Stress-Relieving or Post Weld Heat Treatment

To relieve stresses that remain locked in a structure as a consequence of Post Welding . Postweld heat treating is to provide, in addition to the relief of residual stresses. For example, most ferritic weldments are given postweld heat treatment to improve the fracture toughness of the heat-affected zones (HAZ).
Moreover, austenitic and nonferrous alloys are frequently postweld heat treated to improve resistance to environmental damage. Stress-relief heat treating is the uniform heating of a structure, or portion thereof, to a suitable temperature below the transformation range (Ac1 for ferritic steels), holding at this temperature for a predetermined period of time, followed by uniform cooling. Care must be taken to ensure uniform cooling, particularly when a component is composed of variable section sizes.

Sources of Residual Stress

• Welding The cause of residual stresses that has received the most attention in the open literature is welding. The residual stresses associated with the steep thermal gradient of welding can occur on a macroscale over relatively long distances (reaction stresses) or can be highly localized (microscale). Welding usually results in localized residual stresses that approach levels equal to or greater than the yield strength of the material at room temperature.
• Bending a bar during fabrication at a temperature where recovery cannot occur (cold forming, for example) will result in one surface location containing residual tensile stresses, whereas a location 180° away will contain residual compressive stresses.
• Grinding is another source of residual stresses; these can be compressive or tensile in nature, depending on the grinding operation. Although these stresses tend to be shallow in depth, they can cause warping of thin parts.