Post Weld Heat Treatment (PWHT) Codes

Current design codes for piping and pressure vessels stipulate that PWHT may be necessary if material thickness exceeds an established threshold, typically determined by measuring Charpy energy. Some carbon and low alloy steels qualify for exemption if an appropriate preheating cycle is utilized.

Girth welds in steel pipes have undergone an assessment by Mohr that indicates they may not require PWHT treatment.

What is ASME PWHT?

Post weld heat treatment (PWHT) is an increasingly popular technique to alleviate weld residual stresses in pressure vessels and piping equipment, often specified in code as required for carbon steel weldments with thickness thresholds above certain thickness thresholds to decrease their propensity for crack initiation and brittle fracture. Unfortunately, performing PWHT on large parts is sometimes impractical or costly due to remote locations or large component weight, thus spot or bulls eye PWHT configurations may be employed instead.

These configurations typically utilize soak, heating and gradient control bands sized and positioned to prevent distortion, cracking of adjacent welds, and severe residual stresses from being generated. Unfortunately, due to the complexity of these designs, accurately predicting their thermal-mechanical response makes engineering them extremely challenging. Advanced computational simulation techniques used in this investigation provide analysts a way to ensure that any local PWHT configurations implemented following equipment repairs won’t cause costly additional damages such as distortion and cracking that extend equipment downtime further.

Current design codes for pressure vessels and piping typically mandate PWHT when weldment thickness exceeds an amount determined by material Charpy impact test properties, yet requirements vary from code to code with significant variance between thickness limits specified in various codes. Our investigation revealed that taking an approach similar to that used by PD5500 could help address some of these disparate specifications.

What is the ASME PWHT Thickness Threshold?

As shown in Table 1, while piping and pressure vessel codes typically require PWHT up to 19mm for C-Mn steels, general structural engineering standards such as BS 5400 for bridges or 2633 [25] for buildings may set lower exemption thresholds from PWHT requirements for exemption. These discrepancies likely come down to differences in chemical composition of steels as well as different requirements for fracture toughness (as illustrated).

Post-weld heat treatment helps significantly decrease residual tension by tempering both weld metal and base metal microstructures, thus significantly decreasing residual stresses in the weldment, increasing its strength and toughness, and lowering environmental-assisted cracking risks.

PWHT also can reduce thermal distortion, which is costly for pressure equipment and prolongs repair downtime needed to restore serviceability. Achieve proper PWHT by striking an ideal balance between preheat and cooling rate is key to its success.

Utilizing advanced non-linear FEA simulation, an optimal PWHT configuration for pressure equipment repairs can be determined. By simulating thermal-mechanical reactions of weldments in response to various preheat and cooling conditions, this method identifies optimal local PWHT arrangements that minimize distortion as well as residual stress.

What is the ASME PWHT Preheat Threshold?

Current design codes for pressure equipment and piping specify that PWHT must only be performed if weld thickness exceeds a specific limit, usually determined by Charpy test properties of material or minimum service temperature requirements. This limiting thickness approach has long been utilized by industry, as evidenced by its success at protecting equipment against brittle fracture.

But the use of PWHT consumes significant energy and contributes to greenhouse gas emissions and other environmental concerns, adding additional costs associated with its operation and maintenance. Furthermore, multiple PWHT cycles may be necessary over the service life of equipment – adding further costs associated with its upkeep and management.

As such, there has been much enthusiasm surrounding efforts to reduce the need for PWHT by creating more forgiving material properties. This would enable equipment using stronger carbon steels while simultaneously decreasing both energy usage and risk associated with environmentally assisted cracking processes.

Approaches have been devised for reaching this goal, such as using preheat temperatures and multi-pass welds. In 2014, ASME B31.3 “Power Piping” code was amended with an exemptions table from mandatory PWHT for welds in carbon steel conforming to its P-No 1 material group; under these exceptions CS >25mm (1 in) thick must receive a preheating of 95 degC (200degF) prior to welding with multipass welds being utilized.

What is the ASME PWHT Thickness Exemption?

Current fabrication codes for pressure vessels and piping typically specify that PWHT may be necessary if weld thickness exceeds a given value, with this limit typically determined using Charpy test properties of material. However, requirements vary between codes; some can be more conservative than others.

ASME B31.3 (2014) now allows an exemption from mandatory PWHT for carbon steel materials of material group P1 with a preheat temperature of 95C being applied prior to welding, representing an important shift in good engineering practice. This new provision represents a substantial change in good engineering practice.

PWHT is an energy-intensive process with significant environmental implications, including greenhouse gas emissions. Furthermore, multiple PWHT cycles over the life of equipment may cause warping or distortion that necessitate extensive repairs; composite repairs offer an alternative to PWHT for repairing damaged pressure equipment parts.

Composite repairs offer several environmental advantages over their metal counterparts. Not only can they improve structural integrity and decrease leak risk, but thanks to high-performance materials combining, composite repairs are highly durable repairs that resist age-related effects such as fatigue. As such they help ensure safe operations of equipment long after original PWHT work has been completed.