For HT you need a forge that has several things which aren't as important for forging a knife. The Atlas mini-forge is a nice portable tool for forging small blades, but isn't as good for HT. Building a good forge for HT will give you a great forge for forging, though.
For a forge to do HT it will need:
1) Insulation - A HT forge needs good insulation as well as a refractory coating over the insulation. That is normally a 1 or 2" layer of kao-wool or ins-wool and a 1/4" to 3/8"coating of satanite with a thin coating of ITC-100 over that. It only costs a few dollars more to make a better forge. Money spent on insulation and refractory is money well invested.
2) Burner. A well built venturi burner will work, but a blown burner is far better if you are going to do HT. The blown burner will run at a lower pressure and temperature as well as allow tuning of the chamber atmosphere ( amount of free air/oxygen ). Here again, the extra money spent on a blower will re-pay itself in a better forge and better blade HT. Blown burners are also much easier to make and operate.
3) Size and Shape - A HT forge needs enough room to allow the blade to be evenly heated. No hot spots or cold places. It also needs a chamber that allows the hot flames and gasses to move smoothly around in a swirl. A round or oval works well. A square or rectangle is much less efficient. A 16" section of 8-10" pipe is a good forge body size. many "found" objects will fit in this approximate size range.
4) Temperature readout - No mater how good the forge is for HT, if you don't know how hot the blade is you might as well just use the BBQ grill. A digital pyrometer is a real necessity for doing HT. Since the heart of a PID unit is a digital thermometer, that is what most folks use for the readout device. A PID, proper TC, and TC sheath will cost around $100. The PID will also allow other great things...read on!
Adding PID control to a well built forge will make a good forge a great forge. It adds about $100 to the cost, and makes the forge many times more useful. You will know the exact temperature in you forge, and can hold that temperature within +/- 5 degrees easily. The PID control does not have to be built in initially, but placing a ceramic TC sheath in the initial build is a good idea. The stickies have the build info on my two-stage PID controlled forge burner system. It will hum along with +/- 2 degree accuracy for HT or for forging.
A good forge with PID control will take a few weeks to build and cost between $200 and $400, depending on your skills and scrounging abilities. Just like birth control and nutrition, this isn't a place for "good enough".
What you build into your forge will directly determine what comes out of it.
A few tips:
1)Get good quality parts and materials when building a forge. High Temp Tools and Refractory is a great source for everything you need.
www.hightemptools.com
2)Cure and fire each coating of refractory before putting on the next. You don't need to make it roar like a jet on take-off to cure the satanite, but a low flame for 10-15 minutes will drive out the water and fire the cement on each layer.
3) The blower is important. Just like a sports car, a shiny paint job and leather bucket seats are useless if the engine is from a lawn mower. Don't try and save money on the blower. Using a hair dryer or a leaf blower is a foolish waste of money and time. Get a blower that delivers the air right. You don't necessarily need a $200 forge blower form Centaur Forge Supply, but you want a blower that is able to be speed controlled and delivers enough air volume. something between 70 and 150 CFM is the norm. Many people use a simple light/fan speed control to control their blower motor.
4) Burner size - The size pipe for a good forge burner is important. A 16" long forge with a 6" round chamber can run well off a 1" to 1.5" blown burner. Making the burner too small ( and also too large) will make the whole forge inefficient. Adding a mixing chamber to the burner pipe, and a flare on the end will make the burner much more efficient. These things are discussed in many burner build threads and tutorials. If building a first burner, a 1.25" black iron or stainless burner pipe and a 70CFM blower is a good combination.
5) While all PIDs run about the same, all aren't created equally. A PID and TC from ebay may work just fine, and cost less than $50, but it may not be the right PID for your forge. If you aren't familiar with these devices and using them, getting a unit from Auberins or Omega will cost more but be a good idea. They have superb quality and good tech support.
6) Thermocouples - The TC needs to be type K with 1/8" (or 10 gauge) lead size. The free one that comes with most ebay PIDs is useless for forge operation. The TC should be installed inside a ceramic TC sheath and the leads attached to a TC terminal block. Also, you want ceramic separators/insulators on the leads inside the sheath and the immediate area outside the forge. Lead length can be trimmed, but getting the right length to start with is a better idea. 12-16" leads is about right. Understanding how these things work is how to use and install them right. It takes special wire to hook up the TC to the PID, so make sure you get the proper wire with the parts. Using TC plugs and sockets is a good idea ( they are special just like the wires), as it allows the PID unit to be plugged into other devices as well as simpler replacement of the TC ( they don't last forever). Mount the socket on the PID housing, and put matching plugs on all your TCs. This will allow you to switch the PID from running the forge to running the tempering oven in less than 60 seconds. Planning on future expansion and multi-use of devices is how some shops have their equipment evolve into high-tech and others get filled with piles of old equipment.
All this sounds somewhat different than hooking up most electrical parts because a TC and the parts associated with it are different. All parts are polarized and have different materials in the + and - sides. Every joint in the path from the TC tip to the PID will alter the reading of the TC. Using the right type K wires, a TC block, and TC plugs/sockets will keep any accuracy aberrations down to a minimum. Strong screw pressure junctions are needed to allow minimal resistance changes in the wiring joints. The TC mounting block is very important for this reason, as the leads are hot at that point. Using wire nuts and lamp wire will not work nearly as well.
