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Hey Y'all

I had a thought; does quenching a piece in oil prevent as much firescale, or is all the scale formed in the forge, not the quench? Also, does heating a piece in a clay sleeve prevent scale from forming?

Thanks and Cheers!

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No expert, but from what I know, scale is oxides that form in the forge when the hot metal is exposed to oxygen. In effect rust. The scale is formed in the fire, not the quench, so oil vs water should not matter. Clearly a dip in water could lead to rust after the fact...

 

The clay coating does help prevent fire scale. I found I lost my clay once as I insisted in burring it in the heart of the coals, trying to avoid oxygen. In the last try I just let the clay do its thing and laid the blade on top of the coals.

 

HTHs

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scale is iron oxide - it is formed when the hot steel comes into contact with oxygen. if you run your forge with a reducing atmosphere/ keep the piece out of the air blast, then there shouldn't be much opportunity for scale to form during heat treat, as liquid quenches are pretty much anaerobic, so the only time the blade is exposed to oxygen is between the fire and the quench, which shouldn't be enough time for much scale to form. even during forging, you shouldn't have much scaling while the piece is in the fire - the scale forms when you take it out to work it.

 

i'm not really sure what you're asking, or why? scale is a by-product of forging, but it should have no effect on the finished knife...

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What Jake said.

 

Quoting one of my heroes here, Dr. Mike Blue, "Oxygen, when it gets that hot, gets a little loose. It's not quite sure iron is the best boyfriend it's ever had," while describing the reduction process, making really hot CO gas to pull an O off of Fe3O4.

 

It works in reverse, too, and that's how scale is made - iron, very hot, heating the oxygen near it when you pull it from the forge, will pick up some O and transform to FeO in some combination, and those are the little flakes you get when hammering hot iron/steel at the anvil. Some is unavoidable, but if you grind a blade clean, heat gently to normalize and then quench, the stress of the quench should blast most if not all the very-thin scale off regardless of quench medium. To address your question about clay, anything that prevents oxygen from contacting the hot steel should prevent scale, but clay is porous and not perfectly air-tight. I think it helps, but also carries some consequence if you're planning on applying thin clay to the entire piece and then quenching... it will get incandescent at different temperatures than your blade, which makes judging when the steel is hot enough by eye a more difficult challenge.

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I understand what scale is, I'm just wondering if quench it in water would create Fe (OH)2 and then decompose (almost instantly) into FeO. I know that's similar towhat happens with iron in water (Fe2O3 instead with the iron III ions). I now get that it occurs in the time out of the forge (and in the forge as I found when I ruined a file).

Jake, there's really not much for me to do to reduce the atomsphere, other that shutting off the air, shoving the piece in the charcoal, and covering the top.

Thanks Chris for the advice about clay. What kind of clay could work for heat treating?

Thanks and Cheers!

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You're WAY overthinking what happens in the quench. It's just scale, it's a fact of forging, grind it off like everyone else does. ;) Once it forms the only way to reduce it back to iron is, well, by reduction, which is not something you can easily do on a finished blade.

 

A reducing atmosphere in the forge helps greatly, as has been said. If it's a gas forge, throw a piece of wood or charcoal in there while you're heating it up for the quench. If it's solid fuel (or even gas), search my previous posts about heat-treating in a pipe. That works better than anything besides an inert-gas-purged electric furnace.

 

If you're not going for hamon and you insist on a coating, they make several anti-scale compounds for heat treating that work as advertised. They do cause a serious mess if overheated, but then you know better than to overheat, right? :P

 

The blade should be clean and bright before HT anyway, and what little scale you get before the quench is usually easy to remove with just sandpaper. Even when I do tomahawk blades with an acetylene torch, the scale wipes right off after the quench with a little 400 grit paper, since I take 'em to 400 before HT in the first place. A smooth surface won't scale as much and what scale there is will be much easier to remove afterwards.

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Thanks guys. I guess I am over thinking it, but on a bit of practice (RR spike kinfe), I got really deep pits and I guess I just got the piece too hot and into the air vent too much. Thanks for the tip on polishing before HT. I'll try the pipe. Wish me luck, I gotta get this one done before the 24th for my bro's B-day.

Cheers and thanks!

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ok - this has nothing to do with the quench and everything to do with your forging habits and technique. some things to try to avoid this:

 

1. keep the work out of the air blast - generally you want at least 4" and preferably 6" of fuel above the air outlet.

 

2. wire brush the steel before putting it back in the forge and if you see any heavy scaling. brush loose scale off the anvil while the piece is in the fire.

 

3. wet forge - as you get close to finished shape, dip your hammer in your slack bucket and sprinkle water on your anvil before taking the piece from the fire - this wil vaporise and blow scale off the work with a satisfying pop that sounds like a weld taking...

 

4. forge colder - at the final stages of shaping, you should forge just around critical - this prevents scale from being driven too far into the surface of the steel, stops you putting deep dings in the surface, and starts the thermal cycling process.

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Yep, Jake nailed it: operator error. ;)

 

Scaling alone does not cause deep pits. It takes a burning heat (steel is sparking in the fire), poor hammer control, or an improperly dressed hammer face to do that. Burning the steel can be from either too high a heat OR too much air, i.e. too close to the blast.

 

Do what Jake suggests, and I'll even warn you to be prepared for a few loud bangs when you try wet forging. When the water gets trapped between the hot steel and the anvil, especially if there are hammer marks/pits for it to get trapped in, it will flash to steam and blow the scale right off with a great big "POW!" that can cause soiling of the underwear if not expected. :lol:

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