Reduce PWHT Thickness Requirement As Per ASME B31 3

1. Material Thickness

Petrochemical industries and power generators alike have both expressed an interest in reducing the number of weldments requiring postweld heat treatment (PWHT). Due to its cost and potential outage issues for nuclear plants, various EPRI subscribers suggested it would be worthwhile examining current requirements and exemptions and determining whether relaxation might be feasible technically.

Researchers have concluded that PWHT requirements tend to rely more on industry practices rather than specific metallurgical and structural considerations when establishing requirements, while there can be significant inconsistency between specific Code sections governing PWHT exclusions. As a result of these observations, relaxing existing requirements may be possible for nuclear service applications, however careful review must take place first to ensure adequate crack resistance and residual stress relief are achieved through PWHT treatment.

2. Heat Treatment

Pipe weldments that undergo high temperature PWHT treatments can be costly to operate and outage costs for nuclear plants can be considerable, so it would be ideal if as many weldments as possible were subjected to such treatments as this may reduce outage costs significantly.

Multiple codes, including the B&P Vessel Code Section III and some petro-chemical codes, mandate postweld heat treatment when weldment thickness exceeds an established limit; typically this limit relates to toughness requirements as measured by Charpy energy. The present paper explores whether and where exemption requirements differ across various current codes; we then propose rationalisation options as possible solutions.

Studies of PWHT requirements demonstrate that they tend to reflect traditional practices within industries rather than any specific metallurgical or structural considerations. As an example, for weldments made of CR-Mn steels the lower thickness threshold embodied by ASME B31.3 “Power Piping” and 40mm set by EEMUA 158 could be relaxed without detriment to structural integrity of welded weldments; this would substantially lower PWHT costs and save considerable costs on welding services.

3. Welding Procedure

Pre and post weld heat treatments are essential components of strong, safe, and compliant welds. Preheating involves heating the base material prior to welding in order to reduce thermal gradients and prevent hydrogen-induced cracking; postweld heat treatments (PWHTs) apply controlled heating post weld to ease residual stresses and enhance material properties.

PWHT helps reduce stress while improving ductility and strength to prolong component lifespan, according to ASME Section VIII requirements for carbon steel thicker than 38 mm or alloys prone to cracking. It is mandatory in such instances.

Resistance or induction heating methods provide faster, more precise results than their resistance counterparts. Ceramic mat heaters, induction coils or gas or electric furnaces may all be suitable. You may be able to forgo preheating if using materials under 25mm but check code requirements first. Austenitic stainless steels typically do not require PWHT due to their excellent corrosion resistance and ductility – however ageing treatments or solution applications may be needed to restore toughness; other materials used in nuclear applications might need an additional stress relief and tempering heat treatment treatment process before their final use.

4. Control Thickness

Petrochemical and power generation steel users may see B31.3 and BS 2633 exemption requirements as offering an opportunity to reduce post weld heat treatment requirements, however when compared with carbon-Mn steel requirements such as those set out by B31.1, PD 5500, EEMUA 158 for offshore structures etc this reconciliation may not be possible.

Table 1 illustrates that to reach these exemptions, the required thickness could be as little as 14mm for simple welds with good details and quasi-static strain rates; it would likely be much less in more aggressive welding conditions with gross stress concentrations and gross stress concentrations. Furthermore, fracture toughness requirements do not address weld HAZ toughness requirements and this area warrants further consideration based on an approach taken through fracture mechanics to meet such toughness requirements.