46 Stress Relieving

Stress Relieving is also a process for making material softer.  Stress-relieving is a technique to remove or reduce the internal stresses created in metal. These stresses may be caused in a number of ways, ranging from cold working to non-uniform cooling. Stress-relieving is usually accomplished by heating a metal below the lower critical temperature and then cooling uniformly. Stress relieving is commonly used on items like air tanks, boilers and other pressure vessels, to remove a portion of the stresses created during the welding process.

However, stress relieving does not change the material properties as does annealing and normalizing.  A material can be stress relieved by heating it to a specific temperature that is lower than that of annealing or normalizing and letting it cool to room temperature inside or outside of the oven. This heat treatment is typically used on parts that have been severely stressed during fabrication.

It is worth noting that some welding processes cause stresses in the material that can lead to warpage either after the heat treating process or during subsequent machining operations. If a weldment is to be machined, it should almost always be stress relieved or normalized before the machining process. This is because machining chunks of material from a stressed weldment redistributes the internal stresses and can cause the part to warp. If the stresses are first relaxed, then abrupt changes in geometry after machining are reduced.

Think about this analogy in the past: tighten your arm wrist and hand and fingers in a bodybuilder’s pose. There are a lot of pressures applied to keep the pose rigid and unmoving. When machining a weldment, it is like relaxing one muscle in the pose and seeing how the pose moves to find equilibrium. One muscle can be just one fly cut that changes the stresses holding it all together.

Derived from Heat treating – Wikipedia accessed and available 11 December 2024 and The Virtual Machine Shop (http://www.jjjtrain.com/vms/eng_heat_treat/eng_heat_treat_04.html) retrieved from the Wayback Machine 17 January 2024.