What code are you working to? Are you talking about fillet welds or groove welds? The codes I am familiar with allow a pipe groove weld qualification to be used for plate groove welding, but the plate thickness range is still dependent on the pipe thickness used in the qualification. Also, pipe groove weld qualification typically qualifies for all fillet weld sizes on all plate thicknesses. But again, you need to consult the specific code you are working under.