MadSci Network: Engineering

Re: Will you increase the density of steel if you fold it in production

Date: Fri Oct 31 13:24:02 2003
Posted By: Jeff Yap, Materials Engineer
Area of science: Engineering
ID: 1066851473.Eg

Hi Jeremiah,

Youíve asked a great question, and Iím glad youíre investigating 
information your friends tell you before you accept it as fact.  The short 
answer is that your friend is wrong, but he has the right idea.

The folding he is talking about is an ancient metalworking technique 
that was perfected by Japanese swordsmiths.  They would fold a 
piece of metal hundreds, sometimes thousands of times, and this 
would result in a strong, flexible, yet hard sword that would be very 
good at keeping a sharp edge.  There is quite a bit of metallurgical 
detail, history, and experimentation behind the process, but the short 
version is that they formed swords composed of super-thin layers of 
steel that had soft, flexible cores with hard, sharp edges.  

However, in the situation you describe, the density of the steel will not 
increase.  (Unless during the folding process, you accidentally add 
carbon or make some other change to the material itself.)  The crystal 
structure and the grain size (and thus the strength) will change 
depending on how quickly and to what temperature the material is 
heated (casting, tempering), how quickly the material is cooled 
(quenching), and how the material is mechanically deformed 
(peening, rolling, hammering, drawing, and folding).  Folding the 
material many times under the right conditions will indeed make the 
steel stronger, but the density will not change.  (in the real world, 
each time you heat up and hammer a piece of metal, you'll lose 
some material to slag and evaporation, so your bar of metal will not 
entirely fill the 1'X2'X3' volume but it will also have lost mass, so the 
density won'y have changed.)

If youíre interested in learning more about metallurgy or Japanese 
swordmaking, try looking online.  Here are some links.

Heat treating steel

Heating / Quenching


Work Hardening

Japanese Swordmaking History
Japanese Swordmaking History

Japanese Swordmaking History 2

How Stuff Works - Swordmaking

I hope this helps.  Keep asking questions!

Jeff Yap
Mad Scientist

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