Found this it seemed interesting
Here is my process for using and recycling steel that had a previous application in life.
First off, remove all fasteners, bolts, spacers, etc and get everything knocked apart. With leaf springs it might be easier to manuver them around if you cut them in half after you get the individual leaves apart, use an angle grinder or whatever you are comfortable with.
Next give them a good lick with a wire brush to knock off most of the grit and grime you can before annealing. Then build a decent size wood fire and toss them in, leave em in until the fire dies out. For me this is usually a late afternoon or early evening start and I dig the steel out in the morning.
Wirebrush again, if the steel has formed scale that prevents you getting a good inspection for cracks soak it in vinegar for 3-4 hours and wire brush again.
Now if it is free from cracks and it was a spring, it was probably an oil quench steel. That isn’t always the case 100% of the time though. Cut three small strips off the spring and draw them out to a uniform thickness, I usually will do about 1/4″ to 1/8″ thick by 6″ long so it’s easy to work with. Take each up to non-magnetic/critical temperature, hold there for about 5 min, then quench one in veg oil or similar, one in water, and air cool the last. After quenching, clamp each of your pieces in a vise and use a wrench to snap them in half, take note of how difficult each one was to break and compare the ‘grain’ structure of the metal. The test piece that has the finest grain structure is the one that was quenched correctly.
Now if you are really feeling curious, you can nail down one more characteristic of your steel. This one isn’t really going to help you a lot, but it might be useful.Take one of your test pieces and grind it down to shiney clean metal, then do the same with a chunk of plain ‘ole mild steel. Sit them side by side and spray them with salt water, you can dunk them if you don’t have a spray bottle handy.
If they rust at the same rate, then you’re likely dealing with a 10xx series. If they don’t, then there are likely other alloying elements in your mystery steel.
So now you have a ballpark idea of what you’re working with, what’s next?
Lets define some things before we get too far in the weeds.
It’s important to be aware that steel is not just one element with a few other bits poked into it like a granola bar. It’s more like a cake or a brownie depending on how the heat treatment has been done. You need to understand how to get it all to be uniform, otherwise you end up with some bits being more brittle or softer than others and that means it’s more prone to break.
Steel is made of two primary things at its most basic, Iron and Carbon. The two elements interact and bond together in different shapes that make steel harder or softer. When you add in othere elements to the mix like chromium, manganeese, copper, nickel, etc. it changes how all the elements in the mix ‘hook on’ to one another. While those other alloying elements can dramatically change the nature of the metal, it’s still the carbon and iron that do most of the bonding.
Normalizing is essentially a hard factory reset for an alloy steel. The material is taken up to it’s critical point and held there long enough for the atoms in the alloy to let go of each other somewhat and just hang on to the ones that they are most attracted to. This means that you get an extremely fine and uniform grain structure throughout the steel. It also means that when you do the rest of the heat treatment, you won’t have hard and soft spots. The general process is pretty simple, the steel is taken up to the hardening temperature (just a pinch beyond non-magnetic) and held there until the temperature is constant throughout, 15 minutes per inch of thickness is my rule of thumb, and allowed to cool in air, a moderately slow rate I think, to allow that fine uniform mix of elements in the alloy, aka grain or microstructure.
If you’re recycling steel from something else, like springs and such, normalizing is critical before and after forging. Not doing so will frequently lead to unexpected and undesireable issues in whatever you’re making.
Annealing relieves internal stresses in the material from either intentional or unintentional cold working of the metal. Essentially you take the steel up to a temperature below it’s critical temperature where you’d need it for hardening or normalizing and let it cool slowly, this varies a bit depending on the steel alloy being cooled since there are some steel alloys that harden in air, those you would have to cool at an even slower rate.
The terms annealing and normalizing do have an ounce of confusion between them, and don’t by any stretch, call on me as an expert. My understanding is that Normalizing is going to reset the grain structure of the steel, and annealing will generally get things softer and relieve stresses, largely, without meddling with the microstructure of the steel.
Quenching is done when the steel is evenly heated to a little beyond the temperature a magnet will attract it, commonly known as the critical temperature, then inserted into the quenchant like oil, water, brine, air, or whatever. This cools the steel at a uniform and predictable rate, hardening the steel. If the correct quench medium was used, it will be as hard as it can be, but brittle as all get out, frequently it’s referred to as “glass hard” at this point. Usually I’ll use an old file to make sure that it won’t bite into the material at this point to make sure it hardened up well.
Tempering is the next step, you’re heating the steel just enough to slightly soften it. There is a trade off though, higher temperature tempers rob the steel of that hardness. There is no avoiding this trade off, harder steel holds a keener edge, but is far more fragile. For most common steels used in knives get tempered between 300F to 550F And allowed to slowly cool in the tempering oven or in air. Don’t quench it again if you can help it.
Depending on the steel and the intended use, the temperature you temper will vary depending on the final hardness you want, many of the more common steels will be very brittle at around 60 to 70 hardness RC if all has gone well.
Straight razors, for example, are usually tempered to something around 300F-350F in order to be very hard and hold a fine edge, approximately 58-60 RC.
Skinning knives are often tempered for hardness around the 54-56 RC range, 400F-500F, as they see more aggressive use that is more likely to damage the cutting edge.
Blades made to be struck and beaten on, pried with etc, will often be tempered for a hardness around 48-52 RC range, 500F-650F.
All those numbers vary based on the steel/alloy being used. There are many other variables to the process depending on what you’re using and what you’re doing.
On a final note, motor oil is a terrible quenchant and it has NOTHING to do with toxic smoke. It’s all about the rate of cooling. When you quench a blade, you are trying to get the right mix of pearlite, martensite, and other crystalline structures to form in the steel to give you the most hardness.
Motor oil doesn’t do that, it’s WAY too slow (we’re measuring the rate of cooling in 1/100ths of a second here at least).
Vegetable, canola, and peanut oil, however, are much faster quenches and better able to cool most 10xx, 51xx, 41xx, and many other steels much closer to their ideal rate. They match commercial quenchants like Parks closely too.
That is why they are the preferred.