What is hardening mig welds?

[DanGunit]

Originally Posted by Richard

According to my welding text mig welding is usually done with electrode positive polarity and tig welding on steel is generally electrode negative. The flow of electrons into the positively charged filler wire probably heats it more than the weld pool heats the filler in tig. Maybe this extra heat is affecting the metal.

Originally Posted by Gene Olson

my guess is you are superquenching the red hot material and hardening it.

You harden tool steel by heating to red heat (non magnetic) and quenching.
Some steels harden best in oil, some in water, and some in air.

regular low carbon steel typically does not air harden you can even take a piece heat it red hot cool it off in water and still file and saw it.

However low carbon steel can be superquenched, cooled very quickly, and it will harden. A piece of plain old HR can be forged into a chisel, superquenched and used to cut the untreated portion of stock. (google superquench, a salt solution quench with added soap to change the surface tension of the liquid so it stays in contact with the metal even as bubbles form. (without the soap, the vapor bubbles will insulate the metal from the cooling water, and salt water conducts heat better)

A mig weld has a very narrow heat affected zone. both sides are cold and also conductive radiators. They are still cold when the surface is flash melted and filler added. They quench the weld at a very fast rate and it hardens.
When you gas, or tig weld, you melt the base metal, the metal next to it gets hot and the metal next to that gets nearly as hot, and the metal next to that gets almost that hot . . . until it tapers off.
In a gas or tig weld you do not have that severe temperature difference. There is no possibility of quick cooling all the metal around the weld is warm.

If you preheat the weld area, you won't get hardening. And I would venture a guess that tempering the weld at about 500+ degrees would get rid of the hardening too.

I normally try to stay out of these discussions, but I think a combination of the above two ideas are the main culprits in hard Mig welds.

The biggest factor influencing the weld integrity is the polarity used in the Mig welding process, electrode positive, this means that the electrons flow out of the base metal toward the torch and concentrate the majority of the heat there. This polarity is required by the Mig process to melt the constant addition of filler wire from the torch, but it leaves the base metal colder.

By contrast, the Tig process uses the electrode negative polarity, where the electrons flow from the torch to the base metal, concentrating the heat in the base metal and mimicing the function of the oxy-fuel torch process.

The biggest functional difference is that the Tig and oxy-fuel processes allow you to melt the base metal and filler metal together completely as you progress, whereas the Mig process progresses regardless of weld fusion. The Mig process heats the base metal to a lesser extent, sometimes barely melting the edge of the work pieces, but it melts the filler wire completely until a ball of fluid molten metal falls to the workpiece.

Since the base metal is much colder than the molten ball of filler, the filler is quenched on contact until the base metal reaches sufficient heat to allow proper fusion to occur. Since most sheet metal welding is done in short bursts the base metal doesn't reach sufficient temperatures for proper fusion.

In this video you can see the short circuit welding action and it is plainly evident that the filler is radically heated while the base metal remains relatively cold and the filler piles up on top of it.

http://www.youtube.com/watch?v=UFFSJ...eature=related

It is possible to get more complete fusion with Mig, but it involves running a short wire stickout and a long arc to generate sufficient heat, but the risks are blowouts, burn back to the contactor tip, and drastically increased spatter due to the longer distance the molten metal has to travel to the base metal.

There are certain processes that allow you finer control over Mig welding parameters, such as those developed by Fronius;

http://www.youtube.com/watch?v=_WrhW...eature=related

This process is called CMT (Cold Metal Transfer) and is especially usefull for things like additive manufacturing and cladding where they boast low weld dilution. This is an exaggerated example of the above assertion, low weld dilution means that there is very little base metal melting and mixing with the filler metal. This is important if you want to build up lots of weld metal without heating the base metal very much, or welding dissimilar metals like steel and aluminum where the dilution would cause reactions, but it is the opposite of what we are looking for in welding sheet metal with complete fusion and resulting ductile joint.

Cheers,
Daniel

[hodijag]

Without wanting to state anything scientific, I find I am happy when patching in a panel to tack and fill in with MIG first (easy to add a bit of filler and for tacking when there is a bit of a gap). I then grind the weld and stretch it back (easy because the MIG spots weld bead is unpenetrated and only on one side so the dolly is on the panels). Grinding allows to keep a minimum of added filler material and I then run over the weld with my TIG to fully penetrate and soften up the weld bead (no filler wire).

My experience with this is that the resultant weld appears soft and allows planishing of the weld area without the weld line interfering.

I described the process in a bit more detail on the other site (XK panel repair).

May I suggest to have a go at reheating the MIG weld with TIG or oxyacetylene and see for yourself.
If you also find the MIG weld has softened again after TIGging or oxy-acetylening it over , I would tend to think this would confirm it is the heating/cooling cycle that makes the MIG weld hard.

Kind regards,

Marc