Manufacturing processes
heat treating stress relief is used to reduce the internal material stresses within a part or assembly as a consequence of manufacturing processes. Following manufactuirng processes that include forming, machining, cut, or are assemblies fabricated by welding, will have internal stresses that may cause distorsions. By heating the component or assembly materials to an appropriate temperature and holding for a known period of time to allow the entire part to reach this temperature, most of the interneal stresses can be relieved. The resultant end item will then be less prone to dimensional variation by post manufacturing. Typically, parts that benefit from stress relieving are large and complex weldments, castings which have had a lot of machining, parts with tight dimensional tolerances and machined parts that have had a lot of stock removal performed.
The most common form of pwht is a “stress relief” or “tempering” heat treatment. This heat treatment involves heating the material to a temperature high enough to significantly relax residual stresses from welding, but low enough to avoid metallurgical phase transformation. Heat treatment at this temperature accomplishes the following functions: • reduces residual welding and forming stresses. This is the main benefit of any pwht. Reducing residual stresses improves resistance to corrosion and all forms of stress corrosion cracking. It allows for dimensional stability during machining, and improves mechanical properties like impact toughness and ductility. See figure 600-23 for an example of how various pwht temperatures and holding times affect residual stress levels in weldments.
The stress relief heat treatment is a process that is used to uniform heating of a welded component to a temperature sufficient to relieve a major portion of the residual stresses. This heat treatment is important for components that are subjected to high levels of stress, such as those used in the aerospace and automotive industries. The heat treatment can be performed using different methods, such as furnace heating or electron beam welding. The most common method is furnace heating, which uses an electric furnace to heat the component evenly. Furnace heating is typically performed at temperatures between 500 and 1200 degrees fahrenheit, depending on the type of metal being welded.
Why Stress Relief Heat Treatment (PWHT) is required?
The primary role of post-weld heat treatment is to improve the weldment’s mechanical characteristics and make it outlast for particular service conditions. Pwht helps to reduce the residual stress level and hardness, achieving the required level of ductility in the weldments. This improves strength and resistance to brittle fracture and increases the material’s lifespan.
Date 09-03-2010 15:26 i disagree to some extent with what has been stated. Post weld heat treatment is a controlled process that has to be done properly if the desired results are to be obtained. Different metals respond differently to different heat treatments. Pwht is an essential variable that has to be addressed when the welding procedure is qualified and it has to be addressed by the welding procedure specification. This is true regardless of the welding standard that applies to the work. Pwht can include tempering, stress relief, normalizing, or annealing. Some base metals have to be pwht as soon as the welding is completed and other base metals have to cool to room temperature before pwht can be initiated.
Thermal stress relieving is an annealing process that heats the metal below the austenite phase to reduce distortions or changes in dimensions that might occur after shaping. Stress relieving will reduce distortion and high stresses from flame cutting, machining, welding and other processes that can affect additional manufacturing procedures. Various manufacturing processes such as forming, machining, grinding, heating, and plating leave their characteristic residual stress pattern implanted in the steel. These residual stresses can lead to undesirable dimensional changes, during the production process. Stress relieving, or stress-relief anneal, is done to low carbon grades, in order to remove stresses that have built up in the part from flame-cutting or from mechanical flattening.
Different types of post-weld heat treatments are used for varied reasons and different materials. A) stress relieving - post-weld heat treatment is most generally used for stress relief. The purpose of stress-relieving is to remove any internal or residual stresses that may be present from the welding operation. Stress relief after welding may be necessary to reduce the risk of brittle fracture, avoid subsequent distortion on machining, or eradicate the risk of stress corrosion. B) obtain suitable metallurgical structure - for some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgical structure. This treatment is performed after the weld has cooled, but under certain circumstances, it may be necessary to perform this treatment before it has cooled to prevent cracking.
Stress-relief heat treating of steel is the uniform heating of a structure to a suitable temperature below the transformation range, holding at this temperature for a predetermined period of time, followed by uniform cooling. This article provides information on the sources of residual stress, briefly describes the factors influencing the relief of residual stresses, and discusses the various thermal stress-relief methods. It contains tables that provide a summary of compressive and tensile residual stresses at the surface of parts fabricated by common manufacturing processes. The article presents the temperature range of alloy steels for stress-relief heat treating and describes the importance of stress relief of springs.