bearmtnmartin SuperDork
Dec. 7, 2017 11:39 a.m.

I have a friend who is a very talented machinist. Handy guy to know. He is in the process of building a vacuum oven for heat treating gears and whatnot. His stumbling block is not the actual construction of the thing, but rather the management of the process afterward. What gasses to use for how long and in what circumstance to achieve a certain result. I know nothing. I just learned in fact that this is a thing. But I told him I may know some peeps who may know some stuff......

RevRico UltraDork
Dec. 7, 2017 11:45 a.m.

Not sure if it's the same, but I did own a vacuum oven for a bit. 

No gases were involved with it though, it looked like an over grown toaster oven that you hooked a vacuum pump up to. 

I really don't know how to generalize it more than that, but I could maybe answer some questions if they come up. 


 it was this one. Maybe not the same kind he's thinking of building. 

bearmtnmartin SuperDork
Dec. 7, 2017 11:54 a.m.

Thank you for the suggestion. All I know is it heat treats in the absence of oxygen. 

stafford1500 HalfDork
Dec. 7, 2017 12:52 p.m.

YOu could purge with Argon/Helium/Nitrogen/CO2 or whatever is the preffered non-reacting gas before you pull vacuum and that will give a no/low oxygen environment. Nitrogen, for some steels is bad as it can cause brittleness... it also is 78% of the air we breathe so there is a bit of mixed in with the oxygen.

Suprf1y PowerDork
Dec. 7, 2017 12:57 p.m.

Purge with Nitrogen, carburize with acetylene and quench with helium.

I worked on high vac heat treating equipment for quite ahwile, unfortunately I'm not a heat treater and don't have recipes.

bearmtnmartin SuperDork
Dec. 7, 2017 2:23 p.m.
Suprf1y said:

Purge with Nitrogen, carburize with acetylene and quench with helium.

I worked on high vac heat treating equipment for quite ahwile, unfortunately I'm not a heat treater and don't have recipes.

I believe it is the actual recipies that are the stumbling block, to avoid sending heat treated items to customers without any quality control.

rothwem New Reader
Jan. 9, 2018 8:33 a.m.

Hey, I'm actually a metallurgist, so this thread is relevant to my interests.  

The heat treating is, unfortunately, going to be really different between the different materials.  He should get a copy of the heat treater's guide from ASM as a very basic starting point:

For carbon steels, the general recipe is a higher temp (1500-1900F) anneal/normalizing heat treatment, followed by a lower temperature (500-1300F) temper.  Something like a gear would likely be carburized, where you basically make the atmosphere in the furnace full of carbon by using acetylene or "endo gas".  The carbon is what allows the quenching to produce a martensitic (hardened) microstructure, so the goal is more carbon in the outside of the part for hardness, and less carbon in the middle for ductility and toughness.  The increased carbon in the outside of the part + a quench will make for a nice case hardening effect.  

So, if he's going to be doing any type of case hardening, he needs a hardness tester, or a good relationship with a testing lab.  

For furnace controls, there's a wide variety.  There's simple PID types that work well for ~$100, but a vacuum furnace often has several inputs and outputs that need to be controlled, so you usually need some kind of PLC control on top of that.  A Honeywell HC900 integrated furnace controller is pretty much the "cadillac" option, but you'll pay 100k for something like that.  

Anyways, if there's any specific questions, I can probably answer them.  

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