I thought it would be advantageous to have a thread to reference for the benefit of beginners (or anyone under equipt). In this we are only going to look only at our beginner safe steels. Being that I am highly underqualified to direct anyone on metallurgy, correct me at will, and add what you think, or any questions!
First up; steel selection . What makes a beginner safe steel? The answer is to keep it simple. A general rule of thumb is the less complex the steel; the less complex heat treatment is (with exceptions).
High chromium steels who's carbides require long soak times in order to get into solution are not safe for beginner's. However, Alloys like 5160 with a moderate amount of chromium are easy for beginners to use and are a fan favorite for its attributes such as toughness, wear resistance, and edge retention alongside harder to heat treat steels like 1095.
Alloys like vanadium can actually help keep grain size small.
Manganese can have an effect on the depth of hardness. Low manganese steels are shallow hardening (use for hamons) and classified as a water quenched steel (don't try water). Higher manganese steels are deep hardening and classified as an oil quenchable steel (definitely don't try water).
A lower-high range of carbon content (.75-.84%) can use a slower speed quenchant (such as 120°F canola oil) and are less sensitive to overheating.
So our favorite begginners steels are:
From the 10xx group: from 1075, 1080, and 1084.
Unrelated to those; 5160, 80crv2, 15n20.
How to work these steels
There is no doubt some will want to try forging a blade. Anything heat wise you do to a blade is a part of its heat treatment. These steels need to be forged at what I see as a high orange color to a mid orange color. To me, red is around 1,100°F- 1,300°F. Don't forge anything more than to straighten a blade at this heat. You want to be at a temperature of around 1,900°- 1600°F for forging. This can be tricky to go by. Some claim the steel is "cherry red" others claim it is yellow or orange. We all see it differently.
It is next, but not until we learn the big word below.
"De"as a prefix means "to be away from", or "without" and "calescence" means "to warm up" in Latin. So, "decalescence" means (roughly) "to be without warming up".
Since energy is matter, and matter is energy; the steel will release light and heat energy When heated. When you heat the steel to a certain point, the steel begins to change its atomic arrangement. Such a change requires energy to accomplish, so the steel cannot emit its light energy, and heating may slow down. This creates a visible "shadow" in the steel that can be used as a waypoint in the normalization and hardening processes.
Recalescence is the same thing as decalescence but in reverse. So you see it as the blade is cooling. Here's a video by our own @Wes Detrick (hope you don't mind Wes ).
For a closer look, I'm gonna quote the guy who explained it to me in another thread; Alan Longmire.
"It's not heat, nor is it grains. It's photons and individual crystal structure. When the crystal goes from face-centered cubic to body-centered cubic it takes energy to accomplish, thus the momentary darkening. It does not cool off (much), and when it brightens again after transformation is complete it is because the photons are being emitted again rather than absorbed.
Exactly the same thing happens in reverse when you heat it up. The swirling shadows you see are the crystals transforming from body-centered to face-centered, absorbing energy. This is the dimming via lack of photon release, it is not cooling off.
We're in the realm of subatomic phenomena here; where visible light is due to electrons jumping up or down one step in energy level, releasing or absorbing photons in the process. Matter is energy and energy is matter, light becomes solid and vice-versa. E=mc^2 and all that.
Grains are just groups of crystals growing in the same alignment, not unlike quartz crystals. You can have big ones you can see or tiny ones you can't, but that make up a large mass anyway."
So, if you couldnt make sense of that; the steel darkens or forms a "shadow" at the temperature right before you would be ready to quench. You continue to heat the steel until the shadow brightens until it becomes the same color as the area just outside of the shadow. You want to heat as evenly as possible until the shadow is gone. Heat thicker areas first, and then move to thinner areas. I pull my blade in and out of my forge's hot spot to achieve even heat. Some use a pipe capped on one end inside of the forge to create an even heat.
This phenomenon is best seen in low light conditions and is used for both normalization and hardening.
We're going to skip annealing as I see it as unessesary and difficult for a beginner to accomplish.
To soften the steel for stock removal and drilling as well as grain refinement prior to hardening; we normalize.
Using decalescence, we typically (using these beginner steels) run 3 cycles to refine grain after forging or annealing. To do this, you take the first heat a little above "critical" (the point after decalescence) and let cool in still air until no color is left. I typically quench in oil at this point, others like to wait until it's just about cool enough to grab. Then, another heat is taken to right at critical temp and then allowed to cool in still air. Lastly; the blade is taken to a dull red heat and allowed to cool in still air.
Note: if this project was taken to welding heat or fully annealed more cycles of normalization won't hurt. I typically do 3 sets of each cycle above.
This is just about the same as the second step of normalizing with the addition of quenching.
The above steels can all be heat treated using canola, or peanut oil. You'll need to heat the oil in a metal container to around 120°F. I judge this as uncomfortable to hold my finger in for more than a second. If you wanna get fancy; buy a meat thermometer. Scrap metal can be heated and dunked in the oil to heat it.
Have your oil warm and just a step away. Heat your blade to critical, and without lollygaggin, put it tip first into the oil and make slight cutting motions through the oil with the blade. Wait 12 seconds to pull it out. Any warps you have can be fixed in the temper.
This is what softens the brittle blade and should be done immediately after hardening.
The right temperature for tempering should be decided with the design of the blade in mind. A blade with a lot of force and leverage applied to a robust edge should be tempered hotter for toughness. A chef's knife might be left harder to maintain an edge longer. This is a compromise between toughness and edge retention.
You can temper in a toaster oven, a conventional oven, or even a real tempering oven. If you choose a conventional oven or toaster oven, use a meat thermometer to measure heat. Most ovens are out of calibration, and have temperature swings. To combat this; use a heat sink such as a tray of sand, or a firebrick. Flip the blade each cycle.
The cycles should be one hour minimum for 3 cycles minimum. I do three 2 hour cycles. Leave it alone for one cycle, take it out, and quench in water. Repeat that until you are done.
The point behind cycles is:
When you harden a blade; you heat it to critical which forms a grain structure called austinite. The austinite is converted to martensite when quenched. Some austinite is left. Retained austinite turns into untempered martensite while tempering, so you temper in cycles just to try and get everything tempered.
Now you're done!