What is spring tempered?

I think it is used usually in reference to differentially hardened blades. Differential hardening is a process whereby the edge is hard and the spine and tang are soft. The idea behind this is to give a tough, very sharp, very durable edge to a knife/sword that will also resist bending since the soft parts act like a spring and therefore, may sometimes be considered to be "spring tempered". Maybe I'm misinterpreting this term, though!
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This type of hardening is seen with knives by Ed Fowler and many other smiths, Sean Perkins, most Japanese-style swords, tantos, etc, traditionally made khukuris.

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If you buy a spring alloy, like 5160 that is used for car leaf springs, it is likely to come annealed. In that state if you bend it, does not spring back--it stays bent. You can heat treat it to around 45-50 RC (I don't know just where for 5160) and it will become resilient, like a good spring should (it is "spring tempered"). It will also be very tough and not inclined to crack. You can take the same steel and heat treat it to 58 RC and it is less tough, but holds an edge very well.

With 5160 you can differentially heat treat it so that the spine of the blade is more like a spring temper while the edge is more like a knife temper. You can also do multiple quench cycles to refine the grain structure and cryo quench. You can do way cool things with 5160, particularly for big knives like Kukuris.

 
First of all, this is not a term properly applied to Mad Dog knives. Kevin maintains that the term 'spring tempered' resulted from a misinterpertation of what he said over the phone to some journalist. Kevin calls his knives 'differentially tempered.' This assumes, of course, that there is only one edge on the knife. Double edged or false edged knives are NOT differentially tempered.

For a great discussion including actual methodology, check the site of the late, great Bob Engnath:
http://www.knives.com/claytemp.html

For a description, complete with pictures, of modern Japanese sword making and tempering, check Jennifer Gates Durham's site:
http://www.stanford.edu/~jgates/sword/fujiyasu/swordmk.html

If you want the complete, definitive, from iron oxide sand to computer simulations of the stresses caused by heat treatment, study of Japanese swords, their manufacture and heat treatment, go here:
http://marcia.energy.kyoto-u.ac.jp/~inoue/jsword/jsword.htm

Hope this helps, Walt
 
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