Practical Applications of Quantitative Metallography

Book excerpt: 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.