At what point does heat "damage" steel?

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I'm no metallurgist, but does anyone know at what point do hot tempatures damage a knife blade? I'm asking from a practical standpoint. In other words, if you inadvertently leave your knife on the dashboard of your car and the knife gets real hot, will it damage the steel? By accident I sometimes have let my knife blades get hot, almost too hot to touch, and I was wondering if this alters the strength of the blade steel. Thank you in advance.
-C.B.
 

TheMightyGoat

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I'm not an expert on this, but this is my understanding. Someone slap me if I'm talking out of my ass.

The way heat damages steel is by degrading the temper. The higher the temper on the RC scale - for example, 62 RC - the more heat is required to alter the temper. I don't think what you described would be enough to damage the metal. The most common way someone will screw up temper is grinding the hell out of a blade on a bench grinder. You can take a blade to the bench if you're careful and cool with water, but most of the time it's not a good idea with stuff more delicate than an axe.
 
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From what I remember you would need a heat of around 350 degrees F or higher, depending on the steel, to start to effect the hardness. That is way more than, "almost to hot to touch".
 

Gollnick

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To affect the temper, you have to exceed the final heat-treatment temperature. Very few heat-treatment processes have a final temperature below about 350F.

Water boils a little over 200F. So, as long as the blade isn't so hot water boils on it, you're fine. In fact, putting a blade into boiling water isn't a problem either. Therefore, the dashboard of your car will not affect the temper of your blade.
 

Lenny

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How about stirring the hot coals of a camp fire?
I wonder how long you'd have to leave the blade in to affect the temper.
Lenny
 

MelancholyMutt

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Originally posted by Lenny
How about stirring the hot coals of a camp fire?
I wonder how long you'd have to leave the blade in to affect the temper.
Lenny

Yeah Lenny, that comes close...
don't do that...
 

Gollnick

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I wonder how long you'd have to leave the blade in to affect the temper.

Just as a knifemaker can differentially-temper a blade, you can differentially-damage it. While a quick stir of the coals might not raise the overall temperature of the blade above even 200F such that water dropped onto the blade does not boil, the edge is very thin and, in hot coals, may locally rise well above 350F.
 
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Well guys breif heat exposure will not effect a knife all that much. If you shuffle coals around for a few second at a time that will not effect the blade. Lets not forget that steel does not become as hot as coals just because it touches them for a few seconds.

Now is the blade is covered with some Teflon coating then I would try to avoid the fire with it as it might just burn off or melt. But if you have a BC1 coating it will burn off at 2300 F so you should be good for a while.

So like the guys say keep it under the final temper heat and you are fine. The dash board will not generate enough heat unless you car catches fire and the knife sits in the fire for a while.
 
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I have attached a tutorial by Max Burnett regarding Heat Treatment of blades. He does a great job of describing the process as well as cause and effects of over temp and under temps of the heat treat process.
 

MelancholyMutt

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Originally posted by Nimravus
Well guys breif heat exposure will not effect a knife all that much. If you shuffle coals around for a few second at a time that will not effect the blade. Lets not forget that steel does not become as hot as coals just because it touches them for a few seconds.

You may be right, but I still wouldn't feel good about it...
The same way I wouldn't stick my finger in there for half a second...
 

Bill Siegle

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The quick method to judge messing up the heat treat is did the steel get hot enough to change color? If it did you have probably messed up the temper in the colored area. On most steels a light sraw/brownish color is going to be your tempering range. Get it blue/purple and things are getting bad. A dashboard in the sun probably won't get steel hot enough to ruin temper(could mess up kydex sheaths though). The easiest thing to do if it is a concern is stop leaving your blade in that location.
 
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What puzzels me is why a stainless steel oven rack doesn't go soft or break or go "bad" over time... When steel gets hot does it get softer or more brittle? I've talked to machinist type people who say they heat treat steel but it seems that the effects of heat on steel are part science and partly an "art" as well. Is this so?
 
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Steel becomes britle only if it is rapidly cooled. The faster you cool it the more britle it gets. This is why if some of you own Henckel cooking knives you might just one day break it. Instead of freezing the edge they freeze the whole blade. This make it good in the sense that the whole blade can take an edge and that you will never run out of edge holding steel the down fall to this is that it could just break since it will not allow flex as much.

There is this new Crio freeze technique out there but if I understand it correctly it freeze the edge of the blade and not the whole blade. So edge retention is excellent but the blade can still flex abit before you yield it or fail it.

Getting an edge is one very very hard thing to do you could create a knife that would never need to be sharpened again, the only downfall to this is that if ever you where to try to force the blade or try to flex it it would break since it is so britle. So with knives like in life we must make a comprimise to allow us to have a blade that will cut and not break.
 
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The freezing/cryo treatment is done before/during the hot tempering cycles where you draw the blade back to a usable hardness. All blades are to brittle to use directly after the initial quench of heat treating, even if they are only edge quenched.
 

Gollnick

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What puzzels me is why a stainless steel oven rack doesn't go soft or break or go "bad" over time...

"Steel" is a name for a broad family of metal alloys. Steel oven racks are not made out of the same steel alloy that knives are. And, of course, there's the question of how the metal has been processed, heat treated, etc.

