Post Weld Heat Treatment

PWHT reduces residual stresses, improves mechanical properties and increases corrosion resistance of weldments, which increases their reliability as welded structures. But before beginning PWHT treatments it’s essential to understand all its limitations and potential dangers.

PWHT (Pressure Wash Heat Treating) is the process of heating steel to temperatures that fall below its transformation range and holding it there for an extended period. This technical subject requires specialist knowledge and equipment.

Stress relief

Post weld heat treatment (PWHT), also known as postweld stress reliving, reduces and redistributes residual stresses in weldments while improving their mechanical properties like ductility and toughness. PWHT involves heating metal to an exact temperature for an extended period – however this must remain below its transformation temperature to prevent damage to material properties.

Internal stresses often form during manufacturing processes like machining, stamping and welding. This leads to distortion in components which must be eliminated in order for use. PWHT provides a thermal relief mechanism which can alleviate these tensions without altering their physical properties.

Note that the stress relief process can be affected by its environment. High humidity or exposure to chemicals can alter the PWHT process and diminish residual stress relief effectiveness, increasing weld failure risk as well as corrosion risk.

PWHT not only reduces residual stress but can also protect against corrosion. When exposed to both stress and corrosion conditions simultaneously, metal may become vulnerable to corrosion cracking due to an imbalance between tensional stresses and elastic deformation of its material structure. Stress relief during PWHT prevents this by decreasing tensional stresses while increasing elastic deformation, thus avoiding corrosion cracking altogether.

Mechanical properties

As welding creates large temperature gradients between weld metal and parent materials, residual stresses in weld material may reduce its strength. A postweld heat treatment (PWHT) may be used to alleviate such stresses and enhance mechanical properties – this process is commonly referred to as PWHT.

PWHT involves heating welded material below its lower critical transformation temperature and holding it there for an established amount of time, typically performed on pressure vessels and other pieces of equipment that demand high levels of safety and reliability. It may help reduce brittle fracture in welds while increasing their tensile properties.

However, it should be kept in mind that PWHT can also degrade certain mechanical properties, including impact toughness and weld ductility; nonetheless, its benefits remain considerable.

PWHT has been demonstrated to significantly enhance the mechanical properties of welded NiTi materials, particularly plastic strain localization in coarse-grained regions such as HAZ and FZ, by suppressing dislocation formation and decreasing actuation strain in these regions; an effect attributed to dissolving strengthening g phases during PWHT.

Corrosion resistance

Post weld heat treatment (PWHT), or post weld heat treating is a process performed after welding to ensure compliance with industry codes and specifications as well as reduce corrosion risk. It is an integral step that must take place if materials are to meet industry specifications while at the same time protecting against it.

Heat Treatment involves heating the welded metal to a specific temperature before slowly cooling it, in order to relieve residual stresses produced during welding and prevent hydrogen-induced cracking (HIC). Furthermore, this treatment can improve mechanical properties by creating more homogenous material with reduced hardness.

Normalizing involves heating material at higher temperatures for longer than when annealing; this allows it to achieve a finer grain structure suitable for higher strength applications. However, it should be remembered that certain structures cannot undergo normalizing PWHT due to large and complex-shaped components being incapable of supporting themselves at soak temperature; this would likely cause catastrophic buckling and distortion. Therefore, PWHT should only be performed on components that can easily be supported; an example would be the dished end of a pressure vessel as this type of component would make an excellent candidate for normalizing PWHT. Re-solution PWHT may be employed to dissolve any chromium carbides located along grain boundaries between delta ferrite and austenite which might impair pitting corrosion resistance of duplex stainless steel (DSS) weld overlay.

Environmental conditions

PWHT is an essential step in the design and maintenance of many steel structures such as pressure vessels, piping equipment, bridges and offshore platforms. PWHT reduces environmental-assisted cracking by reducing weld residual stresses and tempering weld metal microstructure, while increasing toughness for improved resistance against loads and environmental conditions that the weld assembly will experience over its lifespan.

PWHT is typically required in structural applications involving carbon steels over a code-defined thickness threshold and low alloy steels to reduce residual stress caused by welding that could otherwise lead to crack initiation and eventual brittle fracture. The process includes preheating, soaking, heating to target temperature, and controlled cooling in order to minimize distortions or residual stresses in adjacent weld areas.

Local PWHT, such as spot or bulls eye configurations, are frequently employed after weld repairs and modifications in pressure vessels and piping systems. Engineering local PWHT configurations carefully is key to avoiding distortion, cracking, and other damages that extend repair downtime; advanced thermal-mechanical finite element analyses of local PWHT configurations and their residual stress states can help achieve this result.