Hey Guys,

This topic has come up a couple of times on JD & BV’s, Jody was there last time, webinar and it is regarding why dual shield flux core (FCAW-G, AWS, or FCAW-GS, Europe) works so well (fuses and penetrates) on hot rolled steel with mill scale left on the metal. Jody mentioned it would be interesting to delve into the chemistry behind it.

I’m not an expert in metallurgy and flux, and don’t purport to be, but here is some info that I hope will add something to the discussion. Feel free to challenge, add to, or refute what I say.

Regarding FWAW-G (GS) (only) there are wires with two types of flux, BASIC flux (which literally means basic from a pH standpoint when the flux material is dissolved in an aqueous solution and are typically calcium based) and RUTILE flux (which is mostly composed of TiO2, titanium oxide, or Titania, and is considered an acidic flux whose components form an acidic solution in aqueous solutions). The wire that was being discussed in the webinar was Lincoln Outershield 71M which has a RUTILE or slightly acidic flux.

Welding fluxes can be divided into three groups:

Acid fluxes with a basicity index of <0.9
Neutral fluxes with a basicity index of 0.9-1.2
Basic fluxes with a basicity index of > 1.2

Generally speaking, as the basicity of a flux goes up, the oxygen content of the weld metal goes down, or vice versa, when the basicity of the flux goes down, the oxygen content of the weld metal goes up. For this reason, BASIC fluxed weld metals make tougher welds (Charpy V notch toughness test) because the oxygen content of the weld metal is lower, whereas RUTILE flux wires make less tough welds (relative to BASIC fluxed wire) because there is more oxygen left in the weld metal.

Said a different way, the rule of thumb is: the better mechanical properties a wire offers, the greater the difference in the way it welds. In short, the elements and compounds used in the flux formulation to ensure a higher performance weld do not usually help optimize the wire’s usability characteristics (ease of welding).

FCAW-G (GS) wires do not provide enough oxygen shielding during the welding process (don’t produce enough shielding gas from the flux) to shield the weld metal so gas shielding is required in addition to just the slag to protect the weld metal from oxidation.

RUTILE flux FCAW-G (GS) wires have AWS5.20 T1, T9 or T12 usability designators. Based on Lincoln’s specifications for the Outershield 71M it meets the T1 and T9 designators. T9 being considered the most common in industry and tends to offer the optimal balance between welding characteristics and mechanical properties.

Basic flux FCAW-G (GS) wires have the AWS5.20 T5 usability designator. This type of BASIC flux inherently contributes less oxygen to the weld metal which results in a tougher weld. Another thing to note is T5 wires offer low weld deposit diffusible-hydrogen levels, because the fluoride in the flux “ties up” the hydrogen, preventing it from contributing to stresses in the microstructure that could lead to cracking.

T5 (BASIC) wires are most commonly used for demanding applications in the heavy equipment and offshore fabrication industries. The high amount of fluorides in these wires are quite detrimental to achieving smooth welding characteristics. Compared to T1, T9, and T12 the difference between T5 wires is “night and day” as the T5 wires provide much more globular transfer, harsher arc, increased spatter and a more convex bead profile with noticeable solidification lines on the weld face vs. RUTILE flux based wires which provide a much flatter bead profile.

Manufacturer’s data sheets provide recommended polarity and positional capabilities of the wire so always consult those.

Regarding the chemistry of how FCAW-G (GS) wires perform so well in fusing and penetrating through steels with mill scale still on:

These wires have fluxes that are inert at atmospheric temperatures but when heated to very high temperatures have compounds that become strong reductants (reducing agents). A reductant gives up electron’s in the chemical reaction and becomes oxidized (the flux) and the oxidant gains electrons becoming reduced (iron oxide, all three forms in mill scale, is reduced to iron). Thus, the mill scale is reduced to iron and the oxygen from the mill scale forms non-metallic oxides with the flux that either vaporize or form very buoyant, much lower density than the weld pool, immiscible solids (not soluble in the weld puddle) and become slag that floats to the top of the weld puddle.

Solid wire short circuit MIG does not have fluxes that are able to effectively “dissolve” mill scale and thus, the potential fusion and penetration issues when trying to weld over mill scale.

