Heatthreating a thin blade


Hey, i want to heatthreat a machete. It is about 3mm thick. I cut out the shape in an old saw and I tried to harden it but it started bending and cracking. Does anybody has an idea how to harden a thin blade without cracking



Hey Yair - this is going to be tough. Heat treating is typically done at at a thicker width so the blade can withstand the shock - this is why many knife makers do their rough grind and leave a good thickness on the blade end before heat treating, and then do a final grind on it after heat treating.

If it’s cracking, unfortunately I would suspect it may be a loss. If there is one crack, there is typically going to be more, or it was just way too hot and now the blade is too brittle.

What I have seen some smiths do when heat treating thinner blades that bend in the quench is try to immediately straighten it out while it still has good heat on it, after you do the initial quench, you can take out the blade and do the “3 pin” method of trying to take out the bend (you see this a lot on the show, Forged in Fire).

The 3 pin method you put 3 pins in a vise, two pins against on jaw, one on the other jaw, and the blade in the middle. Line up the 1 pin to push against the bend of the blade. This can be a little tricky because after heat treat, and before annealing, your blade is brittle and could break completely.

What might help you preventing this is, if it’s a thinner blade, using a torch or something to keep the back end of the blade from heating up much - but then your blade end may get warpy. Thin blades are just tough to heat treat. Maybe someone else on the forums here has tricks.

Good luck!