Hello hello everyone. I want to make a farrier´s brush, and I'm thinking of making it out of used hacksaw blades, annealing, removing the teeth and cutting them in half to make the wires.

¿what type heat treatment should I give to the wires once they are formed into shape, so they don't bend too easily? 
thanks

Welcome aboard Pablo, glad to have you. If you'll put your general location in the header you might be surprised how many members live within visiting distance. Another tip to keep you off the moderators watch list is do NOT use all large bolded font. Please.

 I have no idea how to harden and temper hacksaw blades for what you want to use them for, you'll have to test until you find what works. No need to anneal to grind the teeth off and if you're careful not to let them over heat and run the temper you won't have to heat treat again.

That is going to make a darned aggressive brush. I'd try snapping the blades to the dimensions I wanted were I trying this. I have some 1" wide metal cutting band saw blades gifted to me from a company scrap bin. I'd make a simple gauge to control the width and snap bristles off in the vise. After grinding the teeth off, clamp the main part of the band below the jaws with just the width I wanted exposed, say 3mm. That's just a number I picked for discussion, NOT a recommendation. Anyway with 3mm. sticking above the vise jaws I can snap or shear it off easily using a piece of flat bar laid on the jaws and striking the other end with a hammer. 

You could do similar to split hack saw blades down the center though it'd be a little trickier. Not too though.

Frosty The Lucky.

Remember that a lot of bandsaw blades these days are bimetallic, with a high carbon edge and a lower carbon back. This give good sharpness and tooth durability, while retaining flexibility and lowering the risk of work hardening the blade to the point of failure. You should test harden any blades you plan to use before doing any cutting.

Frankly, you might find it more worth your time to simply buy a brush and spend the time you would otherwise have spent on this project doing more actual smithing.