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Old 12-11-2010, 06:49 PM
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What does welding in two passes mean, precisely?

The garage that I am working in does not have 220v, so I am working with a Lincoln 140 MIG welder (110v). It says it will weld 1/4" and 5/16" steel in two passes. Precisely what does that mean? As you can tell from the question, I'm a hobby welder with no formal training but a good bit of practice.

Let's say I'm welding a bracket onto my frame. The whole idea is to get penetration all the way through. If the bracket design does not permit me to weld both sides of it to the frame, I have to get penetration with the single bead. But the first pass fills the valley between the pieces. How is a second pass going to help? I now have a thick weld bead to get through, and if the first pass that directly contacted both pieces of steel didn't penetrate deep enough, how is laying a second bead on top of the existing bead going to do it? Does a second pass mean to create short beads and go back over them immediately while they are still hot? I haven't tried that, but I can see how that might do it.

If I pre-heat the steel with MAPP gas, will that obviate the need for two passes?
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Old 12-11-2010, 07:12 PM
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It usually applies to thickness of material your trying to weld. If the material is thin, you may get by with one pass. If the material is heavy or thick, then two passes is required. It basically means your going over the first weld and adding more metal filler with the welder, giving it a stronger weld.

Last edited by kleen56; 12-11-2010 at 07:33 PM.
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Old 12-11-2010, 07:15 PM
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the first pass is for penetration (the bead extending out the back side) and the second pass is to get the bead on the near side. More powerfull welders can do both at once.

If you have some scrap, do some practice parts and then try to destroy the parts. if the part cracks at the weld its not a good weld.
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Old 12-11-2010, 09:31 PM
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The main thing to do, is to grind the bracket edge on a bevel.. That way, When you weld you get 100% weld.. Then just put a cap pass over your fill it weld..
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Old 12-11-2010, 09:32 PM
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The rule of thumb for ANY weld is that you only weld a bead the same size as the thinnest material you are welding. In other words, if you are welding 1/8" to 1/4", an 1/8" weld is all that is needed. The first pass, (aka: root pass) is the most important weld pass, it is the strongest pass. If you can only weld one side of something, bevel both pieces to insure penetration. If welding a bracket onto a flat surface, you can preheat the material prior to welding. Even though you are working with a small welder, you can change shielding gas to help with heat and penetration. The place I work, we use Miller 456 welders and a gas mixture of 88/10/2 which is 88% Argon, 10% CO2 and 2% Oxygen. A gas change will make a big difference in welding.
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Old 12-12-2010, 12:07 PM
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multi pass welding adds thicknees and strength to the weld
http://www.esabna.com/EUWeb/SA_handbook/585sa4_42.htm
this shows a 2nd pass added to a butt weld

you could weld 1'' thick steel with that same welder using multiple passes
building each weld up like using rope caulk to fill a large gap
you want each pass to penetrate the previous pass and base metal

pipe welders typically use 3 passes to weld schedule 40 pipe, each pass is ground ot to keep impurities out of the weld
first pass is your root pass, used to fill the gap and get full penetration inside the pipe. this could be 6010 rod or tig
second pass is a filler and can actually be multiple passes, usually done with 6010 rod
finally a cover pass of 7018 for strength

wikipedia has this picture of a root pass (first pass) it is tig'd but imagine a mig weld in there
it also has an unusual bevel (imagine a 45 bevel in there)

and while way to technical this paper shows multiple passes in a piece of 5/8 steel
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Old 12-12-2010, 12:17 PM
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Quote:
Originally Posted by McDeuce
and if the first pass that directly contacted both pieces of steel didn't penetrate deep enough, how is laying a second bead on top of the existing bead going to do it?
a second pass won't make a better weld if your first pass did not have sufficient penetration.

work on your heat and wire speed so that you have penetration on the first pass
pre-heating is more for controling heat on bigger stuff than for a bracket

butt weld 2 - 1/4'' pieces of steel and break it in a vise
then bevel both pieces 1/8'' and give it a slight gap, weld them together and break it in a vise
generally the weld should be stronger than the base, the weld should not fail
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Old 12-12-2010, 12:21 PM
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Quote:
Originally Posted by ogre

wikipedia has this picture of a root pass (first pass) it is tig'd but imagine a mig weld in there
it also has an unusual bevel (imagine a 45 bevel in there)

We were ''never allowed'' to weld a butt that looked like that... The gap is way to tight.. We did all the stainless steel anchor lines..(4'' PIPE) The gap had to be 5/16'' And all TIG welded.. All X-Ray pipe.. And had to be beveled.. No straight cut..

