Macroetching
Metallographic specimens and chemical analyses will provide the necessary detailed information about specific localities but they cannot give data about variation from one place to another unless an inordinate number of specimens are taken.
Macroetching, on the other hand, will provide information on;
- variations in structure, such as grain size, flow lines, columnar structure, dendrites, and so forth;
- variations in chemical composition as evidenced by segregation, carbide and ferrite banding, coring, inclusions, and depth of carburization or decarburization. [1]
- the presence of discontinuities and voids, such as seams, laps, porosity, flakes, bursts, extrusion rupture, cracks, etc.
- 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, etc.
- the location of hard or soft spots, tong marks, quenching cracks, case depth in shallow-hardening steels, case depth in carburization of dies, effectiveness of stop-off coatings in carburization, etc.
- the determination of grinding cracks in tools and dies.
Macroetching is used extensively for quality control in a number of industries;
- the steel industry, to determine the tone of a heat in billets with respect to inclusions, segregation, and structure.
- forge shops to reveal flow lines in setting up the best forging practice, die design, and metal flow.
- foundries to determine the presence of internal faults and surface defects.
- copper industry uses macroetching for control of surface porosity in wire bar.
- aluminium industry, use macroetching to evaluate extrusions as well as the other products such as forgings, sheets, and so forth to identify coring, cracks, and porthole die welds defects.