Post Weld Heat Treatment

Post weld heat treatment (PWHT) is an industry standard method of postweld heat treating metallic parts and weldments at temperatures below their lower critical transformation temperature, usually prior to welding or metallurgical applications such as pressure vessels and piping.

Local PWHT is an approach for minimizing out-of-plane deformation. This method was studied using finite element analysis and local stress distributions were compared with those from furnace PWHT.

Weld Strength

Post Weld Heat Treatment, commonly abbreviated as PWHT, is an important step after welding has been completed that involves heating metal to below its critical transformation temperature and holding it there for a set period of time. This helps alleviate residual stresses within the weld zone while increasing toughness and helping prevent brittle fractures within weld joints. As soon as possible after completion of welding it should take place – its importance should not be ignored!

PWHT is required due to welding processes that create high temperature gradients between parent material and weld metal, creating stresses in the weld material that exceed their design stress limits. PWHT can significantly decrease these stresses, improving strength and durability of welded structures.

Assuring a stable temperature cycle requires that structures or components being weighted receive proper support. Trestles designed to fit the contours of their object at regular intervals will help minimize distortion; this is especially important with long or awkwardly-shaped components as thermal expansion differences between sections can cause them to bend or distort the material over time.

Advanced PWHT furnaces are engineered to be flexible and versatile, capable of fulfilling many different processes such as annealing, aging, normalizing, stress-relieving and tempering. Their double-wall construction limits heat loss while conserving energy; precise control systems follow mandatory heating/cooling profiles mandated by welding codes with thermocouples measuring both internal furnace temperature as well as weldment temperature for precise results.

Welding Residual Stress

Residual stress plays an integral part in the performance of spot welds. Compressive residual stresses push materials together while tensile residual stresses pull apart; compression generally enhances fatigue strength and fatigue life while slowing crack propagation while increasing resistance to environmentally assisted cracking such as hydrogen-induced cracking or stress corrosion cracking.

Tensile residual stresses cause reduced fatigue lives, increased cracking potential and brittle fracture susceptibility in welded structures. Their exact nature and extent depend on several factors including structure geometry, fabrication procedures, welding methods used, postweld treatments applied postweld, service conditions etc.

Forecasting and mitigating stressors is no simple task due to complex nonlinear interactions among various variables. Furthermore, residual stress measurements and analytical predictions often display significant scatter and variability due to many different reasons, including uncertainties in measurement locations/materials used as well as errors introduced into measured data or incorrect assumptions in analytical modeling.

For accurate residual stress prediction and mitigation, it is necessary to gain an in-depth knowledge of the fundamental processes governing their development in welded structures. To aid with this endeavor, this special issue features 13 original research articles focused on mathematical models, experimental techniques and measurement methodologies developed specifically to detect and mitigate residual stresses caused by welding for various joint geometries and welding conditions.

Welding Cracking

PWHT can be performed on many projects, from piping spools and pressure vessels to storage spheres and storage spheres, in specially-erected furnaces on site. When dealing with larger and/or heavier structures such as vessels, however, engineering firms may create tailored furnaces specifically tailored for their customers’ needs.

PWHT is necessary due to residual stresses created during welding thick materials. When combined with load stresses, these residual stresses can exceed material limitations and result in weld failure. PWHT helps reduce these residual stresses; however, microstructural changes occur which decrease a material’s toughness and ductility as a result.

Low-alloy and high-carbon steels, in particular, are more prone to cold cracking due to being more brittle and losing tensile strength when cooling from their weld toe area.

Welding codes require a smooth transition of weld metal to base metal at the weld toe, to avoid “stress riser” formation that leads to cracking. Achieve this goal using appropriate travel speeds and voltage settings as well as weld trestles shaped specifically to the component can help avoid excessive distortion; regular placement should help ensure an even weld.

Welding Fracture

As metal welding produces non-uniform heating and cooling cycles, which leads to the build-up of internal forces known as residual stresses in the material, this may result in distortion and reduced mechanical properties that lead to weld failures and greater cracking potential – necessitating post welding stress relief heat treatment as a crucial way of protecting its strength and performance of welded fabrications.

A PWHT furnace is used to reduce and redistribute residual stresses through repeated heating and cooling cycles, inducing tempering that improves material ductility and resistance to brittle fracture. Such heat treatment is required by many codes and standards in order to guarantee safety of welded structures.

Heat treatment for components can be done in a pwht furnace, either permanently installed, portable, or field-erected. For optimal results, these furnaces must be positioned so as to avoid areas which could produce excessive temperature gradients or differences; otherwise, these regions would cause the component to exceed its phase transformation temperature and lead to unexpected volume and phase changes.

PWHT involves heating parts of a welded structure to high temperatures and holding them there for an extended period. A standard guideline suggests setting aside one hour per 25mm thickness; local PWHT may also be possible, though certain restrictions must be observed.