In the process of learning to make chisels, I decided that I should see if my chisels could match commercial chisels. In the back of my mind, making something and having it work less well that well established tools (in this case, late 1800s English chisels, or earlier or later if the characteristics are similar) isn’t very satisfying. The why of that is because I’ve now thrown away most of the early tools that I made. They were fun to make, but you realize unless it’s going somewhere, it’s just leisure time wasting and you end up using purchased tools. What’s “going somewhere”? Well, you do almost always need to make something lacking to learn what to correct so that it’s not.
I ended up matching or bettering all of the new and old chisels that I have except for japanese types, and perhaps in some cases (For example, HSS chisels always seem to fare better if they’re introduce to trimming metal and wood junctures). But in this case, we’re just talking about normal wood and “normal” use (chisel goes straight into wood or slices it).
How? This process is what I ended up with – it’ll take more than one post, but it’s not that complicated and doesn’t require anything expensive. How good can the results be? As good as commercial heat treatment, and with some steels, maybe better. But more importantly, you’re doing the hardening, so there’s no pooling lots, finding out that someone chose a different spec for hardness because they weren’t paying attention, etc. All I can really do in the shop is snap steel samples and see how small the grain is (which is good, but not always perfect – the composition of the steel itself can make comparing two samples difficult. An example of this is 26c3 or white steel vs. O1. The two former types will have carbides in them (carbon in this case) and larger visible grain than the latter, but samples of 26c3 end up with a better hardness/tougness profile. )
To cap things off, I guess you have to have samples analyzed. I have sent samples of 26c3 and O1 in to be tested by a metallurgist who specializes in knife steels and I can match the results for ideally heat treated O1 and better them for 26c3. The fact that I can means you can, too. There are no secrets here – if I wanted to really perfect this and market it as a proprietary process, it wouldn’t be disclosed.
What Steels?
This is a process for simple steels. That means iron and carbon and a small number of additives that improve iron carbides, but that do not result in free carbides of other types. This includes:
- 1084
- 1095
- O1
- White 1 / 2
- 26c3
This does not include 52100, A2, D2, M2, etc. Anything that involves chromium carbides or more than just a little bit of tungsten (O1 has tungsten, but not in large amounts), vanadium, etc, is not something that this perfects. I have had OK luck with 52100, but it’s a high toughness steel and it probably benefits from a long duration soak before quenching. That’s something you just won’t want to bother with long even if you had bionic eyes – I don’t think good things happen in the open atmosphere with steel held at high temps.
Before the Process – What if You just Want Simple?
Good quality O1 steel from a good name (bohler, starrett, etc), cut something with the grain oriented to the length of the item, heat it to nonmagnetic quickly, then allow it to heat a color brighter (so if red when a magnet stops stick, allow it to get to orange quickly ) and then quench in clean oil until all the heat is gone, wipe off and then temper in a kitchen oven for an hour at 400F.
That’s simple – it works, no expensive oils, no temperature holding. YOU ARE NOT A FURNACE, so you will go off the rails if you read commercial schedules and attempt to duplicate the numbers by eye. All open atmosphere heat treatment before quench involves getting to temp quickly and uniformly, overshooting a little and then quenching.
If your oil is warm when you quench, toss whatever you’re making in the freezer for half an hour when you’re done, or dip it in ice water. You’ll end up with better hardness.
What about the Knife Steels with Lower Carbon?
I don’t use them – if you’re into knives, I don’t know what optimizes them, but nothing in the process will harm them. For tools, steels that aren’t super high toughness but that attain a certain property at higher hardness are better. Toughness can lead to folding or damaged edges hanging on and that’s not great for use in working wood- we want damage that occurs to leave and be sharpened away later vs. straightening an edge and moving on like can be done with knives.
If you’re into tools, don’t allow someone to tell you that you should be using leaf springs or 5160 bar stock. It’s not the right thing for chisels and plane irons (better for a froe, hatchet, axe, machete etc).
