Material and Product Form: Plate, Small- and Large-Diameter Pipe
Plate versus pipe
Plate tests use coupons from flat stock; pipe tests weld all the way around a cylindrical section. Pipe demands root control: gap, root face, and, where the procedure allows it, backing, consumable inserts, or a carefully controlled root pass. An open-root pipe test qualifies a different set of conditions than a plate test with a backing strip.
Diameter categories
Small-diameter pipe is generally taken as about 2 in nominal (2.375 in OD) and under, with large diameter above; confirm the exact cut-off in your code. Smaller pipe is harder: tighter curvature forces constant angle changes, and heat builds quickly, so current and travel speed usually drop. Larger pipe tolerates more current and a wider weave but invites distortion, and thick wall demands good root preparation or penetration suffers.
Qualification ranges
Most codes let a test on thicker material qualify thinner material, and a larger-diameter pipe test qualify smaller diameters down to a limit, not the reverse. Diameter, wall thickness, base-metal group (for example P-number or A-number grouping), filler and position together define what you may weld in production.
Worked example
Qualified in 6G on 6 in schedule 40 pipe with a GTAW root and SMAW fill, do not assume coverage of 1 in pipe or a 0.5 in wall; check the code's diameter and thickness ranges before accepting the job.
Traps
Assuming a plate test covers pipe, that a pipe test covers a different base-metal group, or that any pipe size is covered by one coupon.