I experimented with clay masks for a bit, I didn't get the hard hamon, more of a difference in luster. It did differential harden though, upwards of 10 points Rc, depending on the mask thickness. The real problem is 5160 doesn't like water a bit, forget brine. I actually had one tear itself apart from the edge inward in brine, little hairline cracks spaced about one thickness apart. Hung it on the Wall to remind me.
Temps for SAE 5160, data from "The Heat Treater's Guide" original data from Bethlehem Steel Corp.
Forge above 2000 F (orange-yellow) Normalize: Bring to 1600 F and air cool in still air.(cherry pink) Anneal/spheroidize: Bring to 1380 F and decrease 75 F per hour to ambient.(fire truck red) Harden: Bring to 1575 F and quench in oil until convection no longer visible in tank, Full Hard Rc 62-63 (bright cherry red) Draw to desired hardness in oven for one hour and cool to ambient in still air. (min temp aprox 300 F)
You need volume to take the heat load, unless you don't plan on quenching more than one at a time in a given day.
My tank is made from six inch pipe welded to a base plate, IIRC it holds about ten gallons. I quench vertically, point first. Yeah, it flashes fire, but only until the work is completely submerged, so welding gloves and a leather jacket are usually enough 'armor'. Watch for hot oil splatter, wear that face shield!
2" by .291", evidently a fairly popular leaf spring size, judging from the supply the local shop keeps on hand. (They're really not that bad in the swing, if balanced properly. If you'd like, we can talk about balance for a meg or so.) If you just hold it out in an engarde', it'll get heavy, but you don't do combat with swords by standing around in engarde'. I cheat, I use a mill and fixtures for the swords. I get better results and it's a hell of a lot easier. I use a computer controlled oven for HT, you can't beat +1/-3 degrees off the setpoint for producing reliable repeatable results in production. They've paid for themselves a dozen times over in the decade I've used them. 5160 is the bottom of 'high carbon'. the chrome makes it a little tougher in the forging, just use a bigger hammer.Charly