Black oxide / blackening steel heat treatment

A. Hi Naveen. I don't know much about heat treatment, but it doesn't sound like you're actually talking about heat treatment anyway. "Blackening" and "black oxiding" are slang, not a specification, but in this context they are probably the same thing, and are achieved by putting the parts into an aqueous based solution of sodium hydroxide and oxidizing/blackening agents such as nitrates at about 230°F.

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A. a) Black Oxide is one of several Blackening processes. These are not normally considered heat treatments, and several processes form a black coating (e.g., copper selenide) on steel at room temperature. The hot black oxide process described in MIL-DTL-13924 [on DLA] does not affect the metallurgical properties of many steels and hence is not considered heat treatment. It does involve limited heat treatments, e.g., stress relief and hydrogen relief bake-out, for hardened steels (> 40 HRC).

b) For this 'Mission Impossible' use a force-field from Star Trek. The corrosion resistance of black oxide is primarily due to a supplemental oil or wax. Good protection may require measurable wax or lacquer thickness. Black chromium (electroplated, ~0.00001"), is more protective and can also be oiled. It is normally applied over a chrome or nickel basecoat, but can be directly deposited. Use the 'Jobshops' link below to find a finisher with whom you can discuss your options. In any case, inform the metal finisher of the lead content of your steel, as additional pretreatment is normally required.