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Mig Welding Preparation Procedures


knots

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Weld Fusion/Penetration: When I first started welding using MIG welders I had troubles with failed welds. I quickly learned that you cannot depend on your MIG welding machine to cut through mill scale on the surface of the work piece, and provide penetration into and fusion with the metal below. Over time, experience and research revealed the preparation procedures that I now use whenever I weld. They are as follow:
GRIND OR BRUSH THE WELD AREA CLEAN: Dependable welds cannot be consistently achieved without good cleaning preparation of weld areas.
IF A GOOD STRUCTURAL WELD IS REQUIRED GRIND BEVELS TO PROVIDE GOOD PENETRATION INTO THE WORK PIECES.

IF A YOU GRIND YOUR WELD TO BLEND OR MATCH THE WORK PIECE PROFILE OR SHAPE, GRIND A BEVEL FOR GOOD PENETRATION. If this is not done, when the welded joint is ground, most of the weld will be lost leaving a weak joint.
SELECT THE WIRE SIZE TO MATCH THE WELD SIZE: Fusion at the root of a weld is influenced by wire size The time the arc is held or dwells on the weld without excessive weld metal build up will effect the quality of the weld at it's root. Select a wire size that will allow you to see the fusion and degree of penetration as it occurs. A smaller size wire will give you a bit more time to weld deep into the root of the weld joint. I call this dwell time. Use small wire for small work pieces.
ADJUST SHIELDING GAS FLOW, WIRE FEED RATE , AND WELDING AMPERAGE AS RECOMMENDED BY THE MANUFACTURER OF YOUR WELDER.
BE SURE THAT YOUR WELD WORK PIECE IS PROPERLY GROUNDED. If your wire fails to strike an an arc, check your ground. If your arc is unstable or doesn't sound right, check your ground. It is a good idea to purchase a good quality cast bronze grounding cable clamp to replace the stamped sheet metal clamp shipped with most welders.
Both powered by 220 volts. However neither of my two welders are big machines. They are: an older Millermatic 35, 150 Amp @ 60% duty cycle, Millermatic 185, 150 Amps @ 60 % duty cycle. I run >030 wire in the MM35, and .023 wire in the MM185.
I have had good results with my MIG welds since I started following these rules. The picture below shows last winter's project, my 4x8 heavy tandem trailer. It's maiden voyage was a trip to HGR Industrial Surplus in Euclid Ohio to pick up my Gorton MasterMill. Since then I have also moved a power hammer and loads of other stuff. All of the welds were made with my ancient MillerMatic 35.

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Good start on explaining how to prepare the work for welding for beginners.

But you failed to mention : Keep the wire at the leading edge of the puddle.

You talk about Penetration and fusion in passing. How about commenting on how to get good penetration using Mig ? What constitutes good penetration for this process ?

Seems like there could be a lot more discussion on this subject that would benefit us all. I am not an expert either maybe some of the pro's can fill in more detail.

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You can't weld what you can't see. Auto-darkening lenses are the greatest thing since sliced bread. Even a cheap import is better than a fixed shade for casual use.

You typically MIG weld 12-24" from your nose. If you can't read the fine print at that distance, get corrective lenses. Cheater lenses are made to fit standard 2x4 openings, prescription safety glasses help, but bifocals never let you hold your head right, and contact lenses can cause eye burns. Wear your safety glasses, always! Even under the face shield! (A hard hat is a helmet. It protects your skull from falling objects. They make welders face shields that attach to them for construction site welders. Those are the only real "welding helmets". Rant over.)

If you need more light before you strike an arc, get a 500W to 1000W work light or two, and hang 'em where it will shine on your work, but not in your eyes or at the back of your head. Even with a fixed shade lens, you can get plenty of light by working in sunlight.

Block any drafts or breezes that might disperse your coverage gas. If you can, weld in the flat position, where gravity is your friend. Pros use chains, cranes and powered weld positioners to move the work whenever possible. Race car builders build the frame in a big rotisserie.

