The basic hardening process with a furnace involves three steps. I do not do heavy forging, so if you do, I think you will have difficulty getting the same results as these as it’s no easy task to heat steel to forging heat and then refine the grain afterward.
The Three Steps – All Before Tempering
- Shape, heat, anneal steel. There’s no reason to get complicated here. I shape steel during this. Heating is hot enough to hammer a general taper to the steel to reduce the amount of grinding that I need to do. I do this three times. Hotter than critical temperature by a full shade and let the steel cool while hammering to black. Overheating here may cause headaches later
- If you’re dealing with good stock, nothing is gained here if you’re not shaping.
- Too, you could do a whole bunch of things here, like a single heat and anneal in vermiculite. I don’t do this, but it’s popular and a google search will help you find out more about it.
- Thermal cycle steel prior to quenching. In this case, heat steel until just prior to critical. Realistically, it is probably changing phase but not completely, and then partial quench in oil until the steel is magnetic again. This is quick and there is no holding at temperature – the quenches should be done before the steel becomes completely magnetic. Let the steel cool to air cool or close to it after the third iteration.
- Quickly heat steel to nonmagnetic evenly and as fast as you can heat it. Allow the steel to get to a full color past nonmagnetic and then quench in appropriate oil until fully cool. Or…
- Finish the top part of the quench in quench oil, and then once the item being hardened is mostly cool, switch to ice water
- Same as the prior bullet, but just room temperature water and then toss whatever is being hardened in the freezer
A couple of rules:
- The steel is never kept at high temperature for long. As little as possible. Heat changes the structure of the steel. If steel needs to be at a high temperature for duration to normalize, this process probably will not work well.
- Quench oil needs to be able to transition the first part of the cooling fast enough. The faster transition occurs, the harder and the deeper the hardness of the steel will be. Steels that are made for cooling in oil are more tolerant of slower oils – like O1 steel. If you’re using 1095 or 26c3 or some other water hardening steel, the results will be subpar in a slower oil. The quench oil I use for everything is Parks 50. It’s not cheap and the smoke point is low, but the hardness results are excellent and consistent.
- The first part of the quench needs to be fast. The second part doesn’t need to be as fast, but the lower the terminal temperature, the better the result. If you are finishing steel in ice water or the freezer, it needs to get there very quickly or you’ll get no gain. Don’t put hot steel in the freezer – the hot part of whatever you’re working with will be working against your effort to cool the steel in the freezer. Continuous immediate cooling down to the lowest temperature possible yields higher hardness.
- Chasing higher hardness is also correlated with more warping. If you’re using water hardening steels, you’ll be dealing with warp. You can introduce plates or chilled plates to the middle of this process to reduce warping, and mind bevels on things like plane irons. It’s fine to pre-cut cutting bevels, but don’t go overboard or they will help induce far more warping than you want.
The next part of this is a burden on you. When you are learning to heat treat steel, test it. How? You will end up with small cutoffs or scrap, and when you do, those bits of steel are ideal for testing process. Take something well known, like a quality file, snap it, and take a picture of the grain. Files have excess carbide, so they make a good starting target. The carbides make the grain appear a little bit coarse. If you’re using O1 or 1095 or something with less carbon, you should be able to snap your own samples after they’re cool – and before tempering – and look at the grain with a hand held microscope and see that you can create fineness superior to the file steel.
Take digital magnified pictures of your snapped samples and keep them with process notes. Snapping the steel means just that. Before you temper steel, you can hit it with a hammer or put it in a vise and break it. Files will also break easily. The only steel I’ve found to have toughness right out of the quench that can be unnerving is 52100, but 52100 has surplus chromium carbides and I don’t think it’s as good for woodworking as O1, so I don’t use it. It benefits greatly from a furnace normalizing cycle and I’m trying to avoid a furnace. It’s slower and with 1095 and O1, I can match published schedules for hardness and toughness in combination. With 26c3, I can better published data, and only with 1084 so far in this type of steel have I created a dud in tested samples. I never bothered to follow up because I don’t think 1084 has any advantage over O1.
What you’re looking to do with your making of samples is train your eye at the same time. Training yourself to see what red looks like before steel becomes nonmagnetic. Training yourself to see the color at nonmagnetic transition so that you can heat steel a shade further before quench. Getting repetition.
You need your eyes for this. Little checks and tricks and tempilstiks or whatever are too slow, this is all quick.
If someone tells you this kind of experimenting is a waste of time, then you don’t really need to hear from them. To cycle small samples takes only minutes – it’s an ideal thing to keep your brain working and thinking and analyzing when you may not have the time to do something more substantive in the shop.
If all of this seems like too much, just buy good O1 steel (starrett, precision, whatever it may be), heat it quickly to a shade past nonmagnetic and quench it. You’ll never do anything in woodworking where the results just from that will be lacking.
Tempering is next.
Of Making by Hand with Wood and Steel 