New to welding.

And want the inside skinny on what exactly is flux core wire?

Then stick around.

And you’ll find out all you need to know.

By the end of this article you’ll know what is flux core wire.

When it’s best to use flux core wire. And why.


What Is Flux Core Wire Exactly?

It’s a type of welding wire used in automatic. Or semi automatic welding machines.

What’s Inside Flux Core Wire?

Made as a long tube, your flux cored wire has your weld metal around the outside.

And inside this wire tube are compounds.

And these compounds are the flux.

Inside Flux Core Wire
Inside Flux Core Wire

These compounds when melted in your welder’s arc gives you a shielding gas around your hot metal. And that gas protects your weld from the air.

And the reason your weld needs this external shielding.

Is because without this shield. Your hot, molten weld pool would pick up impurities from the air.

Why is that an issue.

Well, it leads to a poor weld with porosity. Or a failed weld that breaks.

Plus, your flux has special alloys and de-oxidizers.

And they work to pull stuff that would pollute your weld. Out of your weld pool and into something called slag.

What Is This Slag From Your Flux Core Wire Welding?

Slag is key to welding with flux core wire.

And you’ll find it as a hard coating on top of your weld when your weld cools.

And underneath you’ll find your nice smooth weld.

Slag and Direction Of Travel
Slag and Direction Of Travel

Good, well formed slag that simply glides off your weld when you’ve finished your weld is your sweet spot.

You’ll find a document here on the site that goes into much more detail on welding slag.

Follow the link text to find out more.

What About Your Metal And Your Flux Core Wire?

You buy your flux core wire to match the metal you’re welding.

What does that mean?

When you’re welding mild steel or low alloy steel.

You buy flux core wire for mild steel.

Flux core wire for mild steel has mild steel around the outside of the flux.

And that steel acts as your filler material for your weld.

What about galvanized steel?

Look for self shielding flux core wire advertised for galvanized steel?

Welding stainless steel?

Then you buy stainless steel flux core welding wire.

You pick your stainless steel flux core wire to match the type of stainless steel you’re welding. Want to know more about flux core stainless steel wire. Follow this link.

What Types Flux Core Wire Are There?

You’ll find two main types of flux core wire.

Self shielded flux core wire (FCAW-S).

And Gas shielded flux core (FCAW-G). Gas shielded flux core wire you’ll also find called double shielded.

Self shielded flux core wire was introduced to the market in the early 1960s. And Gas shielded flux core in the late 1950s.

About Self Shielded Flux Cored Welding Wire

What Is Self Shielded Flux Cored Wire?

Self shielded flux core welding wire is the wire of choice for the beginner, hobby. And semi professional welder. Because it’s the easiest and most convenient to use.

Self shielded flux cored wire creates shielding gas needed for your hot weld pool from the flux inside the core of the wire.

There’s no need for a tank of external shielding gas with self shielded flux cored wire.

As the flux inside your wire melts.

Your flux forms your shielding gas at the point of your weld.

The gas from the flux protects your weld pool from the air around your weld. Plus the flux works to form something called slag over the top of your weld.

And pulls contaminants out of your hot welded metal.

The metal round the outside of your flux melts and gives you your filler metal.

FCAWS Shielding Gas
Flux Core Wire Making The Shieding Gas

The filler metal in self shielded flux cored wire can be mild steel or stainless steel.

When Is Self Shielded Flux Core Wire Used?

Picking to use flux core wire really depends on the job you’re doing and its location.

Let’s say you’re a farmer.

You’ve broken yard or field equipment. And it’s way in the back 100 acres. So it’s some distance away or tricky to get to.

Long Way Down The Road

And you know up there is pretty windy.

If you were to take your MIG welder and do a MIG welding repair.  You’d need a wind shield. Or tent so the wind didn’t blow away your external shielding gas.

Plus if it was near your workshop. You’d prepare your weld metal real nice for MIG welding. But out there you can’t.

And the broken piece is in an awkward spot on the vehicle.

