Watch For Welding Undercut

Undercut, is when the edge of the weld pass does not fill the edges along the existing metal where the weld is being placed. It does not always just occur in lap fillet welds against the parent metal. In this article I give some clear tips and indicators on how to be aware of undercut and how to prevent it.

First of all I would have to ask the question just for general purpose; do you know what undercut is? When we talk about undercut in the welding world what picture immediately comes to mind?

No silly, it’s not the missing money you thought happened on your last check. You just didn’t add right.

Undercut, is when the edge of the weld pass does not fill the edges along the existing metal where the weld is being placed. It does not always just occur in lap fillet welds against the parent metal.

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Undercutting the weld is not acceptable from the perspective of inspection and can lead to weld failure eventually. When welding under strict code or formal welding guidelines, this seemingly small error by the welder will almost always have to repaired or fixed before the welding inspector will sign off on the weld.

Undercut usually happens on the top side of the weld. The reason for this is simply due to gravity pulling on the molten puddle when the following combination of items are not set correctly: heat (amperage), travel speed, proper placement of either rod angle or filler material.

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Now that we have an understanding of what the problem is, what is the solution?

When trying to correct any problem in welding, always start with one area and work from there.

When trying to correct undercut, the first area of the weld that comes to mind is the heat being used in the process. That term, the heat, refers to the amperage setting being used to burn the rod or filler material. Rod size will dictate your range of welding amps. Normally you will not see undercut on fast freeze rods such as 6010 or 7010. You will normally see this type of problem occur in Low-Hydrogen rods such as 7018 or 8018.

When welding on material that is not extremely thick your rod size or diameter should not be excessive in size. In other words, don’t use 5/32″ 7018 on 2″ pipe or something such as that. The rod is oversized for what the material can take as far as heat output and puddle size.

Burning up the material you are welding on is a major culprit of this problem.

There is no need to burn up your material.

When I am welding plate or pipe where I have to go uphill and fight gravity at some point in the weld process, I always try to use no more or less than a 3/32″ rod when working with stick rod. When we are working with TIG this is not the case but with stick it makes a difference.

Someone recently told me of his welding experience with undercut. I began to ask questions to isolate the problem. The first question I ask is what is the material you are working on? Carbon, stainless, etc. Carbon was the answer. Okay, what position were you in? Overhead, vertical, 6G etc. Was it plate, pipe, or tube? Pipe in a 6G. Okay. What size was the pipe?

2″ schedule 40 I was told. Okay. What size rod were you using? 1/8″ he said.

I said what? Why would you use 1/8″ rod on such small material that can’t take that kind of heat?

That size pipe cannot take that kind of heat. I never use 1/8″ 7018 or bigger on something that small. In the 6G position all of the heat is going to the top part of the pipe. By the time you get to the cap or last series of welding passes, the pipe is extremely hot and at that point there is really no place for the heat to go anymore. Gravity is pulling the weight of that large 1/8″ puddle away from the top where you need it to go in order not to undercut the pipe.

Industrial Welding Inspection of San Diego
9914 Oak Grove Dr. San Diego, CA 91916
(619) 329-5877

Undercut, is when the edge of the weld pass does not fill the edges along the existing metal where the weld is being placed. It does not always just occur in lap fillet welds against the parent metal. In this article I give some clear tips and indicators on how to be aware of undercut…

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