Steel color change - how come?

"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.)

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

Starrett also does so-called 'color case-hardening' of their tools.

This was "The Bullseye Mixture" story in the bedside reader.

The idea is you pack harden items, and then dump them right out of the retort into brine with oil in it, agitated by air pumped through bubblers in the bottom.

Jim

Controlled coloration was often practiced in heat treatment. S&W, for example, used to color the hammer of their hand guns in that fashion. You can expect a wonderful range of colors, blues, greens, reds. It's chemically induced. I have no information on the process, but one of Guy Lautard's (sp?) Bedside Readers has a formula contained within.

Harold

Blacksmiths use these colors to determine tempering temperatures. Take this link:

You can pretty well get as far as "full blue" in a kitchen oven. Some experimentation will be necessary since ovens aren't always that accurate. When I typed "temper colors" into Google, I got 275 hits. My own eye sees some prettier reds than that particular chart shows, but I'm sure you get the idea. If you spray your completed project with clear laquer, etc., the colors will stay a long time. But if you leave the part bare, eventually thicker rust will overtake the original thin oxide. I once made a set of hammers, fullers and hot cuts that had beautiful temper colors, just where I wanted them. They stayed that way until one day when I was demonstrating on a rainy day. The next day it was all over!

Pete Stanaitis

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Peter Grey wrote:

clock hands are blued in this manner. Willian SMith, a famous clock maker suggests to put the hands on a bed of brass filings collected from the lathe or band saw and heat then on the stove top until they turn blue.

The metal must be clean because you are dealing with oxides and you want the metal to all react at the same time. Therefore the metal must be clean and uniformly heated.

chuck

There are loads of proprietary and home brew formulae for different colours on steel and brass. Try googling for "chemical blacking"

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.

Just anecdotally, the most brilliant coloring I've ever seen was on a chromed motorcycle exhaust pipe 'heat treated' by hard riding.

"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?

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.

The oxide doesn't change back to steel.

Correct. It fumes off.

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

I used to do this, as many antique clocks and watches that came through my shop were missing hands. I used a stainless steel container for the brass filings, which slowed the process, giving better control over the colour.

Steve R.

Do you know what's happening with the interaction between the brass and the steel that accentuates the "blueness" (insert Yellow Submarine joke here) of the steel?

Peter

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

That explains why the color disappears as the temerature of the piece goes up.

Peter