Proper grain size determination?

[kevlar49]

Am I doing this correctly? I get an mean intercept of about 0.23 mm measured at 30x. Per ASTM E112, the G0=1=(-3.2877-6.64398log(0.23)). The Q=6.6439*log(30/100)=-3.5, so the ASTM grain size (microscopically determined) is -2.5?

 

[kevlar49]

Is there an error in ASTM E112 2010? should para A1.2.1 "...For microscopically determined grain sizes, l is in millimeters at 100x ...". It currently says 1X.

 

[Buckeyefan]

If I understand the numbers/equation correctly there is no error in the standard, 1X is correct. Using the mean intercept method you only need to use the first equation to calculate your grain size. I believe your length IS in mm at 1X, that is to say although you may view it at 30X you use the "true length" of the line and not the length that it appears to be on the screen. Therefore you are already correcting for your magnification.

I think the second equation you are using is likely used if you are determining grain size by the comparison method at a magnification other than 100X.