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Summary
While the basic principles of quantitative metallography have been firmly established, the practice of such measurements yields highly variable results. Simple laws define the statistical limitation on any measuring method. Serious errors arise from improper selection and preparation of specimens. Anticipated gradients must be encompassed by a planned sampling scheme which guarantees unbiased representation and permits measurement of actual variation in the material studied. Specimen preparation must give a truthful image of the structure. Any free choices by the operator introduce bias. The precision of measurement of one micrograph is primarily controlled by a combination of statistical uncertainty and observational error at the edges of particles. The statistical uncertainty conforms to the ideal when the apparent particle size is small, but observation becomes inefficient as the average intercept width increases and the number of particles in the field decreases. Edge errors are maximum with small particles and vary with the observer or the instrument used. Maximum precision is obtained when these two errors are approximately equal. While quantitative metallography is a statistical, rather than exact, process and subject to serious or fatal errors when practiced crudely, carefully controlled measurements can yield several structural parameters with a precision adequate to satisfy any practical metallurgist.
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Reproduction Note
Electronic reproduction. W. Conshohocken, Pa. : ASTM International, 1970. Mode of access: World Wide Web. System requirements: Web browser. Access may be restricted to users at subscribing institutions.
