Differentionally H/T Dagger?

How would one go about doing this using an open forge heat source and still leave the center in a softened state?

Spin it while in the heat

There are a couple (or more) ways to achieve this. If this is your first attempt at doing this, then coating the spine (in the cast of a dagger, that would be the centerline of the blade) with "clay", then heat in the forge, paying CLOSE ATTENTION until the edges are up to temp, then quench. The idea being that the edges are ground to about the thickness of a dime (you can make the edges thinner, but the thinner the edges, the greater that chances of warping). The thinner cross sections are going to heat up faster to critical temp then the spine/thicker cross sections of the blade. It's going to require that you pull the blade out of the forge to actually be able to see/judge the color(s), and put it back in if the edge areas are not up to temp.... if you try to view the colors in the forge, the whole blade will be hot before any color(s) are visible.

The second option is to preheat the edges with a torch, prior to placing the blade in the forge. This method requires some experience, and "know how" to keep the "spine" area from reaching critical temp.

I know you didn't mention a salt tank, but in my option it's the easiest way to achieve what you're wanting. With a salt tank it only takes several seconds to get the edges/thinner areas up to temp, while the spine remains below critical.

Any direction to take in an attempt to achieve what you're wanting is going to take some experimentation.....don't expect to get it right the first time....unless you're just REALLY lucky.

I agree with Ed exactly. The only thing I'd add, is if you want to do this, choose a steel for it that will help you out with your goal. That means a very shallow hardening steel that will have trouble hardening in thicker cross sections anyway.

I'd personally go for 1075 or W2, probably in that order. Maybe consider 1095 but that would be a distant third choice for me. If I were you, I'd also do the clay down the centerline. If I were me, I'd use my salts and W2 with no clay at all.

Good luck and post pictures!

I would be with John on this. I would choose a simple carbon steel that's low on manganese no thinner that 1/4" thick and forge or grind the blade to maintain the center thickness. The 1075 would be my choice. You will also have to do heat cycles, often referred to as multiple normalizations, to refine the grain. This will require the ability to spot decallesanse in dim light. After sundown is a good time to do this As John said with a thicker cross section down the center and a fine grain structure, which lowers hardenability, you should be able to create a pearletic center line on the blade with hard martensetic edges and point.

You could also try a modification of Ed's method by austinizing the edges and point of the blade, if it's not too long, with a torch and then quenching.

Doug

Never tried this method with a double bevel but if the blade isn't to long it could work.

Heat the whole blade up to cherry and remove from heat then light up a torch and keep the edges hot as the spine cools. Then quench.

I see this is an older thread but at least its still 2016.