Another way to ensure a good bond is preheat, minimum 70F, 200F is better, maximum 500F for preheat and interpass temps. Thicker metal, more heat. Cold starts and restarts are the biggest cause of test failures. Even a wimpy 120V welder can do a more respectable job if you preheat the metal. For autobody work and other sheet metal, a heat gun is fine. For plate, a propane or acetylene torch with a rosebud tip. Anvil repair, weed burner or campfire.

Post heating and slow cooling can reduce weld cracking and warpage. So can "skip" welding, working by jumping around to spread the heat load, the same way you tighten lug nuts on a wheel rim.




The way to tell if your welds are any good is the same way the pros do: test them. All you need is a big vise, and/or an anvil, and a big hammer. Anybody here have access to that kind of exotic hardware?

Get some scraps of whatever you are welding, run a 1" bead with the same joint profile and settings and position, put it in a vise, or on the anvil, and beat on it with a big hammer. Try to stress it at 90 degrees to the weld. Ideally, the base metal should fail and tear out before the weld does. Be warned: cast iron or leaded steel will tear out every time, some metals just are not weldable with MIG.

If the weld cracks down the middle, or separates from the base metal, then you know you have a problem.

Tiny bubbles trapped in the weld bead are known as porosity, kind of a swiss cheese effect. Paint, grease, rust, dirt or moisture can cause it, but lack of shielding gas coverage is the usual culprit on clean metal.

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You can't weld what you can't see. Auto-darkening lenses are the greatest thing since sliced bread. Even a cheap import is better than a fixed shade for casual use.

You typically MIG weld 12-24" from your nose. If you can't read the fine print at that distance, get corrective lenses.




Thanks John For all of that .

There is another question regarding penetration of MIG welds into the parent metal. Whenever MIG welds are discussed it seems to be accepted that deep weld penetration cannot be expected. However I do not recall ever having seen a discussion of what constitutes an acceptable weld penetration depth, or what constitutes "good penetration" for this welding process. I have been left with the impression that so long as the welded area is completely fused all is OK. I have witnessed bar joists being fabricated using GMAW process so am comfortable with the idea that these welds can be considered structural when properly executed.

So my question is: What constitutes good penetration for Mig welds ? Does wire size influence weld penetration, if so how.
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Just a side note before we start. The AWS and ASME do not recognize any standardized welding procedure utilizing short arc MIG as a joining process except for sheet metal, and as a root pass followed by another process, such as stick or flux core, when used for plate or pipe. This is not the be-all and end-all process, it is just fast, cheap and shoddy. It may be acceptable for your purposes, but is never used in critical applications.


Wire size relates to carrying capacity as far as voltage and amperage is concerned. Voltage + Amperage is your heat input, to melt the wire AND the base metal. Bigger wire = more juice. Match the wire thickness to the base metal thickness. Good penetration is getting an equivalent bond that you would expect from stick welding.

The base metal acts as a heat sink, that is why preheating aids penetration. You are moving faster with MIG than stick, so there is less arc preheat, and therefor less penetration. Iron is a lousy conductor of heat, that is why you can hold one end of a bar and forge the other. Don't try that trick with aluminum or copper. (Thawed hamburgers cook faster than frozen. The middle never gets hot before the outside is burnt. Same principle. That is also why you need a soaking heat to forge large stock without cracking.)

Little 120V/20A input welders can only run .023" solid wire. Good only for joining sheet metal, say 1/8" max single pass weld. The equivalent V/A output of 1/16" stick rod. (Can you weld (slightly) heavier metal in multiple passes? Yes. You can dig a swimming pool with a folding shovel, too.) Switching from 75/25 Argon/CO2 mix to straight CO2 and .030" wire will give you a hotter, uglier weld. Now you have gone from MIG to MAG welding, 'coz CO2 is an Active gas. And cheap, too. But they are dumb, cheap machines with limited duty cycle, and can only do short arc, and some flux core. Don't believe the salesman hype, they are meant for autobody and muffler work. They do OK for that.

Move up to a quality 240V/30-50A input unit, and in addition to .023" you can run .035" or .045" solid wire. And the wire selection is now huge: standard mild, CroMo, hard-facing, the list goes on. Now you can run the equivalent V/A of 3/32", 1/8" or even larger stick rod. Join 1/4" plate in a single pass, 1" plate in multiple passes. With different gas mixes and programmable computer chips, you can go beyond short arc MIG and run RMD, pulse spray or true spray arc, which is SCREAMING hot, to join heavy plate in a single pass.