What now?

The weld is vertical and hard to get to.  

You’d use flux core because it’s ideal for this situation.

What Is Self Shielded Flux Core Wire Good At?

You’d use self shielded flux cored wire when weld outside.

Where you don’t want to. Or can’t take a bottle of external shielding gas with you.

It’s one less thing to bring to your weld.

You may find in your particular location or area of the country. It’s tricky to get shielding gas or transport it. Flux core is then your best welding option.

Flux core self shielded wire tolerates ‘dirty’ steel. You’ll get away with more dirt on your steel compared to MIG welding. And it’s better to use when you’ve some rust, mill scale or oil on your base mild steel.

That said. It’s best to have your steel as clean as you can.

But when you can’t get completely clean steel you pick self shielding flux core wire.

And it’s great when you can’t weld on the flat. Then you pick the type of flux core self shielded wire for welding out of position.

What Is Out Of Position Welding With Flux Core Wire?

Vertical welding, welding overhead, welding that is not flat horizontal welding is called out of position welding.

Weld with self shielding flux core and you’ll get a faster weld.

When you compare to solid wire.

You’ll find flux core wire welding best for welding galvanized steel.

Or welding hard to weld steel.

It’s easier to use when you lack welding skill. Because it’s a more forgiving welding process.

And flux core wire welding makes for strong ‘clean’ welds with good penetration.

What About Shielding Gas Used With Self Shielding Flux Core Wire?


If you do you’ll get problems with your weld.

The compounds in your wire’s flux need to react with air. You’ll blocked them if you add external shielding gas.

And if you do then the flux inside your welding wire made to help your weld. Instead contaminate your weld.  Causing a poor weld and cracking.

Picking Self Shielding Flux Cored Wire, What To Bear In Mind?

What’s Your Welder Able To Do?

A low cost flux core only welder?

Or a MIG welder that can weld with flux cored wire as well.

Its wire feed speed, Amp output and voltage needs.

Take a note of the minimum and maximum settings as you’ll need this information when you choose yourself shielding flux core wire.

Think About The Metals You Plan On Welding

How thick is your metal?

Generally you don’t weld thin metals (24 or 22 gauge) with flux cored wire.

Because the welding process it too hot for thin metals.

Thicker metals (18 gauge and thicker) are great for flux core wire welding.

Are the joints straight and clean? Or are they wide and variable?

Wide and variable use self shielding flux core. And welding multiple passes with your welder.

Multiple Passes?

Each weld line is called a pass. Thick metals and wider variable gaps may need another layer of weld – another pass to build up the weld.

If your welder has to be set to the maximum to weld the thicker metals. Then you’ll weld for short bursts. Or risk overheating your welder.

You can buy flux core wire for single pass welding.

Or multi-pass welding. Choose the right wire for the job you’re doing.

Self Shielding Flux Core Wire Needs DC Current

But a welding machine that plugs into a household plug uses AC current.

I hear you say.


But your welder has a transformer that converts your AC current into DC current. You set the right polarity on your welder for the type of flux core wire you’re using.

Self Shielding Flux Core Polarity
Self Shielding Flux Core Polarity

A flux core only wire welder is already set up for MIG gun negative. So you use self shielded flux cored wire.

Disadvantages Of Self Shielded Flux Cored Wire

You get more smoke and fumes with self shielding flux core welding wire.

Keep your head out of the plume rising from the weld.

The welds themselves are not as pretty. But look fine.

Self shielding flux core wire is more expensive to buy.

But if you factor in the saving on a tank of external shielding gas. And a regulator. It starts to compare.

Flux core wire welding produces more spatter. Look for flux cored wires rated for low spatter. And save yourself some post weld clean up.

Or use Anti spatter spray on the surrounding area, (not on your weld joint). To protect your work piece.

Flux cored wire isn’t for thin metals.

You’ll need to remove your slag between weld passes. And when you’ve finished welding by chipping at the slag. Or wire brushing.

