heat coloring steel (intentionally)

If you are talking about the bright blue color seen on clock springs, screws, hands, etc., it is called temper bluing. It is accomplished by heating the clean and polished parts to about 600° F either in air or in a molten salts bath. The color is actually a microscopically thin layer of iron oxide and it is not durable. While it does provide a small degree of rust resistance, It is not suitable for pocket carried hardware as it will wear away quickly. It doesn't work on anything but carbon steel, so stainless is out of the question.

Randy

Yeah, that's what I mean, or like the colors you get when you heat HSS too mugh when you grind it, or just chips from the mill. I have seen SS turn blue from grinding it, so is this temper bluing a different process than just heating the steel till it changes color?

Anyway, I kinda suspected that it wouldn't wear too well, and that the rust resistance would be pretty small.

Thank you,

Gene

Somewhat along the lines of heating metal for the colors, I'm wondering about brushing the almost red hot metal with a brass brush?.. I know it gives a nice looking patina, but anyone know how good it lasts/wears? Thanks..

Temper bluing is just heating the metal in an oxidizing environment until it takes on the color desired. The oxide film is very thin and translucent. Higher heat causes the film to grow thicker, darkening the color toward the blue range at which continued heating makes the film thick and opaque black, as in mill scale.

The color is caused by constructive/destructive reflection between the surface of the oxide and the surface of the metal underneath. That's why the metal surface must be highly polished to obtain bright color. It is the same effect as the colors seen with an oil film on water except the color range is limited. Temper bluing is well known and predictable with iron or carbon steel and has been used by clock/instrument makers and gunsmiths for at least 500 years, mainly for decoration.

While SS will color with heat(as will chrome plate), I suspect it takes higher temps than carbon steel. I have never seem any data published on temper bluing SS, so I don't know if the full range of colors is obtainable.

Randy

Heating in air will give you various colors of oxide coating depending on the temperature and alloy, but they will not be durable or rust resistant. Google 'case hardening` or 'color case hardening` for an interesting alternative process. It involves heating the parts in a crucible with charcoal and salts and is a little too involved for a short post here. There is not much color control and the patterns are random but they can be quite interesting and might fit the look you have in mind. You can see the effect on the receivers of some older firearms.

Pragmatist - "Where is Madame DeFarge now, when we really need her?"

Guy Lautard's Second Bedside Reader had a good explaination of the process, in the story at the back called "The Bullseye Mixture".

Ken Grunke SW Wisconsin