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Post by Robnose on Sept 11, 2015 13:38:07 GMT
First off, I'm not sure if this thread should be here or in general discussions but here's my question:
If a blade takes a set on approximately a 15 degree angle during a flex test, is the heat treat bad or non existent? Is the steel too soft?
I was working on my Sword n Armory Qing Dao and managed to bend the blade with very little effort. I just placed it on the bench and bent it upward with one hand while keeping even pressure on it with the other. It very easily took a set. I think it was a nice looking SLO but since the handle has been trashed I've decided to make a tanto from the top 10" of blade. Should I bother?
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Post by MOK on Sept 11, 2015 16:45:49 GMT
Hard to say, really... How far and how sharply did you bend it?
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Post by Robnose on Sept 11, 2015 19:44:53 GMT
Rise about 2 1/2" run about 18". Felt very much like bending a spoon... if that makes any sense. i think the steel is way too soft for a long blade. I'll probably go ahead with the tanto build if only for fun.
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Post by MOK on Sept 11, 2015 20:19:28 GMT
Yeah, it's probably effectively unhardened if it bends that easily. On the bright side, by the same token it should be easy to work with, though!  |
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Post by Lukas MG (chenessfan) on Sept 11, 2015 20:31:34 GMT
Yeah, I don't think that steel is heat treated at all... I wouldn't bother with turning it into a tanto blade. Decent steel of that size is cheap and sending it to someone for HT shouldn't be too expensive either. I'd rather spend my time making a blade out of a known quality piece of steel and actually having something useable in the end. You could use the SLO for practicing sharpening though.
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Post by Timo Nieminen on Sept 11, 2015 23:29:44 GMT
How thick is the blade where it bent? Is it a sharp bend or a gradual bend? (Photo?)
Since the yield strength (and therefore the force required to put a set in the blade) only increases by about 2.5 times from annealed to quenched and tempered hard (for 1060), there isn't that much difference. What a bend test will tell you is whether or not the blade can be bent as far you tested it.
Note that, contrary to many people's expectations, it's just as hard to bend a soft steel as a hard steel, to a given angle. The elastic modulus changes vary little with heat treatment (and even between steel alloys). What changes with heat treatment is how far you can bend it without it taking a set or breaking, and how the blade fails (taking a set or breaking). (The impact strength (or fracture toughness) also changes, but that's not tested by a bending test.)
But since you're thinking about recycling the blade, why not try to measure the edge hardness? Get a set of hardness testing files, and go for it.
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Post by L Driggers (fallen) on Sept 12, 2015 4:06:37 GMT
As a blade maker I find a soft steel blade a lot easier to bent than a heat treated blade.
You know you may even have a blade made of mild steel.
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Post by Timo Nieminen on Sept 12, 2015 12:02:28 GMT
What do people get with quantitative measurements? Measure the elastic modulus (Young's modulus), and you get 200 GPa or 30 million psi, give or take a few %. So about the same force to bend it by some amount.
Try it. Clamp a blade by the tang, and hang a weight from the tip. A light enough weight so the blade doesn't take a set, and heavy enough so it bends far enough to measure. Measure how much the blade bends. Do before and after heat treatment.
Yield strengths vary a lot.
Mild steel, annealed: 300 MPa
9260, annelaed: 500 MPa
9260, quenched and tempered 200C: 2000 MPa
9260 has the same elastic modulus, so just as hard/easy to bend, but with that yield strength, it survives it.
If the blade only bends a little bit before the elastic limit is reached (i.e., the stress reaches the yield strength), the deformation due to yield can (and often will) make the blade a little thinner and easier to bend at that point, so it bends more where it first deforms, which make it deform more, and bend more easily, and bend more, deform more, etc. So the yield strength can affect attempts to measure the modulus of elasticity. Avoid that by staying below the elastic limit.
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Post by Lukas MG (chenessfan) on Sept 12, 2015 17:05:02 GMT
Timo is quite right, blade flex doesn't change during heat treat, what changes is whether the blade will return to true or not (of course when pushed past a certain point, a HTed blade will take a set as well). Seeing how your blade seems to have taken a set very easily (i.e. stayed bent when only pushed a little bit out of true), it seems to me as though it wasn't properly heat treated, if at all (or as Driggers suggests, it might even be mild steel for all we know). Checking with files is a good idea though.
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Post by L Driggers (fallen) on Sept 12, 2015 20:03:18 GMT
All I know is it's easy for to bend a untempered blade than it is a tempered blade.
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Post by Robnose on Sept 13, 2015 16:28:34 GMT
Ok, I've working on that tanto this morning. Why does it seem I'm getting farther and faster with a hacksaw and file than using a grinder with a zip disc and jigsaw with metal cutting blade. I'm friggin beat out. Had to come in and take a break. Keep in mind my experience with modifying blades is limited to what I can do by hand. I was expecting much faster results with power tools...
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Post by Robnose on Sept 14, 2015 1:05:21 GMT
here it is. I was going to dress it up but I think I'll leave it bare.  |
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Post by brotherbanzai on Sept 14, 2015 13:38:02 GMT
Hey L Driggers,I think there is some confusion coming from different ideas of "bending". Timo is talking about bending only to the yield point, whereas I think you are talking about bending in general. If you take two identical pieces of steel, one heat treated and the other not, and try to bend them over your knee, the un-heat treated one will feel like it bends much easier. Because it does, after it's past the yield point.
Timo is saying that the force required to get them both to the yield point is about the same (before either has taken a set). Once past that point, however, the heat treated blade will get harder and harder to keep pushing while the un-heat treated will get easier.
You've probably noticed this while trying to make small corrections to a piece of unhardened steel. You try to bend it just a little and at first it resists, then suddenly, "oops, now it's bent too far the other way". It had about the same resistance to flexing as a hardened and tempered blade at first, before the yield point. If you had backed off before that point, it would have returned to it's original position, it was still "flexing". Once you went past the yield point, it became easier to move. At that point it was "bending" and would not have returned to its original position.
As a side note; bending is a plastic deformation (once bent, it will stay bent), flexing is an elastic deformation (when flexed, it will return to its original position when the load is removed). It might be clearer to say that two identical pieces, one hardened and one unhardened, will take a very similar amount of force to "flex" (to the yield point), and then the unhardened one will "bend" more easily after that.
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Post by L Driggers (fallen) on Sept 14, 2015 15:24:52 GMT
You may be right, just was making it sound like tempter steel would bent as easy as untemptered steel.
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Post by Robnose on Sept 14, 2015 21:30:47 GMT
"oops, now it's bent too far the other way" is EXACTLY what happens! That's what I meant with the spoon example. I'm guessing it's not hardened steel.
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