Steel color change - how come?

Hi all,
I know that as steel (or some steel anyhow) heats, the surface changes color and if one removes the heat the color will stay on the surface. I've done
this using a propane torch, but isn't easy to do in an even manner. Can anyone explain what's happening to cause this?
I've got some projects where I'd actually like to maximize and control the surface coloration. What I can do to make it as pronounced as possible? Are there different types of steel that would be more or less prone to this? Any techniques where this coloration can be more easily generated or saved? Any websites that discuss this?
Thanks,
Peter
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"Peter Grey" wrote: I know that as steel (or some steel anyhow) heats, the surface changes color (clip) Can anyone explain what's happening to cause this?(clip) ^^^^^^^^^^^^^^^ As the steel is heated, an oxide layer is formed on the surface. Light falling on the surface is partially reflected from the front of this layer, and more is reflected from the back of this layer/front of the steel. If the thickness of the layer is 1/4 wavelength thick, the light from the front and the light from the back will interfere and cancel. The eye will then see a color that is the complement of that wavelength. (The same mechanism produces the colors in an oil slick.)
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Thanks. To your knowledge, are there any steels that are more prone to forming oxides on the surface as they're heated? This is not a structural application so the mechanical properties of the metal are secondary to its appearance.
Peter
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IIRC these colours occur about 100C higher for stainless steel. Not what you asked but it might be worth noting.
Peter Grey wrote:

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Peter Grey wrote:

Just anecdotally, the most brilliant coloring I've ever seen was on a chromed motorcycle exhaust pipe 'heat treated' by hard riding.
--
Fred R
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Leo Lichtman writes:

More to it than just that. The thickness (1) equilibrates instead of just growing, and (2) differently so with differing temperatures, and (3) reversibly so since the thickness follows the temperature both up and down.
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"Richard J Kinch" wrote: More to it than just that. The thickness (1) equilibrates instead of just growing, and (2) differently so with differing temperatures, and (3) reversibly so since the thickness follows the temperature both up and down. ^^^^^^^^^^^^^^ Your additional explanation answers some of the questions I have had, but it raises others that I wish I had answers to. We know that steel oxidizes in contact with air. We know that chemical reactions are accelerated at higher temperatures. So, I always assumed that as the steel is tempered, the oxide film grew continuously, but more rapidly as temperature went up. According to your esplanation, this is not true--instead the film thickness reaches an equillibrium thickness depending on temperature. Now I think I understand why the temperature can be judged by the color, and time does not enter into it.
Now the part that baffles me. If the film thickness tracks the temperature reversably, as you say, why doesn't the film/color disappear as the steel cools?
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Leo Lichtman writes:

I suppose the process "freezes" at relatively cool temperatures, and if cooled quickly enough you freeze the film thickness characteristic of the recent hot temperature.
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It seems to remain even if one lets the steel cool down by exposure to room temerature air. I haven't had to quench the piece in order for the color to remain.
Peter
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The oxide doesn't change back to steel.
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Correct. It fumes off.
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That explains why the color disappears as the temerature of the piece goes up.
Peter
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Whaaa?! You do realize ferrous oxide has a *melting point* in excess of 2,000F, don't you?

Maybe under heavy reduction, where the hydrogen and, more importantly, carbon in the flame are able to actively reduce the oxide from FeO to Fe (such happens readily with copper metal), but not under any other circumstances.
Tim
-- Deep Fryer: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
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Just using a propane torch the blue will disappear as the I continue to hold it to the steel piece I'm heating. It doesn't come back as it cools.
Peter
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See what else Tim said. The color disappearing doesn't mean the oxide does.
Steve
Peter Grey wrote:

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Leo Lichtman wrote:

