Stress relieving is an intermediate heat treatment procedure to reduce induced residual stresses as a result of machining, fabrication and welding. The application of heat to the steel during its machining or fabrication will assist in removing residual stresses that will, unless addressed during the manufacturing by stress relieving, manifest themselves at the final heat treatment procedure.
It is a relatively low temperature operation that is done in the ferrite region, which means that there is no phase change in the steel, only the reduction of residual stresses. The temperature region is usually between 800xF to 1,300xF.
However, the higher that one goes in temperature, the greater the risk of surface oxidation there is. It is generally better to keep to the lower temperatures, particularly if the steel is a “pre-hard” steel. The hardness will be reduced if the stress relieve temperature exceeds the tempering temperature of the steel.
There is a general rule of thumb for time at temperature. It must be stated that the time is taken when the part is at temperature, not when the furnace is at temperature. The time at temperature for the processes of full anneal (not spheroidize anneal), normalize and stress relieve is 60 minutes at part temperature per one-inch of the maximum cross-sectional area.
Stress relieving is carried out on metal products in order to minimise residual stresses in the structure thereby reducing the risk of dimensional changes during further manufacturing or final use of the component.
Benefits
Machining, and cutting, as well as plastic deformation, will cause a build up of stresses in a material. These stresses could cause unwanted dimension changes if released uncontrolled, for example during a subsequent heat treatment. To minimise stresses after machining and the risk for dimension changes the component can be stress relieved.
Stress relieving is normally done after rough machining, but before final finishing such as polishing or grinding.
Parts that have tight dimensional tolerances, and are going to be further processed, for example by nitrocarburising, must be stress relieved.
Welded structures can be made tension free by stress relieving.
Application & materials
Stress relieving does not change the material’s structure and does not significantly affect its hardness.
Hardened and tempered parts to be stress relieved must be treated at a temperature around 50°C below the temperature used for previous tempering to avoid an impact on the hardness.
Stress relieving before nitrocarburising should be executed at temperatures >600°C.
Copper and brass components can also be stress relieved. For stainless steels a high temperature solution heat treatment is normally necessary.
Process details
The stress relieving temperature is normally between 550 and 650°C for steel parts. Soaking time is about one to two hours. After the soaking time the components should be cooled down slowly in the furnace or in air. A slow cooling speed is important to avoid tensions caused by temperature differences in the material, this is especially important when stress relieving larger components.
If necessary, stress relieving can be performed in a furnace with protective gas, to protect surfaces from oxidation. In extreme conditions vacuum furnaces can be used.
The temperature for stress relieving copper parts is, depending on the alloy, 150-275°C and for brass components 250-500°C.
Mechanical stress relief treatments are a process of stress re-distribution during which the high peaks of an existing residual tensile stress are removed by controlled localised yielding.25 Externally applied loads in the same direction as the residual stress cause yielding in regions where a high stress already exists.
The release of this external load takes place elastically and results in a reduced peak value of residual stress. Although mechanical stress relief can be used to relieve residual stresses produced by welding, there are no accompanying metallurgical benefits as with thermal treatments. They can be effective where a thermal treatment is not practicable, but cannot be considered a complete alternative. Benefits can be gained from the redistribution of local stresses around notches and stress concentrators, providing improved fracture strength at low temperatures and improved fatigue life in these regions. This phenomenon is known as prestressing. However, a comprehensive knowledge of the responses of steels to such treatments is essential, and great care is needed in determining the direction and magnitude of the applied loads.
The stress relief process is performed by heating in an oven or furnace to a temperature below the critical temperature (different materials have different temperatures) and holding it at that temperature long enough to achieve the desired reduction in residual stresses. Temperature, time and time held at the temperature are key process variables, if necessary stress relieving can be performed in a furnace with protective gas, to protect surfaces from oxidation. In extreme conditions vacuum furnaces can be used.
The advantages of stress relief include:
It relieves internal stresses without causing any color change.
It returns the material to a strength level approximately equivalent to where it was prior to forming.
It increases mechanical strength of the material.
Welded structures can be made tension free by stress relieving.
Stress relieving is a heat treatment process in which a metal is subjected to a constant temperature that is below the metal’s critical temperature, followed by controlled cooling.
Drawing, forming and machining induce stresses in materials. A stress relief operation is typically used to remove internal (residual) stresses that have accumulated in the material. These stresses can cause loss of tolerance, cracking and distortion, and contribute to in-service failures. For these reasons, stress relieving is often necessary or even mandatory.