Way back, when I was first working out forging, I knew a blade- and a lot of other things- had to beheat-treated, but didn't really understand the why and only a fuzzy idea of how. So did some reading(couldn't find a lot at the time,pre-internet it was books only and not yet a lot of them). A lot of the stuff I could find was more industrial-oriented, but got the idea.
I've mentioned this job before, basically two steps: bring the piece up to critical temperature and
quench in the proper medium to harden; then heat again to a lower temp to temper*. Nice and simple;but the first times you do it... not just stressful on the steel.
I'd learned to forge and grind and polish blades fairly well, but knew had to heat-treat them properly before they'd be ready for real use**. So gathered my special material. Which in this case was some old motor oil.
A quick digression: the proper medium to quench in depends on the steel(and sometimes
on the end use); for blades even a steel rated as water-hardening usually gives best results with an oil quench; a steel that hardens beautifully in water in a large structure may well crack if made into a blade and water-quenched, it just cools TOO fast and the shock is too much.
There are oils made specifically for hardening different types of steel, but I not only didn't have any I didn't know where to get them and couldn't have bought them if I had(two little kids, etc.) So this attempt started with motor oil. And I didn't know that it was best to warm it either(though if I were smart enough I'd have figured that part out on my own). In any case, I had about two quarts of oil in a bucket, and two small blades for the trial.
Lit the forge, worked it to a clean fire and put the first piece in. This part, no problem, just work
the blade back & forth, turn it over to keep the heat even, get to that proper shade of red that meant critical temperature***. This was a short blade to make this easier, and it only takes a few minutes to get it there, and then pull it out of the fire and plunge it into the oil.
Oh yeah, it smokes and stinks, and I was standing there wondering if it would crack from the shock. It didn't. A piece that comes out of the quench looks gray and scaled(especially from dirty oil), and you have to clean off the oil and scale and shine it up to see if there are any flaws. There weren't, so came the tempering.
Turn the blower back on, low, and start working the blade back & forth and turning it to heat it evenly. You have to keep it clean as you do this, so you can see the color of the steel**** to tell how hot it is, how hard it is. (very)Generally speaking, for a general-use knife with steel like I had you looked for a dark yellow-light bronze shade which indicates around 450F. The level of stress I was feeling as I worked that blade around was incredible; it was so damned important
to me that this work. What I had read said that it was best if you didn't have to quench it again to stop the heat, to time it so when you took the blade away from the fire the color would not continue to darken. Amazingly, I managed to time it right, and let it cold down completely, and test it.
The first test is with a file; a sharp file should almost cut the edge of a properly hardened blade;
if it actually cuts metal away the blade is too soft and won't hold an edge well, if it won't cut
at all(just slides off) it may be too hard and the edge will chip or crack in hard use. So clamp the tang in a vise and try it, and it seemed just about right. So the next is to clamp an inch or so of the point in the vise and see if it'll flex without breaking, not only to make sure it's not too hard but to find if there might be a flaw hiding inside from the forging or hardening. I cannot
describe the elation as I flexed that blade back & forth like a spring, tough enough that I couldn't
bend it, just coming back to straight when I took the pressure off. Effing incredible, and I'd done it. Myself. With my home-built forge and charcoal and a piece of coil spring. If there'd been a woman handy...
When I calmed down a bit I did the second blade, and it worked too! I was damn near floating above the ground I was so high. And when sharpened they actually cut, and held the edge well! Just bleepin' amazing!
A somewhat disgusting number of years has gone by since then, but I still remember it. Lots of knowledge and experience gained, and materials; a few years later I found a place in Tulsa that sells industrial oils and got a five-gallon bucket(of which I still have about two gallons) of light quenching oil; I found places where I could buy oil- and water-hardening tool steel of different alloys for downright cheap, though I still used spring stock for a lot of blades; I found places to get coal instead of charcoal briquets for the fire. I made a lot of things in time to come, small and large, but I still remember the feelings when I heat-treated blades for the first time.
*Quick repeat from years ago: the quench- if you have the temp right- freezes the steel structure in a very highly stressed state, makes it brittle-hard; the tempering heat relieves enough of the stress to remove the brittleness but still leaves it stressed- hard- enough to be tough & flexible AND still hold an edge in use.
**Realistically, back when in the historical periods I was interested in- especially the early ones- the steel I was using, just ground to shape, would have made a better blade than most of humanity would have access to for centuries to come; but to help that steel come to its real potential...
***One of the tricky things, the difference between a little too cool where it won't harden fully and a little too hot which coarsens the structure and makes the blade less than it can be. To get a bit poetic, I came to describe it as 'the moment when that slight shadow in the steel disappeared', which if you actually watch the piece heat is true.
****One of the things I later did was temper in the oven; clean off the oil and shine the piece, and you could set it in at a known temperature. By the way, another method was to heat an iron block really hot and then work the blade on the surface to heat it, or lay a piece of iron pipe-an arch, if you will- over the fire to heat and work the piece on it.