Heat treat oven nitrogen results

[JTknives]

So I got everything hooked up and plumbed in. I ran 2 tests last night, one 15n20 and another on D2. The 15n20 got a 5 min soak at 1450° and the D2 got a 30 min soak at 1850°.

The 15n20 came out of the oil black but wiped right off and was gray color. No scale or pitting from scale.

Befor

After

Close up

Grain/skin test

I was most worried about the D2. 1/2hr at 1850° Is really harsh on steel. I ran the test like with out a purge. I heated to 1850 and turned on the gas. Once the temp was stable I opened the door and put the steel in. I'm guessing all the hot nitrogen left the oven when I opened it. I was trying to simulate how I would do a blade.

Befor

After

A quick wire wheel and clean metal was exposed but it was dark. Kinda like gun bluing but there was no pits or scale flakes. You can see a texture but that's the surface grind that was there befor heat treat.

Grain/skin test

So I would say it's looking good but How does it compare to your results.

P.S
Hardness tests befor any sanding
15n20 measured 65rc+ And the D2 measured 60rc.
I then sanded the surfaces lightly to remove dark colouring. The 15n20 was the same and the D2 jumped to 64-65. I know from reading that gas shielding does not prevent decarb 100%

 

[Ken H>]

JT, that looks pretty darn good to me. Provided the process doesn't require too much N2, I'd say you got a good setup there. Next to try with SS at 1975⁰F {g}

 

[Busto]

Jt
That looks about right since photos can be a little off just angle and lighting. I understand the apprehension of soaking at 1850 for 1/2hr that is harsh but we do that when in the foil so the first couple of times makes you hold your breath. The hot GN2 took what little bit of atmosphere that was left in the furnace and that lurking around the door and consumed it with the dry heat. The 15n20 looks great kinda scary when you see all that black burnt oil out of quench huh...maybe not 100% but close enough to not have to spend hours wrapping. At this point you can fiddle with gas settings SCFH for the size of your furnace I started at 8SCFH and settled at just a hair over 5-1/2SCFH.
On another note...when I'm doing a batch I will select 2-3 blanks and RC check and if they are in spec I will Glass Bead Blast all the blanks to a uniform finish before grinding. I leave the tang at the blasted finish so it is slightly rough for epoxy adhesion.

 

[kuraki]

Looks great to me.

 

[JTknives]

I tryied 15n20 at 5CFH (C02 reading) for 1/2hr and when I pulled it out it had a weird puffed out scale surface. I then bumped it to 7CFH for the next 15n20 and D2. My flow tube in the meter has argon, CO2 and helium. The CO2 and argon lines are the same up to around 15CFH.

 

[Busto]

Looks like that bump in flow helped and again its about your furnace and what it likes. Kinda like matching bullets and powder to the barrel each one may be little different it will tell you what it likes.

 

[jdm61]

Nitrogen may be good for most blades, but if you are HT'ing a pirate cutlass, I would think that AAAAAARRRGON would be preferable.

 

[JTknives]

Looks like that bump in flow helped and again its about your furnace and what it likes. Kinda like matching bullets and powder to the barrel each one may be little different it will tell you what it likes.

Are you using argon or nitrogen? I tryied some AEB-L tonight at the same flow rate and got some scale pitting around the edges. The temp was 1950° and I'm wondering if maybe the thin stock heated befor the chamber could flush out or the nitrogen stopped working at that higher temp?

 

[Busto]

I use Nitrogen...Did you flood the chamber for a couple minutes before putting the blades in? I know you stated initially you waited for the furnace to reach set heat then turned on the gas which is ok as long as you give a couple minutes to flood before then adjust down to the hourly set rate of flow.
I do high heats on AEB-L not seen that problem, but you are fine tuning your system right now I think you're close just need to dial in routine and flow rates.

 

[JTknives]

I did not flood the chamber for that long. I turned the gas on and opened the door and put the steel in. My thought was once I open the door all the hot nitrogen would rush out.

 

[Busto]

Yes and No...You want a Nitrogen rich environment yes some gas is leaving and no not all the gas has escaped but is reducing the amount of oxygen atmosphere trying to enter. By reducing that small amount of oxygen entering you are reducing the possibility of heavy oxidation and scale. Think of a fan at an open window with enough flow to keep flies from getting in.

 

[mknife]

It's very interesting, thank you for sharing.
Can I see some photos of the entire setup?

 

[JTknives]

I will post some photos later when I get to them.

I was thinking about the issue about purging the chamber of oxygen when the blade first enters. And if you think about it practically my oven is right around 1/2cf. Gas at say 5cfh is actually 20cfh at temp (1800°-2000°) blades heat very fast at that temp. So 20cfh/.5cf oven = 40 oven fills per hr. that means every 1.5min the oven is getting a 1/2cf of nitrogen. And like was stated above its most likely diluting the oxygen. So in 1.5min we have cut the oxygen to 10% and in 3min it's 5% and 4.5min 2.5%, 7min 1.25%, 8.5min .75%, 10min .375%. So the blade is exposed to a good amount of oxygen for a rather good amount of time. From experance most thin blades come up to temp (1500° 15n20) in 2min. So a 5min soak has 3min of at temp time and is exposed to around what 2-7% oxygen.

My thought is to build in a purge system. If you had a small tank in the gas line befor the oven and then a valve between the tank and oven. This tank say holds 1CF of gas at 1ATM pressure. This can be calculated by the pressure and the tank size. But say once you hit the valve a 1CF shot of gas entered the oven. This now turns into 4cf which is 8 oven fills. That now moves our oxygen to what .2% (20,10,5,2.5,1.25,.75,.375,.1875) at start of heating. And this is counting on a perfect mix when injected. It could push more oxygen out the front of the oven becaus the nitrogen is pushed into the rear.

I sealed around all my seams with high temp silicone so the gas can only come out the front where the door is.

Am I off my rocker or does this seam practical?

 

[Salem Straub]

Sounds like you are talking about effectively a shielding gas accumulator. Shouldn't be hard to do at all.
Of course that shouldn't be taken as definitive proof that you're not off your rocker...

 

[lonepine]

What if... you put a chunk of high carbon steel (cast iron???) in the furnace as a sacrificial piece to give the oxygen something to work on rather than the blades?
- Paul Meske, Chippewa Falls, Wisconsin