Stress Relieving / Sub Critical Annealing

Stress relieving / Sub Critical Annealing

Stress relieving is carried out on metal products so as to minimise residual stresses in the structure thereby reducing the risk of dimensional changes during further manufacturing or final use of the component. It is carried out below the critical temperature and is therefore also referred to as ‘sub-critical annealing.’

The stress-relieving temperature is normally between 550 and 680°C for steel parts. Soaking time is about one to six hours depending on section thickness. After the soaking time, the components should be cooled down slowly in the furnace or in the air. Slow cooling is essential to avoid stress build-up 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 a protective atmosphere ie Nitrogen, to protect surfaces from oxidation. In extreme conditions, vacuum furnaces can be used.

The temperature for stress-relieving nonferrous metals such as aluminium, copper, brass, etc differ from that of steel but is usually much lower.

Benefits

Machining, cutting, welding as well as plastic deformation, will cause a buildup of stresses in a material. These stresses could cause unwanted dimension changes if released uncontrolled, eg: during a subsequent heat treatment. To minimize stresses after machining 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, eg: by nitrocarburizing, must be stress relieved.

Welded structures can be made tension free by stress relieving.

Application

Stress relieving does not change the material’s structure and does not significantly affect its hardness except when applied to heat treated components.

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 ensure that the hardness is not compromised.

Stress relieving before nitrocarburizing 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 followed by a rapid quench is usually required.