Heat threating thin metal

My neighbour askesd me to heatthreat an old machette. it is about 1,5/2 mm thick. I tried heat threating an old sawblade but it cracked and bended a lot. does anyone has an idea how to do this?

When it bends and cracks there are a few bad things going on that I have learned from experience.

Bending/warping comes from getting the metal too hot, it actually has the reverse effect of heat treating and causes metal to become brittle. The cracks are coming from cooling the metal too quickly, metal shrinks and expands when transitioning between extreme temperatures, but it also makes a difference in the steel too. Some steels just won’t take to tempering.

Play around with it and experiment some, I am hardly an expert so let me know how it goes.