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Jeff Pringle
Industry Professional
Location: Oakland, CA Joined: 19 Nov 2005
Posts: 145
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 Posted: Sat 30 Sep, 2006 8:27 am Post subject: |
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| Quote: | | Looking back to what I initially wondered/suggested, this could indicate that grain flow doesn't necessarily survive phase changes, recrystallisation and such during the forging of a blade, in which case the impact of such factors on the final properties of a blade might be neglible. |
Every time you raise the temperature of steel above the critical temperature, you start to get a whole new set of Austenite grains, and when it drops back down you get another whole new set of Ferrite/pearlite/cementite grains, so you'd think there would be no residual effects from prior deformation or grain structure. However, all the carbides don't dissolve immediately (or at all, depending on the temp & alloy), and they act as nucleation sites for the next set of grains, as do inclusions and the previous set of grain boundaries, the practical result of which is you can increase or decrease alloy banding in both hypo- and hypereutectic steels of various alloys by the way you thermally cycle during and after the forging.
Once a bar does have some variability in it, you have to hold it at a high temp for a long time so that diffusion will level things out again.
A properly forged and heat treated blade will not have grains that are enlongated in the direction of the forging, though, since the deformation occurs in the austenite several grain 'generations' back from the martensite in the finished piece. The forging vs. milling of industrial parts does not necessarily hold as an analogy for blades. Yes, they can have structures that are aligned in the direction of forging, but they ain't exactly 'grains' due to forging.
I'm no metallurgist, but I did read Verhoeven's 'metallurgy for bladesmiths' , and have made a lot of crucible steel and watched (via polishing and etching pieces during the process) the initial dendritic alloy segregation morph as the ingots get slowly forged into bar and blade, so the above should be roughly correct, if not in technical detail. 
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This is a standard topic
