Betydningen af PWHT i rørsvejseprocesser

Currents generated by high-frequency coils generate the necessary heat, and then cool it via convection.

Studies have demonstrated that weld residual stress and HAZ fracture toughness concerns do not provide sufficient technical justification for applying PWHT requirements to pipe materials with wall thicknesses of less than 5/8 inch.

1. Relieves Residual Stresses

Welding creates large regions of high tensile residual stresses that may contribute to environmental assisted cracking or brittle fracture in welded steel assemblies. Post-weld heat treatment (PWHT) relieves these tensions and allows the pipe to continue operating safely in service.

Conventional PWHT involves subjecting the welded weldment to temperatures that exceed its minimum transformation temperature for optimal weldment strength, corrosion, fatigue resistance or hydrogen damage protection. Temperature selection depends on desired tempering effects as well as required material resistance levels against corrosion fatigue or hydrogen damage.

When using local PWHT to reduce residual stresses in a small area, choosing the geometry of the heated zone geometry is of critical importance. Studies have revealed that variable width heated bands reduce stress magnitude more effectively than constant width circumferential bands due to elastic analysis showing greater residual stress after cooling from their use versus variable width heated zones.

These findings suggest that current PWHT exemption limits in piping codes may not be accurate and that it may be possible to reduce PWHT requirements without impacting weldability or fatigue performance of materials, especially since existing codes don’t mandate fracture toughness testing on welds thinner than 5/8 inch.

2. Strengthens the Weld

Post Weld Heat Treatment (PWHT) is an essential component of welding processes. PWHT alleviates residual stresses in materials that could otherwise lead to distortion and cracking; furthermore, PWHT increases strength by eliminating stress concentrations that could otherwise weaken welds under dynamic loading conditions and render them brittle and fail.

PWHT involves heating metal to a specified temperature for a predetermined amount of time and with various techniques such as annealing, normalizing, quenching and tempering to increase strength while decreasing risk. The goal of PWHT is to strengthen welds while decreasing cracking risks.

However, when these treatments are performed incorrectly they can actually weaken the weld. This happens when thermal gradients aren’t controlled properly; when this happens the weld experiences higher temperatures in some spots than others and is forced to expand and contract at various rates, leading to less resistance against stress corrosion cracking and hydrogen-induced cracking.

PWHT may or may not be required depending on the type of pipe, metal weld material used and other factors. Pipes made of carbon steel typically require PWHT while stress relief heat treatments before installing welds may be necessary before installation to ensure sufficient strength against environmental stresses.

3. Reduces the Risk of Cracking

Post-weld heat treatment is an integral component of steel fabrication. This controlled heating and cooling process relieves residual stresses, tempers the weldment microstructure, removes diffuse hydrogen, reduces susceptibility to stress corrosion cracking, increases strength and hardness as well as providing increased strength. When performed improperly or completely skipped altogether, weldments become susceptible to trans-granular cracking that could catastrophically fail structures such as pressure vessels or major pipe components.

PWHT welding techniques can also reduce cold cracking in pipe components by increasing weldment ductility; to accomplish this goal, filler metals with low diffusible hydrogen designators values (e.g. H4 or H8) should be utilized.

Studies have shown that using a preheat can significantly relieve residual stresses, thus decreasing fatigue cracking risk. It should be remembered, however, that PWHT only makes an impactful contribution when subjected to loads of sufficient magnitude alternating over time.

Based on this research, PWHT becomes less significant as pipe diameter increases for any given wall thickness, suggesting that current code exemptions limiting when PWHT must be performed should be revised without impacting standard power plant piping schedules.

4. Reduces the Risk of Defects

Many piping systems must operate in hostile environments with high temperatures and pressures, where welds may experience thermal fatigue or fracture failure without PWHT treatment. When subjected to PWHT however, their metal can withstand this stress better, decreasing risk of failure while improving system safety and reliability.

Though PWHT provides many advantages, there is the risk that it could be overapplied or improperly applied in certain instances. An overdose may reduce tensile and creep strengths while diminishing notch toughness; incorrect application may even cause cracking and trans-granular cracking of concrete surfaces.

Therefore, various codes pertaining to piping and pressure vessels often have differing requirements when it comes to PWHT requirements. For instance, ASME BP&V Sections I and VIII exempt weldments made from P-4 and P-5A materials of less than 5/8 inch in thickness from this requirement; while other codes such as ANSI B31.3 only require PWHT on weldments between 4-inches in thickness and less.

These differences could be attributable to different professional bodies drafting the codes and being guided by their engineering experience and practice. However, it has been discovered that weldability tends to increase with increasing diameter of pipe materials; thus PWHT becomes less critical with thick walled weldments than thin-walled ones.