ASME B31.3 Post Weld Heat Treatment Requirements For Carbon Steel Pipe

Since 2014’s edition of ASME B31.3, thick carbon steel pipe has been exempted from post weld heat treatment (PWHT) requirements with only multi-pass welding required and no preheat requirement necessary. There have been concerns raised that such exemptions may be unsafe.

This paper, sponsored by EPRI and written by RRAC team members, is intended to support changes to minimum PWHT temperature requirements of B31.1 and B31.3 codes as well as similar changes elsewhere in codes.

Minimum Preheat Temperature

ASME B31.3’s minimum preheat temperature requirements differ significantly across codes and standards, possibly reflecting different service conditions addressed by each standard or code. Real concerns emerge when requirements of one code or standard seem excessive from a technical perspective – an example being post weld heat treatment (PWHT) requirements for carbon steel materials.

The PWHT temperature requirement set forth by the code is close to their lower critical temperatures for carbon steels, potentially leading to reheat cracking of weld zones and weakening their strength significantly. Studies have revealed, however, that hardness and toughness of P No 4 welds may still be maintained after PWHT at temperatures lower than what is specified by current code requirements.

Though there may be concerns, strong support exists for maintaining current preheat and PWHT requirements for P No 4. A study sponsored by EPRI found that PWHT could be performed at temperatures lower than what are currently required in B31.1 and B31.3 codes while still producing acceptable material properties – providing more design flexibility, more margin of safety for dissimilar material joints, and eliminating cracking at weld regions in susceptible P No 4 materials.

Maximum Preheat Temperature

ASME B31.3 requires that materials meet a minimum preheat temperature at which post weld heat treatment (PWHT) can begin. PWHT is a controlled process in which weldments are heated up below their lower critical transformation temperature for a specified amount of time to reduce residual stresses and microstructural changes caused by welding; additionally it may help increase hardness by increasing hardening processes during PWHT treatment.

The 2014 edition of ASME B31.3 introduced several significant revisions to its PWHT requirements for process piping, particularly carbon steel weldments that meet ASME Section VIII Div 1 requirements that previously needed postweld annealing (PWA).

This change was implemented following a request by RRAC, who highlighted how ASME B31.1 and B31.3 PWHT temperatures are close to their materials’ lower critical transformation temperatures resulting in unnecessary losses of hardening capacity due to over-heating. To address this issue, the report recommended changing PWHT requirements between 1200-1300 degF. Additionally, 150 degrees should be set as maximum above PWHT, more in line with BS 2633 requirements.

Minimum Holding Time

Post Weld Heat Treatment of Carbon Steel Piping by Post Weld Heat Treatment (PWHT). This is intended to avoid environmental assisted cracking of carbon steel pipe joints and also to decrease residual stresses created by welding while increasing toughness of weld metal. In addition, PWHT helps mitigate environmental degradation and protect against corrosion.

PWHT requirements vary by code and material group. For instance, B31.3 specifies minimum holding times of 15 minutes for low alloy steels and 20 minutes for 2.25%Cr-1%Mo steels respectively based on maximum carbon levels and weld thicknesses permitted within each group; this minimum holding time requirement applies equally to both single pass welds as multi pass welds.

PWHT may not always be necessary; for example, in the case of welded steel vessels such as oil storage tanks. Though designed to withstand pressures up to 5,000 psi, risk of brittle failure remains due to extreme temperature gradients and stresses at weld joints.

PWHT processes can be costly and inconvenient; for this reason, some jurisdictions provide exemptions for carbon steel piping. Furthermore, B31.3 code series requirements aim to strike an efficient balance between safety and efficiency when it comes to PWHT requirements.

Maximum Holding Time

ASME B31.3 is an American National Standard that covers requirements for piping used in petroleum refineries; chemical, pharmaceutical, hydrogen, textile, paper and pulp mills, nuclear reactors, semiconductor plants as well as power generating facilities; natural gas compressor stations as well as power generation plants. It addresses materials and components for design fabrication assembly erection inspection inspection testing of pipework.

As in the case of welds, preheat temperatures should be adjusted based on the thickness of each weld. Thin welds may require lower preheat temperatures than thicker welds for their respective welds; however, the time necessary for each stage remains constant.

Prior to revision, mandatory PWHT weld thickness limits were established based on good engineering practice and experience. As fracture mechanics has progressed since this initial implementation of these limits was done so in accordance with good engineering practice and experience; they have since been revised in light of computed toughness-based exemption curves requiring higher minimum wall thickness requirements in carbon steels than their older equivalents did.

PWHT welding in most cases now requires 19mm as its maximum weld thickness – significantly lower than general structural codes like BS 5400 for bridges) or other specialized codes like ASME B31.1 which govern pressure vessels, power piping, and API 650 which govern construction of large storage tanks.