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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 MTA so that it is often a crucial part of examination and investigation.
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Examples
Examination of mounted, ground, and polished metallographic sections enables the following to be evaluated:
Fully Annealed Copper
Carbon steels, stainless steels, non ferrous alloys, and reactive metals such as titanium, zircon – The microstructure of these materials can be examined for quality, mechanical processing, and heat treatment.
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 heat treat condition.
Coatings
The thickness, adhesion, and integrity over 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. MTA has carried out this type of examination on coatings such as electroless nickel, electroplated nickel, silver, and gold. Plasma sprayed and weld deposited layers have also been evaluated.
Weakness in ion vapor deposition coating. 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.
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.
Coatings
The thickness, adhesion, and integrity over 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. MTA has carried out this type of examination on coatings such as electroless nickel, electroplated nickel, silver, and gold. Plasma sprayed and weld deposited layers have also been evaluated.
Weakness in ion vapor deposition coating. 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.
Example Failure Analysis
Failure of Gear Teeth due to Incorrect Heat Treatment
Tooth hardened on the crown instead of on the
flanks which are the wearing surfaces. x5
Section through tooth flank showing heavy deformation of the normalized grains. x50.
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