"Dual Heat Treatment" - How is it done?

[zzyzzogeton]

I saw on web site for a non-BF vendor (so no free advertising allowed ) where they are saying that their swords are dual heat treated - like low 50s on the edges and upper 40s for the core.

Other than the Elmer Fudd answer of "vewwy, vewwy carefuwwy", how is it done?

 

[J W Bensinger]

It can be done by applying a heat sink to the edge, and a heat source to the spine-or like ya said, "vewy, vewy cawefuwwy" lol.
I do all my single edged swords that way, and can get away with it on double edged stuff (which I very rarely do-just not my gig) if the fuller is broad (i find the fullering helps keep the heat off the edge, since the thin section of the fuller bleeds off some heat, and keeps the edge from getting too hot when you're drawing back the spine).
Here's a ginunting done with torch and heat sink paste, thus the clean line:

And the finished sword as part of an espada y daga set:

 

[J W Bensinger]

I I remember correctly (and I may not, errors are my own) Tinker Pearce had his swords tempered pretty hard and drew the center/fullered section to a spring tempered. I've done it on a broad, thin-fullered migration period viking sword but on narrow fullered, later period swords it's not easy to do.

 

[Timo Nieminen]

There are various ways in which it can be done:

(1) Differential quenching, which can be done in different ways:
- Insulate the part of the blade that should be softer (usually with a mix of clay and ashes) so it will cool down more slowly when the blade is quenched. This is the traditional Japanese technique, and modern differentially hardened katanas are usually done this way.
- Only quench the edge. Can be done by only putting the edge into the quenching fluid, or by pouring water onto the edge.

(2) Differential tempering, as described above.

(3) Differential auto-tempering. Quench briefly, and the body of the blade, away from the edge(s) will be hot enough to temper the body.

Variable hardness can also be done through lamination or other composite construction (e.g., inserted-edge), with a uniform heat treatment. Can also combine lamination with differential heat treatment.