One topic I will mention is fluidity of the weld puddle, which is the reciprocal of viscosity. Thus, as fluidity goes up, viscosity of the weld puddle goes down, and vise versa, as the fluidity of the weld puddle goes down, the viscosity goes up. Fluidity of the BASIC flux weld puddle is lower than the fluidity of the RUTIL flux weld puddle which makes it easier to make good welds with RUTILE flux wires (out of position welds better and gives flat bead profile) vs. BASIC flux wires.

Also, there are a couple of things to note regarding “wettability” of the substrate (work):

The surface energy of the substrate (the work) must be higher than the surface tension of the molten metal for the weld metal to “wet out” on the surface of the work. I believe this is why mill scale is so “non wettable” because the surface energy is so low relative to the molten weld metal surface tension it will not “wet out”. The welding flux increases the surface energy (in a number of ways, removing or dissolving the oxides being one of them) of the substrate (work), such that it overcomes the surface tension of the weld metal (negative gradient in surface tension / energy) and causes the weld pool to “wet out” on the substrate.

NOTE: Surface tension applies ONLY to liquids and surface energy applies ONLY to solids, but are measured in the same units. Thus, if the surface tension of the puddle minus the surface energy of the substrate (work) is negative, surface energy of the substrate is higher than the surface tension of the molten weld metal, the surface of the substrate (work) will wet out with the weld metal.

This could be another topic all on its own. I know your eyes are rolling back in your heads about now.

One thing I’ll also note is the surface tension of molten metal is is a function of temperature. As the temperature of the molten liquid increases the surface tension decreases. I notice this when I’m brazing and the temperature is hot enough to melt the silicon bronze but not hot enough to cause the surface tension to be below the surface energy of the substrate, but as I focus the arc on the ball of silicon bronze, it will all of a sudden wet out on the substrate because the surface tension is lowered enough that the surface energy of the substrate is higher. This is at least what I believe is happening. Would be nice if Jody’s metallurgist friend comes on and we can discuss this type of stuff more.

I know I’ll never give Jody more than he has given me, through is content, but hope this helps out. I can provide references for all I have discussed here.

DZ

FIXING A TYPO (basic flux puddle fluidity is higher than rutile flux puddle...corrected in paragraph below):

One topic I will mention is fluidity of the weld puddle, which is the reciprocal of viscosity. Thus, as fluidity goes up, viscosity of the weld puddle goes down, and vise versa, as the fluidity of the weld puddle goes down, the viscosity goes up. Fluidity of the BASIC flux weld puddle is HIGHER than the fluidity of the RUTILE flux weld puddle which makes it easier to make good welds with RUTILE flux wires (out of position welds are better/easier and gives flat bead profile) vs. BASIC flux wires.

DZ, thanks for a very informative and interesting post.

As far as I understand it, this applies to stick welding rods too. 7018 is has a basic flux and produces a tougher weld than 6013 which has a rutile flux and is typically easier to weld with. At least that's how I learned it here in Germany where 6013 is a popular rod for many applications.
I've seen some multi-pass welds at my previous workplace where the first passes were done with MIG for speed but the cap was done with 6013 for a smoother bead appearance.

I'm a bit confused though as you said that basic wires offer low weld deposit diffusible-hydrogen levels. When it comes to stick rods it's the other way around: 7018 (basic) needs to be kept dry because it otherwise will absorb hydrogen and leave it behind in the weld. Rutile rods on the other hand such as 6013 don't have to be kept in a rod oven. Have I understood you correctly?

That's an interesting comment you make about getting silicon bronze to wet out when TIG brazing. I have to say in my experience I cannot get the silbro to wet unless I puddle the base material. This puddle may be tiny but until it's there, the silbro just pearls off the surface. This has made me always wonder if it really is brazing at all. With MIG brazing you at least have the reducing effect (cleaning) of the DC+ arc which I think must promote wetting.

Cheers!

Hey thanks for the clarification with the hydrogen. I understand what you mean now.

Look forward to opening those links and having a read.

