The multifaceted fracture surface that you describe sounds familiar. I've
looked at quite a lot of blister steel and have also done a few
cementation experiments using 19th century British and Swedish wrought
iron. Contrary to popular belief, it is possible to carburize the wrought
iron all the way through. Quite a few pieces of historic blister steel
that I've analysed did not have a distinct low carbon core (aka sap),
these were the bars with more than 1% carbon content. The method commonly
used in Sheffield to produce high carbon blister steel was to put the bars
through two cycles of cementation, producing what was known as 'double
converted' steel. Obviously, this steel was too hard to forge, but it
formed the feedstock for the crucible furnaces.
I think the reasons why the Sheffield steelmakers used 'double conversion'
was to fit in their production cycle and to avoid taking the furnaces much
above 1100 degrees C. Maybe the guys at Williamsburg were running a
slightly higher temperature and/or a longer time period than that used in
Sheffield?
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