Tempering when kiln is too hot

[FunkCoaster]

I have S35VN steel, making myself some leather cutting tools.
Going by the sheet: http://www.crucible.com/PDFs\DataSheets2010\dsS35VNrev12010.pdf , I chose to temper at 600F for 2 hours, twice.

The problem:
600F is not attainable for a while after hardening.

My kiln is cooling from the hardening cycle at 1950F. It takes somewhere around 90 minutes to cool to 600F.
My kitchen oven maxes out around 500F.

My metallurgy-fu is moderate at best, but I feel like the best solution is:

- temper at 500F in the kitchen oven until the kiln is cooled to 600F
- cool the blade to room temp
- transfer to the kiln at 600F for 2 hours
- air cool
- kiln temper for 600F for another 2 hours

This would avoid an overly brittle steel hanging around and risking cracking. Is this right? Better options?

Please note when responding:
My question is not about the "ideal" scenario, which likely means spending another $700+ on a second kiln. I am a hobby maker, so budget matters.
The question is about the best way to maximize the current equipment.

 

[DevinT]

Do a sub-zero quench, leave it in the sub- zero quench until the furnace gets down to temp.

Hoss

 

[S.Alexander]

You should be putting it into cryo, or subzero treatment right after the quench. Let it sit there overnight and then temper.

On another note, why are you tempering so high?

 

[FunkCoaster]

Responding to DevinT and S.Alexander:

First, thank you for your help!

An excerpt from the pdf from crucible:

Temper: Double temper at 400-750°F (200-400°C). Hold for
2 hours minimum each time. (See Table) A freezing treatment
may be used between the first and second tempers. Freezing
treatments help to attain maximum hardenability and must
always be followed by at least one temper.​

The statement:
"You should be putting it into cryo ... right after the quench"
conflicts with the pdf, which says to do a temper after quench, followed by cryo/sub-zero. Do you know of any references for this procedure?
My reasoning suggests that an immediate cryo might reinforce the hardness without any stress relief, raising the possibility of cracking.

S.Alexander - the chart on the pdf indicates that 600F tempering, following a 1950F hardening would provide a ~59 Rc hardness.
Do you suggest that I shoot for a higher hardness? Using 400F would target ~60.5 Rc. Harder than I intended, but I'm curious what you might think would be best.

 

[DevinT]

If you temper before cryo/sub-zero there will be little or no benifit in doing a cryo quench. Continuous cool down is the best way.

Hoss

 

[S.Alexander]

Those heat treat specs are not knife specific. Devin has it right, quench to cryo, then temper.

For leather cutting implements harder is better. It will hold an edge longer. 400 degree temper seems optimal for this steel IMO.

 

[P.Brewster]

Turn off the oven, then put a BIG block of steel or aluminum in the oven. It will drop the temperature in 5-10 minutes. Quench the block in water and repeat if needed.

 

[Lo/Rez]

For your application temper it as hard as possible.

 

[butcher_block]

im like hoss quench then LN over night. in the morning i fire the kiln back up to 400f and temper keeps the stress low on the blades and the kiln

 

[FunkCoaster]

Thanks for the advice! I don't have a LN setup, so I'll be shooting for dry ice and acetone to get at least a sub-zero treatment post quench.

And yes, I suppose I should go for a higher hardness on leatherworking tools to get a crazy sharp edge.

 

[Lo/Rez]

Unless I was going to make a camp knife I would temper any knife I made to the highest hardness possible. It does not make sense to leave anything on the table when it comes to cutting ability. Especially when it comes to these 'super steels'.