HDB

I've done a bit of research.

Ferritic and martensitic steels tend to become more brittle the colder it gets. You can temper the steel more to make it more suitable to the cold, but this works only to a small degree. 

The clue to cold "proof" steels is the Face Centered Cubic structure of steel: Austenite. Steels in their austenite form are both tough and strong in very cold (~arctic) conditions. They aren't affected to cold the way 'classic' ferritic/martensitic steels are.

Is this due to the nickel content? Yes, indirectly. Nickel is an austenite-stabilizing element,  but so is manganese. So both Nickel and Manganese are important.

Sources:

• http://www.bssa.org.uk/topics.php?article=41
• ASM - Metals HandBook VOL 4

Would it make sens to temper a knife/tool more if it's intended to be used, let's say, in the Arctic? The cold would make the steel more brittle so tempering at a higher temperature to increase tensile strength and toughness would make sense, wouldn't it?

My question was about quenching from the austenitic temperature (hot quenching), not cold quenching (cryogenic treatment, a concept I'm familiar with). I was just wondering what the temperature shock would do to the steel and N₂. Not that it would have any practical use... :-)

What would happen if one would quench in a very cold liquid? Let's say liquid nitrogen. Would it crack into a million pieces, explode,...?

I use a car fan with 12V power supply. Easy to control with potmeter (or pwm) and a lot of wind!

​That's what I would do also. Heat treating ovens / kilns/... should use a PID/SSR solution. The self learing algorithm of the PID is perfect for the application.

• Use YouTube! Walter Sorrels (a popular American sword/blade smith) has a series on hardening knife steels (O1, D2, 1095,...).
• You need to know about martensite, austenite, ferrite,...
  My advince: read Metallurgy of Steel for Bladesmiths & Others who Heat Treat
  The pdf above is one of the best manuals on practical metallurgy
• Forget the steps you need to take (normalize, quench this, temper that) but understand what they are. If you understand them, you will automatically know what to do when. That way you will understand the reasons of warping, scaling, the concept of differential hardening (often used to get a hamon).
• Use magnets for detecting critical temperature. Never try to get the steel hotter then necessary. This will increase grain growth.
• Use a metal cannister for the quenching medium and preheat your medium to 60°C. 
• Google for datasheet 5160. You will get PDF's with scientific information about the steel and how to heat treat, machine, etc. The graphs are very interesting and tell you temper (temperatures,...) to a certain hardness (~HRC, rockwell scale).
• Read a lot

​Lignite indeed.

Regards,

HDB

The local Brick factory here (Belgium) used brown coal, which they pulverized. The brown coal dust then was transported (using a pipe) with air nozzles every couple of meters (to keep the dust flowing).

The pressurized dust then was ignited and burned with a forced air/venturi combination. They heated very large kilns this way.

A: Content big cylinder: 6inch²*Pi*11 = 1243.44inch³

B: Content small cylinder: 3inch²*Pi*11 = 310.86inch³

C: Content insulation:A-B = 932.58inch³

So you would need:

932.58/5 = 186.516inch³ fireclay

932.58/5*4=746.064inch³ vermaculite

Of course there's also the addition of water, but this will evaporate.

Why not use kaowool? It's more resistent against shock, temperature,...

What's the forge for? Forging or smelting,or,...

Since nitrogen (N2) is lighter than air, why not contruct a bell in which you put the conveyor / induction coil? Hot nitrogen (due to heating) will rise, but not escape (the bell prevents it), except from the bottom (but not to much, since the lighter nitrogen will have the tendency to float/rise on air).

Then use some sort of PID-controller to add nitrogen when concentration changes.

By the way,

The carbon buildup you talk about sounds like scaling to me, not carburization imho.