Depending on the Edition of ASME Section IX, review QW-451.1 for WPS qualification ranges. See Note 6 (2013 Edition)
(6) For test coupons over 6 in. (150 mm) thick, the full thickness of the test coupon shall be welded.
For the 1.5 " to 6" coupon thickness the maximum qualified for a WPS is 8"
Beyond the 6" coupon thickness, the coupon thickness qualified is 1.33T as the maximum thickness.