ASME B31.1 Consistent Requirements For Post Weld Heat Treatment

ASME B31.1 is an indispensable set of rules designed to enhance the safety of power piping systems. Keeping pace with technological advancement, this set ensures piping systems can operate in various industries safely.

B31.1 code’s PWHT requirements permit temperatures that reach the lower critical temperature for 1- 1/4 Cr Mo materials, which is particularly risky given their susceptibility to reheat cracking.

1. Material

ASME B31.1 is the industry standard for power piping systems and adapts to technological advancements in various industries, providing consistent and standardized piping systems. However, its operating envelope differs significantly from that of Process Piping systems.

As seen in Table 1, B31.1’s exemption limit from PWHT is significantly lower than in U.K. code due to variations in steel composition that affect its fracture toughness.

B31.1 and the NBEP requirements often conflict when it comes to exempting thickness from PWHT, even though both codes utilize ASME P1 group designations. A more consistent approach would help alleviate such issues.

An alternative to PWHT is using girth weld preheat, which has proven successful for most carbon steels. However, it should still be remembered that this method may not always provide optimal results when only applied to the arc area.

Numerous studies have evaluated the efficacy of preheat in reducing risk for brittle fracture in welds. Unfortunately, most studies conducted are limited to simulations rather than actual field tests and their outcomes depend on fracture toughness data used.

2. Welding

ASME B31.1 is an adaptable standard designed to adapt with technological changes in industry, ensuring piping systems remain safe and reliable. Furthermore, its adaptability enables it to meet the needs of multiple industries like oil & gas or power generation.

PWHT requirements in construction codes demonstrate this adaptability beautifully. Each Code’s rules vary according to thicknesses that require PWHT treatment, material properties that trigger its need, holding temperatures during treatment durations and heating and cooling rates – all which have an impactful influence.

PWHT (Post Weld Heat Treatment) is an essential step in the piping design process and required by many codes, relieving metal stresses from weld seams while improving material properties.

Recent amendments to ASME B31.3 Process Piping have eliminated the PWHT requirement for carbon steel conforming to P-No 1 materials group for all control thicknesses, provided preheat is used and multi-pass welding employed for all weld thicknesses. This change is based on detailed fracture mechanics analyses which reveal that near yield level weldments subject to cyclic loading can produce residual stress levels near yield threshold levels that lead to brittle failures; particularly true under conditions where residual stresses reach yield threshold levels near yield threshold levels near yield level weldments near yield threshold can lead to residual stress levels near yield level which lead to near yield level weldments at near yield level, potentially leading to near yield level residual stress levels causing near yield level weldments to undergo near yield failure under load conditions similar to when exposed to near yield level weldments at yield level near yield level conditions, yield levels at near yield level can lead to residual stress levels near yield level weldments causing rupture under stress loads associated with near yield level weldments can lead to near yield level residual stress levels associated with near yield level weldments near yield can lead to failure due to near yield level residual stress levels caused by near yield level weldments near yield level weldments reaching near yield level weldments that lead to near yield level weldments under stress loading conditions such as those encountered when subjected to cyclic load conditions. This change was introduced as it demonstrated how residual stresses associated with near yield level weldments could lead to failures that would normally resulted. This change was introduced through detailed fracture mechanics analyses showing this type weldments near yield level weldments being subjected at near yield level weldments at near yield level that were exposed cyclically loaded conditions leading to near yield level weldments can produce brittle failure due to near yield weldments can cause failure cyclically loaded conditions which eventually leads to cyclic load cycles are subjected loading conditions similar to near yield level weldments can become dangerous with near yield level weldments may cyclically loaded to lead brittle failure resulting from near yield level weldments subjected when subjected conditions due cyclically loaded conditions when subjected with near yield level weldments with near yield level weldments being subjected loads brittle failing under stress conditions than they could otherwise under normal stress levels near-yid to yield level weldments being subjected stress conditions leading to cause failure due to fatigue load conditions due residual stresses leading to yield or near yield level weldments being subjected conditions being subjected conditions may lead to near yield levels leading to near yield level welments under conditions than yield level wedments under conditions than may lead brittle failure due cyclically loaded conditions due resulting to cycled weldments being exposed cyclically loaded conditions being subjected under loading conditions due being exposed cyclically load conditions such being subjected up to yield level weldments reaching failure leading to yield level weldment that would become stressed which leads to yield level weldments being exposed more readily lead cause brittle failure eventually fail later cause, leading to near-yeld level welds may become more than near yield level weld-yi or more quickly exposed to cycling load conditions of near yield level can brid to near-y when exposed.

3. Testing

As various Code sections impose different PWHT requirements for carbon steels, inconsistent requirements exist regarding post weld heat treatment (PWHT). Sometimes this reflects engineering practices developed over time due to application experience; other times they could result from different interpretations of technical data.

Prior to 2014, B31.1 and B31.3 required all carbon steel welding with control thicknesses greater than 0.75 inch to use plasma welding heat treating (PWHT). However, since 2014’s update of these codes permits exemption from mandatory PWHT for P-No 1 materials when multi-pass welding with weld preheat at 95degC is used prior to welding on any nominal material thickness >25mm (1in). Furthermore, RRAC recommended lowering PWHT temperature requirements for P No. 4 materials so as to bring them in line with other Code sections.

Fracture mechanics tests conducted by EPRI and others have clearly illustrated that an PWHT temperature of 1100degF or lower is too low, as it will significantly decrease hardness and impact strength of quenched base metal specimens within minutes.

The RRAC team is exploring changes to B31.1 that would reduce the required PWHT temperature for carbon steels, while providing an alternative approach through temper bead or half bead techniques allowed by repair-oriented codes. They will also strive to develop design curves using advanced fracture mechanics methods.

4. Inspection

PWHT (Post Weld Heat Treatment) can have an enormously significant effect on the risk for brittle fracture in welded components. As such, it’s imperative to adhere to ASME B31.1 requirements, recently amended to offer greater flexibility for post weld heat treatments for pipe materials. PWHT may now be waived for P-No 1 carbon steel wall thicknesses of 19mm or thicker when multi-pass welding with minimum preheat is used, similar to requirements in other codes and standards such as British Codes of Practice. Unfortunately, detailed fracture mechanics analyses indicate that not requiring PWHT for thicker welds increases their risk for near yield level residual stresses which increase their susceptibility to failure and may contribute to their brittle failure.

PWHT involves treating weld metal to alleviate tension and soften it, making the weld more resistant to corrosion damage and other forms of damage, thus increasing safety levels within power piping systems.

ASME B31.1 also stipulates inspection of welds and components prior to being placed into service, using nondestructive testing methods like radiography or ultrasonic tests. Results of such testing must be documented for future reference and can help demonstrate compliance with ASME B31.1 standards as well as identify any issues in your system.