Quenching

Hello,
What happens if steel (O1) is heated beyond critical temp and then quenched? I have been trying to heat treat some steel and it doesn't seem to get hard
enough. Is it possible that I'm over heating it? Other than the magnet trick is there other inexpensive ways of determining quench temp?
Thanks,
Eide
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Hey Eide, nice to hear you are into the thick of it. It sounds most likely that you are quenching at too low a heat. If the metal comes out of the quench and you can still cut it with a file this is certainly the case. If you overheat and quench, several things might occur, none of them good. Slightly overheated steel (50?-100?f) usually comes out of the quench with an "orange peel" surface texture. Sometimes you can still get a workable tool in this scenario. When overheated much more than 100? most likely the steel will crack or split. Just say "sh-- Da--", and start over. O-1 seems especially susceptible to cracking when overheated. Another problem with overheated steel is warpage. This can be severe enough to make a tool unsharpenable. Trust me, if the blade is severely warped it is a waste of time to try and compensate by grinding. Often the tool design,(shape and cross section) is the cause of warpage and not overheating. This should NOT be a problem with a simple plane blade shape. O.K. those are the problems. So how do we get it right? Without special gear you need to train your eye to discern subtle changes in the heat glow of steel. The magnet trick is certainly helpful, but the point at which the steel appreciably hardens is really a bit higher,50? or so. I think the best place to start might be to get some 1/2" O-1 drill rod and do some testing. This material is inexpensive compared to the flat stock. First cut several 6" lengths of rod for samples. If you do not have a pyrometer then use your magnet to indicate an "almost there" temperature. One sample at a time, progressively raise the quenching heat, (note the heat color) until the sample comes out file hard. Don't stop there! Keep doing samples until the steel comes out cracked or split. Hell, burn the shit out of it and see what happens! If you do this enough times your brain WILL remember the right color and you should be able to get consistent results with out a lot of expensive gear. In dim, indirect natural light the spectrum of incandescent colors produced when heating steel is quite broad and directly corresponds to temperature. Avoid florescent lighting as it limits the spectrum. Also avoid bright direct sunlight. I can't tell you how many things i have ruined by being fooled by ambient light. Eide, you seem determined to do this. This heat treating thing on simple shapes is NOT rocket science. Judging heat and color is an intuitive skill and it just takes paying attention to get it right. So get on it and let me know how it turns out.
Glen G.
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I've never done this, so take it for what it's worth:
I've got an _old_ book on heat-treating that recommends taking a rod of tool steel and heating the _tip_ to white hot, then quenching the length, the purpose being so you can recognizes the effects of heat treating all the way from way-burned steel to steel that's too cold to do anything.

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...

That might explain it. I've been trying not to get much hotter than non-magnetic. Have you used the "crayon" (or whatever you call them) to determine temp?
Thanks for your comments. I think I'll hit the drill rod a few times and see what I come up with!
Eide
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The crayons are not much good that hot. Work on the calibrated eyeball, unless you have a chunk of change to spend on a pyrometer. The calibrated eyeball is cheaper, faster to use, and more portable.
--
Cats, Coffee, Chocolate...vices to live by

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Eide wrote:

How about firing cones for ceramics? They come in a range of temps and are fairly accurate and dirt cheap. Yalu Pages under 'ceramic supplies'. Just set the cone on top of the workpiece and shake it off before you quench. Remember that thin stuff loses heat faster than thick stuff, it's a surface area to volume thing, and plan the quench accordingly.
Charly
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Thanks Charley,
Can you expound on:

My steel is 1/4 and 3/16 thick. How do I plan my quench? (I never thought I'd ever have to say that!)
Eide
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Eide, If you are using an electric kiln for heat treating the "cones" Charly suggested will probably work. If you are using a coal forge i think they will not help much as they need to remain vertical and undisturbed. The "crayons" you mentioned are called Tempil Sticks these do work pretty well. The mark liquefies at the designated temp. They also make a pellet version that is probably easier to observe. I agree with Charly about, "planning your quench". Have your quench medium warmed up in advance and positioned as close as possible to your heat source. Rehearse your moves so you know just how you will grab the work piece and get it into the quench swiftly. With something like a plane blade, plan to plunge the blade edge first into the quench tank. Some of this seems obvious but when your steel is READY! you don't want to be searching for your tongs. There is a "ready" indicator that I have used with some success. It is a bit difficult to observe but.... after the steel has lost it's magnetic properties (lower critical) and the heat continues to rise there is a moment when the heat glow seems to drop. Kind of like a shadow passing over the work. This has something to due with latent heat i think. Anyway, if you are working in a dim light this phenomena can be observed and as the heat seems to rise again, it's TIME! Anyway, stop worrying about it so much. Just get it hot and dunk it! You will get it to work........eventually. There WILL be some scrap metal but that's the game dude.
Glen.
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Thanks all for your suggestions. I'll keep making scrap until I get it right!!!!
Speaking of scrap - how many times can I requench O1 steel before all of the carbon is gone?
Eide

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You can quench it as many times as you like. Quenching doesn't remove carbon. What removes carbon is heating steel in an oxygen atmosphere, ie the carbon burns out. That starts at the surface (decarburization). The scale on the surface is iron oxide with the carbon having burned and outgassed as carbon monoxide. As long as you don't hold steel at high heat for a long time, only the surface is affected. Grind that off and you're good to go for another attempt.
Gary
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Yeah, the phase change to austenite (gamma) requires energy. So you see a momentary cooling of the bulk material when it occurs. The reverse change, from gamma to alpha (ferrite), releases energy, so you'd see that as a momentary brightening as the steel cools slowly through the transition temperature.
By quenching, you freeze the structure in the gamma phase before it has a chance to do the alpha transition. That's what makes steel hard. By cooling slowly through the transition temperature, you allow the alpha transition to occur, and the steel is soft (annealed).
Gary
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Gary, Thanks for the science behind the phenomena. It's been a while since I picked up a treatise on heat treating and all my alpha's and beta's are a bit confused. Actually I never could keep all the nomenclature clear in my head. After you get the "trick" the words don't really matter though, don't you think.
Glen.
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