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Metallography
Metallographic sectioning for the quality control of materials and coatings as well as failure analysis, is an important technique that has been optimized by our laboratory so that it is often a crucial part of examination and investigation.
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Examination of mounted, ground, and polished metallographic sections are shown below:
Fully Annealed Copper
Fully Annealed Tantalum
Welding
Polished and etched cross sections through weldments enables defects such as lack of penetration, lack of side wall and inter-run fusion, porosity, slag inclusions to be determined. The weld profile, including possible excessive undercut or crown, and over-penetration can be evaluated. Possible weaknesses in the heat affected zone, such as martensitic hardening in carbon steels, and carbide precipitation in austenitic stainless steels can be observed.
Poor fit-up and lack of penetration in a TIG orbital weld in 316L stainless steel tube. The austenite grains have grown in the heat affected zone of the parent plate.
Castings
Defects in castings such as inter dendritic shrinkage, gas porosity, slag entrapment, and residual piping can be observed as well as microstructural details.
Coatings
The thickness, adhesion, and integrity of a coating can be evaluated under an optical microscope, while verification of coating composition can be verified using energy dispersive X-Ray analysis under the scanning electron microscope. Our laboratory has carried out this type of examination on coatings such as ion vapor deposited aluminum, electroless nickel, electroplated nickel, silver, and gold coatings. Plasma sprayed and weld deposited layers have also been evaluated.
The individual nodules growth within the coating have left a path for corrosive gas diffusion to the substrate. The deliberate peening of the coating surface was not able to close the gaps between nodules.
Weakness in ion vapor deposited coating
Trans-granular, branching cracks through the annealed grains of austenitic 316L stainless steel are typical of chloride stress corrosion cracking at temperatures above 65°C.
Fractography
Examination of metallographic sections can determine chloride stress corrosion cracking in austenitic stainless steels, hydrogen embrittlement in high tensile fasteners, fatigue in components under cyclic stress and brittle fracture of materials with poor ductility.
Failure of Gear Tooth due to Incorrect Heat Treatment
The tooth was hardened on the crown instead of on the
flanks, which are the wearing surfaces. x5 magnification
Section through tooth flank showing heavy deformation of the normalized grains. x50 magnification
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