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QW 403.8 is the standard thick range shown in Table 451.1.
QW 403.6 only applies if supplementary essential variables apply. When this is the case, the minimum qualified thickness alone is at issue:
For example, if you qualify on 3/4" plate, your WPS qualification range would be 3/16" to 1.5" provided supplemental essential variables do not apply.
Using the same example, and if supplemental essential variables do apply, your WPS range would be 5/8" to 1.5".
QW 401.3
When essential variables are qualified by one or more PQRs and supplementary essential variables are qualified
by other PQRs, the ranges of essential variables established by the former PQRs are only affected by the latter
to the extent specified in the applicable supplementary essential variable (e.g., essential variable QW-403.8 governs
the minimum and maximum thickness of base metal qualified. When supplementary essential variable QW-403.6
applies, it modifies only the minimum thickness qualified, not the maximum).
QW 403.6
The minimum base metal thickness qualified is the thickness of the test coupon T or 5/8 in. (16 mm), whichever is less. However, where T is less than 1/4 in. (6 mm), the minimum thickness qualified is 1/2T. This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic or P-No. 10H material is solution annealed after welding.