About Gas Shielded Flux Cored Welding Wire

What Is Gas Shielded Flux Core Wire?

The clue is in the name.

Gas shielded flux core wire needs external shielding gas from a compressed bottle or tank of gas.

Typically, gas shielded flux core wire uses pure Carbon Dioxide. Or an Argon and Carbon Dioxide mix.

Gas shielded flux core wire also has the name double shielded flux core wire.


Because you’ve shielding from external gas. And the gas made in the flux of the flux core wire.

What Are The Types Of Gas Shielded Flux Core Wire?

You’ll find gas shielded flux core in two main types.

There’s gas shielded flux core wire made for pure carbon dioxide gas.

You’ll find types made to use with an Argon and Carbon Dioxide mix.

And you’ll find it for welding mild carbon steel. And for stainless steel.

What Does The Gas Shielded Flux Core Wire Classification Mean?

The classification of gas shielded flux core welding wire tells you how you use. And with which type of external shielding gas. 

Gas shielded flux core wire with the classification of E70T-1C.

The C tells you use this gas shielded flux cored wire with pure carbon dioxide. If your wire classification ends with -1C/M. This tells you that you use your wire with carbon dioxide gas. Or you can use with an Argon/CO2 gas mix.

What’s Great About Gas Shielded Flux Core Wire?

You’ll find gas shielded flux core wire’s slag easier to get off your weld.

It’s also a better flux core wire for welding thicker metals. Because gas shielded flux core welding wire gives you better weld penetration. You can weld deeper into your metal.

You’ll find it great when you’re welding out of position. – anything not flat or horizontal.

And known for its higher deposition rate means you can weld faster with gas shielded flux core wire.

What’s Not So Great About Gas Shielded Flux Core Wire?

You’ll get more spatter.

Gas shielded flux core wire shielded with pure carbon dioxide gas gives you more spatter.


Those sparks you see jumping from the weld are molten pieces of metal.

They land, solidify and need cleaning up.

Yes, you’ll get a less stable arc with pure CO2. But good weld penetration of the metal you’re welding. And that helps when you’re welding thick metals.

Compared to gas shielded flux cored wire for an Argon/CO2 gas mix. You’ll get a better arc. Less spatter but less penetration.

What Else to Know About Flux Cored Welding Wire?

Buying Your Flux Core Wire

You’ll find flux core wire sold on spools by weight. – 1lb., 2lb., 10lb. and above.

Or by spool size.

Your welder takes 4 inch spools?

Then you’ll find 2 pound spools are the equivalent.

8 inch spools equal a 10 pound spool.

You can use flux cored wire in a Flux Core only welder.

Or a MIG welder that you can set up to feed flux core wire.

Flux Cored Wire In A Flux Core Welder Or A MIG Welder

Both a Flux Core welder. And a MIG Welder use electricity to produce an arc.

And feeds your wire.

The arc gives you heat. And that heat you use to weld your metals together.

And your flux cored wire act as your flux cored electrode.



It’s a key part of your electrical circuit.

Your arc forms at the tip of your flux core wire.

And the heat from your electrical arc melts the end of your flux cored wire.

Your filler metal melts and your base metal melts.

The base metal is the metal you’re welding.

The metals flow together and at the same time compounds from your flux core wire form a gas. And that external gas protects your weld from the air around your weld.

Illustration of the MIG Process
Picture of the MIG Process

A ground clamp completes your electrical circuit.

You fix your ground clamp to your work. Or the welding table you’re welding on. And a lead from your ground clamp goes back to your welder. And from there to ground via your electrical outlet.

Want to read more more on flux core welders? Click on the linking text.


If it’s a MIG welder you have and would like to know more follow the linked text.

What Size Of Spools Will Your Flux Core Wire Welder Take?

The smaller portable, low cost flux cored wire welders generally won’t take more than a 2 lb. spool of wire.

And there’s no point you buying spools larger than your welder can take.

What Drive Rolls For Flux Cored Wire?