Leo, the film thickness does not track the temperature. The oxide grows faster at higher temps as you say. The oxide stays around--it doesn't go away as the temperature drops.
One factor confusing this issue is that the colors don't keep going. Once the oxide gets thick enough, the colors go away, but the oxide keeps getting thicker.
Steve
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Here's the deal. Take a piece of iron. It has a perfectly clean surface, I mean the oxygen and nitrogen molecules of the air are bouncing directly off the pure iron (and occasionally iron carbide and others) surface. Now at room temperature, a few of these do bust up on the iron surface and oxidize it. Like aluminum, this layer is invisible, but because it's somewhat indifferent, it doesn't change the chemistry of the iron and you don't notice it. This layer is only as thick as it is because the oxygen molecules can't get past it at this temperature.
Ok, so let's raise the temperature. Radiation heats the air molecules near the steel to the same kinetic energy, that is to say, hot air's molecules move faster. So they hit the iron surface with more energy, and occasionally one will pass through the oxide layer and oxidize more iron. (It probably passes by diffusion, where the surface oxidizes to magnetite - Fe3O4 - or rust - Fe2O3 - which is then passed backwards to the metal, which reduces Fe3O4 and Fe2O3 back to FeO, at the price of more Fe metal being burned.) Just as carburization can diffuse hardening carbon (or nitrogen in some cases) only so far, likewise the oxygen only goes so far through the oxide. It's always passing through, even at room temperature, so the response of oxide growth is probably logarithmic - it tapers off quite quickly as thickness rises, but never stops completely. It's just that thermal response is exponential, so it'll take about two million years to eat a tin can, while at orange heat, your tin can will hold molten aluminum for only about fifteen minutes!
A temperature of 350F for a few minutes produces a nice light straw color (hard to spot because the oxide takes time to grow, and you can't anticipate it because this is the first interference layer, around 80 nanometers thick?), but the same temperature extended to an hour gives a purple coloration.

Actually, they come back several times, but each time harder to see because the light has to travel through more oxide thickness. If you heat a shiny bar from one end in plain air, you'll see the first layers, yellow, purple, blue; then a darker run of yellow, purple and blue, and so on for maybe three or four total modes. What's happening is light is spending 1/2, 1 1/2, 2 1/2, ... wavelengths inside the thickness of oxide (1/4, 3/4, 1 1/4, ... wavelength thick layer, for the wavelength of light *passing in the medium* (light slows down per the index of refraction), of the *canceled frequency*). Thicker layers attenuate more, so it quickly (500nm?) starts looking black. Fe3O4 is a wonderful electromagnetic-absorbent material, after all. No wonder the military uses it...
Tim
-- Deep Fryer: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
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Yup. What Tim said.
Steve
Tim Williams wrote:

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"Tim Williams" wrote: Here's the deal. Take a piece of iron. It has a perfectly clean surface, I

^^^^^^^^^^^^^^^ I followed your explanation with great interest. You seem to take me back to some the beliefs I held originally, and abandoned briefly as this thread unfolded. I take it that you do not agree that a certain color on the surface is correlated with a certain temperature in the metal. As I thought originally, the color DOES correlate with film thickness, which, in turn, is dependent on the time/temperature history.
Since "drawing the temper" of steel, as done by blacksmiths, using the surface colors, is a way of raising the steel to the desired temperature, why does it work? Wouldn't a longer time at a lower temperature produce the same interference colors as a shorter time at a higher temperature? Could it be that the time/temperature history produces the same effect on color that it does on hardness? Or am I completely off the track here?
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Thanks.
It can't be, since I've torch tempered and oven tempered metal myself. The oven pieces come out considerably darker, in the purple range as I mentioned.

Because it works in the short term. As you expand time exponentially, you get more "out" of it, but after a while, to get even a small amount out, takes a very long time. The endpoint is arbitrary; by eye, over a few minutes, you'll probably spot between 300 and maybe 500F (SWAG). In the oven, you get a bigger change out.
Why does it work as far as the metal? Two reasons. For one thing, it's just simple carbon steel, you can't really go wrong with it (short of overheating before quench, which makes it crunchy no matter what!). Number two, the reactions in the metal, where bainite and whatnot break down to more stable phases during tempering, is the same kind of time-temp governed reaction as the oxidation is. It might not proceed at the same rate (it would be interesting to compare this!), but who knows.

Exactly! So it may be that my purple blades at 350F for an hour are overtempered, while the yellow-for-a-few-minutes torch tempered jobs are undertempered. This is where some imperical evidence comes in handy. The last blade I tempered was the brass handled knife here:
http://webpages.charter.net/dawill/Images/BrassKnife2.jpg
Now I've sharpened this good enough to shave with (not real comfortable, but it cuts the hair smoothly anyway..), and in the process I don't notice much of a burr turned up so it must be pretty hard. That's fine with me since it's so thick it'll "never" break.
I tempered that to 350F for an hour and it came out purple (splotchy mind you, fingerprints for instance are prime spots to prevent oxidation). It seems to be simple 1080-1090 (used to be a chisel), nice yellow-white bursts on grinding, same as a file.
Tim
-- Deep Fryer: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
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