Metallographic Analysis
Macroetching, which is the etching of specimens for macrostructural examination at low magnifications, is frequently used in the study of weld structure, definition of weld penetration, dilution of filler metal by base metals, entrapment of flux, porosity, and cracks in weld and heat affected zones.
Due to the portable nature of the equipment, and the high accuracy in flaw detection and sizing, UT is often deployed to detect internal discontinuities in welded plate and pipes on site, and for measuring wall thicknesses of in-service pipes, boilers and pressure vessels.
Macroetching will also provide information on variations in structure, such as grain size, flow lines, columnar structure, dendrites, etc.; and variations in chemical composition as evidenced by segregation, carbide and ferrite banding, coring, inclusions, and depth of carburization or decarburization.
The primary objective of metallographic examinations is to reveal the constituents and structure of metals and their alloys by means of a light optical or scanning electron microscope. Examination of metallographically prepared specimens are conducted using an advanced high magnification (up till 2000X) optical light microscope.
Determination of planar grain size, characterization of the two dimensional grain sections as revealed by the sectioning plane, is conducted.
This is a systematic manual point counting procedure for statistically estimating the volume fraction of an identifiable constituent or phase (e.g. ferrite in duplex stainless steel) from sections through the microstructure by means of a point grid.