardness. However, the effect of the process depends on the temperature and the previous condition of the material. In cold-hardened or heat-treated materials, increasing temperature can lead to partial loss of the hardening effect or microstructural changes.

Yield strength and tensile strength

Changes in yield strength and tensile strength depend on the material and process parameters. The purpose of stress relief is not to increase strength, but primarily to reduce residual stresses. However, excessively high temperatures or excessively long heating times can affect mechanical properties, especially in previously hardened materials.

Dimensional stability

One of the most important practical effects of stress relieving is increased dimensional stability. This is crucial in the production of machine frames, molds, dies, shafts, plates, large forgings, and other precision-machined components.
If a semi-finished product contains significant residual stresses, removing material during milling or turning can disrupt their balance and lead to bending or twisting of the machined part. Stress relief performed at the appropriate stage of the technological process can reduce this phenomenon.

Post-weld stress relief annealing

Weldments are among the most important applications of stress relief treatment. During welding, a small area of ​​the material is intensely heated, while the rest of the structure remains significantly cooler. Subsequent cooling and material shrinkage cause residual stresses to develop.
In appropriate materials and applications, post-weld heat treatment can reduce welding stresses and stabilize the structure. However, the parameters of such a process should not be selected solely based on the general temperature range; they must take into account the material grade, component thickness, joint type, and the requirements of relevant standards and process documentation.

Stress relief annealing after machining

Stress relieving can also be used between machining steps. It is particularly useful when producing large or precision parts that require significant material removal during roughing.
A typical process might involve roughing, stress relieving, and then finishing. This allows any distortion resulting from stress redistribution to occur before the final surface and dimensions are achieved.

Application of stress relief annealing

Welded structures

The process can be used in large frames, bodies, tanks, machine structures and other welded components where dimensional stability and reduced post-weld stresses are important.

Forgings and forged parts

Large forgings can contain stresses resulting from both plastic deformation and uneven cooling. Appropriate heat treatment can be one step in preparing the material for further machining.

castings

Stresses can develop in castings during solidification and cooling, particularly in components with varying wall thicknesses. Stress relieving is used, among other things, to increase the dimensional stability of castings and other large components.

Molds, dies and tools

In the case of molds and dies, dimensional stability directly impacts the accuracy of manufactured parts. Stress relief between machining steps can reduce deformation that occurs after intensive milling or other mechanical operations.

Energy and machinery industry

The process is also used in the production and repair of industrial installation components, housings, shafts, pipelines and other components if the technology of a given material provides for stress reduction by heat treatment.

Stress relief annealing and other types of annealing

Stress relief annealing should not be confused with normalizing, softening, recrystallizing, homogenizing, or spheroidizing annealing. Individual processes are conducted at different temperature ranges and are designed to achieve different technological effects.
In the case of stress relief, the main task is reduction of residual stresses with the smallest possible change in the structure and properties of the materialIn other annealing processes, however, changing the microstructure may be the primary goal of the treatment.
A comparison of individual processes is presented in more detail on the website comparison of steel heat treatment methods.

Advantages of stress relief annealing

• reduction of residual stresses,
• improving the dimensional stability of elements,
• limiting deformation during further machining,
• reducing the risk of deformation after welding, forging or casting,
• the ability to stabilize large and complex elements,
• preparation of the semi-finished product for precise finishing.

Limitations of stress relief annealing

• incorrect temperature may change the microstructure or properties of the material,
• the process does not always lead to complete removal of residual stresses,
• large components require a long and controlled heating and cooling cycle,
• parameters appropriate for one type of steel do not necessarily have to be appropriate for another,
• In some alloys, the effect of temperature on corrosion resistance, hardening state or previous heat treatment must be taken into account.

Stress relief annealing is an important heat treatment process used to reduce residual stresses generated during welding, casting, forging, rolling, straightening, machining, and other manufacturing processes. Properly selected temperature, soaking time, and heating and cooling methods increase the dimensional stability of the component and reduce the risk of subsequent deformation.
Stress relief parameters should always be selected based on the specific grade and condition of the material. High-alloy steels, steels that have been previously heat-treated, and non-ferrous alloys require particular caution, as excessively high temperatures can cause property changes beyond the primary purpose of the process.

Methods of heat and thermochemical treatment of steel

Softening is just one group of processes used to modify the microstructure and properties of steel. Depending on the desired effect, normalizing, stress relieving, hardening, tempering, and thermochemical processes such as carburizing and nitriding are also used.

Steel heat treatment methods

• Normalizing steel – grain refinement and microstructure unification.
• Stress relief annealing – reduction of residual stresses.
• Softening of steel – reducing hardness and improving susceptibility to further processing.
• Full annealing – austenitization and slow cooling.
• Isothermal annealing – transformation carried out while holding at a properly selected temperature.
• Ball annealing – spheroidizing – change in the morphology of cementite or other carbides.
• Recrystallization annealing – removal of the effects of crushing and restoration of plasticity.
• Homogenizing annealing – reducing segregation and homogenizing the material composition.
• Hardening and heat treatment – shaping the microstructure and properties by hardening and tempering.
• Tempering steel – controlled modification of steel properties after hardening.
• Induction hardening – surface induction heating and rapid cooling.
• Flame hardening – surface hardening after local heating with a flame.
• Oversaturation – obtaining a supersaturated solid solution by an appropriate thermal cycle.
• Aging – controlled phase separation leading to material strengthening.

Methods of thermochemical treatment of steel

• Nitriding of steel – diffusional enrichment of the surface layer with nitrogen.
• Carburizing steel – diffusional enrichment of the surface with carbon.
• Carbonitriding – simultaneous enrichment of the surface layer with carbon and nitrogen.
• Nitrocarburization / nitrocarburization – enrichment of the layer primarily with nitrogen and carbon.
• Drilling – diffusion saturation of the surface with boron.
• Diffusion chrome plating – diffusion enrichment of the layer with chromium.
• Diffusion aluminizing – diffusion enrichment of the layer in aluminum.