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Metal Casting Technologies : Dec 2009
46 www.metals.rala.com.au BacktoBASICS Carbon and low alloy steel 2% Nital -- Add 2ml Nitric acid to 98 ml alcohol (ethanol or methanol). Used by dipping or swabbing from a few seconds to 1 minute. For carbon steels, this reagent gives maximum contrast between pearlite and a ferrite or cementite network. It also reveals ferrite boundaries and differentiates ferrite from martensite. Picral - Add 4 gm Picric acid to 100 ml ethyl or methyl alcohol (95% or absolute). Etch by dipping or swabbing for a few seconds to 1 minute or more. This reagent is not as good as Nital for revealing ferrite grain boundaries, but gives superior resolution with fine pearlite, martensite, tempered martensite and bainitic structures. Detects carbides. Vilella's Reagent -- Add 5 ml HCl, 1 gm picric acid to 100 ml ethyl or methyl alcohol (95% or absolute). Etch by immersion until structure is revealed. This reagent is used for revealing austenitic grain size in quenched and quenched and tempered steels. Picric Acid -- Add 2 ml HCl and 1 gm picric acid to 100 ml alcohol. Etch by immersion until structure is revealed. This reagent is used for etching retained austenite and the delineation of the prior austenite grain size in quenched and tempered steels. Aluminium alloys Hydrofluoric acid -- Add 1 part HF (48%) to 200 parts of water. Etch by swabbing or immersion. This reagent is a general purpose etch for aluminium alloys. It outlines light-tone constituents and grain boundaries. Keller's Reagent -- Add 5 parts HNO3 (conc), 3 parts HCl (conc), 2 parts HF (48%) to 190 parts of water. Etching is by immersion for 10 to 30 seconds followed by washing in warm water and blow-drying. Do not remove etching products from surface. With copper-containing alloys, this reagent reveals dendritic segregation (coring) before solution heat treatment, grain contrast and absence of coring after solution heat treatment, reduced grain contrast and presence of precipitate after precipitation heat treatment. It outlines grain boundaries and constituents and increases contrast of some constituents. Figure 6. Sulphuric acid -- Add 20 parts H2SO4 (conc) to 80 parts of water. Heat solution to 70oC and immerse for 30 seconds. Rinse in cold water. This reagent darkens Fe-containing constituents. Caustic etch -- Add 1 gm NaOH to 100 ml water. Etch by swabbing for 5 to 10 seconds. Constituents Al2Cu or Al8Mg5 or not attacked and outlined. Copper alloys Ammonium peroxide -- Add 20 ml NH4OH, 8 to 20 ml H2O2 (3%), 8 to 20 ml H2O. Use by dipping or swabbing for up to 1 minute. This is a general-purpose etchant for most alloys and should be freshly made. Reveals coring in bronzes. Ammonium sulphate -- Add 10 gm (NH4)2S2O8 to 90 ml H2O. Application and purpose are similar to above. Alexander's reagent -- Add 30 ml acetic acid (75% aq.), 20 ml HNO3, 30 ml acetone. Apply by dipping or swabbing for up to 20 seconds. This reagent is used for complex aluminium bronzes, giving preferential etching and defining various phases. Acidic ferric chloride -- Add 5-20 gm Fe Cl3, 5-50 ml HCl, 100 ml H2O or ethyl alcohol. Apply by immersion or swabbing until structure revealed. This reagent darkens beta phase in alpha- beta brasses and aluminium brass. Figure 7. Safety precautions All chemicals are potentially dangerous; formulating and using etchants requires thorough knowledge of the chemicals involved and the proper procedures for handling and mixing. ■ Acknowledgement The photomicrographs appearing as figures 6 and 7 are by J. Adamowski and Dr R. F. Cochrane respectively and are copyright DoITPoMS Micrograph Library, University of Cambridge Fig 6. Al-Mg-Si alloy -- etched with Keller's Reagent Fig 7. Phosphor bronze -- Ferric Chloride Reagent REFERENCE 1. Australian Standard AS 2205.5.1-1997 2. ASTM Handbook- Vol. 9. Metallography and Microstructures, ASM International, 1985. 3. Aluminium Casting Technology- 2nd Edition, American Foundrymen's Society, 1993. 4. Typical Microstructures of Cast Materials -- The Institute of British Foundrymen, IBF Publications, 1981.
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