You’ve heard of flux core wire and may even have seen the lower cost flux core only wire welders out there in the market.

And you’re thinking of getting one.

But first you want to be sure what is flux core wire, know when it is best to use it and to find out why you would use it.

New to welding or perhaps you’ve a little knowledge then this article is for you.

For the beginner, home hobbyist and those doing light vehicle repairs. Enjoy!

What Is Flux Core Wire Exactly?

At the most basic level it is a type of wire used in a welding machine. Sold on spools by weight – 1lb, 2lb, 10lb and above.

Or by size. Your welder takes four inch spools? Two pound spools are the equivalent.

Eight inch spools equal ten pound spools.

Flux core wire is used in a Flux Core only welder, or you may have a MIG welder that can be set up to feed flux core wire or solid wire with shielding gas.

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

A welder uses electricity to produce an arc. The arc produces heat and that heat is used to weld metals together.

The flux core wire is fed through your Flux Core or MIG welder depending on which you have and acts as the electrode.

An arc forms at the tip of the wire and the heat from the arc melts the end of the Flux core wire providing metal for your weld and gases to protect your weld from the surrounding air.

Illustration of the MIG Process
Picture of the MIG Process

A clamp is fixed to your work piece or the welding table and leads back to your welder thus completing the electrical circuit.

That is a quick overview of what happens in the welders. If you would like to read more, I have an article on the site that goes into more detail on flux core welders you can take a read through the linking text.


If it is a MIG welder you have and would like to know more follow this linked text.

Flux Core Versus Solid Wire

The alternative to using flux core wire in a welder is to use solid wire. For those welders that support solid wire a tank of compressed gas – normally Argon and Carbon Dioxide must be used for the weld to be successful.

Solid wire as its name implies is solid and made of the metal you plan to weld – well it does contain some other stuff but that is a whole other conversation.

Flux Core How Is It Made And What Is Inside It?

The flux core wire is designed as a tube with metal around the outside. Inside are compounds – called the flux.

Inside Flux Core Wire

These compounds are designed so that when they are melted in the arc they form a shielding gas that protects your weld.

Shielding gas is literally gas that shields your molten weld pool from contamination from the air, which would lead to a poor or a failed weld.

The flux also contains alloys and de-oxidizers that pull contaminants out your weld and into the slag.


A hard coating formed over the top of your weld that is chipped off once cooled leaving you a nice smooth weld.

Slag and Direction Of Travel
Slag and Direction Of Travel

There is a document on the site that goes into much more detail on what slag is. Follow the highlighted text to learn more.

Flux Core Wire Types

There are actually two types of flux core wire.

Self shielded flux core wire (FCAW-S) and Gas shielded flux core (FCAW-G) (also known as double shielded).

Self shielded flux core wire was introduced to the market in the early1960s and Gas shielded flux core in the late 1950s.

Self Shielded Flux Core Welding Wire

Self shielded flux core welding wire is the wire of choice for the beginner, hobby welder and semi professional welder because it is the easiest and most convenient to use so we’ll take a look at that first.

Self shielded flux core wire creates the shielding gas needed for the molten weld pool from the compounds within the core of the wire.

The flux burns and the shielding gas forms at the point of the weld.

The self shielding flux core wire has to react with air for the flux to provide the protection for the weld pool and form the slag.

FCAWS Shielding Gas

When Is Self Shielded Flux Core Wire Used?

Which you choose to use, solid wire plus compressed gas or flux core depends on the job and location.

Let’s say you’re a farmer with yard or field equipment that has broken down way in the back 100 acres so is some distance or difficult to get to.

long Way From A Gas Distributor

Up there it’s pretty windy and to MIG weld you’ll need to take a wind shield or tent so the wind doesn’t blow away your shielding gas.

If it was near your workshop you would be able to prepare the metal real nice for welding but out there you cant.

And the broken piece is in an awkward spot. What now?

The weld is vertical and hard to get to.

Flux core is ideal for this sort of situation.

What Is Self Shielded Flux Core Wire Good At?

Used to weld outside where the wind could cause you problems with solid wire and the wind blowing away your separate shielding gas.

With flux core there is no shielding gas to take with you – one less thing to bring.

You may find in your particular location or area of the country it is difficult for you to get shielding gas or transport it. Flux core then could be your only option.

