Hi, i've found this text at http://www.epa.gov/tri/lawsandregs/alloys/alloysreport.pdf "Types 420 and 420F, despite having a higher chromium content than type 410, do not have an appreciably higher corrosion resistance level. Type 420F is almost identical to type 420, except that there is an addition of sulfur to improve machinability. This results in a slight sacrifice of corrosion resistance." Even if it doesn%B4t say anything explicit about the F, i've also read that this is a free-cutting tool steel. May be the F stands for free-cutting. If you find something more acurate post it here, ok? Best Regards, Miscow.