Metallography reveals a metal’s grains and phases by cutting a small sample, grinding it flat through progressively finer abrasives, polishing it to a mirror and etching it briefly with a dilute acid that attacks grain boundaries and different phases at different rates. A hand lens of 11 times magnification shows grains of 0.040 mm, but finer structure such as pearlite or martensite needs a microscope. It is the only test that shows why a part is hard, soft or brittle, because it shows the structure itself.
Part of Testing and Quality
Metallography is the preparation and examination of polished and etched metal cross-sections to observe grain size, phases, case depth, inclusions and defects.
What The Structure Tells You
A prepared section shows what no external test can:
- Grain size. See Grain Boundaries for why it matters.
- Phases. Ferrite and pearlite in a slow-cooled steel, martensite needles in a quenched one, as in Phase Diagrams.
- Case depth after Case Hardening.
- Cold work or recrystallisation, which separates a properly annealed piece from one that was not.
- Slag inclusions, pores and weld defects, the flaws of bloomery iron and cast metal.
What You Need
- A hacksaw or fine saw, for cutting without overheating the sample (keep it cool, as heat changes the structure you are trying to see).
- A flat, hard surface for grinding, and abrasive papers in coarse to fine grades, or sand sorted into grades.
- A polishing cloth or leather with rouge (iron oxide), as used for lenses in Lenses.
- Etchant and glass or ceramic dishes.
- A lens or microscope.
- Water, clean cloth and alcohol.
Method
- Cut a small piece through the region of interest, with water or oil cooling.
- Grind flat on the coarsest paper. Then change to the next finer paper and rotate the sample 90 degrees, grinding until all scratches from the previous stage are gone. Repeat through every grade, rinsing the sample between grades so coarse grit is not carried forward.
- Polish on cloth or leather with rouge and water until the surface is a scratch-free mirror. Rinse and dry with alcohol.
- Etch. The Advanced Materials article lists acids and alkalis for etching. For steel the standard etchant is a few percent nitric acid in ethanol, called nital. Dab or immerse the sample for a few seconds, rinse at once in running water, then alcohol, and dry. For copper alloys a ferric chloride solution is used. The surface should turn from mirror to a faint matte grey.
- Examine under a lens or microscope. Re-polish and re-etch if the result is unclear.
Seeing Grains: Magnification And Scale
The Advanced Materials article’s magnifying glass gives a visual magnification of M = 1 + D / f, with D = 250 mm (see Magnifying Glass). A lens of focal length 25 mm gives M = 1 + 250 / 25 = 11. Grains of 0.040 mm then appear 0.040 × 11 = 0.44 mm across, large enough to see. Grains finer than about 0.01 mm need a compound microscope.
Worked example, grain size by intercepts: at 100 times magnification, a line drawn 100 mm long on the image covers 100 / 100 = 1.00 mm of metal. It crosses 25 grain boundaries. The mean intercept length is 1.00 mm / 25 = 0.040 mm, or 40 micrometres. Repeat on three lines in different directions and average the results. A smaller number means finer grain and a stronger steel, as the Hall-Petch relation in Grain Boundaries explains.
Calibrate scale with an object of known size in the same view, for example a fine wire of measured diameter, since magnification estimates are rough.
Macro-Etching Needs No Microscope
Cutting through a forged billet or a weld, polishing it flat, and etching shows flow lines, weld shape and large defects to the naked eye or a lens. It also shows the case in a carburised part as a different shade at the edge.
Etchants are corrosive and some are unstable
Nitric acid in alcohol burns skin and eyes and gives off toxic fumes. Mix small amounts fresh, adding the acid slowly to the alcohol, use the low end of the usual few-percent range, keep it in an open container and never store it in a sealed bottle, since it can build pressure and become hazardous as it ages. Do not use stronger solutions. Wear eye protection and gloves, work in a ventilated area away from flame, and rinse spills with plenty of water. Avoid picric acid etchants, since dry picric acid is explosive. Cutting and grinding make sharp edges, and dust from abrasives should not be breathed.
Common Mistakes
- Overheating while cutting — changes the structure you want to study.
- Skipping a grade of abrasive — deep scratches remain through polishing.
- Over-etching — a black, featureless surface; re-polish and try a shorter etch.
- Looking at a smeared surface — soft metals can smear and hide grains.
What’s Next
- Learn next: Grain Boundaries — reading what you now see
- Combine with: Tensile Testing — linking structure to strength