Of Making by Hand with Wood and Steel 
Thanks for the overview and insight. You mention if your quench oil is warm, put the piece in the freezer afterward. Why? Related question, do you have an opinion on cryo treatment in general? Commercial cryo is around -190C I think. I’ve seen reference online to “shallow” cryo at-80 degrees C, and dry ice in alcohol gets to -90C. Not difficult if a person has easy access to materials. Just spitballing here.
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Actually, your two questions are generally linked together. The lower the terminal temperature when cooling before temper, the harder the tempered tool will be. It may only be a point or so, but you want that point for woodworking (e.g., a 61 hardness tool of the same alloy and tempering quality is going to be nicer and more stable than one at 60).
there’s a lot of press given to cryo, but for woodworking tools, the big gain from it is the same – it improves terminal hardness by creating a very low temperature end point and making sure the microstructure is converted as much as possible to martensite (which is what we temper and what makes a stable final microstructure). If the temperature isn’t driven low, some of the structure that would convert to martensite (austenite) isn’t converted and austenite is soft.
You can get some of the benefit without having nitrogen, though, by quickly finishing something in cold water or in the freezer – as soon as possible (like immediately is good). The trade off when hardening things by hand is that steel that warps will warp a little more when the terminal temperature is cold and achieved quickly.
This effect varies by steel and by terminal temp (as in, for some steels, you can wait longer and let the steel sit for a while, dump it in liquid nitrogen and still have a better result – as much as 2 points gained in hardness.) For the plain steels we’ll use and either an ice water finish or a freezer finish, we want to cool the tool as much as possible in the quench oil, then perhaps go to water and toss the tool in the freezer right away for half an hour. It’s free to do and it adds about a point of hardness from just letting most plain steels cool after warmed oil. If you have a target hardness, even if you don’t want an extra point, it’s better to complete the conversion and temper back.
I’ve unintentionally tested this early on by slightly underhardening O1 – to the point that right out of the quench, it was about the right hardness (around 60 or 61). the iron isn’t quite as good (it’s easier to get a deflection in it, but not any easier to sharpen than an iron that’s fully hardened and then tempered back).
Most of the stuff advertised about cryogenic treatment making tools tougher is nonsense, especially for woodworking. It makes them harder (increased strength) and less tough. The woodworking community uses the term “tough” a lot where it makes no sense. Even Lee Valley’s ad copy refers to V11 as being very tough (or listings of it), but it’s a relatively low toughness steel – its virtue if there is one for planes and chisels aside from wear resistance is that it can attain relatively high hardness for a near stainless steel – it’s “strong” even though it’s not very tough.
I probably won’t be able to make a dent in terminology, but what we like for tools is as fine of a structure as possible and just enough toughness at higher hardness – toughness allows a tool to deform and not fail, but we would prefer a tool that doesn’t deform.
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Larrin thomas wrote a nice long article about this – it may be beyond what you want to read. Cryo does have other purposes perhaps in complex steels, but we can’t really heat those well in the open atmosphere. And it allows a maker to more or less overheat steel before quenching a little bit and then go to liquid nitrogen for terminal low temperature and get better hardness than you can get without LN (or the same – whereas overheating steel in a furnace a hundred degrees or so above suggested without LN can result in reduced hardness – the liquid nitrogen converts the austenite that’s formed and “saves the say” so to speak whereas not going to a low temperature would leave a lot of unconverted austenite).
The names aren’t that important for us other than to refer to them along the lines of “we change the steel to something that’s not magnetic when we heat, then we quench and we want the stuff that wasn’t magnetic to be converted to something else. If a lot of the nonmagnetic stuff is left, the steel won’t be as good for a keen hard knife or woodworking tool”.