Self Shielded flux core is more tolerant of ‘dirty’ steel as it will tolerate higher levels of rust, mill scale or oil on the base metal.

Use of solid wire plus compressed gas by contrast demands clean steel.

Although it is always best to clean your weld as much as possible before you start.

Self Shielding Flux core is chosen because there are versions great for out of position welding.

Out Of Position Welding?

Vertical welding, welding over head, welding that is not flat horizontal welding is classed as out of position welding.

Welding with self shielding flux core produces faster welds – compared to solid wire.

Flux core is better for galvanized steel or welding hard to weld steel.

Is amenable to the skill or rather the absence of skill of the user than welding with solid wire and shielding gas.

Flux core creates strong ‘clean’ welds with good penetration.

Self Shielding Flux Core Wire For Particular Metals

It is important to match the flux core wire type to what you are welding.

Welding low carbon or galvanized steel? Look for Self shielding flux core wire advertised as suitable for low carbon or galvanized steel? Welding stainless steel? Look for Self shielding flux core wire suitable for stainless steel.

Using Shielding Gas With Self Shielding Flux Core Wire?


Doing that’ll produce problems with your weld.

The compounds that need to react with the air are blocked by the addition of shielding gas.

This blocking results in the materials that are designed to help your weld stay instead inside your weld and contaminate it. Causing a poor weld and cracking.

Choosing Self Shielding Flux Core Wire, What To Bear In Mind?

What Kind Of Welder Do You Have?

A lower cost flux core only welder or a MIG welder that can use solid wire and a bottle of shielding gas that is capable of welding with flux core wire as well.

What Is Your Welder Capable Of?

Its wire feed speed, Amp output and voltage needs. Take a note of the minimum and maximum settings as you will need this information when you choose your self shielding flux core wire.

Think about the metals you’re planning on welding.

How thick is the metal. Generally thin metals (24 or 22 gauge) is better suited to solid wire and shielding gas.

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

Are the joints straight and clean or are they wide and variable. Wide and variable will push you toward self shielding flux core and 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 will be only able to weld for short burst or risk overheating your welder. You may need to accept that your welder is not up to that particular job.

Flux core wire comes in single pass flavors and multi-pass flavors. Choose the correct wire for the job you are doing.

Practice before you weld in earnest and be sure you’re making good welds.

Self Shielding Flux Core Needs DC Current

However, any machine that plugs into a household plug uses AC current. The welder has a transformer that converts the AC current into DC current.

The correct polarity needs to be set on your welder for the type of flux core wire you are welding. This is particularly true with MIG welders that can weld with solid wire and flux core – make sure of the polarity before you weld.

Self Shielding Flux Core Polarity
Self Shielding Flux Core Polarity

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

What Size Of Spools Will Your Welder Take?

The smaller flux core wire welders, that are designed to be portable and at the lower cost end of the market, generally won’t take more than a 2 lb. spool of wire. There is no point in buying spools larger than your welder can take.

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

This is really only a concern for MIGs that will also weld with solid wire as often smooth rolls are provided with the MIG. Knurled drive rolls are an optional extra.

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

If you have a flux core only welder then the drive rolls will already be suitable for welding flux core wire.

Knurled Drive Roll
Knurled Drive Roll

Make sure you’re feeding through the right sized groove of the drive roll. .030 on the 0.030 side and 0.035 on the .035 side.

Softer than solid wire flux core wire can be deformed or crushed if the incorrect drive rolls and or tension is used.

Flux core is more prone to crushing because of its center containing the shielding compounds; take care with your wire tension.

Keep your welding cord as straight as you can up to the torch head to get the best wire feed.


Check the stickout recommended for the wire and your welder is correct. Stickout is the amount of flux core wire extending out past the end of your contact tip before you start to weld.

Welding Wire Stickout
Welding Wire Stickout

The wrong length particularly too short a stickout can result in burn back (the flux core wire burning onto your contact tip) and incomplete slag coverage. Or difficult slag removal.

The right stickout improves the deposition rate – the amount of metal going into your weld. And results in a stronger weld.

Flux Core Wire Sizes

Flux core Wire is available in several wire diameters. For example 030, 0.035, 0.045 0.068, 5/64, 3/32. Look at which diameters your welder uses.

