Weld Overlay – PWHT Considerations

When PWHT is required for the base metal, it is recommended to use low carbon grade filler metal for the weld overlay to avoid the carbide precipitations or Sigma phase formation.

PWHT may be in the carbide precipitation range for unstabilized Austenitic chromium–nickel steels, as well as within the range where a sigma phase may form, and if used indiscriminately could result in material of inferior physical properties and inferior corrosion resistance, which ultimately could result in failure of the vessel. [1]

Note: When PWHT of the base metal is required, PWHT does not have to be performed for welding attachments to the overlay/clad when the actual overlay/clad thickness is 3/16 in. (4.8 mm) or greater. [2]

When the overlay/clad is less than 3/16 in. (4.8 mm) thick, a specially qualified WPS shall be provided to verify that the attachment weld does not affect the base material. [2]

[1] ASME BPVC SEC. VIII DIV.1 - UCL-34
[2] API RP 582- Normative Appendix B

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