I've seen that video before of the guy brazing the chassis, love it. For the sake of clarity, I think it's important to distinguish between brazing with a flame and brazing with an arc.
When brazing with a flame you definitely don't need to (and should not) melt the base material and some application of flux is essential. There is actually an exchange of atoms between the braze material and base material at their boundaries (in german they refer to this as the diffusion zone, probably the same in english). This exchange of atoms provides a tremendous bonding strength but it's not actually a fluid mixing in the same way that two molten metals might mix to form an alloy.
When brazing with a TIG torch and silicon bronze, you can't use flux because the arc exceeds the working temperature of the flux right away. In the absence of flux, I don't know what allows the silicon bronze to effectively wet out during TIG brazing with a DCEN arc. I didn't know about the significance of the ratio of surface tension to surface energy, really interesting. It's certainly been my experience that the silicon bronze won't flow until the workpiece (e.g. steel) begins to puddle.

What process were you using to braze your bible?

Josef,

Concur on on the frame brazing video...it is mesmerizing. Great clarification on the flame brazing vs arc brazing. You could very well be right (my eyes could have been deceiving me). I'm new to the welding and metallurgy game, and to TIG brazing, so I'm a bit behind on the learning curve. I'm going to stay on it until I understand the science. I was using DCEN TIG to braze the Bible.

During my research I've found quite a bit in the literature on flux assisted TIG to significantly improve weld penetration (link below) in austinetic stainless steel welds. I need to peel that onion back more but here is a link to one paper if you are interested:

http://weld.npust.edu.tw/ezfiles/196/1196/img/2292/AMR2011.pdf

Appreciate the discussion.

DZ

Lincoln Excalibur 7018 MR SDS shows it has CaF (calcium fluoride) in it:

https://www.lincolnelectric.com/assets/US/EN/MSDS_lib/ZLE_SDS_CN-EN-200000000523.pdf

Hi DZ

1. Thanks for confirming that 7018 has fluoride in it like T5 flux cored wire. Cool to know.

2. My suspicion that silicon bronze doesn't flow when TIG brazing until the workpiece puddles may be mistaken, is just my experience and definitely contradicts what most people say about TIG brazing, including Jody in his videos. And as JD said in a previous post... Jody is always right! And JD is always right too, so that makes Jody always really right! ;-)

3. I just scanned through that paper on TIG welding with flux, very cool, thanks for sharing.
So they are using silica powder and titania powder. The titania is like the flux on a 6013 stick rod, rutile, and develops CO2 gas during welding. I've been told that CO2 during MIG welding reduces weld puddle surface tension and assists penetration so that makes sense. This is why it's not advised to MIG weld steel with straight argon. The penetration is really poor.
Silicon will scavenge oxygen out of a weld puddle and float to the top as glassy slag. This is why solid MIG wire has silicon in it. You can see little glassy bits on the top of a fresh MIG weld. That's the silicon having done its job. As far as I know, TIG wire doesn't need this because an inert shielding gas is being used so there's way less available oxygen which needs cleaning up. That's why you don't get those tiny bits of slag on a TIG bead the way you do with MIG.
Silicon also improves fluidity of molten metals and I would guess that's why they put it in silicon bronze rod.

I wonder how the tungsten electrodes looked once they were done making those weld tests with the flux... probably pretty rough... that's definitely a job where you don't want to use your expensive fupa gas lense.

Really interesting how the surface tension gradient on the weld puddle affects convection which affects penetration profile. Full penetration on 6 mm plate at 180 amps is remarkable and the difference in bead profiles with and without flux is extreme. Makes me wonder if there are other possible ways to achieve this surface tension gradient, for example through manipulation of the arc characteristics rather than with flux and the major disadvantages it brings.

Cheers

Thank you both, DZ & Josef, for this intriguing discussion into flux! I am guilty of not fully understanding the chemical specifications of the differences between flux and only using my experiences on the job welding. I am learning so much from this conversation and it really makes me want to dig in to this information. Thank you for the links to articles on this topic. I will be reading them over and over until they stick in my brain. Cheers to both of y'all!!

If this had been in the Chemistry classes in college maybe I would have paid attention! I enjoy nerding out about welding like this! Thank you DZ and Josef for the topic! Very informative and interesting!