You’ll need knurled groove drives rolls for your flux core wire.

This is really only something to think about if you’ve a MIG welder. Because a MIG welder more often uses solid wire. And has smooth rollers. Knurled drive rolls are sometimes an optional extra.

Smooth drive rolls won’t feed flux core wire properly.

If you’ve a flux core only welder then you’ll have knurled drive rolls.

Knurled Drive Roll
Knurled Drive Roll

Check you’re feeding through the right sized groove of your drive roll. 0.030 on the 0.030 side and 0.035 on the 0.035 side.

Flux core wire is softer than solid wire. And you can crush it in the wrong groove. Or if you use too much wire tension.

This because the center of the flux cored wire is softer with the compounds inside.

Keep your welding cord as straight as you can to get yourself the best wire feed.

What Stickout Does Flux Cored Wire Need?

Stickout is the amount of flux cored wire extending out past the end of your contact tip before you start to weld.

Welding Wire Stickout
Welding Wire Stickout

The wrong length of Stickout. Particularly when you’ve got it too short. Results in burn back.

What’s burn back?

Your flux core wire burning onto your contact tip. And bad slag coverage. Or difficult to get off slag.

The right stickout improves your welding wire deposition rate.

Flux Cored Wire Deposition Rate – What Is That?

Your deposition rate means the amount of weld metal deposited. And you’ll find it’s measured in pounds per hour.

A part of your welding wire filler metal you lose in your slag.

But you need your slag and so that’s fine.

When you use Self shielding flux core wire. 65% of your filler metal makes it into your weld.

With gas shielded flux core wire 75% to 85% goes into the weld.

And so gas shielded flux core wire has higher deposition rates.

What Flux Core Wire Sizes Will You Find?

You’ll find flux cored wire in several wire diameters.

Like 0.030, 0.035, 0.045 0.068, 5/64, 3/32.

Check what flux core wire diameter sizes your welder uses.

And you’ll find your welder uses specific sizes of flux cored welding wire. Depending on the gauge/size of metal you’re welding.

What Is A Flux Cored Wire Parameter Chart

The wire manufacturer publishes the wire parameters chart. And it tells you what your flux cored wire can weld.

Metals thicknesses. And the Amps needed by your welder to weld with that wire.

If it’s ok for single pass or multi-pass welding.

And the shielding gas – if needed for your flux core wire.

You can use the chart to find out how suitable the flux core wire you plan to buy is for your welder.

YouTube Video What Is Flux Core Wire Welding?

I’ve discovered a great 5 minute video that describes flux core welding. And what is flux cored wire.

Highly recommend viewing.

What is Flux Cored Arc Welding? (FCAW)

Video Credit:

And if you want to learn more, you’ll find an article that goes into much more detail on the flux core welding process. “What is Flux Core Welding for Beginners and Everyone Else“. Check it out.

Storing Your Flux Cored Wire

Store your flux cored wire in a dry enclosed area. And keep your wire in its original packaging until you’re ready to use.

Keep your flux cored wire away from water and moisture.

It may not be something you think about.

But when you move flux core wire from a cold storage area. To where you’re welding. And it’s warmer where you weld. Condensation can form inside your flux core wire packaging. 

To stop this. Move your wire to where you’re welding at a cooler time of the day. Or give yourself 24 hours for your wire to acclimatize before you use it.

Flux cored wire exposed to moisture will rust your wire. And you’ll get porosity in your weld. Or a poor weld that could fail.

Take out your flux core wire from your welder when you’re done welding. And you don’t plan to weld for a while.

Because your wire can rust on your drive roll. And in your liner up to your torch head.

Take care that your wire doesn’t jump off the spool. And secure the end of your flux cored wire.

Store your flux cored wire in a sealed plastic bag. And get as much air out of the bag as you can before you seal.

What To Do Next

Now you know what is flux core wire and when best to use. And can think of situations you would like to use it in.

Have a look at some of my other articles here on the site.

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