I understand the basics- use a carbeurizing flame, heat till non-magnetic, quench in oil.
Would it be smart to coat the piece in brazing flux to protect the surface? Or would that just add to the cleanup later?
I wish I paid attention better, or remembered better, when they showed us this in machine shop class 40 years ago.
-Ed
As discussed in another thread, coating with Boric Acid is a good idea because it comes off easily with a Scotchbrite pad. I haven't used brazing flux but it might be the same thing. The boric acid actually works well and I've been using it for 20 years.
Basically, you take some boric acid powder (termite killer) and make a thin paste using rubbing alcohol. You don't need denatured alcohol, although it won't hurt. You want the paste thin enough to spread but not so thin that it runs. Add alcohol sparingly as the powder will thin out fast; if too thin, add more powder.
Use some wire (I use rebar tying wire) and tie it to the part. Some folks tell you to make tight wraps to cover the part with wire; I see no reason for this and just tie the wire so it hangs on to the part well. Cover the entire part with your boric acid paste; I just use an acid brush for this. Then heat it to critical temp.
Using color to heat a part is reasonable but cherry red to me and cherry red to you may differ. Lighting conditions also make this a very subjective thing. I use a Mapp gas torch (or two if the part is big) to evenly heat the part until it looks cherry red, then start to introduce it to the magnet until it no longer attracts. Then I plunge and twirl it in ATF until it stops bubbling.
Once the part is cool enough to handle it is up to you how you want to temper it. I have been advised that the part should go into the tempering oven as soon as it comes out of the quenching oil. This is the way you should do it. However, I usually take the wire off and use Scotchbrite under running water to get the boric acid stuff off so the part is clean. Then I use lacquer thinner to completely clean the part before putting it in the oven. Oil from the quench or your hands can stain the part in the oven so I want it clean.
I normally use a toaster oven with a glass door so I can see colors easily. I normally preheat the oven to 350-375 degrees or so and put the part in, then watch for the color change. Naturally, thinner sections of your part will draw back faster than thicker parts so if you want a differential temper, where one part must be softer than another, then an oven is not the right tool. Use a torch and temper until your thin part (like a cutting edge) reaches your desired color (like amber), then heat the rest of the tool to purple or blue or whatever you need. Just stay away from the thinner part and it should work fine. This is not an exact thing but differential tempering this way does work.
Hope this helps.
