Macroetching, also known as deep etching, involves etching specimens prepared with a suitable acid or reagent for macrostructural examination at low magnifications and rating by a grades series of photographs showing the incidence of certain conditions such as: cracks, pipe, center voids, center unsoundness, pinholes, porosity, white band, chill structure, dendritic structure, inclusions, hydrogen flakes, segregation, banding, grain size, mold slag, and other discontinuities or defects such as laps and seams.
Other applications of macroetching include:
- Development of grain flow in a forged part or fastener, which is a critical property for the integrity of the forging.
- Study of weld structures, definition of weld penetration, dilution of filler metal by base metals, entrapment of flux, porosity, and cracks in weld and heat affect zones, etc.
- Evaluation of heat treated parts to determine locations of hard or soft spots, tong marks, quench cracks, case depth in induction hardened or carburized parts, and effectiveness of stop-off coatings.
- Evaluation of machined parts for grinding cracks in tools and dies.
The most common reagent for macroetching iron and steel is a 1:1 mixture, by volume, of concentrate hydrochloric acid and water. Other reagents included: sulphuric acid, ammonium persulfate, and nitric acid.
Macroetching Examination Specifications
- ASTM E381: Standard Method of Macroetch Testing Steel Bars, Billets, Blooms, and Forgings
- ASTM E340: Standard Test Method for Macroetching Metals and Alloys