Originally Posted by
v2505
what do you look for on the back that shows good penetration? I just built a handcycle for my so, and never welded before. I have to agree it must be an artform. My welds were ugly but they seem effective. In the spots that I had the ability to inspect I could see a discoloration on the back side of the weld.
It depends what you're welding and what kind of joint it is. You will always see discoloration from the heat but full penetration is indicated by some actual lumpiness from some melting of the metal.
For butt joints on thicker material (3mm plus) you bevel the edges a bit to make that happen. On a fillet joint on something that thick you would not normally go all the way through but this is made up for by the extra metal added by the fillet.
On a bike frame it's very hard not to go all the way through because the metal is very thin. But you're looking for just a bit of lumpiness on the back but no more, and it should be fairly even all the way around.
If in doubt the best thing to do is a break test on some offcuts of the same material of whatever it is you're welding. It should be impossible to break the weld-- the test piece should break somewhere else first. This won't find all possible defects and mistakes you can make but it's a good start and will show up lack of penetration.