In your opinion, is MIG an acceptable way to weld structural members, or trailers, or pieces where failure would produce catastrophic results? And I am just talking about MIG, and not FCAW.
I have run some hot welds with MIG, but always wondered how it REALLY compares with stick or even FCAW. And if there are limits where the failure level is lower than with other processes for the same amount of weld metal deposited.
MIG is a low hydrogen process. The gas keeps water vapor away from the melted metal. The most common wire for steel is ER 70S-6. That would be the equivalent of using 7018 with a stick welder. And you are sure that the flux has not adsorbed any water as there is no flux. So in my opinion MIG is good for structural assemblies.
A very common wire is a 70xxx wire. So the deposit is rated at 70kpsi, same as the 70xx rods. The xx is the alloying elements, we generally used the S6 because the extra silicon made it flow out better.
The other half of the equation is getting a good bead flow out on the two sides of the joint. And that means getting the right amount of heat on both sides. So bad technique can give you bad results. Just that bad wire feed generally looks better than bad stick.
So if you know what you are doing, use good wire, set the machine properly, you get an excellent weld.
Low hydrogen is unlikely to make any difference for S275 structural steel - "mild steel". Also for SHS's (squares, rectangles) to S355, as are thin. ???
I've seen plenty of MIG (not-)welds springing apart on construction sites. As you say, run the welds hot and see "fluid" edge tends to reassure there won't be the problem.
Not MIG in moving air / in the wind - so not on site. Stick - they say weld in a protected environment - but in reality the weld seems good so long as you are able to keep the weld run smooth (keep the electrode in the same place as the wind blows you from side-to-side). What is the experience on this?
MIG welds done in a workshop seem tough - as someone else said, ER70-6 is good for general structural. ???
IMO as a novice MIG weldor I'd say the most critical factor is the proficiency of the guy holding the gun. I remember taking a 2-day MIG class at the Crucible
and teacher had us weld, then put one part in a vise and beat the attached part 'til it failed, then look at the joint. The very fact that the welds could fail in this way indicated to me at least that TIG was a better process. By the time the classes were over only two of us (out of maybe ten) were sufficiently proficient to not have our welds fail this way..
MIG in a breeze doesn't work so well, as your shielding gas gets blown away really easily. Lots of porosity will be the typical result, with dramatically lower strength. With MIG, your torch cup will usually be about 5/8 inch or so away from the weld, giving lots of room for your shielding gas to get blown away. Flow rate is adjustable, so often people may try to conserve gas by turning it down too low for welding position and / or conditions.
Many people buy "point-and-squirt" mig machines and think welding is really easy- they don't know what to look for in the puddle to ensure a good weld. Construction site equipment can get left out in the rain, leading to rusty or contaminated wire (if they leave the spools in the machines).
Stick (SMAW) is usually the preferred process for outdoorsy conditions in our shipyard since it produces its own shielding gases and the slag coat protects the new weld. We also have a saying- 100 years of tradition unhindered by progress. Sometimes stick really is the way to go, though.
I can say that I have welded MIG for thousands of hours. I was always amazed when a weldment would just pop off. Looked beautiful, but there was a plane right along the parent metal where it broke off. I know that if I am welding for strength, it takes a definitely different movement, pauses, tip angle, push or pull, heat settings, wire speed, puddle size, direction of travel, and a couple of other things. And then, there's just the slam bang of welding 500 spindles into railings for a fence where failure is not critical. And then, I'd see a loose spindle or three waving there with a beautiful little MIG weld, but holding nothing together.
What size and type of electrode do you use? What amps? Any 6mm 7028 by any chance? If so, never seen one of these in real life, have to confess... What is your main joint type and welding position - well at least for structural steel?
In way of thanks (?!) in advance - here's bit of metallography on some early-stage fatigue cracks of dip-transfer MIG welds
directionality in weld - just that no 6micron diamond before 1micron :-( hence fine lines)
There's a big difference between breezing along doing a spindle every ten seconds, and welding something that has to stand for 100 years, hold pressure, or something that will support people or things. When I want to weld something that HAS to stick, I use stick, and preferably 7018. Mig has been described as a hot glue gun, and that description is accurate and appropriate. It takes welding for thousands of hours to really understand some of the intricate nuances involved, and situations where failure can be either insignificant or life-threatening to those close by.
Stick: 3/32", 1/8", 5/32" are common sizes, 7018, 8018-C3, 11018-M,
12018 are the typical wire types.
3/32"= 75 to 85 amps. 1/8"= 105 to 120 amps. 5/32= 135 to 150 amps.
Amperage is a guideline- position and welder skill dependent, unless working to a heat-control requirement that imposes joules per inch limits.
Nope- I've never run 7028, not sure if we have that in our shipyard.
If you can imagine a way to join two plates together, in positions the Kama Sutra books won't talk about, you'll have a pretty good idea.
Toss a mirror in the mix and you have my typical day.
Butt joints with various bevel preps, lap joints, T-joints, plug welds, you-name-it, we will find a way to produce a 100% joint strength weldment. We'll put down over sized fillets on partial-penetration joints, add fillet reinforcements to full-penetration joints, whatever it takes. And if the engineers call for a non-full-strength joint, we ask them about it just to make sure.
I guess you could say it comes down to mostly fillets and butt-joints with bevels. The two plates could be at almost any angle to one- another, too.
I omitted thickness- take all of the above and apply it to any thickness base metal from 1/8" to over 3".
(no directionality in weld - just that no 6micron diamond before 1micron :-( hence fine lines)