Antikythera Fragment #4 - Ancient Tool Technology - The First Hardened Steel?

Antikythera Fragment #4 - Ancient Tool Technology - The First Hardened #steel
One of the key tool technologies that needs to be explored around the Antikythera mechanism is the simple hand held file. So this is the second of 2 Fragment videos relating to the making and hardening of a set of custom files, using materials and processes consistent with the period. Thanks for watching!
The first video where I make the files can be found here: • Antikythera Fragment #...
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Abbreviated Transcript:
00:09 As part of the process of investigating how the Antikythera Mechanism was made, I've made a set of hand cut files, using mild steel as a substitute for the wrought iron that was likely used in antiquity. But of course when it comes to hardening those cutting surfaces, there's a problem. And its the same problem as that faced by the original maker more than 2000 years ago. Mild steel, has insufficient carbon to simply heat and then quench harden.
00:37 So in this video I'm going to use the ancient process of case hardening to harden the file surfaces. It involves heating the work in close proximity to a carbon source, to absorb carbon into the surface prior to quenching.
01:54 The charcoal was then ground into a fine powder, in preparation for making the carbon pack. I mixed the carbon powder with salt and flour in the proportions onscreen, and then turned into a paste with a small amount of water. The paste was then packed ar ound the file surfaces, and then left to thoroughly dry. Now a reasonable degree of case hardening can be achieved by simply heating this carbon pack, as-is.
02:59 But to optimise the chemical reaction, the process is best conducted in an airtight environment. And traditionally, this was achieved by enclosing the carbon pack in clay. Again for convenience, I'm using a modern propane furnace to do the heating, but a charcoal furnace from the period would have been quite capable of raising the clay to a red heat. And its at this red heat temperature that the transformation begins
03:44 The metal is now in what's known as the Austenite phase, and has a strong affinity for more carbon. At the same time, the carbon pack is reacting at the steel surface generating a surplus of carbon that diffuses into the body of the steel. Effectively carburising the metal from the outside in. The depth of this carburisation is dependent on time.
04:05 The longer that I leave it in this heat, the deeper the resulting layer of high carbon steel. So while that's cooking, I'd like to show you the tests that I did earlier to figure out just how long the steel
needs to remain in the heat. I started with the same mild steel stock as the files, using a control piece and 4 carbon packed pieces. All were enclosed in clay, heated to a red heat, and then quenched in room temperature brine.
05:06 The Austenite has been transformed into a hard crystalline microstructure known as Martensite. That appears as a silver-white ring around the darker low carbon interior. And you can clearly see it progressing towards the centre of the metal, as a function of time. The longer the material spends in the heat, the
deeper the layer of martensite. Now a cutting tool like a file doesn't really need the hard layer to be particularly deep.
07:13 Today, modern steel composition means that there are several ways to form tempered martensite, in addition to the process that I've shown in this video. And we use it in much of our day to day life, mostly without even being aware of it. But in the ancient world, before it even had a name, this is one of the ways
that it was formed. Using a simple process, and the simplest of ingredients.
Antikythera Fragment #4 - Ancient Tool Technology - The First Hardened Steel