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Post by Deleted on Dec 6, 2009 7:38:44 GMT
A basic question. How hot must the blade be for its heat tempering to be ruined? Can it happen in sunlight? Can it happen while rubbing it with leather?
I tried to polish the blade by rubbing it with leather belt that I hold in my hand and realised it gets warm quite fast. If I had continued I think it could have been hot. So is this dangerous to the blade?
I know there are better methods to polish but I am now interested to know the safe temperature.
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SlayerofDarkness
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Post by SlayerofDarkness on Dec 6, 2009 8:05:32 GMT
Hey Rauta. ;D I'm no expert, but I seem to recall that it's around 400*. It's DEFINITELY higher than would be safe to touch, though. So, no, the sun can't do it (at least not within our atmospere. ), and I honestly can't see how a bit of leather could. I'm a bit (okay, way) out of my area of expertise (which is, um, non-existant. ) with this, though, so don't quote me on it. ;D However, I will be VERY much surprised if anything less than 250 is a problem, and mildly surprised if the 'bad temperatures' start below 350. I'll look forward to hearing an expert's opinion, though... BRENNO!!!! SAM!!!! Thanks for bringing up a good question! Now you've gotten me intrigued, lol. -Slayer
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Post by Deleted on Dec 6, 2009 8:20:04 GMT
Depends where you are in the world, I have left steel out in the sun that got hot enough to literally fry an egg on, though that is nowhere near 400 degrees, I wonder if time left in heat, even a lower heat would affect temper.
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SlayerofDarkness
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Post by SlayerofDarkness on Dec 6, 2009 8:26:32 GMT
Wow... that's hot. The time vs. temp thing is pretty insightful too... I'm looking forward to non-middle of the night tomorow when I'm sure that one of the resident sword-guy'll comment. ;D -Slayer
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Post by Deleted on Dec 6, 2009 11:09:15 GMT
Tempering at 700 F produces a hardness of 52HRC. That's what a good swordblade normally has. In order to destroy this hardness, you'd have to hit over 700 F and I doubt you get it that hot. the harder the blade, the easier it is to tuin the temper. The normally highest hardness for blades is about 59HRC which requires a temper of 400 F. So you'd need more than that to destroy it. Since your sword is very likely not that hard you can go over 400 F without fear of messing up your blade's temper. To actually make a blade too soft to take an edge you'd have to hit over 800 F which is not gonna happen during polishing ;D
Edit: that's for 5160 steel.
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Post by Dan Davis on Dec 6, 2009 13:13:57 GMT
And that, my friend, is wrong. On several points.
As BW pointed out: this is a matter of time versus temperature. You can temper steel at 250oF long enough to drop the Rc into the spring range, if you have the patience for it.
Then there are the factors of localised heat versus general temperature, you can get small regions of a piece of steel very hot while the overall piece is still cool enough to hold in your hand. This usually happens in regions where the steel is very thin (like the edge of a blade).
And then there is the fact that the answer is different for different steels. For example, any stainless steel that CAN be hardened and tempered will lose that temper if left in the hot sun long enough, it is the nature of the metal.
And yes, you can hit temperatures above 800oF when polishing, even when polishing by hand, although it takes some effort.
basic rule of thumb when polishing: Steel goes from cold to warm very slowly, and from warm to GODDAMMIT THAT'S HOT!! in a split second. Stop when it gets WARM and let it cool.
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Post by Matt993f.o.d on Dec 6, 2009 13:28:28 GMT
Hey Dan, with regards to stainless losing its hardness in the hot sun, is this a result of the chromium migration you mentioned in a separate thread?
During its lifetime, any knife will be subject to varying ambient temperatures, it is unnavoidable. Does this mean that over a long period of time a stainless blade will gradually decrease in measurable hardness?
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Post by Deleted on Dec 6, 2009 15:59:55 GMT
What I wrote were generalizations of course, I know you could temper a steel long on a low temperature to get the same result as a short temper at rather high temperatures. I meant temper times like one hour and a half. Except for that I meant general temperature, like in an oven. I don't really see where I was wrong... Also, how the hell do you get 800 F from HAND polishing?? I can't believe that
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Post by Deleted on Dec 6, 2009 17:20:26 GMT
It doesn't have to get to 800 F to ruin the temper
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Post by Deleted on Dec 6, 2009 17:27:55 GMT
It doesn't have to get to 800 F to ruin the temper So what? I didn't say that. I said that over 800 F it's too soft to take an edge. The previous hardness is already messed up by then.
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Post by Dan Davis on Dec 6, 2009 18:13:49 GMT
Hey Dan, with regards to stainless losing its hardness in the hot sun, is this a result of the chromium migration you mentioned in a separate thread?
During its lifetime, any knife will be subject to varying ambient temperatures, it is unavoidable. Does this mean that over a long period of time a stainless blade will gradually decrease in measurable hardness? Yes, largely due to chromium depletion. C-D occurs in high-chromium bearing alloys because of variations in temperature through out the blade. This happens all the time and the mean differential in temperatures only affects the speed at which it occurs. Also chromium migration occurs from high to low temperature. As the edge of the blade heats up more (thinner cross-section, smaller heat sink, lower volume for heat transfer) the chromium in that region becomes depleted over time. Since Chromium accounts for at least some of the hardness in stainless alloys, this results in a net loss of hardness at the edge. In real terms, though, you're far more likely to see C-D stress cracking and failure long before you notice the loss of edge retention capability.
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Post by Dan Davis on Dec 6, 2009 18:21:34 GMT
What I wrote were generalizations of course, I know you could temper a steel long on a low temperature to get the same result as a short temper at rather high temperatures. Perhaps, but you did NOT characterize you statements as generalizations. But you did not say that, either. No? Then don't worry about it. You get that by energetic polishing work. I have taken edges of blades all the way up to dull red on a polishing stone. Stop by sometime and I'll show you.
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Post by Deleted on Dec 6, 2009 18:36:29 GMT
No, you're right, I did not say those things even though I should have. Sorry for that. But even without these additional statements it was clear what I meant. Usually, you will not get the blade too hot from hand polishing. Sure, if you're REALLY enthustiastic you can but seriously, who takes edges all the way up to glowing red?? Kinda stupid, isn't it. You should have noticed loooong before that you're getting too hot. Especially if you're working without gloves which I think you should to have a better feeling for what you're doing.
Anyways, even though that last comment wasn't meant as a serious offer, I'd still like to do that, you know. Unfortunately, the US is a bit far away.
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Post by Deleted on Dec 6, 2009 19:19:47 GMT
green: a 25°F increase in temperature will produce a 1 HRC drop in hardness. blue: a time factor of 10x will produce an approximate 2 HRC drop in hardness.
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Post by Dan Davis on Dec 6, 2009 20:04:26 GMT
Of course I was serious, otherwise I would not have offered.
I'm not sure where you are in the world but if you ever get this way let me know and we can visit.
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Post by Deleted on Dec 6, 2009 21:11:59 GMT
That's a very good graph quasihodo, thanks.
Well, I'm in Germany, but if I ever manage to come to Texas I'd love to visit you. Thanks very much for the offer.
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Post by Deleted on Dec 6, 2009 21:12:50 GMT
No it was only clear to you what you meant without those other clarifying statements.
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Post by Deleted on Dec 6, 2009 21:14:08 GMT
Oh, well, I guess I'm not perfect after all.
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