Most knife makers will recommend buying new steels as it is takes the guesswork out of the process. But, if you are like many you have a lot of steels lying around or can’t obtain new steels here’s a quick process for determining the quality of unknown steels. If you do this you’ll want enough of the same steel to make many knives. It may take a few to get the procedure down.
A quick walk through my shop and you’ll notice, there’s a lot of scrap steel lying about. I even have a section under one workbench just for knife steels. So how do you determine whether or not the steel you have is worthy to make a knife from? Keep in mind, there’s charts that have the items and what they were made from. They are for rough reference only, the items in some cases aren’t close to the actual steels used. Here’s the way I would determine it.
The first thing you can do is a spark test. There are spark charts available but I really like to keep pieces of know steels for comparison. You can use an old file and a piece of mild steel , they will be your extremes. If your mystery steel sparks like the mild steel, well that is what it is. If it sparks like the file then you probably have a higher carbon steel. Some steels that are heat treatable will have unusual sparks. If you are unsure then take the next step.
The second step is to either cut or forge out a thin section, 1/4x 1/8 x 3” is a good size to start with. Use a torch or forge and heat it up checking with a magnet periodically. When the magnet no longer sticks to the steel (don’t burn yourself) quench in water.
Take the piece and place it in a vice. Use a hammer and give it a tap (where your safety glasses). If the steel bends over then it wasn’t steel that you can use. If it breaks clean, inspect the break. The exposed area should look like satin. If there is a grainy texture then there is a problem. Repeat the procedure with another piece. Be careful with the heating temperature as overheating will cause grain growth. You are trying to determine the proper temperature for hardening. Grain growth will make a poor performing blade.
If everything tests out good you are ready to make a test knife. Forge or grind a blade out and heat treat it. This time however DO NOT QUENCH IN WATER! You will want to quench in oil. The best of the quenching oils for carbon steel is Parks 50, for the beginner canola oil will work fine.
I keep a blade with several cracks on my workbench with *do not quench in water* as a reminder. After many hours into the blade to have it ruined by a water quench is disheartening.
While you are getting things ready for the quench preheat your oven (I keep a yard sale toaster oven in my shop). Start it out at 375F.
Heat your blade up slowly checking it with the magnet. When the magnet no longer sticks heat the piece up a little more then quench the blade only point first into the oil. Be careful not to let the blade lean to one side and watch for flair ups. When the blade is cools down the oil will quit boiling and any flames should go out pull the blade out.
Take the freshly quenched blade and test the edge with a small file. Do not bear down on the file. The file should skip over the edge. If the file cuts into the blade you may have done something wrong or it really wasn’t a good steel. If the file skips then using your belt sander or hand polish up one side or both being careful not too over heat the blade(remember it is very brittle at this point). Do not do this on a hard wheeled grinder as you risk breaking the blade (yes, I had that happen). When it’s polished put it in the preheated oven for an hour. Compare the test blade to the tempering color chart. It should be a straw color. If the blade is paler then turn the oven up 25F and repeat the tempering process. Keep repeating until you reach a straw color.
Finish the test knife out. Put it through cutting test, I like to use hemp rope on a cutting board. It’s good to have a factory knife for comparison. Generally I’ll see about double what a factory blade does but that will vary with the quality of the factory blade and the steel you used.
Rule #10; "I can make that" translates to; "I'm to cheap to buy it new."