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heat treating my homemade anvil


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Sooo when i make the next one the process should be?...

-since i don't have control over whether the furnace is running rich or lean to avoid the decarb, which is a drag to remove.
-bring it up to temp, non magnetic or 1500 ish.
-don't let it soak.
-take it out and quench or should i normalize once then quench?


Lemme preface this by saying that you've heat treated one more anvil than I have, so all I'm going on here is what I think I know about metallurgy and heat treating.

I think you need to let it soak at least a little to let that good austenitizing temperature penetrate into the steel. Not all the way through, I suppose, but I'd think preferably an inch or so. Otherwise you'll end up with a thin, hard skin over a relatively soft center. That'd be better than nothing, but not ideal.

One possible, at least partial solution to your decarb problem might be to coat the anvil with an anti-scale compound. Boric acid (readily available as roach killer) dissolved in alcohol and painted on, then allowed to dry until the alcohol evaporates, will help. You can do the same thing with borax dissolved in hot water. But beware: as I understand it they both become corrosive to steel above 1600 F.

Whether you need to normalize depends on what you've done to the steel up to that point. If you've been welding on it, you'll probably have some fairly large grain in the heat affected zone that could use a normalizing cycle (or better yet, two).
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Wish I could find the thread where we were talking about whether steel could be up to temp on the outside and not on the inside. Specific research and experience says that when the outside is up to temp, so is the inside. The research indicated that the core never lagged by more than 25 degrees. Anyone know where that thread is?

Matt: Have you tried any of those anti-scale brews? Sounds interesting. There are commercial compounds for that.

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I don't know about that Grant. After watching the video of your induction forge I'd say the 1(2?) 1/8" rd was near yellow on the outside and the center was still black.

Of course a furnace isn't going to heat steel anywhere near as fast as induction.

Do you have a special procedure you follow with the induction heater or do you use it for heat treating.

Frosty

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Grant, I just learned about them; I plan to try boric acid and alcohol on a blade this weekend.

The borax slurry was recommended by a metallurgist who used to supervise heat treatment of F/A-18 landing gears. He said he used a paint made of borax and solvent (he didn't say what solvent) on the gears during heat treating. Of course he also used a fancy protective atmosphere in the furnace, but that's not practical for most of us. The boric acid and alcohol idea came from an engineer who's into bladesmithing, and a goldsmith confirmed that it's a standard anti-scale dip in his profession. I'd post links, but I don't want to offend anyone. You can find the threads pretty easily with Google.

I'm aware of the commercial compounds, but cheap and improvised are my middle names. :)

Edited by MattBower
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Wish I could find the thread where we were talking about whether steel could be up to temp on the outside and not on the inside. Specific research and experience says that when the outside is up to temp, so is the inside. The research indicated that the core never lagged by more than 25 degrees..


Found it. http://www.iforgeiron.com/forum/f57/generating-voids-middle-13262/

Of course that's possible with a material that's sufficiently thermally conductive, especially if the heat source isn't extremely hot (O/A flame, electric arc, etc.). (For these purposes fast induction heating is like a really hot external heat source, because it acts almost entirely on the skin of the steel.) I just didn't realize steel was that thermally conductive. Edited by MattBower
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Grant, I heat treated a small blade today, using a slurry of boric acid and 90% isopropyl alcohol as an anti-scale compound. I painted on several coats. With gentle heat the stuff dries very quickly, and although the coating it was fairly fragile it adhered well. I got a little worried when I stuck it in the forge, because it immediately started bubbling. But it settled down as the steel started to heat, and it seems to have done a pretty good job. I'll get a better sense of things when I start sanding, but the steel looked really clean coming out of the quench. I also used a reducing atmosphere.

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Yes, borax in water was another possibility. I liked the alcohol and boric acid idea because alcohol evaporates fast -- i.e., my "anti-scale" dries fast -- and I figured boric acid might bubble less than hydrous borax. I'd have to say that latter idea didn't really pan out, though.

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Final update on the boric-acid-as-anti-scale idea: combined with a reducing atmosphere, it seems to have worked exceptionally well on the blade I mentioned above. I had sanded to 220 before heat treating. Afterward, although there were some variations in color on the surface of the steel, everything felt very smooth. I dropped back down to 100 grit to remove the discoloration, and that only took a few minutes. There was no sign of any pitting, no scale adhering to the blade, nothing, really. The biggest problem was some removing some glassy flux residue on parts of the tang that didn't get to hardening temps. Boiling water was the quick and easy answer to that. I'm definitely going to continue experimenting with this for simple carbon steel blades; I think it saved me quite a bit of sanding -- and I really hate sanding.

This probably isn't the thing for high alloy blades that need hotter HT temps, because the boric apparently will become corrosive a little above 1600 F.

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  • 1 month later...

downsfish,
I think your anvil will be just fine judging by the hardness figures you posted. With 0.3 carbon and 1.1 manganese, I think your anvil would benefit from some work hardening. I run the numbers you gave us and came up with a carbon equivalent (Deardon-O'Neil formula) similar to 4140. Sounds to me you did everything pretty right on and you could cold work your working surfaces a little bit with a pneumatic "welders scaling gun" or preferably with a rounded/blunt chisel bit. That's all I would do...looks good enjoy!

