Stereological Point Counting Protocols for Tool Steel Phase Fractions
Systematic ASTM E562 manual point counting protocols quantify tool steel carbide and austenite volume fractions within verified statistical error boundaries.
LePera Etchant
Meaning
Metallographic tint etching reveals phase distributions in high-strength steels by depositing thin tint films of varying thickness onto constituent microstructures. Chemical formulation for lepera etchant relies on sodium metabisulfite and picric acid dissolved in water to differentiate martensite from ferrite. Material labs use this chemical treatment to measure phase fractions in battery structural frame welds and die-steels.
The action stops when constituent layers reach optical thickness thresholds, failing to tint non-ferrous alloys or fully austenitic structures.
Phase Contrast
Microstructural constituents absorb reactive sulphur ions at different rates depending on local crystal orientation and carbon saturation. Using lepera etchant produces white martensite alongside tan ferrite under standard light microscopy. Quantitative image analysis depends on this color differentiation to compute transformed phase percentages without scanning electron instrumentation.
Ferrite Distinction
Retained austenite appears clear alongside un-tempered martensite, requiring cross-verification when both constituents inhabit the same sample. Microstructural examination of structural battery enclosures confirms whether thermal cycles during laser welding generated brittle quenched zones. Accurate identification prevents premature field failure in load-bearing frame assemblies.
Etching Duration
Immersion time governs film deposition thickness and optical reflectivity across the polished specimen face. Over-etching darkens all iron phases simultaneously, erasing color differences between adjacent grain boundaries. Proper application requires fresh chemical preparation because aqueous sodium metabisulfite degrades within hours through atmospheric oxidation.