Masking A2 for heat-treat?

I have a small part that needs threads on one end after hardening. Is "stop-off paint" a way to mask the area so it doesn't get as hard?

Not finding a lot of info...

stop off is for carbon migration issues it doesn't control heat and cool cycle which is what makes shit hard
i'm sure more knowledgeable people will step up

Never heard of this paint.
More normal would be induction or very careful and fast with torch.
Induction not so good for one off unless you like to make your own coils and cool down stuff. With such it is possible to keep one end or section cold but I've only seen this done in very high volume shops.
Change a coil, all goes who knows. Cut some parts and check, adjust the heat and travel on the cnc induction hardener , try again...
There is just harden the whole thing and thread with CBN which is fun all in itself.
You are sure the threads have to be done after hard? A2 is fairly well behaved in standard HT with know growth and can be comped. Maybe they should be ground?
Bob

Terry Keeley said:

I have a small part that needs threads on one end after hardening. Is "stop-off paint" a way to mask the area so it doesn't get as hard?

Not finding a lot of info...

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You will not be able to tap a hole in the A2 after heat treatment.

Stop-off coatings only work were the part requires carburization or nitriding before heat treatment.

All the tapped holes in A2 I put in before heat treatment.

CarbideBob said:

Never heard of this paint.

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Not going to work. A2 hardens just by air cooling after the heat soak. Nothing you can do will prevent the area where the tapped holes go from being just as hard as everywhere else.

Put the threads in first, wrap the part with ss foil, put a piece of paper inside to consume the residual oxygen and heat treat. The wrap will prevent any scaling and your parts will be nice and clean.

Terry Keeley said:

I have a small part that needs threads on one end after hardening.

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Why after?
A2 doesn't move in heat treat. Thread it and forget it.

Or grind the threads after (if they are external).

Okay, okay, I just luvs me a thread grinder. They make beautemous threads.

Or with an orbiting EDM if there internal, cos' every shop has one of those machines

I think the real question is why op needs done after hard in A2?
Plus we do not know inside or outside threads or thread function or tolerance.
Perhaps the part not a nice round shaft but more a potato chip or fin with tits or holes in the ends.
If internal how big the hole?
Bob

CarbideBob said:

I think the real question is why op needs done after hard in A2?

Bob

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The engineer probably hasn't figured out where to put the tapped hole yet.

Make the part from 8620. Use stop - off paint on the areas you want soft. Carburize, quench, temper, then do your final machining on the soft areas.

Terry Keeley said:

I have a small part that needs threads on one end after hardening.

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I am assuming it is a male thread since he said on and not in.

I think we need more definition as to what the part actually is.

Thanks for the replies. It's for a buddy's scale motor (hobby chit - sorry).





The original crank (top) wasn't hardened, thread is on the end. He keeps the boss' on for grinding then cuts them off and threads the end. Prolly coulda made it a little longer and threaded before HT but he's here now.

I'm hearing there's no real way to mask the area but he could possibly draw it back even more to single point thread? Maybe grind the thread or even cut it at 60 Rc?

Here's what it goes in if interested.

There's no practical way to draw just the end of such a small A2 part to a significantly softer temper than the rest. I had a friend just go through that exercise with a knife blank. You can heat a spot red hot, but it's going to air quench again.

If you drew the whole thing to RC55 or so, you could try hard turning, but that part is going to be awfully small and floppy for that kind of action. And hard turning threads is worse than just hard turning to dimension.

If you must put the threads on after heat treat, thread grinding is probably your best bet. Get a toolpost grinder and dress a wheel to 60 degrees. I would not be surprised if you have to redress before you finish the threads, so you're going to have an additional challenge of matching up a redressed wheel with your partially ground threads.

Much better to put the threads on before heat treat if you're using A2. I think it would be a lot easier to make and clock/indicate a jig that fits on the threaded end to assist in post-HT grinding of journals, than to make the threads post-HT.

You could harden and temper the whole works. Then wrap wet rags on the crank leaving just the spigot exposed. Heat the spigot to reduce its hardness to maybe Rc 45 or so by heating the spigot to 1200 and allowing it to cool. This wet rag masking works. I’ve done it on O1. It should work well on A2. In theory you are supposed to hold the tempering heat for a prolonged time. In practice that is not necessary. To convince yourself try it on a piece of scrap which he undoubtedly has. It’s a lot easier on O1 as the tempering temperature is a lot lower. You’ll know in a half of testing how well it will work.

Denis

Would a copper cap protect it from hardening? I supply a customer with threaded copper studs to protect internal threads from hardening during heat treat process. Keeps them from having to anneal afterword. But the threads are done before ht so not sure how much difference it makes. Also I don't know the material being treated other than a carbon steel.

Hodge

People. Its made from A2. AIR Hardening tool steel. This stuff will harden fully just sitting in still air. To make it soft it must furnace cool. How you make one end of a part cool slowly over several hours while the rest of the part cools in 1 hour or so? Tain't natural. Not gonna work, no way no how.

Grind the threads or Furnace anneal the part back soft, thread then re heat treat and hope the anneal did not mess the grain size and give you crappy properties on the second hardening.

dgfoster said:

This wet rag masking works. I’ve done it on O1. It should work well on A2.

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Denis, I'm pretty confident this will not work on A2. Just letting a red-hot spot cool down naturually will air quench A2, and it will end up back at RC60+. O1 (or W1) behaves very differently from A2 in this respect.

Well, there is well thought theory and there is practice. I knew the only way to answer the question satifactorily would be actual hands-on testing.

I just went out and quickly hardened a 1" dia by 1/4 thick disc of A2. I did the hardening "wrong" because I did not hold the part at or above the transformation temperature anywhere near the recommneded time. But I know from experience torch hardening lots of O1 that you can get good hardening (maybe not maximal) just heating the piece to non-magnetic temperature (usually a fairly bright red) and then quenchining it. In the case of A2 it air quenches. So, I let it air cool for a few minutes----maybe 3 or 4. Again "incorrect" by the books and not a good idea for ideal results. Then I shallowly surface ground it to get the scale off and tested it on my Wilson Hardness tester which is calibrated and I know is accurate. The disc tested 50 to 52. Maximal hardness? No, likely because of the extremely brief holding time. But clearly significantly hardened. I did a half dozen tests to ensure consistency which was confirmed. Then I tempered the part all "wrong" --- I torched it to a purple color which is likely on the order of 650F or so. I again gave it a light grind and again Rc tested it. 40 was the result. It clearly tempered. Higher temps would have tempered it more. Rapid cooling from a non-transforming temperature did NOT cause it to revert to its hardened state. You would not expect it to do so.

Conclusion: It would not be ideal to do as I suggested and rag shield the crank while temperiing just the spigot. But it is very likely to work. If the OP is in a pickle to get it done, I'd strongly suggest more closely replicating the crank with a larger piece of A2 round stock. Harden it. Rag shield most of it and try to torch harden the end taking it to about 1100F as determined with a Tempil stick. Let it cool and test it with a file if that's all you have and try ctting the needed thread. Sharp carbide and good technique and you should have beautiful shiny thread.

Denis