Post Weld Heat Treatment Requirement

Post-weld heat treatment (PWHT) is required by fabrication codes for certain structures to reduce welding residual stresses and temper hard, potentially brittle microstructure regions, but is an expensive process.

Table 1 below displays the minimum thickness requirements above which PWHT must be utilized for various pressure vessel and piping standards, and this article reviews them along with suggesting potential rationalisations of these requirements.

1. Reduces the risk of brittle fracture

Brittle fracture is a catastrophic failure mode for vessels, tanks and other components that often leads to significant loss of personnel life and property. Brittle fracture can be avoided through proper design, fabrication and inspection practices such as specifying minimum Charpy energies of materials; designing to avoid high stresses; stress relief post weld heat treatment of thick sections; adopting fabrication and inspection methods which minimise defects while increasing chances of finding them; conducting proof tests as part of regular maintenance operations and so forth.

Operating equipment and piping within its designed limits is crucial, particularly when operating temperatures fall below its lower design temperature (LDT or MDMT according to ASME codes). Existing carbon steel equipment should undergo a Brittle Fracture Assessment as detailed in API RP 579 Fitness for Service Part-3: Assessment of Existing Equipment for Brittle Fracture.

This assessment must be based on sound technical justification rather than assumptions, using fracture mechanics to define safe working limits for temperature and pressure conditions of equipment. A risk-based approach is far superior to simply adhering to minimum thickness criteria set by codes which often rely on subjective experience instead of engineering justification.

2. Reduces the risk of distortion

Many codes and specifications require PWHT to reduce stress in welded joints, thereby decreasing fracture risk while increasing distortion and warping of pressure equipment, which in turn compromises its dimensional accuracy, structural integrity, and increases the risk of leaks.

Es is possible to mitigate distortion during heat treatment by providing proper support and cooling of material during processing, typically via trestles shaped specifically to fit a component and spaced at regular intervals around its diameter. The number of required trestles will depend on its size, shape and thickness.

As further safeguard against distortion, make sure the PWHT temperature does not go beyond what was originally specified for tempering steel. Higher temperatures can cause temper embrittlement or oversoftening and this could lower its strength below specified minimum levels resulting in distortion that reduces its strength to below the specified minimum value. Therefore it is advisable to carry out mechanical tests after PWHT equipment processing to confirm its strength retention.

3. Reduces the risk of warping

PWHT involves heating welded material to high temperatures before slowly cooling it back down again, which may result in warping or distortion to pressure equipment, leading to reduced structural integrity, leaks or failures and greater energy use and contributing to greenhouse emissions. Furthermore, it requires large amounts of energy usage which contributes to greenhouse emissions as well as other environmental concerns.

PWHT processes have traditionally been utilized to provide stress relief, modify weldment microstructure and diffuse hydrogen for corrosion and oxidation damage resistance. However, studies have revealed that similar benefits can also be achieved at lower temperatures than typically employed.

Although PWHT can provide many advantages, current codes vary greatly in their exemption requirements from PWHT. This variation often stems from engineering practices and experience within different industries rather than from specific metallurgical or structural considerations; consequently, there can be confusion and conflict among code requirements regarding exemption from PWHT for pipe weldments using P-4 and P-5A materials across different codes.

4. Reduces the risk of leaks

PWHT requires supporting equipment during its exposure to high temperatures in order to prevent it from distorting excessively, often by means of trestles shaped to fit its shape, size and thickness. As part of this process, material is heated to high temperatures before gradually cooling back down at regular intervals around its perimeter in order to redistribute stresses within it and cause weakness to form that could reduce weldments strength or even lead to leaks within its structures.

Different codes impose differing PWHT requirements, with certain specifications exempting certain materials or weldments from it altogether due to factors like thickness. Such variations in requirements are likely the result of diverging engineering practices and application experience rather than differing interpretations of technical data or experimentation; it is advised that if a material or weldment will be used in nuclear service its exemption criteria be reduced down to zero regardless of thickness.

5. Reduces the risk of corrosion

Corrosion can damage equipment, pollute soil and water supplies, release harmful toxins into the air, weaken structures like tanks and pipes more likely to fail and increase risks to equipment and personnel. By taking proactive measures against corrosion, you can lessen these risks to equipment and personnel.

atmospheric corrosion occurs when metal objects are exposed to oxygen and moisture conditions, also known as uniform or general corrosion as the process takes place across its entirety. Other forms of corrosion such as pitting, crevice and stress corrosion can be more unpredictable as they form at specific spots on a metal object.

Protection from corrosion depends on both metal composition and protective methods used. Cathodic protection involves coating metal with more active elements, like zinc, which will corrode and oxidize as it reacts with its surroundings to protect its underlying metal composition from rusting. Other ways of prevention may include controlling chemical levels in air or water sources or using materials that offer greater abrasion resistance such as using anticorrosive coatings and paints.