Optical 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.

Examination of mounted, ground, and polished metallographic sections enables the following to be evaluated:

 

• Carbon and stainless steels – The microstructure of austenitic stainless steels such as 304L, 316L, the precipitation hardened stainless steels such as 17/4, and 17/5 PH, ferritic and martensitic stainless steels can be examined for steel quality, heat treatment.   

 

• 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.

 

• 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.

 

• 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.