The goal of heat treatment is to make the metal harder. This is not magic. It's accomplished by encouraging the formation of a tigher crystal structrure. There are several ways to do this. Passing a huge electrical current through the metal can. Exposing it to a powerful magnetic field can. But heat treatment is something that you can do with relatively simple equipment. A typical heat-treatment process is to heat the metal to 1600F, cool it very quickly, heat it back to 1400F, cool it very quickly again, heat it to 600F, and then allow it to cool slowly. Heating loosens the bonds within the metal matrix and encourages the molecules to move around. Cooling causes them to settle down into a crystal matrix. To understand it, measure a cup of flour out of a sack of flour. Now sift it. Measure it again. It's more than one cup. How can this be? Because the particles of flour aren't so tightly packed anymore. If you were to introduce some mechanical energy, perhaps by tapping on the side of the measuring cup, you could ecourage the particles to pack together again. Heating and cooling the metal is like tapping on the cup. It encourages the molecules to settle into a tighter pack. And yes, the blade does shrink during this process, though not within the ability of most people to measure.

The heating can be -- and often is -- done by putting the metal into a hot fire, a forge for example. You can judge the temperature by the color of the metal. The cooling can be done by plunging the material into cold water. But these methods, while still used by many knife makers, are poorly controlled and produce inconsistent results. Modern heat-treatment is done in strictly-controlled industrial ovens. The temperatures, the rates of heating and cooling, and the times are all exactly controlled. This gives very good and very consistent results.

And heat-treatment isn't a new idea either. Men have been heat-treating metals for well over a thousand years.

It's only recently, though, that modern science has come to understand exactly what happens during heat treatment, about the crystal structure of the metal and so forth. Our modern understanding of heat-treatment, metallurgy, and crystallography have enabled us to formulate steel alloys that respond especially well to specific heat-treatment processes. Our modern facilities allow us to confect those steel alloys accurately from highly pure materials. And our modern process control capabilities allow us to heat-treat that steel with great accuracy and repeatability.

For our ancestors, though, heat-treatment must have seemed like magic: you apply a little fire, a little water, and a piece of metal dramatically changes its character. That must be magic, eh? And to further contribute to the mystery, the magic didn't always work very well. It could have been inconsistent composition of the metal or impurities in the metal, poor control of temperature, poor control of the rates of temperature change, all of these factors affect the outcome of the heat-treatment process and none of these factors were understood or well-controlled. For them, the only available explanations were, "The god's didn't favor us that day... or maybe the eye of newt wasn't newty enough." As a result, many myths and legends and superstitions arose surrounding heat-treatment, especially for edged weapons.

Imagine an ancient blacksmith who, on a Monday, hardens a sword using the process his father taught him and that usually works well. But it doesn't work very well that day. Why? Maybe because the metal contained some impurities. Maybe because he got the fire just a bit to hot this time. Maybe he was distracted by that attractive lady in the short dress who walked past the shop and so he ended up leaving the blade in the fire a few seconds longer than usual this time. Who knows? He certainly doesn't. All he knows is that it didn't work very well that day and the resulting sword isn't very good. On Tuesday, as he's about to heat treat his next sword, he feels the call of nature and decides instead of wasting time running out to the outhouse, to relieve himself in the bucket of water in which he'll be quenching the blade. For whatever reason, better steel, a slightly cooler fire, better timing, whatever, the process goes especially well that day and the sword is superb. But, on Wednesday, for whatever reason, the process fails again. He may sit down and ask himself, "Why did Tuesday's sword turn out so much better?" He may remember that on Tuesday, he urinated in the water. And without a better understanding of metal and heat-treatment, he may conclude that there's something magic about urinating in the water and insist on doing so from now on. And he'll teach that to his son who will teach it to his son and so forth.

Ancient documents are full of stories and formulas about adding magical elements to the water or the fire, about magical songs or incantations to sing or say or dance during the process (which may actually help by giving more consistent timing), about how heat-treatment must only be done by the light of a full moon (which may add consistency to the judgment of metal temperature by color), and so forth. There are even stories, probably with some basis in fact, about swords, still red-hot from the fire, being quenched by being plunged into living humans, usually war prisoners or slaves (which may give some consistency to the cooling process).

Today, our modern, consistent, pure alloys and our carefully-prescribed and fully-controlled heat-treatment processes, we get much more consistent results. But, you know what? There's still a certain element of what might still be called, "magic." It's not really magic. It's just variablilty that remains in the process. For example, a sudden power surge could cause the oven to go a little over the desired temperature or to rise a little faster than expected. So, there is still some variability in the heat-treatment results and still the need to test at least some blades occasionally.

Many modern steels are formulated specifically to respond especially well to specific heat-treatment protocols. Many modern alloys are very sensitive to heat-treatment processes. If they're heat-treated properly, the results are exceptionally good. But, if the heat-treatment process is off even a little, the results may be exceptionally poor. So, control of the process becomes very important and quality control through testing becomes even more needful. This is part of why some knifemakers will sing the praises of a certain alloy and make wonderful knives from it while another equally skilled maker may curse the same alloy and claim that it doesn't heat treat well.

Whatever the metal, when you heat it up, you run the risk of exciting those molecules and messing up their crystal structure. But it takes hundres of degrees to do that.
 

DEA

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it's quite difficult to get teflon to burn or melt
you'd need temperatures of 350 degress celcius and above to achieve that, and i doubt the handle material will be intact when the blade hits that kind of temperature

that said, burning teflon is not good for the health
 
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Temper temps are different for almost every different steel, 440C is down low at about 450 degrees F, ATS is tempered much higher, near 900F................you have to get the temps up near those temperature to affect the temper of your knifeblade, which is not likely in most real life situations.
 
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Very helpful information given by all.
Gollnick, great illustrations, you must be a teacher....
As a knife nut, its good to know these things about the inner workings of "the blade" and to become more knowledgeable about steel. Thanks for the information.
 
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