Stress Relieving / P.W.H.T.
Stress relieving or post weld heat treatment 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.
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 nitro carburising, must be stress relieved.
Welded structures can be made tension free by stress relieving.
Post weld heat treatment (PWHT) is a controlled process in which a material that has been welded is reheated to a temperature below its lower critical transformation temperature, and then it is held at that temperature for a specified amount of time. It is often referred to as being any heat treatment performed after welding; however, within the oil, gas, petrochemical and nuclear industries, it has a specific meaning. Industry codes, such as the ASME Pressure Vessel and Piping Codes, often require mandatory performance of PWHT on certain materials to ensure a safe design with optimal mechanical and metallurgical properties.
The need for PWHT is mostly due to the residual stresses and micro-structural changes that occur after welding has been completed. During the welding process, a high temperature gradient is experienced between the weld metal and the parent material. As the weld cools, residual stress is formed. For thicker materials, these stresses can reach an unacceptable level and exceed design stresses. Therefore, the part is heated to a specified temperature for a given amount of time to reduce these stresses to an acceptable level. In addition to residual stresses, microstructural changes occur due to the high temperatures induced by the welding process. These changes can increase hardness of the material and reduce toughness and ductility. The use of PWHT can help reduce any increased hardness levels and improve toughness and ductility to levels acceptable for design.
The requirements specified within various pressure vessels and piping codes are mostly due to the chemical makeup and thickness of the material. Codes such as ASME Section VIII and ASME B31.3 will require that a specified material be post weld heat treated if it is over a given thickness. Codes also require PWHT based solely on the micro-structural make-up of the material. A final consideration in deciding the need for PWHT is based on the components' intended service, such as one with a susceptibility to stress corrosion cracking. In such cases, PWHT is mandatory regardless of thickness.