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The Absolute Last Final Word on Anhydrous Borax Flux

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20 hours ago, teenylittlemetalguy said:

The melting point gets lowered with increasing carbon

Congrats! Our discussion has covered a lot of ground and I believe it is worth it and come to a good place. You have now presented a hypotheses. It makes no difference if I or anyone else believe it or not. Its your puppy, It is tangible, and most important, you now have something you can experiment with and, just perhaps, do it in an objective manner. Most important, this now has nothing to do with common sense, its focused completely on the chemistry and physics of the interaction of carbon and iron. I hope you will keep us informed on your results.

Your hypothesis:

1: Carbon in solution with iron will affect its physical characteristics.  True

2: Will carbon, when applied to iron and not in solution affect its physical characteristics?  

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Well my head is totally spinning ! I now realize I should have paid more attention in science class all those many years ago in school ! :blink: I'm going to be another year deciphering all this lol. Thank you gents , I've been attempting to follow this conversation all along I haven't added anything because I don't know anything about it yet ! Again thank you for your knowledge . 


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According to the Engineer’s Toolbox, the autoignition temperature of charcoal is 349°C/660°F,  when the steel is still at a black heat. Sodium tetraborate melts at 743°C /1,369°F, around a dark cherry red.

Taking these two  facts into consideration along with the additional facts (a) that the carburization of iron to produce steel is almost always done in a hypoxic environment (think crucible steel or blister steel) and (b) that forge welding is a solid-phase process rather than liquid phase (thus rendering the melting point irrelevant), I suspect that what’s really going on is this:

First, the carbon in the charcoal dust ignites, converting the available oxygen to carbon dioxide and thus preventing it from reacting with the iron to form scale. Since the carbon is locked up in the carbon dioxide (or carbon monoxide, if there’s not enough oxygen for full combustion), it’s not available to dissolve into the metal. 

Second, the borax melts, dissolving any remaining scale within the joint and forming a glassy coating that prevents additional oxygen from reaching the steel.

That’s just my hypothesis, though, and I’m open to other possibilities. 

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While I'm perfectly content using the 20 mule team borax off the shelf, I'd be open to tossing a little charcoal powder in there just for giggles. I have noticed that welding spring steel to itself is tricky and while I don't need to do that very often I'm open to play around with flux additives.... I have plenty of charcoal dust...

At the end of the day I am in neither the "flux is magic and adding some proportion of something or other is going to make it stick every time" nor the "I don't need flux" camp (for the sake of clarity I'm not saying any of you guys are either).

I rather like flux and I'm... frugal... so I buy the cheap stuff at the grocery store and that's all I've ever used. The foaming of hydrous borax never really bothered me much because I learned from M. Aspery's "Scarf Theory" video to bring the material up to a welding heat (or just under), brush, flux, back in the fire briefly, then weld.  That's all I've ever done (excluding damascus) and there is very little time for the borax to foam everywhere when applied at a welding heat.

It's already been said, but you really don't need to have even a basic understanding of science to understand how to forge weld. What you need to do is practice. I try to do at least one weld of one type or another every day I'm in the shop. Sometimes that means taking two pieces out of the scrap bin, welding them together and tossing them back where they came from.

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5160/9160 leaf spring is notoriously difficult to forge weld to itself.  I have heard it said that it has something to do with chromium content, but don't have the metallurgy to back that up.  I've had plenty of success welding it to other steel, or even wrought iron, but otherwise it is a real challenge.  If you come up with some sort of aggressive flux that works better I'm all ears.

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Thanks for that JHCC. I'll stick to practice, clean fire, Neutral/reducing fire makes a good forge weld, and a borax based flux seals, cleans scale and for unknown reasons to me reduces the FW temp, and charcoal does nothing. I believe this applies to all fuel.

I'm not sure about a FW being a  solid-phase process. Not my area of strength, but when your iron is in a neutral/reducing environment, the surface is actually moving. It looks very similar to the puddle you get with ox/acetl welding. Also i learned that if your iron soaks at this temp for a short bit, it is like this through and throughout. This state is called liquidious(sp) and this is where you want to be when doing a FW. When its solid, there is no liquid look to it. When its a liquid throughout, it slumps or melts. When its liquidious throughout you see this "puddle look" on all surfaces and it maintains its shape. Also, a good way to check your temp is to lightly touch your two pieces and if they stick to each other, you are ready. 

Beyond that, I dont know much about the  solid-phase process

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Well the metallurgists consider it a solid phase weld and I've seen Billy Merritt weld at temps I would consider too cold for proper forging. The 2300 degF commonly given as welding temp is several hundred degrees below melting temp.  "The Solid Phase Welding of Metals", Tylecote, has a lot of information on how it actually works.  

I was first taught to weld, (by an ABS MS), to see the stuff molten on the surface; but I don't know if that is steel or molten scale or ?.  Removing that liquid doesn't prevent welding---which it does when welding with torch or arc... (If I recall correctly aren't you a sling it off before welding person?)

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I am, and slinging the gradoo gets rid of the liquid flux and whatever is in it. The surface still looks like what you see When doing an ox/acetl weld.