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  • 3 weeks later...

Decarb is associated with carbon diffusing out of the steel. You will get a nice gradient that is easily predictable from the math in Practical guide for metallurgist published by Timken (it is an easy download.).

Boric acid (and I am sure some other magic pixie dust) mixed with an organic solvent makes for a nice decarb stop off. The base in the commercial stop off we used was toluene but alcohol would work well. It is a lot cheaper and easier to deal with too.

Trouble with induction is that is generally a surface effect - you have to use a low frequency to drive the heat thru the entire thickness. The issue could be differential heating causing high residual stresses or other causes.

Scott

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Final update on the boric-acid-as-anti-scale idea: combined with a reducing atmosphere, it seems to have worked exceptionally well on the blade I mentioned above. I had sanded to 220 before heat treating. Afterward, although there were some variations in color on the surface of the steel, everything felt very smooth. I dropped back down to 100 grit to remove the discoloration, and that only took a few minutes. There was no sign of any pitting, no scale adhering to the blade, nothing, really. The biggest problem was some removing some glassy flux residue on parts of the tang that didn't get to hardening temps. Boiling water was the quick and easy answer to that. I'm definitely going to continue experimenting with this for simple carbon steel blades; I think it saved me quite a bit of sanding -- and I really hate sanding.

This probably isn't the thing for high alloy blades that need hotter HT temps, because the boric apparently will become corrosive a little above 1600 F.


WTG - I knew it would work. Try allowing it to dry a bit more. You were driving off residual water in the isopropanol when you stuck it in the forge. Try slowly heating to above 300F.
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  • 2 weeks later...

Decarburization, as Kenzie noted is indeed due to the diffusion of carbon from the surface of steel into the atmosphere surronding the steel. It is a little bit like osmosis, except that the carbon doesn't just exist as carbon but will subsequently combine with oxygen in the atmosphere to form carbon monoxide and carbon dioxide. Since the movement of carbon within the steel happens by diffusion, the temperature will control the rate at which carbon leaves the steel surface. Time at temperature will control the depth of decarburization. In large forgings, most of the decarburization occurs during the forging step since that stage has the highest temperatures. Some additional decarb will occur during heat treatment if that operation is performed in an oxygen rich envirionment. On our forgings we expect to have 1/8-3/16 inch of totatl decarb after heat treat, assuming no machining prior to heat treat.

The hardness results reported by Downfish would suggest that his anvil experienced only minimal decarburization.

Since decarburization is driven by the desire for carbon to combine with an element it is more attracted to than iron (generally oxygen) decarb can be prevented by heating in an atmosphere that is already carbon rich or absent of oxygen. Alternatively, a physical barrier can be created between the steel surface and the oxygen rich environment.

For those curious about my background, I am a plant metallurgist working at the Scot Forge company in Wisconsin. I've been here for the last 6 and a half years and prior to that worked at Timken in the bearing devision for two years. I've been forging myself for about 12 years.

Patrick

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  • 4 weeks later...
  • 7 months later...

Thanks again for all the amazing help/info, :) and as an update I've started on my second anvil this one patterned after Mr. Hofi's. I'm still in the shaping process which will probably take a couple more weeks to complete since I peck away at it when we're slow at work.

I'm thinking once I get it shaped and ready to HT I'll coat the top with a boric acid mixed with toluene solution, then tack a 1/4" plate cap on top of that to help keep off the decarb. Then before I chuck it in the flume I'll knock off the cap so the water will hit the anvil instead of the cap.

Any suggestions? Did you notice me learning, I'm asking questions before I do it instead of while and after. ;)

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It's cool that you're in a position where making and heat treating your own anvils is actually practical and cost effective. If this is the same sort of steel you used last time -- around 0.4% C, with high manganese and significant bits of chromium and vanadium -- it'll probably make a pretty darned good anvil, assuming you get it hardened properly.

One thing to remember about the boric acid is that it attacks the steel pretty aggressively starting at around 1600 F or so.

Good luck. Let us know how it goes.

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Yup, well kinda, to roll the rebar we start with a 5 1/2" X 28' +or- billet. The smallest we roll is #3 bar (3/8") I can't remember exactly but 1 billet ends up being about 150 or so pcs of #3 bar 180' long, the biggest is #18 bar (2 1/4") which ends up being 1 bar 180'. Since the #18 bar doesn't have near as much reduction/work happening to it, these billets require a substantially different chemistry to end up with similar strengths, to do this we test the "heat" several times before we "tap" it and add whatever alloys are needed. When the chemistry in a "heat" is off or unsuitable those billets are cut up, taken out to the scrapyard, and remelted. I told you all this to say that the chunck of billet I pulled out of the scrapyard was no good for rebar but obviously good for an anvil.

While I'm on this subject I think it's a good time to say that rebar comes in different grades 75,60,40, and "no grade" depending on what the customer orders. The stuff you find at the big box stores is usually "no grade" because it's the cheapest which is also the weakest, in fact when we roll "no grade" we use old nearly worn out rolls and grind out the identifying Mill marks.

I hope that answered your question, if not I'll try again.

Scott

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