For woodworking tools and a lot of our knives, we use steels that are martensitic steels. The meaning of this is relatively simple - most of these steels start as some composition when we get them if the steel is not hard, and the composition remains the same, but the structure is something else. When we harden them, the objective is to transform what is in the steel from a prior (micro) structure to a martensitic structure that is hard and brittle, and then to temper that martensitic structure so that it is slightly less hard and much tougher.
You can know nothing about all of this until you need to solve a problem. You can know nothing about all of it entirely if you follow the simple advice on heat treating O1 steel to make woodworking tools by just buying good stock, heating, quenching and tempering.
So, if the terminology is a pain, this is just a limited amount of it, and I never sought it out until pushing to get better results, either. if you start with the simple case mentioned in the prior sentence, you will find improvement from that isn't that easy because good stock and an appropriate heat and temper will make a tool that matches commercially heat treated tools.
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We typically receive steels like O1 or W1 or 80crv2 (probably pfeil's steel) or 1095, etc, in a spheroidized condition when buying high quality bar stock. This is a microstructure that allows the steel to be as soft as it can be and machine easily. If you are using a computerized furnace on a timed schedule, you may have trouble heat treating directly from spheroidized. However, the method's discussed in this topic will tolerate it without issue. The other commonly sold condition is annealed, which generally means the steel was cooled at a rate that prevented other structures. You can put these aside for now - one really isn't better than the other unless you have problems with coarse spheroidized stock, but put that out of your mind for now.
In the most basic process of heat treating, we heat the steel until it hits a state that is nonmagnetic, and usually a little bit past that - but it varies based on the alloy. No worries, the variation for practical purposes is simple and what you do with one steel vs. another should not imply more than about 10 seconds difference in effort in this step.
Once the steel is fully magnetic, it's transformed to a nonmagnetic structure referred to as austenite, which is why you see suggestions to use a magnet. To use only a magnet is incomplete on most steels and can result in underhardening, but we will use a magnet on the end of a metal rod (like a pickup magnet tool) as part of the process. There are exceptions, such as 1084 steel, which do well if they are heated just past nonmagnetic and are then quenched quickly. Some others will be outright failure.
At the point this transformation and heating are complete, we quench oil or water hardening steel in either a medium oil, a fast oil or brine. I've never seen a reason to use plain water on tools, but cracked plenty of tools trying it. Oil use is more common than brine.
There are two important steps in this procedure when quenching:
1) getting the steel transitioned down to near the temperature where martensite forms as quickly as possible. With a water hardening steel like W1 or a historic cast steel, this may be 1 1/2 to 2 seconds. Crudely, this is getting the steel in the quench evenly and cooling the color out of it
2) if you have successfully gotten steel past the point where undesirable things occur, it will be in a range where martensite forms. All of the steels that I would use and discuss form martensite at room temperature, somewhat above, and somewhat below. I have read that you have about two minutes to finish what you're doing here unless you are going to resort to liquid nitrogen. LN isn't practical for the average person because you need a dewar to hold it, a source that will sell it to you and it does not last in the dewar indefinitely. I guess it can be dangerous, too.
If you get deeper into a bladesmithing community, you will start to find jargon, but there is already more jargon in this description than you need to heat treat steel. Larry Williams did an excellent job in his moulding plane video and got me success on the first try. Tips from others later claiming to have better information were actually worse and the information was relayed from folks who had read a lot about heat treating but did little to differentiate O1 vs. 1084, and so on. The nuances are small if you are heat treating by hand and eye with a magnet, they're easy to understand, easy to perform, but not optional - failure can be the result.
Lastly, if you start reading around and you see one person who puts O1 into liquid nitrogen immediately, or a freezer (me), and another that says to terminate a quench at 200F (hard to measure!) and then allow steel to air cool, both may be suitable. You have to avoid reading a source and believing it's the only way one way or another because my forge heat treated O1 sample nailed book for commercial treatment specs going from hot to the freezer in 30 seconds, and I've seen and done samples that come out warm and finish in still air, an they were also fine. There is a lot of "you can't do" in the knife community which may be true, or it may be "i haven't done it, or read it can't be done, so you can't either".
What do you need to know from all of this:
* When we heat steels we're going to use, they need to go to a nonmagnetic structure, and potentially some past, but not at really high heat for any duration
* Depending on the steel (we will specify), the first part of the quench must be done quickly to black color and well beyond that, and the second part is less of a rush in most cases and provides a lot of options. The second part of the quench to cool can be done "very fast to very cold, too", there's no complicated start and stop needed. The first part gets past forming structures we don't want, and the second part establishes structures we do want