Thermal Cycle Scale?

[theWeatherman]

So I have found that I get this "thin scale" when I thermal cycle my blades. I put the blades in, hit the temp and then when it hits temp take them out and hang them so they can cool enough for me to touch and then continue again.

This scale is really brittle, it starts to crack and then fall off each time I thermal cycle. I am using an EvenHeat oven. I am at 5800 feet in elevation.

I was hoping you guys could tell me what it is and how to prevent it. I don't think it is actual scale. It seems to only happen on my 1095 blades and not on my O-1 blades. This "scale" if it is scale doesn't come off when I soak the blades in vinegar either.

Ideas?

 

[EdCaffreyMS]

What temp are you using? How long are you leaving the blade in/at that temp?

 

[theWeatherman]

What temp are you using? How long are you leaving the blade in/at that temp?

Thermal cycle temps are 1600, 1500, 1400, 1300. Leaving the blade in only until the oven hits the desired temp and then out. The blades are soaking for about 4 hours and some of the scale is starting to rub off, so it might be just normal scale from what some of my maker friends are telling me, but I haven't heard of it being described like this before.

 

[EdCaffreyMS]

Ok. Just wanted to see where you were temp wise. I thermal cycle a bit differently....i never allow the temp to be more then 1350F.....for carbon/alloy steels. Once you hit that 1400F mark, the scale is going to start forming. It's generally much worse in a heat treat oven versus a forge, simple because you have more free oxygen in an oven....which is the reason scale forms.

 

[KenH]

more free oxygen in an oven

How about that - sure makes sense, but never even thought about that. Thanks for that tidbit.

Ken H>

 

[Knifemaker.ca]

Try preheating the kiln and then putting the blade in. A few minutes to equalize and out. This should drastically reduce your time at temperature.

 

[theWeatherman]

So I tried both ATP641 and a thin wash of Santinite. I find that the Santinite works way better for the thermal cycle process. The ATP641 flakes off when cooling and needs to be re-applied every single time you take the knives out of the oven.

 

[KCorn]

Off topic but I just have to ask....is that the San Juan river in your profile picture?

 

[theWeatherman]

Gunnison River: Colorado USA

 

[Josh Dabney]

Brian,

Scale is a fact of life for us even when thermal cycling. There's no reason to worry about scale on the cycling heats but if you must then foil wrap to defeat the scale.

Don't take your blades too close to finished before cycling. Grind it say 90%

Then do your thermal cycling.

Then grind it to where you want it for hardening.

Then sub critical anneal, 1200 for 1 hour then air cool.

Then harden.

No more worries about scale forming while cycling and your blades will be stress free for the hardening cycle.

That's how I do it.

-Josh

 

[theWeatherman]

Josh - I hear you. Scale is a pain in my opinion. Plus you start hearing people talk about decarb and whatever. What it comes down to for me is what makes my life easier working on the knives?

By preventing as much scale as possible it limits my time at the grinder after HT, but what it really does help with is thermal cycling and then allowing me for a clean blade for putting clay on the blade for hamons, which is important to me. I want to work smarter not harder. Scale is like 70RC if I remember correctly.