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Questions regarding chemical reaction that occurs when Bluing Steel

2007

I am a junior engineer at a long established manufacturer of small formed metal parts. The parts are used for cutting, so the cutting surface is chromed, the body left bare steel. In some cases the bodies are "blued". I have been tasked to work on the process, as we are looking to blue more parts, but no one involved in the operations fully I want to fully understand the underlying process. The two ways we currently blue are:

1. After heat treat, the parts (an 86XX alloy) are put in a salt tank (at 525 °F) for one hour. The salt tank contains both potassium nitrate and potassium nitrite. Apparently if the temperature is below 525 °F the steel will not blue, and above 525 F the bluing effect diminishes.

2. Parts are placed in a bake oven (used mainly after chrome plating), again at 525 °F for one hour. There are no chemicals involved, on the air in the oven.

I was under the impression that bluing, like with guns, was formed from the potassium nitrate, but if this were the case, the cutters would not blue in the bake oven.

So my questions boil down to:
1. What is the chemical reaction involved (what is the stoichiometry)?
2. Obviously endothermic, as 525 °F is the minimum temperature, why does the bluing effect diminish at higher temperatures?

Any insight would be greatly appreciated.

Luke Hagen
manufacturer (in house plating) - Portland, Oregon

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2007

Steel in the oxidizing salt bath forms Fe₃O₄, magnetite, which is blue-black.

The chromium plating heated in air oxidizes only slightly, forming a thin film of ~transparent Cr₂O₃. The blue is interference coloring which varies with oxide film thickness. Some of the incident light passes through the oxide, reflects off the metal below and passes back out through the oxide. An observer see both this light which has passed through the oxide twice and light reflected from the top oxide surface. The thickness of the oxide controls the amount by which the light passing through the oxide is out-of-phase with and thus interferes with the surface light off the top surface.

Interference coloring is much used for coloring titanium. It also serves as a rough measurement of oxide film thickness:

http://fabweb.ece.uiuc.edu/gt/gt/gt7.aspx

The oxidation itself is exothermic but is rate-limited by diffusion through the oxide, which has a thermal activation energy.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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2007

Ken,

Thanks for the response. Perhaps I wasn't clear regarding the use of the bake oven. It is primarily used for the chrome, but is also used to blue the steel chassis of the parts in a secondary operation, again at 525 F.

Is magnetite forming in this process too, with the steel oxidizing from the oxygen in the air as opposed to the salt bath?

Luke Hagen
manufacturer (in house plating) - Portland, Oregon

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2007

Yes, up to 1058 °F (570 °C), the oxide on steel heated in air is primarily Fe₃O₄ (although amorphous at lower temperatures, so technically not magnetite), with a very thin top surface (~5% of the thickness) being hematite, Fe₂O₃. At higher temperature, wüstite (FeO) is the primary oxide.

However, the oxide formed at 525 °F, even for 1 hour oxidation, is too thin to give much bulk coloration. The interference color observed is a physical effect and is similar on all oxide-forming metals. The film thickness determines which portion of the rainbow spectrum is observed in white light. On steel, it is more commonly referred to as temper color. Described by Michael Faraday in 1822.

http://www.anvilfire.com/FAQs/temper_colors_hardness.htm
http://www.nasascale.org/howtos/termpering.htm

The blue color corresponds to a first order interference from an oxide thickness of about 0.07 micron on iron or steel. Also true for non-oxide films; e.g., Si₃N₄ seen here

http://www.ee.byu.edu/cleanroom/color_chart.parts/sin_chart.jpg

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

Is this the same mechanism that happens when you get that wonderful blue color on the chips on a lathe when cutting steel. I always have wanted to figure out how to get that on metal parts.

Michael Michalski
- Salt Lake City Utah
February 14, 2010