[Woodcarver] performing cryogenics (freezing at 300* below zero) onwood carving tools.

[Bill Judt]

Mike:

The explanation you provided is exceptionally clear to me. Thank you  
for sharing it. Metallurgy is such a fascinating science. It's no  
wonder that talented blacksmiths enjoyed their craft and were held in  
such high regard.

Bill

My books are for sale at: http://wwwoodcarver.com/Books/Books.html
W.F. Judt,
46 Harvard Cres,
Saskatoon, Saskatchewan,
S7H3R1
PH: 306-373-6649
Email: bjudt at sasktel.net
Website: http://www.wwwoodcarver.com


On 12-Dec-05, at 6:09 AM, Mike Allen wrote:


> Hi:  A good example of cryogenic treatment is the Liberty ships in  

> WW1.  As high-carbon steel is quenched, the high temperature phase,  

> called austenite, transforms into a low (room temperature) phase  

> called martensite, which is very hard and brittle.  If you temper  

> martensite, you reduce the hardness slightly, but increase the  

> toughness or ductility (decrease the brittleness). It's common for  

> some of the austenite to not transform during cooling/quenching,  

> because the final temperature was not low enough.  Later, in use,  

> if the steel is cooled to a very low temperature, the retained  

> austenite will transform to martensite, creating very hard and  

> brittle areas.  The Victory ships broke up in the North Atlantic  

> because the freezing water temperature was cold enough to cause  

> this transformation.  This lead metallurgists to study the  

> phenomenon, which we now know as the ductile/brittle transformation  

> temperature.  For some steels, you have to go very low, cryogenic,  

> to force this transformation.  Remember though, the new martensite  

> isn't tempered and will be very brittle.  It would be better to  

> force this complete transformation when the knife is first heat  

> treated,  then tempering  would restore the toughness you need for  

> a good edge tool.  Mike

> ----- Original Message -----