MIL-STD-867 Temper Etch InspectionThis standard establishes requirements and test for performing temper etch inspection of carburized steels and tool and low alloy steels, which are heat treated above 1.24 x 109 Pascal PA 1.26 x 102 kilogram - force/square millimeter Kgf/mm2 or 1.8 x 105 pound - force/square inch psi. This standard applies to aircraft and missiles structural and propulsion system components when MIL-HDBK-6870 for guidance only and/or MIL-STD-866 are reference documents in the solicitation or contract.

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MIL-STD-867C

5.4   Acid etches visual inspection.  Inspection of etched surfaces shall be performed under light resulting in a minimum of 2153 Lux (200 foot candles) at part surface.

5.4.1   Rejection criterion.  The presence of the following indications shall be cause for rejection.

a.   Un-tempered martensite as evidenced by light gray to white areas surrounded by a light brown to black border.

b.   Over-tempered areas as evidenced by light brown to black indications darker than surrounding areas.

c.   Acceptance/rejection standards.  The presence of the following indications shall be cause for review to established acceptance/rejection standards (See:

4.1.3.1).

(1)   Non-carburized/de-carburized areas.  Non-grinding related: These areas will appear unusually lighter in color than adjacent carburized areas.

(2)   Improper carburization.  Non-grinding related:  These areas are isolated or complete areas of carburization not required by blueprint.

5.4.2   Acceptable color.  If no damage was produced by the pre-production operations, the surface will be a uniform gray.  This discoloration is not detrimental to part usage.

5.5   Baking.   After the final etch, all parts processed by etch methods in

TABLES  I, II and III shall be baked at 191 ± 14°C (375 ± 25°F).  Under direction from the applicable engineering organization the temperature of this bake may be increased to a maximum of 28°C (50°F) below the final tempering temperature of the part.  The bake time shall be a minimum of four (4) hours.  Such baking shall be initiated within eight (8) hours of initiation of the etch inspection process and before any plating operations.

Parts processed by etch methods in TABLE IV will not require a bake.  All furnaces used shall be certified to SAE-AMS-H-6875.

5.5.1   Bake temperature limitation for cold worked parts.   Parts that have been cold worked (e.g. shot peen, cold rolling) shall be limited to the 191 ± 14°C (375 ± 25°F) bake temperature.

5.6   In-service quality checks.

5.6.1   Solution concentrations.  Process solutions shall be analyzed at a frequency determined to assure the solutions are maintained in accordance with the limits of this specification.

5.6.2   Sensitivity.  A standard test piece, which contains representative grinding burns will be processed each day that the etch is operated to verify that the etching solutions and techniques are adequate to detect grinding burns.  The test piece shall be either 300M steel heat treated to 280 - 300 KSI ultimate tensile strength or 4340 steel heat treated to 260 - 280 KSI ultimate tensile strength or as specified in the contract.

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