Article 3 - Heat treating carbon Steel

**Samurai Stu** · 02-11-2010, 20:50

steel selection and preparation
One of the most common steels available for forging is 5160 and is a good way to start. This comes from car leaf springs. Use an angle grinder to trim it length ways to strips of 25 - 30mm. Simply place the end of the length in your forge using the rest as a handle to secure the work. What also works very well is old files. when i say old I mean old not used. Newer file are merely case hardened mild steel and lose all hardness characteristics when heated to forging temperature. Old Pferd or sheffield files work nicely.

An old file forged into a bowie knife

To harden any carbon steel is quite simple.After forging (or grinding), heat the whole blade evenly by passing it in and out of the forge to non magnetic temperature. Keep a magnet of any kind near by to test. A speaker magnet works well because it is strong. Once the blade has been normalised (the name of this process) leave it to cool for about 5 min. Then repeat two more times. This process de stresses the steel from the forging process thus enabling hardening to work more effectively without making the blade brittle. Turn off the forge leaving the blade inside. This slow cooling is called annealing. This is to soften the steel so we can work it without blunting all our tools. When cold you can work the blade with a grinder or file to clean off the scale and reveal the steel.

Blade cleaned and ready for hardening

The blade, ground and filed to the final desired shape it is now ready for hardening. Hardening is the process of quenching the blade to achieve a serviceable hardness so the blade takes and keeps an edge. Re heat the blade evenly to non magnetic (critical) temperature. This is important as too high a heat will result in a blade that is not as hard as it could be. You will feel the steel lose its magnetism within a few heats of critical temperature. Try not let the tip over heat. When ready quench the blade in a suitable solution. Motor oil works fine or a professional quenching medium. I use pig fat mixed with transmission oil. Quench the blade one third of the edge into the oil. This is called a partial or selective quench. This gives the edge its hardness while the back retains a softer medium keeping the over all blade form being brittle. When done the edge can be tested with an old file. I use the corner and try to file a gouge out of the edge. If it does the steel needs to be hardened again. If it skates across the surface it is sufficiently hard. The file is quite hard and is a good gauge of the hardness of the edge.

Not the same blade but this is the quench.

Quickly grind the blade bright again removing the blackness of the quench and then place the blade in an oven at 190deg C. Preheat the oven for best results. This process tempers the steel destressing the newly hardened edge to prevent cracking or chipping during use. Do this three times for one hour each leaving to cool to room temperature during each cycle. If you wish to coax out extra hardness you can quench the blade in cold water between each cycle. This is risky but can result in a fantastic performance edge.

Now do your final grinding and polishing and you should have a very tough knife that will survive a bend test.

I hope this was interesting. Thanks.