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I think that your grain is a little on the course side but it can be difficult to tell some times when you get a ragged break like that. If what you were saying is true about the file skating across the edge but the blade bent to 45? then I would agree that you did have differential hardening but that would also be influenced by the make-up of the 1080. The 70? oil might be a factor too. It might seem counter intuitive but oil at that temperature cool more slowly than oil heated to around 120?-130? which could account for pearlite or at least a mixed structure in the spine.
Doug
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