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So I had this little fluxcore welder that I played around with a little the first time I redid my truck. Now i'm using a MIG with argon/CO2 shielding gas but I still wonder about that little fluxcore. I have been hearing that when welding with a fluxcore, all the welds must be sandblasted or rust will creep in from the flux. Is that true? Sounds logical to me.
any insight?

thanks.
Colin

FCAW should be cleaned before being topcoated. The "flux" is the same material as the coatings on SMAW electrodes. It is fairly inert material and degrades mostly into ash and CO2 during the welding process. Most flux is cellulose (dead trees, paper) with stabilizers. Structural steel procedures call for the aggressive removal of welding slag prior to treatment or top coating. Welds are wire brushed or media blasted (which peens and evenly distributes stress). I have seen a lot of welds in my day and I don't remember seeing one that looked like the slag promoted rust in it's self. The slag and resulting voids can help in retaining water and mineral salts that promote rust. There are a lot of naked welds in the world. Slag is also fragile and subject to cracking (or popping off) due to flex or impact which can let water or other nasty stuff under your finish. If you want it to last it should be cleaned and sealed. FCAW or dual shield is spec'd in most structural steel not GMAW(MIG). GMAW welds have no impact value (Charpy). Second spec'd is good old SMAW(stick).

Colin,

This is my take on the situation. Generally speaking, MIG is the process...you can MIG weld with solid wire and shielding gas OR you can MIG weld with a flux core wire that supplies the shielding medium. For the non-technical, both are MIG welding.

When you see welders building a high-rise, this is usually done with flux core if they are MIG welding it. Open welding with a shielding gas is usually not done as the cool summer breeze blows the shielding gas away from the weld puddle.

If you used your machine with the flux cored wire, and it was set properly, you will probably remember that the thick covering of slag almost popped off 100%, by itself, leaving a very clean nice looking bead.

I don't know what kind of welder you had that ran flux cored wire, but with the addition of a gas valve and bottle of shielding gas, it too could have run solid wire perfectly fine.

Some of the big box stores sell little 120 volt machines quite reasonably and offer a "kit" as a upgrade that includes the gas valve and stuff to run solid wire.

I am not a professional welder, but I think the theory about the creeping rust is unfounded when using fluxcored wire.

Stuart

Spot on Stuart.

AWS (American Welding Society) classification for welding procedures are:

GMAW Gas Metal Arc Welding (commonly MIG)
FCAW Flux Core Arc Welding
SMAW Shielded Metal Arc Welding (commonly stick)
GTAW Gas Tungsten Arc Welding (commonly TIG)

and 100 or so other recognized procedures...

whoa.
okay.
There we go.
Thanks guys.

Colin

just my 2 cents here. i am finishing my first weld class. since flux is there to basically clean a weld surface and assist in penetration it would seem that rust may occur in areas where flux went and weld did not since that metal would be exposed to some degree ?

The main purpose of the flux in FCAW (flux core) or SMAW (stick) is to decompose into CO2 gas during combustion and shield the molten weld puddle from the oxygen and nitrogen contamination of the weld puddle. Run a GMAW (MIG) bead with no shielding gas and the result is a weld bead more pocked marked then the surface of the moon. Secondly the decomposed flux (slag) protects the cooling weld bead from reacting with O or N. It also helps the weld bead cool at a slower rate which prevents brittleness. Any decomposed flux (slag) in the weld bead is called "inclusions" and are considered weld defects. The primary result of O or N contamination is voids or holes in the weld bead which reduce strength. The flux is not there to clean weld surfaces as it would be in a noncolesing process such as soldering or brazing. Paint, rust and other contaminates are removed by the welding heat as the arc is 10000 degrees and the weld puddle is close to 3000 degrees. Different types of electrodes will have various materials added to the metal alloy and flux to aid in contaminate removal. Some electrodes have vary little added and depend on clean base metal. Anything added would be considered a contaminate in its self. I did a quick check at the American Welding Society (AWS) and a welding engineering text (Blodgett) and when dealing with mild steel (90% of all steels) flux(slag) inclusions are not a major corrosion concern. The voids and porosity of the slag is a concern because the resulting voids allow other corrosive contamiates in. That said in certain enviroments elements exist that will combine with oxides in the slag and promote corosion. Aggresive cleaning is always reccommended.

O and N are reactive gases which readily combine with other elements and the heat generated during the welding process aids this process. Noble gases such as argon and helium are inert and are also used as welding shield gases. CO2 is reactive, but in such a way that it aids in penetration and considered positive in many welding processes. A small amount (2%) of O is sometimes added to gas mixtures used to weld stainless steel.

The body of knowledge is huge and there are just about as many misconceptions as truths.

Welding is all about HEAT, too much and you get holes. Too little heat and you get no penetration and no strength.

Jim

Good stuff Jim, thanks.

Brian

Can you still "spot weld" with .025 wire and no gas? I've just bought my first MIG, haven't gotten the gas yet. Tried some sample welding on a piece of 22 guage without the gas...The weld looks good to me. I'm diffently going to purchase the gas this week and plan on using it but wanted to see the difference between the two. I will say that the difference between fluxcore welding and straight wire welding is like night and day. Fluxcore does splatter more and not as clean.