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Heat treated electroless nickel coloring as gun finish.

January 12, 2009

I am in the process of opening a gunsmithing/firearms restoration shop, and I am developing a finish for gun frames that resembles the rainbow effect of case hardening. However, I do not want to get into heat treating specifically, as that that level of metallurgy is beyond my skill set, so I want to develop a lower temperature finish that can be applied to all gun steels indiscriminately. That said, my research and limited experimentation has led me to heat treated electroless nickel, and my questions are as follows:

(1) As I understand it, the rainbow-like discoloration of EN is a function of temperature; does anyone have a chart or list that shows the relationship of specific colors to specific temperatures?

(2) Does the phosphorous content of EN affect coloration and the temperature needed for a specific color?

(3) Can EN plated aluminum parts be heat treated to the point of coloration?

(3) Can the heat treating be applied by molten potassium nitrate (500-625°F nitre bluing tank bath)? Or will the bath negatively affect the nickel in some way? To get the rainbow effect, will I need to quench the part, or will air cooling be sufficient?

(4) And finally --but probably most importantly-- are the temperatures required for EN coloration high enough to affect the hardness and/or strength of the underlying steel to any significant degree?

Michael Broerman
gunsmith - Salmon, Idaho

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First of all, EN is an engineering coating, not a decorative one. That said:
1. I very much doubt there is a chart for colors for the same reason.
2. Not only P content but contaminants or gases inside the furnace or on the EN surface may affect its color.
3. Yes. To attain a specific color maybe not.
4(3). Never tried it.
5(4). For a decorative finish on a collection firearm which is not a matter of life or death, I am sure EN+HT is quite safe over most steels. Some treated, high carbon high strength steels may suffer hydrogen embrittlement, softening or even cracking during temperature change.

Guillermo Marrufo
Monterrey, NL, Mexico
January 15, 2009