Post-Weld Heat Treatment Exemptions in the ASME Code

Post-weld heat treatment (PWHT) reduces the risk of brittle fracture by eliminating residual stresses and tempering hard, potentially brittle microstructural regions. Current codes have different requirements for exempting PWHT depending on chemistry, inherent Charpy test property requirements, or through inspection codes allowing allowable defect size restrictions (Table 1).

Limiting Thickness

Current Codes require PWHT for weldments on pipe materials with thicknesses less than 0.625 inch; however, the limit specified by these codes often conflicts with metallurgical considerations and structural design requirements. An EPRI investigation has provided evidence supporting an increase in this limit without jeopardizing safety or reliability.

Research was conducted to assess existing code requirements and exemptions related to welding of P-4 and P-5A chromium-molybdenum steels used for nuclear service applications, with particular attention paid to their origin and technical basis by interviewing leaders of code bodies as well as reviewing documentation available to us.

Studies conducted on various codes revealed that their thickness criterion are often determined more by traditional practices within specific industries rather than on scientific analyses of metal or mechanical considerations, which resulted in huge variations between them; currently the limiting values vary greatly between codes with power generation industries having much higher limits compared to petrochemical ones; contrary to research showing weld residual stresses decrease as pipe diameter increases and HAZ fracture toughness drops off accordingly.

Charpy Test

The Charpy test is a standard impact test designed to measure how much energy was absorbed during fracture of material samples, typically consisting of a pendulum with an impact hammer swinging and hitting machined specimens that may or may not contain notches facing away from the pendulum; energy absorption levels are measured and converted into notch toughness values.

This test can identify the Ductile-to-Brittle Transition Temperature (DBTT) of metals, an essential characteristic in critical applications like welding and pressure vessel manufacturing. In addition, the test also serves to indicate an overall toughness rating of material as well as detect any localized brittle zones that might be vulnerable to PWHT treatment. Automating the test with the roboTest testing system provides accurate results with reduced operator influences such as hand temperature/humidity variations as well as off-center or angled specimen insertion; providing reliable data with improved sample throughput/error rates than manual testing methods alone can do – creating more throughput with reduced errors overall.

Fracture Mechanics Assessment

Fracture mechanics entails two properties of materials; their strength and toughness. Strength refers to how well a material handles loads, while toughness indicates how effectively it prevents cracks from propagating.

Fracture mechanics has become an industry standard and is recognized in codes for pressure vessels, boilers and piping. At present, this code allows operators to bypass post weld heat treatment (PWHT) if their as-welded fracture toughness meets or exceeds maximum weld thickness consideration.

Engineering Critical Assessment (ECA), which utilizes fracture mechanics theory, allows engineers to make this evaluation. A high fracture toughness value indicates enhanced resistance to crack growth, good damage tolerance and fatigue performance – all essential characteristics in designing, fabricating and operating structures or components effectively and affordably. A higher value can save costs by prolonging inspection intervals or decreasing unexpected failure rates due to increased durability.

Erandid

Exemption is a privilege that allows an individual, group, or organization to avoid certain duties and liabilities. Exemption may involve tax relief (relief from taxes), or exclude amounts from being subject to taxes altogether (non-tax-exempt status). Exemption typically applies to people, groups, organizations and properties alike and appears frequently within legal practice as well as other fields such as business.

Post-weld heat treatment (PWHT) may be necessary for certain welded steel assemblies in order to reduce the risk of brittle fracture by reducing residual stresses and tempering hard, potentially brittle microstructural regions. The need for PWHT depends on its fracture toughness at welding temperature.

PWHT requirements differ between codes due to each being developed by different professional bodies, taking account of extensive experience and engineering practices. To create some consistency across codes for PWHT temperatures, McEnerney7 suggests raising them for P No 4 materials by 30degF like in CEN approaches in order to facilitate unification.