Full disclosure here. I'm not a metals engineer and this info came to me from Turley Forge, fall 1979. He may have changed his view after, I do not know. I offer this simply as other "stuff" to consider. 

Here's a wiki definition of liquidious:

"The liquidus temperature, TL or Tliq, specifies the temperature above which a material is completely liquid, and the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium. It is mostly used for impure substances such as glasses, alloys and rocks"

the last sentence is critical. When I first read this I thought it didn't apply to iron because of the impure bit. And then it dawned on me wrought contains silicon and other impurities and assuming the added chemicals in modern alloy steels are considered " impurities", then wrought and alloy steels MAY qualify for this definition and fits what Turley said. Perhaps iron crystals in solution at the melting point enables steel to maintain its shape whilst being a liquid throughout at the same time. I've never known Turley's teachings to be off base, but this could have easily been a speculation on his part.

The part I have a hard time with wrt solid phase welding, I've always assumed this implies a fair amount of force to make it work. I don't know how much force especially at a forge welding temp. I do know that you can lightly touch two pieces of steel at this temp and they will fuse. I've always assumed this was the two surfaces described above flowing together. Hit it too hard and they bounce apart. I do recognize the problems with any assumption.

Thomas, do you by chance have any sources 

On 10/18/2021 at 11:02 AM, ThomasPowers said:

metallurgists consider it a solid phase weld

I too know many, including myself, who FW at low temps. Again, an assumption, perhaps early in a liquidious transition, and with a light touch and timing, the weld takes at a lower temp. It seems to me that a solid state weld takes force and the lower the temp, the more force needed. This goes against my experience and what I've observed when others FW at low temps. A light touch seems to be the rule.

A further thought, we do know that when steel transitions from one state to another, both states exist for a period of time. That's why time at a specific heat is important.

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Well as I understand it; making a  solid phase weld has three components: temperature, pressure and cleanliness.  Max any of them out and you can get a weld.

Blacksmiths do mainly temperature with a bit of the other two when forge welding. (The more you can increase the other two the lower temp you can weld at.)

Vacuum welding maximizes cleanliness and is know to be an issue in spacecraft.

Pressure; a great example is explosive welding, but galling of bolts is a more common one.

The main sources I would recommend would be "The Solid Phase Welding of Metals", R.F.Tylecote and speaking to various Metallurgy professors until you find one conversant with that part of metallurgy. 

I'll check my copy of tSPWoM and see if the bibliography has any suggestions.

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Thanks,Thomas. I did some searching last night and found out a FW is in fact one of the types of solid phase welds. 




I guess an old dog can learn new tricks, And thats a good thing. 

Im still looking for information on liquidious and steel. 

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Well us old dogs can teach the young ones some things too; note in that second source they say that forge welding can only be done with low carbon steel!  I think I know more folks doing it with HC than only with LC; of course I came into blacksmithing through bladesmithing and so I know a lot of folks welding billets including billets only of differing HC steels.

Reading old sources is fun; as what works, works; but the explanations of why it works can be "out of touch with reality".  (They also have a tendency to leave out things so common they don't need to be documented. )

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As I understand it, the "liquidus" (not "liquidious") of a material is the temperature above which the material melts. The "solidus" is the point below which the material crystalizes. The area in between is known as the "freezing range", and in this range, the material exists as a mixture of solid and liquid, like a slush. In other words, when a solid is heated, it starts to melt when it reaches the solidus temperature and is completely melted when it reaches the liquidus temperature; when a liquid is cooled, it starts to solidify when it reaches the liquidus temperature and is completely solid when it reaches the solidus temperature. When the solidus and liquidus are the same, the material is called "eutectic". Most pure metals are eutectic (such as pure iron, at 2800°F), as are some alloys (such as iron with 4.3% carbon, at 2066°F).

The liquidus and solidus for steels vary somewhat, depending on the content of carbon and other alloying elements. However, even the lowest solidus temperature (~2400°F) for forgeable steel is higher than the 2300°F usually cited as the highest white heat for welding. 

There is a common impression that the surface of a piece to be welded becomes liquid, but I suspect that this derives from the shimmering effect of the heated air between the workpiece and the viewer's eye. That is, the refraction of the image changes as it passes through eddying air currents at different temperatures, much as the heated layer of air just above a road reflects the light to make it appear that there is water on the road.

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On 10/20/2021 at 11:27 AM, ThomasPowers said:

low carbon steel! 

Lol, I noticed this type of strange thangs  in all three addy's I posted. 


22 hours ago, JHCC said:

material melts

Thats not what I found for the definition of liquidus. And thanks for correcting my spelling! I did find conflicting definitions that support you. However, following the golden rule of ask ten and take the consensus, it seems liquidus is the max temp a solid can exist and the minimum temp at which a liquid can exist. I take this to mean both exist at the same time. Call it slush. 

Thomas, it doesnt appear to be a term found in the phase change charts for ferrous metals, but this "slush" state does exist. Its the transition between two states and, as best I can figure, is the reason for holding our steels at a specific temp for a particular amount of time. First its one, then both states exist, then it becomes the other.

So, perhaps a FW at a red heat is a pure solid phase weld and the other a slushy mix where two liquids flow together.

And with that fishy statement, 3 daze is all we can stand.  ;)


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