Good post..Ogre..
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Old 12-12-2010, 12:24 PM
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Quote:
Originally Posted by NEW INTERIORS
We were ''never allowed'' to weld a butt that looked like that... The gap is way to tight.. We did all the stainless steel anchor lines..(4'' PIPE) The gap had to be 5/16'' And all TIG welded.. All X-Ray pipe.. And had to be beveled.. No straight cut..
Agreed, there is no root opening showing in the picture.
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Old 12-12-2010, 12:27 PM
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Quote:
Originally Posted by ogre
a second pass won't make a better weld if your first pass did not have sufficient penetration.

work on your heat and wire speed so that you have penetration on the first pass
pre-heating is more for controling heat on bigger stuff than for a bracket
Pre-heat works on all thickness of material. Heating the material before weld is also used for removing the moisture from the metal, not just for "bigger stuff" penetration purposes.
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Old 12-12-2010, 12:28 PM
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Quote:
Originally Posted by S10 Racer
Agreed, there is no root opening showing in the picture.

On a 5/16 root pass on 4'' stainless steel.. You start with a 5/16'' gap.. But by the time you get the root all the way around.. You Won't have a 5/16 gap anymore.. It will be a tight gap by the time you make the root tie in at the end..
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Old 12-12-2010, 02:08 PM
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Quote:
Originally Posted by NEW INTERIORS
On a 5/16 gap, on 4'' stainless steel, You start with a 5/16'' gap.. But by the time you get the root pass all the way around.. You Won't have a 5/16 gap anymore.. It will be a tight gap by the time you make the root tie in at the end..



This is a little different then what the op is asking,, Just wanted to put this out there..
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Old 12-12-2010, 02:10 PM
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S10 is right about the preheat and it is especially important for those low powered MIG welders using very small wire. Preheat will definitely improve penetration (a lot) and will remove hydrogen from the base metal, also it will help control warpage. The biggest problem however with these little welders on anything thicker than something like body sheetmetal is that the heavier base will act as a heat sink, especially critical on the first pass, and chill the low power weld way too fast. This causes thermal shock to both the base and the bead which can cause brittle welds and unseen underbead cracking, penetration will also be greatly affected by the rapid chilling of the weld. This is usually a problem on only the first pass or two but if that all-important root pass is brittle and/or cracked then the weld is very likely to fail. Preheat is not however a substitute for proper weld preparation and the bevel mentioned is not optional if welding thicker than body sheetmetal because the combination of low power and small wire will not be able to penetrate all the way through even with preheat.

As for preheat in general, think about the thermal shock that occurs when that arc is started and several thousand degrees of heat is introduced instantly into the cold base metal in a tiny spot. Until the surrounding base heats up there will be extreme differences between the molten puddle and the cold base which can be very detrimental to both the base and the weld bead. This critical condition exists only a very short time but significant damage can occur during that time. A little demo for preheat I have used for years is to have someone butt weld two pieces of 2"x 1/4" (the thicker the metal the more pronounced the effect) with a small MIG without a bevel and without preheating. Then weld two identical pieces exactly the same except to preheat to about 400 deg (just to hot to touch with your bare hand) and test both welds by bending. Usually the difference in weld quality is quite apparent before even testing and the "cold" welded pieces will break much easier. 400 deg of preheat obviously is still a big difference between the temperature of the base and molten puddle but that 400 deg can make all the difference between success or failure!
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Old 12-12-2010, 02:18 PM
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If his bracket is no bigger then 1/4'' He shouldn't have a problem... Anything bigger then 1/4'' he will, And I agree with oldred about the thermal shock.. But on a car, I really don't see the need for anything bigger then 1/4'' for any brackets.. IF he bevel grinds and weld's it right, He shouldn't have any problems..
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Old 12-12-2010, 04:05 PM
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Even 1/4" is going to be a bit thick for a 110 machine and .023 wire without preheat, personally I would not weld anything thicker than body metal without heat using those restrictions (110 welder, .023 wire). While certainly possible to weld it by just beveling and omitting the preheat it would still benefit greatly to take a minute or so and warm it up first, it could very well mean the difference between success and failure. That little test using 1/4" plates welded the same except for preheat is usually quite an "eye opener" for most people who are used to using more powerful machines and larger wire which would weld 1/4" with no effort at all.
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