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Old 3rd December 2006, 07:15 PM   #2
Jeff Pringle
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I think those lines are the remnants of the primary dendrites, the first steel crystals to solidify from the melt. If the melted steel is cooling very slowly through the liquid/solid transition, the 'trees' can get pretty big. They make it through the forging process to one degree or another depending on size, how much elongation happens and if the ingot is heat treated to diffuse the alloy content or not.
Figure 1c on page twelve of Figiel's "On Damascus Steel" shows an etched ingot, the dark lines you see on that face are the primary dendrites.
I'll see if I have a photo to post...
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