Quality Control

Process quality is monitored by testing of coupons or test pieces which are run in the retort with the production materials during the process cycle. These test pieces are used to determine quality and depth of aluminum diffusion, and should be of the same or similar base material as the production parts. These pieces should be located in the hottest and coldest sections of the furnace loads. These locations should be determined prior to actual processing of parts through furnace surveys.

After the processing has been completed, the test pieces are removed from the retort and examined according to ASTM C 664-87, "Standard Test Methods for Thickness of Diffusion Coating." There are two procedures for measuring the thickness of diffusion coatings. Method A is the determination of the dimensional-change thickness, defined as the difference in the part before and after coating. This method employs a micrometer. Method B is the determination of total coating thickness, defined as the distance between the observably unaffected substrate and the exterior surface of the coating. This method employs standard metallographic specimen preparation equipment for mounting, polishing, and etching as well as a metallographic or optical microscope for observation of the diffusion layer at 100* and 500*.

Quality control reporting should include the maximum, minimum, and average coating thickness (in mils) of the individual measurements made on a specific part of the specimen surface. If Method B is used, a photomicrograph showing the bounds of a typical area of the coating is recommended. Depending on base materials and processing conditions, thicknesses from 1 to > 40 mils (25 to 1000 pm) can be produced.

Additional mechanical testing may also be appropriate for processed materials which may undergo mechanical property changes due to the temperatures encountered in the process cycle. This testing could include hardness/microhardness measurements, as well as tensile testing to determine yield strength, ultimate tensile strength, and percent elongation.

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