Hi Jeff,
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Originally Posted by Jeff Pringle
I think we have to be careful here: yes, they did not have a scientific understanding of carbon and it's specific role in how steel behaved in the quench. However, they had generations of empirical knowledege that said things like 'steel that looks like this, behaves like so;' to such a degree that when metallurgy and metallography became the way to understand steel they were checking the new theories against the eyeballing of fracture surfaces by the old foundry guys. They could give you the carbon content down to tenths of a percent or better just by looking at a quenched and broken surface. All that knowledge dissappeared in the last hundred years, but I think it's safe to assume it went back a thousand or more, and they had a good idea of how to treat the hypo- and hyper-eutectoid steels as they came from the furnace, if not precisely why.
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I absolutely agree with you that they had lots of empirical knowledge, but perhaps not quite as much as we may think. I am open to be persuaded to the contrary, but would like to know more of their methods.
All the same, they were very good. For one, I never cease to be astonished as to how Japanese swordsmiths managed to identify the high carbon steel for the edges - It was done as you say, by breaking bits of steel and examining their surface. However, we must remember that good steel or swords were the exception and not the rule, which strongly argues for a lottery factor in their methodology.
Carbon was identified as an element at the end of the 18th century and from that point on the metallurgy of steel advanced in leaps and bounds. Once an accurate analysis could be made, all sorts of indirect qualitative tests could be standardized against laboratory results and this is how those very savvy tradesmen did their seemingly unbelievable assessments. For example, if one has a good collection of steel samples of known composition then with a simple grinder spark test one can identify an unknown sample with astonishing accuracy. But without those reference samples it becomes much more difficult.
With bloomery steel made by solid state reduction, the resultant was nearly pure iron which had to be carburized. This was done by heating in a carbon rich environment and the iron absorbed the carbon. My suspicion is that although they did not know what exactly they were doing, they could correlate the end result with carburization time. But in the absence of accurate temperature and furnace atmosphere control, it must have been an uncertain process.
Here is an interesting link onto 18th century steel making:
http://www.staff.hum.ku.dk/dbwagner/....html#Heading1
Cheers
Chris