The Guide to Weld Discoloration Levels on Inside of Austenitic Stainless Steel Tube, D18.2:1999, can be purchased from the American Welding Society.
Visit this link for details:
http://www.aws.org/catalogs/index.html
However, if I remember correctly, this chart identifies discoloration as a result of a certain percentage of Oxygen contaminating the sheilding gasduring welding.
Please share with us your findings on this matter as it is a fairly common topic with much interest.
Good Luck!
Tim
This came of the following website http://www.survivalistbooks.com/faq/metalfaq.htm
Pale Yellow 350º F
Straw Yellow 400º F
Yellow/Brown 450º F
Red 500º F
Violet 550º F
Dark Blue 600º F
Light Blue 650º F
Blue/Gray 700º F
Gray 750º F
However, I think it references carbon steel. It would seem that stainless steel, being more resistant to scaling in general than carbon steel might have an even higer temperature for the given color.
Here is a tidbit excerpted from a larger article at.
http://www.ncsg.org/members/sweeping/10-96.htm
It seems to cut to the chase as much as can be done.
The following is taken from information gathered at the Nickel Development Institute in Canada.
"The oxide layer does not penetrate the thickness of the steel; it is only on the surface. The topmost few atoms may be involved in the oxidization, but this is insignificant with respect to the entire cross-section. This oxide layer does not penetrate the thickness of the steel; it is only on the surface. The topmost few atoms may be involved in the oxidation, but this is insignificant with respect to the entire cross-section.
This oxide layer is not formed just by heating. In fact, such a layer is always present, and simply results from chemical interaction of chromium on the very surface of the steel with the oxygen in the air. If you were to scratch the surface or rub it with sandpaper you would "erase" this layer, but a new oxide film would form essentially instantaneously. This oxide layer is one of the things that makes stainless work. It forms a protective film that separates the steel from the corrosive effect of air, moisture, and other things on the outside. Of course, this layer can be penetrate or temporarily rubbed off, but it always comes back and always is the
first line of defense.
(This also addresses one of the questions that has been tossed around the industry for years. When you brush a stainless flue with a wire brush you are scratching off a protective oxide film. But the film returns immediately, so you haven't done any damage. You would have to clean a chimney many times before you had any significant effect on the thickness of the stainless!)
The oxide layer on stainless that has not been heated is so thin that it doesn't interact with the visible spectrum, so we don't typically see colors. When stainless is heated (beginning at about 500 degrees F) the oxidation reaction is enhanced, and the layer becomes thicker. As the thickness of the layer increases, the wavelength of light transmitted changes, and hence the color we see.
Therefore, the development of different colors is related to the temperature the stainless has been exposed to. However, there is no simple one-to-one relationship between a particular color and a particular temperature. The thickness of the oxide layer is influenced by the amount of time it was heated and by environmental factors. A sample of stainless that was heated to 600 degrees for a long time may display the same color as one heated to 800 degrees for a shorter time. In addition, the quality of the air and what contaminants are in it can also affect the thickness of the layer.
The development of "pretty colors" on the surface of stainless is NOT in any way indicative of the grain structure of the underlying metal, or any other metallurgical condition other than the oxide film itself. It would not be responsible to conclude that "blued" stainless has been sensitized, or exposed to any particular temperature, or even that a chimney fire has necessarily occurred. The only thing that can be concluded with reasonable certainty is that the stainless has been
heated, which for a chimney is not a big surprise"*******************************************
Now that we have some sort of an an answer as to color of oxides (on stainless), that it's not necessarily an indicator of anything in particular. What gives pause is that in your initial discription if I understood it, you stated that your running your stainless tubing at temps considerably higher than they are rated. When doing this, I don't think anybody saying anything about color would give me peace of mind about the situation.
My question to you would be, Why did you select a material that does not match the service conditions?
Just wanted to provide an update. The particular matreial in question is SA312 type 254 SMO, which has an temperature exposure limit of 1100 degress F. The piping was subjected to temperatures of at least 2200 degress for approximately 4 weeks. Yes..... we did have a failure. I have the pictures if anyone is intrested.
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