What metallography reveals
The properties of a metal come from its internal structure. Two parts with the same chemistry can behave very differently if one was overheated, quenched too slowly or rolled from dirty steel. Metallography makes that structure visible.
Microstructure. After polishing and etching, the phases present (ferrite, pearlite, bainite, martensite and others) and their proportions show the heat-treatment condition.
Grain size (ASTM E112). Grain size affects strength and toughness. Many specifications for forgings, bearings and gears set a minimum grain size number.
Inclusion rating (ASTM E45). Non-metallic inclusions come from steelmaking. They are rated by type and severity. Bearing, spring and forging steels often have strict limits because inclusions start fatigue cracks.
Sample preparation matters
Results are only as good as the polish. Each sample is sectioned, mounted, ground and polished through progressively finer stages following ASTM E3, then etched with the reagent suited to the alloy. Poor preparation can create false inclusions or hide real ones.
Used together with
- Hardness testing, to link structure to measured hardness
- Spectro chemical analysis, to confirm the grade
- Metallurgical failure analysis, where microstructure is usually the decisive evidence
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