Differential Clay Heat Treating Options

[brujahbattalion]

I've done quite a bit research into different traditional methods of clay heat treating; however, I haven't been able to find any type of scientific comparison between strength processed through different methods. For example, I've read that removing clay from the spine of the sword will (obviously) harden it and minimize warpage but would that increase or decrease the overall strength of the weapon? Anyways, my question boils down to wondering if anyone's compared impact resistance of a sword/knife with a hardened spine, unhardened spine and a completely hardened(edge-to-spine) knife.

I was going to do the test with some 4140 and Satanite, but I figured I ask here first.

[Daniel]

I've done quite a bit research into different traditional methods of clay heat treating; however, I haven't been able to find any type of scientific comparison between strength processed through different methods. For example, I've read that removing clay from the spine of the sword will (obviously) harden it and minimize warpage but would that increase or decrease the overall strength of the weapon? Anyways, my question boils down to wondering if anyone's compared impact resistance of a sword/knife with a hardened spine, unhardened spine and a completely hardened(edge-to-spine) knife.

I was going to do the test with some 4140 and Satanite, but I figured I ask here first.

There are several difficulties with your question. What do YOU mean by strength? Strength vs. impact resistance? What steel?

[brujahbattalion]

"There are several difficulties with your question. What do YOU mean by strength? Strength vs. impact resistance? What steel?"

Sorry, maybe I wasn't clear. I'm interested in "wondering if anyone's compared impact resistance" and I'd be doing "the test with some 4140 and Satanite". I'm not trying to offend, I'm just not really sure how to further clarify.

[Bithabus]

I've read that removing clay from the spine of the sword will (obviously) harden it and minimize warpage but would that increase or decrease the overall strength of the weapon? Anyways, my question boils down to wondering if anyone's compared impact resistance of a sword/knife with a hardened spine, unhardened spine and a completely hardened(edge-to-spine) knife.

This is the part that is unclear. In the first sentence what do you mean by 'strength'? Thee are many different types of strength. In the second sentence, are you asking for a comparison between 3 swords: one with a hard spine and soft edge, one with a dead soft spine and hard edge, and one through-hardened? Why would anyone make a sword with a hard spine and soft edge?

When you ask whether anyone has done "tests for impact resistance" what do you mean? Usually impact resistance testing is done with single samples of a specific steel at a specific hardness. Are you talking about doing destruction tests on whole swords?

I think the answer (if I understand your question) is that with a good modern steel and modern heat treat doing a differential hardening will be an improvement, but not by much.

[Daniel]

I am still unsure about the question.

Strength in engineering terms means the amount of energy beofre the material moves. This is the same as hardness. The harder a material is the greater this type of strength.

Strength in common terms might mean how much energy before total failure. This can be measured with impact resistance.

But these two are not the same. What do YOU mean?

[ridgit]

Why would anyone make a sword with a hard spine and soft edge?

Because they don't know what they're doing.

I've done quite a bit research into different traditional methods of clay heat treating; however, I haven't been able to find any type of scientific comparison between strength processed through different methods. For example, I've read that removing clay from the spine of the sword will (obviously) harden it and minimize warpage but would that increase or decrease the overall strength of the weapon? Anyways, my question boils down to wondering if anyone's compared impact resistance of a sword/knife with a hardened spine, unhardened spine and a completely hardened(edge-to-spine) knife.

I was going to do the test with some 4140 and Satanite, but I figured I ask here first.

I think I understand your question. What you are talking about is what sword people refer to as "toughness" which is the metal's combination of hardness, wear resistance and overall ability to take a beating.

The problem here is that different heat treating methods work differently on different metals. The Hamon method that you are referring to for instance requires the metal to first be made in a very specific way. Watch a documentary on Japanese Samurai sword making for details.

Damascus/wootz steel works on the same concept of Japanese swords (soft metal to flex and take a beating, hard metal to hold an edge), but in stead of a soft upper and hard lower with gummy core, damascus has the soft and hard metals mixed together microscopically, giving the entire blade extreme hardness and flexibility. Likewise, putting a hamon on Damascus may be possible, but would be pointless. As far as heat treating it, I know nothing about it, but Daniel makes the stuff, so he's the one to ask.

As for comparing these metals, it's sort of like czars, paper, rock. Save for wootz/damascus, I don't think there's anything of the ancient world that could stand up to the stuff the Japanese were using but I am no expert either.

Japanese steel oddly compares to modern stuff. I never saw it, but my friend said he saw a documentary where they compared a newly made Japanese sword that was made by Japanese samurai sword smiths using the traditional method with a sword made of S-7 with newer CNC heat treating methods (sounds like a bright knight if you ask me. Maybe Daniel knows something about it.

The main difference they were able to notice was that the S-7 sword vibrated on impact while the Japanese sword didn't. However, they also supposedly sighted that Japanese swords, as tough as they were, still broke allot while S-7 can be bent to a perfect "U" shape without breaking. I have a sword made of S-7 and can tell you that it's true. I think the old angelsword website even has a video of Daniel doing it.

[brujahbattalion]

"Why would anyone make a sword with a hard spine and soft edge?
Sorry, should have elaborated here. All 3 samples would have a hardened edge. Sample 1 would have a hardened edge and clay coated (soft) spine/core. Sample 2 would have a hardened edge, a soft core (the sides where a fuller would be) and a hardened spine. Sample 3 would be completely through hardened. (All 3 samples are 4140 steel and all 3 would be furnace heat treated and tempered.)

"However, they also supposedly sighted that Japanese swords, as tough as they were, still broke allot while S-7 can be bent to a perfect "U" shape without breaking."

S7 is some pretty cool stuff (but damn expensive). I have yet to handle anything thin enough to hand bend. Looking forward to it though. : )


"I think the answer (if I understand your question) is that with a good modern steel and modern heat treat doing a differential hardening will be an improvement, but not by much. "

This is essentially what I was looking for. I'll probably run the tests once I get some free time and maybe post the results if anybody's interested.

Thanks for the help guys.