Batch Quenching

[imill3567]

I almost always heat treat 6-12 knives at a time when I fire up the HT oven then work my way through finishing each knife. My question is for tips or methods for quenching more than one blade at a time. I always austenitize and quench one at a time because I don’t like opening the door to grab one and subjecting the next to the subsequent temp swing. Any good methods for doing even just two at a time to cut my HT time in half? Wire through a hole in the tang seems doable but not ideal, maybe a pin diameter threaded rod with jam nuts for each knife? Any ideas would be helpful. Thanks!

 

[E.Carlson]

I dont think the temp swing is gonna cause you any problems. I normally do around 6 blades at a time. Open, grab one with tongs, close the door and quench. 10 sec. In oil then into plates. The oven comes back up to temp real quick. I'd say about 5 minutes between blades.
If you're doing a bunch just keep an eye on the oil temp, you can use 1 liter bottles filled with frozen water to cool it down if needed.

 

[imill3567]

E.Carlson thanks for the response but what about the soak differences? Your first blade will get the required soak say 15min but by the 6th blade you’re up to a 40min soak given 5min between each.

 

[Stacy E. Apelt - Bladesmith]

Once the temp has soaked at the target point subsequent extra time isn't a big deal. The temperature of the blade will barely swing in removing a blade and closing the door. The rebound time will likely be about the same as the time to quench and check a blade.

Multi-quenching blades without BIG industrial equipment is not good. Putting a bunch of blades on a wire and quenching them is sure to get uneven hardening. Even with a space between the blades, the cooling curve on the inside spaces will be slower than the outside.

 

[Natlek]

Once the temp has soaked at the target point subsequent extra time isn't a big deal. The temperature of the blade will barely swing in removing a blade and closing the door. The rebound time will likely be about the same as the time to quench and check a blade.

Multi-quenching blades without BIG industrial equipment is not good. Putting a bunch of blades on a wire and quenching them is sure to get uneven hardening. Even with a space between the blades, the cooling curve on the inside spaces will be slower than the outside.

15 min. soak VS 40 min. soak time on 1500 F ....What about decarburization? Every time you open door fresh air will take place inside ?

 

[E.Carlson]

15 min. soak VS 40 min. soak time on 1500 F ....What about decarburization? Every time you open door fresh air will take place inside ?

What about it? Never had a noticeable difference between a single blade or 6. Goes back on the grinder anyway.

 

[Natlek]

What about it? Never had a noticeable difference between a single blade or 6. Goes back on the grinder anyway.

I mean , how deep it will be decarburization ? 40min. on 1500 F is long time. it is more time then we soak stainless steel in foil . Especially if it is thin steel for kitchen knife ? Never mind , I will make some test to find that answer my self . . . .

https://www.phase-trans.msm.cam.ac.uk/abstracts/M0.html

 

[Stacy E. Apelt - Bladesmith]

The decarb at HT temps is pretty shallow. 45 minutes would not be significantly deeper than 15 minutes. Once a layer is formed, it tends to shield the steel underneath. It should come off easily in normal post-HT grinding and finishing.

 

[weo]

but what about the soak differences?

I guess that depends on what steels you are using and how long of a soak you need. I use simple steels (1080/15N20) so my answer might not be applicable, but when I've had more than 2 or 3 blades to do, I'll put in 2, then when I take one out to quench, I'll put another one in.
What I do is open the door, take out the blade, close the door while putting the blade in the oil, after 6 seconds, take the blade out and put it in the aluminum quench plates, then put the next blade in, and repeat when the blade in the quench plates is cool enough to remove without gloves.

What about decarburization?

I've recently been using an anti-scale coating (ATP-641) and this really helps to minimize decarb. But, IMO, this is only something to worry about if you are forging your blades and bevels close to final dimensions. Doing a quick google-fu search I found a research paper that estimated decarb depth from 800C to be 0.35mm or 0.013" (https://www.forging.org/uploaded/content/members/public/PDF/Role of Stable Carbide Formers on Decarburization Depth.pdf), which would be removed during most post HT grinding.

One thing to note, in the study above, their procedure was to have each sample soak for 2 hours, much longer than what we knifemakers do, so decarb should not be as deep in practice.