I am calling on the group to help me out with a question that came up the other day. I am working on a project that has dozens of welded tubes in many clusters. It's an airframe. As I was inspecting a finished weld I noticed a very slight undercut. The undercut is estimated at about 1/2" long and 1/32" wide and parallel to an otherwise beautiful weld. Now I know it's not right to weld on top of a weld so my question is can I parallel a weld bead over the slight undercut to reinforce the thined wall?
I'm using TIG so I think I can keep the heat undercontrol. Learning TIG has been one of the more interesting things I have challenged myself with. I really enjoy it.
Shafts don't warp as easily as tube, but you still have to be careful about warpage. Normally to counter the effects of warpage you preheat the piece before you weld it and then slow cool it so the heat is more uniform throughout.
How deep was the undercut and how sharp. Check the specification to see if it's acceptable or not before you go doing extra welding on it. Having said that, a TIG run over it probably won't hurt it.
I need clarification. What is the primary reason for not welding over an existing weld? Does it weaken the weld or is it a warpage issue. I know when I O/A weld the heat gets all over the place and distortion is harder to control. With TIG I don't see the expansion I do with gas welding. I also read that one should never grind a weld. Here is a scenario that I am faciing. I am making a set of straps out of 4130. Basically its a strap wrapped around bushing stock then edge welded. The edge welding does not add any strength to the fitting does it? My information is the edge weld seals the fitting from weather, so whats wrong with dressing the edge welds with a little filing and a vertical belt sander? I'm at the point where I need explanations of these axioms of welding.
I think many of them are "axioms of FAA inspection" more than they are axioms of welding, as such. At least most people welding 4130 who post here seem to be mostly making airplanes, and have their own set of axioms which have that flavor; not a field I know well. Presumably they don't want welds ground so that they can see evidence that the weld was put in correctly, not done wrong and ground to shape - grinding itself may also provide a place for a crack to start. A lot of the more arcane things thay don't want you to do are based on one or more accident investigations where "some little thing" lead to a catastophic failure. I doubt that they do a very good job of explaining all the "whys" when they write down the "whats", so you might want to ask those questions to someone who should know the whys (hopefully an inspector would, an experienced airframe mechanic should, probably many in the EAA do, etc).
There certainly is no general axiom against grinding welds in the equipment maintenance welding field, which I do know pretty well. And welding over welds is common in all sorts of heavy plate work (though one does want to get the flux off first, if using a process with flux). My axioms tend to be "use 6010 or 6011 until you're up out of the rust", and "hardface only when the thing being hardfaced is already sharp". Neither has much application to airplanes.
It doesn't weaken the weld, but it does increase the weld stresses in that one area. If anything he problem is that it mmakes that one section of weld stronger than other areas.
A smaller consistent weld is often stronger than a larger inconsistent weld. The inconsistent weld creates stress risers where it is stronger or weaker. A consistent weld distributes forces evenly over the whole area.
Heat distortion is your main enemy when working with tubing. Too much weld on one side will seriously distort the tube to one side.
You can grind welds all you want as long as you don't remove the metal you need for strength or introduce sharp fissures that will act as stress risers.
I don't see any problem with your cleaning of the weld. What you want to avoid is grinding critical structural welds flat so they look pretty. Often you will seriously reduce the strength of a weld if you grind too much away.
On heavy plate this isn't as much of an issue.
The best solution is to make your welds so pretty that it would be a crime to grind them.
Of course if you are talking about aircraft parts then you can't grind any weld that will need to be inspected. The Inspectors want to see your weld "as is".
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