A process of preparing metallic specimens and welds for macroscopic or microscopic examination.
In examining for exceedingly small discontinuities or for metallurgical structure at high magnifications, specimens are polished, etched and examined by microscope to reveal the microstructure of the base metal, heat-affected zone, fusion zone, and weld metal. Procedures for selection, cutting, mounting and polishing metallographic specimens are given in
ASTM E3 Standard Methods of Preparation of Metallographic Specimens. Recommended chemical solutions for etching various metals and alloys and safety precautions in handling etching chemicals are given in ASTM E407, Standard Method for Micro Etching Metals and Alloys.
Preparing Specimen for Examination
A weld can be etched by filing a top-to-bottom section of the weld with a coarse file, followed by finer files. File marks should crisscross the previous file marks. After filing, the surface should be polished with emery paper, starting with No. 1 and ending with No. 000 to achieve a high polish. The polished surface must be kept free of fingerprints, grease or dirt. After polishing, an acid solution (nitric acid and water for
iron, steel, copper, brass and bronze; hydrochloric acid and water for aluminum) is applied with a brush to the clean surface. Etching reagents for various metals are
shown in Table E-4.
This solution immediately attacks the oxide, exposing any adhesion, blowholes, lack of metal, or other irregularities. The maximum time for the solution to act is fifteen minutes. The etched surface is then washed with water and dried with alcohol.
The test should be preserved by wrapping it in a layer of transparent polyurethane, or a similar protective covering, for subsequent microscopic inspection.