New Welding Thread ?'s for beginners...MIG

[Matt@RFR]

Key:

...so that the metal pieces don't move (as much) from the solidifying puddle.

For terminology, please see the attached drawing. It represents a section view of a fillet weld, also called inside corner weld.

Undercutting would happen, most likely, at the toe of the vertical leg of a fillet weld. It's when some of the base metal is pulled or washed into the weld, leaving a step at the toe of the weld. It's caused by lots of things...too hot, too fast, too shaky, etc.

Are the stringers the welds that go north-south in the pic? That is what I used on my cart before laying the circles (well, more like bloby puddles) on top of them.

The very same.

What you're talking about is a root pass, and a cover pass. You would probably have been fine running that in one pass, as long as you moved ahead and buried the arc in the root before filling up the rest of the joint, but it's hard to say without seeing your weld prep.



(EDIT): On that drawing, the 'fusion' arrow should be pointing at where the toe meets the base metal. I also wanted to mention that welds are sized by the length of the shortest leg, NOT the throat.

A good rule to live by is the weld should be as big (leg length) as the base metal is thick (thinnest member).