Oven foil vs shielding gas

I would not expect much (if any) reduction in element life using inert gas. The Oxide layer, once established, should not decompose at normal use temperatures and it will see Oxygen every time the oven is opened anyway.

Using cheap N2, it should not be a problem running a higher flowrate. With a timer and a solenoid valve, it should be relatively easy to bypass the needle valve and give a short blast of shielding gas at high flow immediately after the door is closed. That should be pretty easy to set up, given that you built the oven and know how it is wired.

I seem to recall your oven being fairly large: roughly 6" x 6" x 27"? That's "about" 1/2 cu ft, so at 6 CFM, you'd expect to halve the O2 content of the oven atmosphere about 12 times an hour or once every 5 minutes.

Immediately after closing the oven, you'd have 21% O2, after 5 minutes; 10.5% O2, 10 minutes; 5.5% O2, 15 minutes; 2.75% O2, 20 minutes 1.3% O2 and so on.

If you put in a 5 minute purge timer that gives about 25 CFM (the upper range limit on my welding Argon flowmeter), you'd halve the O2 every 72 seconds or so and you'd be down to about 1.2% O2 when the timer timed out.
 
The thing your forgetting is the expansion of the nitrogen gas as it heats. That 6CFH gas turns into around 24. My math says heating from 70° to 1500°-2000° will expand the gas to around 4 times the starting volume.
 
timgunn1962 said:
I would not expect much (if any) reduction in element life using inert gas. The Oxide layer, once established, should not decompose at normal use temperatures and it will see Oxygen every time the oven is opened anyway.

Using cheap N2, it should not be a problem running a higher flowrate. With a timer and a solenoid valve, it should be relatively easy to bypass the needle valve and give a short blast of shielding gas at high flow immediately after the door is closed. That should be pretty easy to set up, given that you built the oven and know how it is wired.

I seem to recall your oven being fairly large: roughly 6" x 6" x 27"? That's "about" 1/2 cu ft, so at 6 CFM, you'd expect to halve the O2 content of the oven atmosphere about 12 times an hour or once every 5 minutes.

Immediately after closing the oven, you'd have 21% O2, after 5 minutes; 10.5% O2, 10 minutes; 5.5% O2, 15 minutes; 2.75% O2, 20 minutes 1.3% O2 and so on.

If you put in a 5 minute purge timer that gives about 25 CFM (the upper range limit on my welding Argon flowmeter), you'd halve the O2 every 72 seconds or so and you'd be down to about 1.2% O2 when the timer timed out.
Click to expand...

It seams like the math is closer to completely changing out the oven atmosphere every min+. 24cf/60sec=.4cf/min flow
 
Mea Culpa. I did indeed neglect the expansion.

I think you'll still get the exponential decay in the O2 content (albeit with around a 1-minute half-life on 6 SCFM of gas flow) rather than a complete change, as I can't see any way to stop the purge gas mixing with whatever is in the oven.

The ratios still seem valid, so 1-minute of high-flow at 30 SCFM would bring the half-life down to about 12 seconds and get below 1% O2 by the end of the purge.

What is actually needed is likely to depend on the details of your process: do you load multiple knives? Do you take them out individually for quenching, or do you quench several together?

My experience is limited to hobby-scale HT, with maybe half a dozen blades put into the oven together and taken out individually for plate quenching. With foil, it doesn't matter much how many times the oven is opened, so long as the temperature recovers before the next one comes out.

Doing things the same way with gas, by the time the 6th blade is taken out for quenching, it would have had Oxygen contact for 5 purge times, however long they might be.
 
If the oven chamber is positive relative to ambient air pressure, will much air actually enter the chamber on door opening? Then my really stupid question, would an air knife (or nitrogen knife) over the door, minimize/prevent that completely?
 
Ok...what I have experienced with multiple blades coming out of the oven to plate quench is that the amount of chamber purge gas is enough to keep ambient air out of the chamber long enough for me to extract 4-6 kitchen blades and get them positioned for plate quenching. I'm quenching 2 blades side by side on my quench plates and I have the plates close enough to cut down on extra wasted motions. I can do more smaller blades but this is a true example for Large Blades. Once the chamber is empty I will let the gas purge for 2 minutes then close the door and wait for the temp to ramp up then I reload the chamber and reset the PID for the next sequence.

Unfortunately I was unable to find any recent photos of blades coming out of the furnace (Thanx for nothing PB)...I thought I might have some saved to my PC but again alas I didn't take enough photos to consider saving them here either...I'm not a Big Fan of photos (cuz I suck at it...don't laugh...ok a little laugh is ok) I will try to capture some here in the near future.
 
i wonder why Argon gets used as inert gas for shielding if the Nitrogen can be used as well.
Did you already ruled out any possibility Nitrogen could embrittle the steel as well as the coils?
 
I would be more worried about Hydrogen embrittlement, but we're dealing with Dry Nitrogen as a shielding gas. Hydrogen embrittlement rears its ugly head with the austinetic 300 series stainless we dealt with a lot of these issues at NASA. The heat treat lab (metallurgy lab) used both argon and GN2, but it was up to the Customer to provide material specs and reason for the desired gas.
 
nitrogen does not react with steel unless its atomic nitrogen. Meaning a single nitrogen atom not a paired n2. there is a few ways to do this, one way is to heat nitrogen to over 33,000 kelven lol or use ammonia NH3 in a heated environment. the heated steel splits up the NH3 and the hydrogen fly's the coop and leaves atomic nitrogen for a micro sec. its just enough time to diffuse into the hot steel. Reading also points to saying moisture in nitrogen could/can form ammonia that could then split and release atomic nitrogen but that's above 1850deg. This is why we want to use nice dry nitrogen.
 
I have not seen any adverse effects of using GN2 that would warrant a change to Argon.
 
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