Correlative microscopy for quantification of prior austenite grain size in AF9628 steel

The characterization of prior austenite grain size is important for an understanding of the microstructure property relationships in steels. In this study, the prior austenite grain boundaries (PAGBs) were delineated in a low alloy high performance steel, AF9628, with two different methods: (i) conventional approach via etching, followed by optical or scanning electron microscopy, and (ii) analysis of electron backscattered diffraction (EBSD) data. Correlative microscopy was employed to obtain images of the same region of the specimens with the two independent methods. The threshold method of delineating PAGBs in EBSD orientation maps revealed a large fraction of the PAGBs that were delineated by the conventional approach. However, many PAGBs were not delineated in the EBSD maps. On the other hand, the twin boundaries within the prior austenite grains were not delineated by the conventional approach and many of these boundaries were delineated in the EBSD maps. The net result was that the measurements on EBSD orientation maps resulted in an overestimation of the prior austenite lineal intercept grain size by approximately 21% and an underestimation of the surface area of PAGB per unit volume of material (S-v) by approximately 17%. The errors in the measurements on the EBSD orientation maps were discussed in the context of the probability density function for misorientation angles in cubic metals (i.e., Mackenzie plot), which suggests that the overestimation in the prior austenite grain size measurement would be expected to be even higher (specifically 47%) for the general case of random texture of prior austenite grains with no twins. In view of the results of this study, it is suggested that the use of threshold method for delineating PAGBs and determining prior austenite grain size or S-v may be sufficient for examining relative trends, for example, as a result of heat treatments. However, the conventional methods to delineate the PAGBs may be required when quantitative values of microstructural parameters are of interest.