متطلبات ASME PWHT

ASME standards help companies ensure the safety and effectiveness of mechanical systems. Furthermore, they assist them with operating equipment efficiently and reliably while reducing downtime while ensuring regulatory compliance.

Post Weld Heat Treatment (PWHT) requirements depend on material type and thickness; generally speaking, thicker walls require PWHT; however, there may be exceptions available for certain materials.

Minimum Temperature

The minimum temperature that must be applied during PWHT depends on the type of steel being used and typically its thickness. As a general guideline, BS EN 13445/BSPD 5500 specifies welds up to 19mm thick must be PWHTd; however ASME VIII mandates welds of up to 15mm thickness require PWHT. As the temperature increases, material strength decreases so ensuring adequate support during PWHT is essential; typically this is accomplished using trestles that fit components perfectly and spaced appropriately during PWHT is effective.

EPRI-funded tests have demonstrated that PWHT temperatures of 1100 deg F can significantly alter the hardness and toughness properties of quenched P No. 4 materials, and that this temperature falls close to the lower critical temperature specified in B31.1 and B31.3. If minimum PWHT temperature could be reduced down to this point, additional margin would be gained.

PWHT poses another potential complication by distorting the base metal and creating distortion, leading to weak welds and compromised structural integrity. Therefore, it is vitally important that during heat treatment a sufficient soak time of one hour per 25mm thickness of base metal thickness be established and adhered to.

Minimum Time

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Post weld heat treatment (PWHT) is an integral component of manufacturing pressure vessels and piping, helping reduce residual stresses, control material hardness and increase mechanical strength. If performed improperly, residual stresses could combine with service load stress to exceed material design limitations and cause weld failure or brittle fracture. Current design codes in the pressure vessel industry stipulate that PWHT may be necessary when wall thickness exceeds an set value determined by Charpy test properties of material – however this approach can be overly conservative, with different codes using different limiting thickness values determining required.

Maximum Temperature

Power piping requires maximum temperatures that depend on both material type and job specific factors. For instance, for 10 in. O.D. Schedule 80 (0.594 in) ASTM A335/A335M grade P22 seamless piping materials that requires postweld heat treatment of around 1300 degF held for 4 hours post weld is generally advised; this could differ slightly based on other job-specific demands.

Welding is a time-intensive and rigorous process, often affected by environmental conditions. To ensure welds can withstand pressure and corrosion in nuclear power plants where high standards of safety and reliability are essential, we adhere to stringent industry standards.

Current design codes cite limiting thickness criteria as one of the key determinants in whether a weld requires postweld heat treatment (PWHT). This criteria takes into account both material Charpy test properties and anticipated service temperatures as part of its determination process.

However, this method has its limitations and should only be used with extreme care and accuracy. Misinterpretations of results could compromise safety and performance; to understand its restrictions before using it. Thankfully, there are ways to enhance its accuracy.

Exemptions

Recent updates to ASME B31.3 Code have allowed for exemptions from postweld heat treatment on carbon steel pipe for all control thicknesses and weld types, yet this change must first be considered by end users before being implemented successfully. Current requirements rely heavily on factors like alloy level and preheat conditions – these do not comply with fracture toughness curves found within these codes.

Industry experts were polled to ascertain the source of these requirements and it was discovered that they are often determined by traditional practices in specific industries rather than technical data, design calculations or experimentation. Furthermore, requirements differ greatly between codes; this article reviews exemptions from PWHT in several current piping codes while offering some rationalisation solutions.

CP-4 and CP-5A materials used in nuclear service qualify for exemption based on their ability to function at low temperatures, which reduces their risk of hydrogen delayed cracking (HDAC). Furthermore, they contain lower carbon contents than other carbon steels and are usually welded using low hydrogen electrodes or bare wire processes which do not require PWHT; consequently these exemptions should be reviewed since they could lead to inefficient welding and substandard welds.