There’ll be a minimum and maximum thickness of metals recommended for your particular welder along with specific sizes of flux core welding wire to use.

Wire Parameters Chart

It is a crucial to double check the wire parameters of the flux core wire you plan to use. As a minimum confirm what metals is it suitable for welding, whether it is suitable for single pass or multi-pass welding.

The wire parameters chart tells you what the wire is capable of and how suitable it might be for your welder. I have an article on the site that goes into much more detail on this check it out.

Disadvantages Of Using Flux Core Wire

  • The self shielding flux core welding process produces more smoke and fumes.
  • Keep your head out of the plume rising from the weld.
  • The welds themselves can be less pretty welds.
  • Self shielding flux core wire is more expensive to buy, however when you factor in the saving on the cost of needing a tank of gas and a regulator, its less so.
  • Flux core generally produces more spatter so look for flux core wires rated for low spatter to save yourself a lot of post weld clean up. or use Anti spatter spray on the surrounding area, (not on your weld joint) to protect your work piece.
  • Flux core wire is not ideal for thin metals solid wire is a better choice for 24 to 20 gauge mild steel.
  • Examine what your welder recommends.
  • Slag must be removed between weld passes and at the end by chipping at the slag or wire brushing.

YouTube Video What Is Flux Core Arc Welding?

I’ve discovered a great five minute video that describes flux core welding and what is flux core wire. Clear, concise, it illustrates some of the points I have raised in this article. I highly recommend viewing.

What is Flux Cored Arc Welding? (FCAW)

Video Credit:

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

Gas Shielded Flux Core Welding Wire

Gas shielded flux core wire is as its name implies, needs to be used with gas. This gas is provided from a compressed bottle or tank of gas. Typically this gas is pure Carbon Dioxide or an Argon and Carbon Dioxide mix.

Hence the term double shielded.

Gas shielded flux core also comes in two main types.

Gas shielded flux core wire manufactured for use with pure carbon dioxide and gas shielded flux core wire to be used with an Argon and Carbon Dioxide mix.

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


Those sparks you see jumping from the weld are molten pieces of metal that land, solidify and need cleaning up.

And a less stable arc but good penetration of the metal you are welding.

Gas shielded flux core wire for an Argon/CO2 mix gives you a better arc, lower spatter but less penetration.

Gas shielded flux core slag is easier to remove, it is better for welding thicker metals and for welding out of position.

Deposition Rate – What Is That?

Deposition rate means (the amount of weld metal deposited in a time period measured in pounds per hour)

A portion of the metal is lost in the slag. With Self shielding flux core wire only about 65% of the metal available actually makes into the weld.

With gas shielded flux core 75% to 85% goes into the weld, hence its higher deposition rates.

The rest goes into the slag

Gas Shielded Flux Core Wire Classification

The classification of the wire tells you how the wire can be used. And with what kind of gas.

For example gas shielded flux core wire with the classification of E70T-1C the C signifies this flux core wire must be used with pure carbon dioxide. If the wire classification ends with -1C/M this means that the wire can be used with carbon dioxide gas or an Argon/CO2 gas mix.

Storage Of Flux Core Wire

Store flux core wire in a dry enclosed area and keep the wire in its original packaging until you are ready to use.

Keep the flux core wire away from water and moisture in the atmosphere.

It may not be something you think about but moving flux core wire from a cold storage area to where you’re welding particularly if where you weld is significantly warmer, condensation can form inside the packaging.

To lessen or prevent this, move your wire to where you are welding at a cooler time of the day or give 24 hours for the wire to acclimatize before you use it.

Flux core wire exposed to moisture can cause your wire to start to rust and will cause porosity in your weld or poor welds that could fail.

Significant Time Between When You’re Next Going To Weld?

The best idea is to carefully remove the flux core wire from your welder. As the wire can rust on the drive roll and in the liner leading up to your torch head.

Remove the wire taking care that the wire does not jump off the spool and secure the end of the wire. Remove as much air as you can and store in a sealed plastic bag,

Last Words

You should now have a better appreciation of what is flux core wire and when best to use. And can think of situations you would like to use it in.

I hope you’d like to look at some of my other articles here on the site, on the different welders available to the home, or small workshop user.


Start with the Basics Understanding Flux Cored Wires – The Fabricator

Getting to know Flux Cored Wires – The Fabricator