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Heat Treating 17-4ph to H900 condition

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Jim_Z

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Hello out there in Hobby Machinist land. I recently had the good fortune to acquire a Kerr Insultherm 900 electric kiln for my shop. The unit will heat up to 2000 F and has a 7 x 7 x 11 inch capacity. It will work nicely for preheating some mechanical assemblies that go together with a bit of hot and cold. I am also thinking that it would be good to heating 17-4PH SS to the H900 condition. 17-4ph at H900 has a hardness of about 45Rc and is pretty tough, ie, a good material for tools etc. What I am wondering is what can be used to prevent oxidation scale on the material during heat treating. The stainless steel foil can be a bit expensive for a hobby shop. Any ideas out there? Can I use borax on SS?

Thanks Jim_Z
 
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Purge the atmosphere in the furnace with nitrogen or argon gas.
 

Jim_Z

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The furnace is not a pressure vessel, like the vaccuum braze furnaces we had a work, but could a whisper of argon gas thru the thermocouple hole in the top of the furnace do, maybe with an initial large volume purge, then backed of to a whisper like 1 or 2 cfm or something?

Jim_Z
 
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The furnace is not a pressure vessel, like the vaccuum braze furnaces we had a work, but could a whisper of argon gas thru the thermocouple hole in the top of the furnace do, maybe with an initial large volume purge, then backed of to a whisper like 1 or 2 cfm or something?

Jim_Z
That should be all it needs. Probable about half of what goes thru a MIG or TIG torch.
 

Tony Wells

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I've never observed what I would term "scale" on 17-4 in any condition except a large rough mill run. And it was as rolled, way hotter than 900°F. H900 will turn a purple-gold with surface discoloration only, and with a little tweaking can get up to 48Rc. It's a one hour process, but the temperature can be +/- 10° or 15°F, I don't recall. That's where the tweaking comes in. It's an air cool process, so really as long as you don't overcook it, it's hard to mess up. Of course, it's still well within machinable range, even at it's max, unless you are taping tiny holes. With it already aged, you can get a corrosion resistance close to T304 and machine a finished part. Most of the time, I prefer machining it after aging. Definitely on the softer agings.
Just my opinion, but I wouldn't bother with a controlled atmosphere cooking 17-4. Handprints show up nicely if you have oily fingers loading the furnace though, so clean the parts well, and wash your hands!:eagerness:
 

Jim_Z

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I've never observed what I would term "scale" on 17-4 in any condition except a large rough mill run. And it was as rolled, way hotter than 900°F. H900 will turn a purple-gold with surface discoloration only, and with a little tweaking can get up to 48Rc. It's a one hour process, but the temperature can be +/- 10° or 15°F, I don't recall. That's where the tweaking comes in. It's an air cool process, so really as long as you don't overcook it, it's hard to mess up. Of course, it's still well within machinable range, even at it's max, unless you are taping tiny holes. With it already aged, you can get a corrosion resistance close to T304 and machine a finished part. Most of the time, I prefer machining it after aging. Definitely on the softer agings.
Just my opinion, but I wouldn't bother with a controlled atmosphere cooking 17-4. Handprints show up nicely if you have oily fingers loading the furnace though, so clean the parts well, and wash your hands!:eagerness:
Tony

Thanks for the pointers, I will give it a try with the furnace and clean hands.

Jim_Z
 

P. Waller

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Funny stuff about the hand prints, this occurs in the aluminum anodizing process as well, permanent fingerprints left on parts to be found by the end user.
 
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