Industrial 480V/100A and bigger machines can run 1/8" wire and join 1" plate in a single pass, but you have to wear a moon suit and shade 14 fiberglass hood (the plastic ones melt), so lets not go there.

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WOW! John.
Your inner welder is showing through. Great advise. I'm constantly attempting to convince students to NOT purchase that $100.00 battery charger that has welding machine painted on it.

Your advise to stay on the leading edge of the puddle is correct. I'm going to add that pulling the wire with the arc following does give better penetration into the base metal. Better penetration is due to the wire/arc directly on the base metal. Pushing the wire with the arc in front actually pushes weld metal under the wire and hinders the penetration. Pushing does give the bead a flatter and better looking shape. Pulling the wire gives a narrower and taller bead.

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  • 3 weeks later...

Good stuff John. Los Angeles city welding codes are some of the strictest in the country (so i have been told) yet i see the structural ironworkers using mig guns to weld structural members. They are using heavy duty flux core wire (i think its 1/8). I always wondered how they get away with this. They use the portable lincoln wire feeders run off a power source. You would think that these strict codes would require stick.

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Very nicely put John. This is the best and right advise. Everyone should take note of this and follow it. Like I say " There is a reason why they do not put up high rise buildings with 110 MIG welders" These are the reasons why.

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Good stuff John. Los Angeles city welding codes are some of the strictest in the country (so i have been told) yet i see the structural ironworkers using mig guns to weld structural members. They are using heavy duty flux core wire (i think its 1/8). I always wondered how they get away with this. They use the portable lincoln wire feeders run off a power source. You would think that these strict codes would require stick.

Flux core is not MIG. It meets the code, they are not getting away with it. Most likely they run 1/16" wire.
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Flux core is not MIG. It meets the code, they are not getting away with it. Most likely they run 1/16" wire.


It is frustrating that most people think MIG welding is anything that comes off a spool and out of a gun.And this leads to a lot of mis information. Because someone in their garage can not make a good weld with a 110 wire feed machine from costco or home depot they automatically think that all wirefeed is the same and therefore bad. Those people should stick with stick :P I have done a lot of earthquake retrofittinig of buildings here in Ca. and have only used wirefeed. Dualshield in the shop and fluxcore for outdoor installations. These moment frames are all inspected with UT. Similar to X-ray. Very stringent on the requirements.
Rob
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  • 1 month later...

One thing that I try to do when mig welding is to push the puddle, this tends to keep the arc out in front and allows for better penetration, keep moving and tie in to both members that are being joined. "Pushing the puddle" = the direction of travel- lets say you are welding from right to left, you would point the gun to the left- point the gun in the direction of travel. I have seen lots of welds fail that looked good but did not tie in to one side, just laid on there

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Flux core is not MIG. It meets the code, they are not getting away with it. Most likely they run 1/16" wire.



Most likely 5/64 NR 232 check out the parameters 365 amps 22 volts 8.7 lbs/ hour deposition rate- thats hot! Most guys are probably running around 300 amps still real hot http://igor.chudov.com/manuals/Innershield_flux_core_wires.pdf
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Has any one experience using the MIG Gas Saver system being promoted on the following site ? www.netwelding.com/prod02.htm

They claim that the volume of shielding gas stored in the hose between the regulator and the machine connection causes a surge in gas which sucks air into the arc area when the weld is initiated. They say this causes unstable starts, porosity, and excessive splatter at the beginning or the weld and wastes a significant volume of shielding gas.

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  • 2 weeks later...

One thing that I try to do when mig welding is to push the puddle, this tends to keep the arc out in front and allows for better penetration, keep moving and tie in to both members that are being joined. "Pushing the puddle" = the direction of travel- lets say you are welding from right to left, you would point the gun to the left- point the gun in the direction of travel. I have seen lots of welds fail that looked good but did not tie in to one side, just laid on there

Dragging will give better penetration; although marginal. Pushing will give a wider flatter bead and dragging will narrow more convex bead.
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