Is there any way to un-work harden Stainless Steel? I have a piece that seems to work harden at the drop of your hat. I already made one piece from it but changed the design. The second on hardened up while drilling a hole. I need to tap the hole. Any way to save it?
Chuck
I can't help with the annealing but can offer a couple points based on experience working stainless at work.
1. Slow on spindle speeds. You don't want to ever let a tool rub when machining stainless, plus you want to minimize heat build up.
2. While you want a slow spindle speed, feed aggressively. The idea here is to keep the tool cutting dee under the already hardened stainless. Further never stop until you hit depths stopping will almost certainly leave a work hardened bottom.
3. If this is a through hole try drilling for the back side if possible. The hardened area is very thin and the drill will break through from the back side. Use a good drill that isn't brittle as you can snap a drill bit fairly easy doing this. Even with a good drill bit follow all safety suggestions.
4. You may need to use a carbide drill to break through for the front side. Again go slow on the spindle speed and feed aggressively.
5. In your case if the hole is deep enough I'd try tapping the hole before I got too worked up over drilling deeper or annealing the part. The hard point is generally at the bottom of the hole. Use a good sharp tap. Ideally a HSS one with some sort of coating. A bottom that is worked hardened is really only a problem if the hole isn't deep enough.
6. Of course use plenty of cutting oil / lube for the above operations.
7. Sometimes you can get lucky by doing this. Say you have a 3/8" hole, take a carbide end mill or drill of say an 1/8" to 3/16" and try to break through the hardened sit puff in the center, then try again with your original but resharpened drill bit. The end mill of course wold need to be center cutting but ideally this would be a single flute drill for hard metals. In the long run it is better to drill stainless with a series of drill bits until You get to size. This will allow you to maintain a good feed rate as the drill bits become larger.
8. Lastly, some machinist will want to close their eyes at this one but in difficult situations I've used an end mill to effectively grind a hole through hard metals. The flutes will tend to chip and break off but if you keep going the carbide will defeat the hard steel. It is an ugly way to do things but if an EDM or the proper straight flute drills don't exist it can get the job done. I've only done this twice in my entire life, as a last resort, so I don't recommend it when other avenues exist. In both cases the parts where too hard too tap so this wasn't a goal, just looking for a simple through hole.
9. These sorts of problems are a good reason to build/buy an EDM machine. Use it in the same manner as described above, burn a pilot hole through and then try to clean up the hole with a conventional drill.
Of the above I've done the drill from the back side many times, it is great solution if you can layout and drill that hole accurately. Unfortunately that isn't always possible on many parts. In any event of the hole is deep enough just try tapping it for the intended threads.
