Unique Original Articles » Easy Steps to Identify Undercut and How to Stop It.

Easy Steps to Identify Undercut and How to Stop It.

Author: Michael

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.

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.

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.

If you would use 1/8" 6010 5p+++++ on the root and hot pass and the amperage is around 85 then the rod that would burn in that same heat range when going to the 7018 on the filler and cap would be 3/32". This is a good gauge when welding from fast freeze to low-hydrogen.

If the amps are to high for the material to handle you will most always undercut the material. This is true on plate as well. There is no real need to drop down below 3/32". Drop down five amps in heat range if the problem persists.

The rod angle will also have a great bearing on the way the metal lays in the pass as well.
Rod angle should be close to a 90 degree angle when compared to the pipe or plate. If anything the rod should be larger than the 90 degree estimation but almost never less. Of course there are always some exceptions but this serves as a general rule.

Travel speed will make a difference as well. A slow travel speed will naturally result in too much build of metal and heat at the top of the weld causing it to sag. Gravity is working against you at this point. Too fast a travel speed will result in not depositing enough weld metal as you make the pass.

Through proper heat (amps), rod angle, and rod placement. Try adjustments within these three areas and go from there.

Thanks for your time and until next time, take care!
If you found this information helpful then you will want to visit us on the world wide web at http://www.thewelderslens.com. We produce real time welding videos that teach the proper way to weld in may different processes where you actually see the weld as it goes in. Stop by today and receive a complimentary gift and get on our mailing list. Thanks from Michael D. Treadway owner/operator.
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