TUTORIAL POST #5
This post will cover the heat treating of both the blade and spring. The information is for cpm154, if you are using a different steel you will need different heat treat specs.
The first step is to clean both the blade and spring. I use rubbing alcohol. This removes any contaminants that could affect heat treat. Even finger prints can have a negative affect. Once cleaned I recommend using surgical gloves to handle the parts. Next you will need to place both the blade and spring into a foil envelope. I buy the foil in bulk and make my own envelopes to size. You can also buy premade ones. Regardless of what you choose make sure your foil is rated to at least 2000 degrees f. Apparently there is some foil that can only handle 1500 f. Avoid this kind. If you are making your own envelope double up on your seams to help keep everything air tight. you can put multiple pieces in one envelope but don't stack anything. It is very important that all pieces lay flat so they can be properly quenched later on. You should have something like this.
I have multiple blades and springs in each one of these. Yours will be much smaller if you just make one knife.
Now place the envelope in your kiln at room temp. The first step is to heat treat both the pieces the same. Below is my heat treat recipe for cpm154.
place envelope in kiln at room temp
ramp as fast as possible to 1440f
hold at 1440f for 11 minutes
ramp as fast as possible to 1950f
hold at 1950 for 25 minutes
Now put on some long welding gloves and remove the envelope with a set of pliers (don't use your favorite pliers for this). immediately close the kiln door. Leaving the door open at high temps can cause stress on the coils due to the extreme temp difference between the kiln and the ambient air. Now quench the entire envelope between two plates of aluminum (1 inch thick). I have fastened my plates onto a woodworkers vise as shown in the above picture. I got this idea from Jeremy at homestead knives. It works great for doing this alone. You don't need a ton of force just press down firmly on the top plate. While you are doing this blast in compressed air from all sides. Having a vise also makes this easier. Continue applying pressure and blasting air in for 45ish seconds. At this point remove the envelope while wearing gloves (its still about 200f) and use tin snips to cut the foil open. Your envelope will be burnt looking and will have sealed itself around the steel parts (like a vacuum sealed steak). At this point both pieces will be 60HRC, verify with a hardness tester if you have one. If any air got in you may have some discoloration. This is not a huge deal but will affect hardness readings until it is ground off later.
I only have 1 kiln so I fire up my kydex toaster oven and let it get to 400f. I then place both the blade and spring inside and start a timer. The spring must be tempered at a much higher temp than the blade. This will result in a softer material with spring property. I set my kiln to 1175f and wait for it to cool down with the door closed (about 1 hour) By placing both pieces in the toaster oven at 400f you immediately start tempering them. This prevents cracks that can result from the steel being in such a volatile state. Its always best to temper asap after the quench. 400f is the actual tempering temp for the blade. This will not be the final temp for the spring but it wont do it any harm to throw it in while you wait.
Once the kiln reaches 1175f remove the spring from the toaster oven and place it in the kiln. it will sit like this for 2 hours. After 2 hours remove it and let it cool to room temp then place in the kiln at 1175f for another 2 hours.
The blade will go through the same steps, just at 400f. It will finish up before the spring because there was no delay in its tempering process. If you are impatient you can likely remove the spring from the kiln when the blade has completed its second 2 hour cycle.
Once both pieces are done tempering surface grind them together again to remove any scale and to ensure they are at the exact same thickness and perfectly flat for final assembly. During the heat treat process it is possible for slight irregularities to develop. This will correct them. At this time hardness test the blade (expect 59HRC) and the spring (expect 47HRC). If you don't have a hardness tester just cross your fingers.
My process does not involve cryo treatment as I don't have the means to do it at this time. If you wish to cryo you will need to adjust accordingly. If anyone has any opinions or advice on cryo I'm all ears.
From my research the spring is functional from 42 to 49 HRCish. The softer it is the less springy and the harder it is the more snap it will have. A harder spring is also more likely to fail. I haven't played with this at all. Mine have come out at 47 and I'm happy with the results so far.
It is beneficial to practice all the heat treating steps in advance to avoid any confusion during the actual process. Its fairly time sensitive. Once the steel is removed from the kiln you want to quench it within 20 seconds or so. Have the air compressor ready and be sure its at full pressure.
If you don't have a kiln just mail your parts out to a heat treat company, sit back and relax
As always feedback is welcome. I am no expert, this is just what has been working for me so far.