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A very common welding certification test for boilermakers is a 2 inch schedule 120 tube welded in a 6G position using Tig for the root and second pass, and stick welding for the filler passes and cover passes.
Sound Simple? If you think it's simple, you are wrong.
A 2 "pipe done in a 6G position means that the pipe is placed with the axis of the pipe running 45 degrees from parallel.
That means you have to be able to position yourself in a way that allows you to move as you weld to move from the bottom, up the side, and to the top. That sometimes means starting out in the kneeling position, moving to a halfway standing bent over position, and then standing. All while trying to maintain a consistent arc length while moving at a steady even speed. This is the kind of welding that separates the men from the boys.
Whoever selected the 6g position for welding certification tests was either entirely incompetent or an evil genius.
6G welding certification tests for boilermakers are used because they test the welders skill and because a 6g weld test qualifies for all other positions.
Its impractical to test a welder in the field doing the exact job that will be done once the welder is certified.
The 6G test is often considered the best way to cull the not so great welders and hire the best ones.
(although a 2G along with a 5G might be better)
Another reason 2 inch pipe is used for a weld test is that boiler tubes are often the same diameter as the test and they are also often welded while installed and next to other boiler tubes. Welding a 2 inch tube is hard enough with nothing in your way. Put another boiler tube just inches away on each side and the weld is much harder to do.
Testing using a 6G test gives some indication that a welder can weld from both sides of a joint.
I took a 6g test once and then welded a whole bunch of 2g and 5g tube joints in a fab shop. Never did a 6G for the whole year I worked there. So in my case, a 2g and a 5g test would have been more representative of the work done.