PWHT - ON OLETS   ( ASME 31.3 )

K331.1.3 Governing Thickness:

 When components are joined by welding, the thickness to be used in applying the heat treatment provisions of Table 331.1.1shall be that of the thicker component measured at the joint, except as follows:

In the case of fillet welds used for attachment of external non-pressure parts, such as lugs or other pipe supporting elements, heat treatment is required when the thickness through the weld and base metal in any plane is more than twice the minimum material thickness requiring heat treatment (even though the thickness of the components at the joint is less than that minimum thickness) except as follows:

(a) not required for P-No. 1 materials when weld
throat thickness is 16 mm (5⁄8 in.) or less, regardless of
base metal thickness.

(b) not required for P-Nos. 3, 4, 5, 10A, and 10B materials
when weld throat thickness is 6 mm (1⁄4 in.) or less,
regardless of base metal thickness, provided that not
less than the recommended minimum preheat is applied
and the specified minimum tensile strength of the base
metal is less than 490 MPa (71 ksi).

(c) not required for ferritic materials when welds are
made with filler metal that does not air harden. Austenitic
welding materials may be used for welds to ferritic
materials when the effects of service conditions, such
as differential thermal expansion due to elevated temperature,
or corrosion, will not adversely affect theweldment.