MIL-DTL-13924D
3.
REQUIREMENTS
3.1 Materials. The materials for the blackening processes shall be selected by
the contractor. The selected materials shall result in black coatings meeting all
the applicable requirements of this specification.
3.2 Preparation of basis metal. Prior to the application of the black coatings,
the basis metal shall be thoroughly cleaned. The cleaned surfaces shall be free of
rust, scale, grease, oil, paint, or other foreign matter, and shall pass the water
break test described in MIL-C-53072. Cleaning materials and methods shall be at the
option of the contractor. The cleaning process shall be performed without
measurable abrasion or erosion.
3.2.1 Stress relief. Unless otherwise specified for a particular end item
specification or drawing, after forming and hardening, and prior to cleaning and
coating, objectionable residual stress in ferrous alloy parts having a hardness
greater than 40 HRC shall be relieved by suitable heat treatment. The temperature
shall be such that maximum relief is given without hardness being reduced to less
than the specified minimum. Stress relief is not necessary where it has been
demonstrated that coating has no harmful effect on the coated part.
3.2.2 Coating as a final process. Unless otherwise specified, the black coatings
shall be applied after all machining, forming, welding, cold straightening and heat
treatment have been completed.
3.3
Application of black coatings. The coating shall conform to the class
specified. The specified black coating shall be applied under controlled time and
temperature conditions. All equipment together with solutions or baths shall be
properly maintained and kept free of dirt or possible contaminants. The selected
process shall not reduce the hardness of the parts being processed or expose the
parts to temperatures in the temper brittle range of the material, nor shall it
cause embrittlement of the steel.
3.3.1 Surface attack. The process shall not result in any attack of the surface,
either pitting or intergranular.
3.4 Alkaline oxidizing solutions (classes 1 and 2). Classes 1 and 2 oxide coatings
shall be formed from a boiling alkaline oxidizing or alkaline chromate oxidizing
solution, respectively.
3.4.1
Rinsing.
All classes of black oxide coated pieces shall be rinsed in cold
water.
3.4.2 Chromic acid dip. After the cold water rinse (see 3.4.1) the pieces shall
be dipped for a minimum of 30 seconds in a 0.06 percent solution (8 oz. chromic acid
per 100 gallons water) of chromic acid maintained at a temperature of l50 to l90°F
(66 to 88°C) and a pH of 2 to 3. After the chromic acid dip, parts shall be dried
without further rinsing by using warm dry air.
3.5 Fused salt oxidizing (class 3). The temperature of the molten oxidizing salt
of class 3 shall not be higher than 900°F (482°C). After suitable immersion, the
treated parts shall be withdrawn, cooled from eight to ten minutes and rinsed in hot
water, l90°F (88°C), followed by thorough cold water rinsing to effect complete
removal of blackening solution. After rinsing, the parts shall be dried by warm dry
air and given a chromic acid dip as outlined in 3.4.2.
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