Quantitative aspects of grain size measurement

Grain size measurement is directly dependent on the ability of the microstr ucture to be revealed in a form that is representative of the material. A s ingle phase, equiaxed ferritic steel was used throughout the present invest igative work, this material being chosen because of the apparent simplicity of the microstructure. The lineal intercept, circular intercept, and plani metric measurement techniques were used. All the results are reported using the ASTM grain size number, G, Two aspects of grain size measurement are r eported in the present paper. The first is the impact of missing boundaries on grain size measurements. The etching techniques established within indu stry to reveal microstructures often only partially reveal grain boundaries . An experiment is reported where the impact of missing grain boundaries on grain size measurements is assessed and hence the importance of revealing all grain boundaries is determined. An image analysis system was used to co mpletely reconstruct the microstructure in a binary form, then to remove a known percentage of the boundaries, followed by measuring the grain size us ing the different techniques. The selection of the boundaries to be removed was done randomly to allow for any bias. The results reported show that, e ven with up to 20% missing boundaries, the impact on the grain size measure ment was not significant, giving a difference of similar to0.5 grain size u nits. Sampling is the second factor studied. In order for measurements to b e representative the number of grains within a field of view from each spec imen, the number of fields of view per specimen, and the number of specimen s have to be considered. From an analysis of the results of the characteris ation of the ferritic steel it was clear that the number of specimens used for measurement was the most important factor regarding microstructural rep resentation.