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An improved procedure based on the intercept method of measuring the grain size of single-phase microstructures has been developed that provides a quantitative description of the grain size yet is fast. The procedure combines the statistical advantage of using large numbers with the advantage of interpreting the data as a normal distribution, as verified by the chi square test. Application of the procedure to ferritic microstructures representative of best and worst case conditions indicates that accuracies on the order of 3 percent at a 95 percent confidence level can be achieved. In addition, the procedure is sensitive enough to distinguish a randomly mixed duplex grain structure. The measurements associated with a two-phase microstructure can be more precisely quantified through determination of volume fractions using a Pp measurement based on the use of an appropriate grid network, the coefficient of variation statistic, and the Poisson distribution. The analysis is demonstrated for an a priori system for which the percent accuracy and confidence level can be specified for the volume fraction measurement simply from calculation of the average value for Pp.
metallography, microstructure, grain structure, grain size, quantitative analysis, statistical analysis, normal density functions, chi square test, ferrite
Research Engineer, Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, Pa.