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How to do a differential heat treatment on a double-edged blade

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Hello. I'm new to the forum, and relatively new to bladesmithing. At least new to creating knives with a little bit of knowledge.


A friend of mine is using my forge (and what little I know) to make a Tai Chi sword from a piece of rebar. We first flattened it (meaning I held it and he did the beating--it's his sword, after all), straightened it, and he has been working it over on a belt sander to clean it up.


I know rebar isn't the greatest steel for a blade, but it's got some spiritual significance for him. I understand how to harden it, but how do I temper the center of the blade without softening the edges?


For heating, I have a coal forge, and an acetlyene torch. For quenching, water (I can make brine), and used motor oil.


Thank you again,



Edited by Buck Hedges
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I've pondered this question, and the only repeatable solution I have come up with is to clay the blade in the Japanese fashion. It is possible to bring the edges up to austenizing temp before the core comes up to temp, but it is tricky, particularly with a long blade. Another solution is to make the edges out of high carbon steel but have a mild steel core, then it would kinda happen automatically. There are some good arguments against having a differential heat-treatment on a blade in the first place... a differentially hardened blade tends to bend when flexed, but is hell to break...


Rebar could be almost anything, it may or may not harden.

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Unless you have a super high carbon( 1045) rebar from some Government defense project , I dont see you getting it to harden any way. Structual steel ( incl rebar ) is designed for tensile strength, you want it to be bendable with out snapping. SO while Im not the end all authority, you may have to try a surface hardening ( case hardening) to get any kind of real edge on it.

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You've got several issues going on here. First, rebar is typically made from scrap and can be a combination of different steels and the hardening of it will be iffy at best. Secondly, you will be guessing at your H/T temperature with your current equipment and none of your possible quenching materials are good choices.


I would suggest that you make this sword a "wall hanger" until you do A LOT of research on H/T'ing various steels.


I'm sorry to "rain on your parade" but H/T'ing a sword is a very complicated matter with much more to it that can be explained here on a thread this way.

If you want to experiment with this blade then clamp a piece of 1/2" x 1/2" piece of any steel to the spine (on both sides) of the blade leaving the cutting edge exposed. After doing some research, H/T the blade this way (as best you can for rebar) and test it as is.


Good luck.



Edited by Gary Mulkey
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Basically what everyone else said. Chalk up this blade to practice. I hope you haven't spent too much time on it. Essentially, welded-up coathanger wire would be a better choice of material.

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Thank you for all the advice.


Fortunately, this isn't my project, and I've told my friend that rebar isn't the greatest steel. He doesn't intend ot use it for combat, mostly for Tai Chi/energy work.


With that in mind, I HAVE hardened a pair of hayhooks I made from rebar. It's not the greatest quality steel, to be sure. I think I will try either the Japanese clay method (which I've had mixed success with in the past) or clamping a bar down the middle of each side.


In this project, I'm just providing tools and guidance. He's got lots of grinding and polishing to do before he even gets near the forge again. Not to mention making the hilt, guard, pommel. I'll try to post some pictures the next time he and I get together.

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