I can’t hang in a conversation with most people who read a lot about hardening things but don’t do it because they refer to all kinds of stuff that’s on charts (“how do you know you’re getting to AC3?” or something like that -I have no idea what they’re talking about) and it’s not relevant to us hardening in a garage. We’re sort of doing a different thing that fully does all of the things on the schedule, but not for the same time durations or at the right temperature – it’s outcome based more than academic. I believe this outcome based experimentation is why the older sheffield tools were so good without necessarily knowing why. When you’re viewing outcome, then you don’t limit yourself to something that may not be relevant to you and there’s no advertising – outcomes are results, not smudged by advertising speak).
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Thanks again! I’ll read the info on cryo. It is more than I need to know, but it’s interesting.
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Hi David,
Great subject, as usual! Wondering if you’ve ever played with the quenching angle – by that I mean the angle at which the generally longer than wide tool is introduced to the quenching liquid? You don’t seem to talk about this subtle issue. When I made a few half-round hand-stitched rasps a few years back and quenched them vertically, they warped horribly to the thinner cross-section side, so I countered this by tilting the tool blank so the thinner cross-section side was (I think upwards) of the flatter side. I this way, the quenched tool remained more or less straight. Didn’t matter much for a rasp, as compared to a chisel or plane iron, but it was a way to control the warping. Compared to your oil quench, I was using a fairly high salt/water brine quench as I had seen that the older English file makers used a brine solution strong enough to float an egg. I got this factoid from the Wilf Davies file maker video I purchased just prior to making my rasps. Also, I was using old softened files as my rasp blanks. So that might give an indication of the steel type I was playing with.
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Hi, John – I think that dealing with warping is probably a lost art. As you say, the less stable steels (which are pretty much all of them that heat treat well in the open atmosphere) will warp a whole lot if cross sections aren’t just a simple rectangle or square. One thing that comes to mind for me is the bevel on plane irons. You can induce a warp toward the flat side of a tool but with water hardening steels, even in parks 50, you can quickly add too much, and it varies by steel (so 26c3 and O1 need a different bevel side to manipulate that – quench is still straight in).
I haven’t had to do anything with unusual cross sections and usually leave significant bevels (like on all bevel edge chisels) to be post heat-treat work. There are other things to consider with water hardening steel, of course (like the fact that it doesn’t really fully harden to the center of anything other than thin cross sections), so I’m not terribly familiar with rasps and things that would need to be hardened with a complicated cross section (anything other than even opposing sides is “complicated” as far as warping goes). My paring chisels have to be quenched in a quench that’s long rather than tall (I don’t have a tall quench) which presents a problem because if you just put them in slowly, they will banana due to one side being cooled before the other as they’re introduced, so I plunge them and turn them back and forth as fast as possible. They still warp some.
What you say makes a whole lot of sense, though – it’s the interesting part of making, to solve a problem by discretion and experiment rather than complex technical analysis and overcomplication (or worse, settling on changing geometry to give in to hardening, or changing to a steel that’s stable that really doesn’t make a very nice chisel or cutting tool).
Most files are close to 26c3 from what I can tell – they do not fully through harden in slow oils and some don’t fully harden to much depth (1095 is the same – 1095 at 400 temper in my cold quench finish came out at 63.1 and relatively low toughness due to the high hardness – but 1095 hardened in cooking oil just doesn’t ever deliver enough initial hardness to temper back to something with good strength). I recall talking to george that industrially, you have a very short period of time after quench that steel is malleable and the files are straightened after quench. There is very little time to do that, so when I’ve had something that warps, I usually just reharden (as there’s also little time to finish in water and then go to the freezer). What I’m getting toward, is I wonder if the makers of rasps deal with it as a combination of technique and quick straightening). I can say for certain that within a couple of minutes, and maybe less, the window to tap steep straight without tempering is gone, and with files and probably rasps, you can’t tap the cut teeth, anyway.
Some of this may have been solved with early rasps and files by surface carburizing while the core of the steel was left lower in carbon . That would also allow for some adjustment after tempering without breaking the tool. I don’t recall, though, rehardening *any* file (even indian ones) into chisels that wasn’t the same grain all the way through.
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