siman said:
/\ Wrong:biglaugh:
Freezing contracts metal.
Heat expands metal.
Cryo treating metals is only to bring the grains closer together to hinder to a tolerable point (greater than that of which the stock casting could do) of expansion of the metal grains....too much expansion (from heat so to speak) causes cracks/breaks.....
Forging is pressing a cast instead of just pouring molten metal into a shaped cup. Cryo freezing is like "forging after the fact". Freezing kind of brings all that metal closer together, like that of forging
-Jonathan
please allow me to retort. heat makes metal expand, cold makes it contract, we are in agreement on this point. but cryo treatment is differnt than that. the purpose of cryo treatment is to further temper the metal. in the case of steel this means changing austenite(which all steel contains) into martensite. austenite is a solid made of carbon and iron, it is relativly soft and ductile. martensite on the other hand is hard and strong. martensite is bigger than austenite, hence steel parts that are cryo treated grow in size.
from the Machinery's Handbook, 27th edition, Industrial Press 2004, Page 545. under the heading "Stabalizing Dimentions of Close-Fitting Machine Parts":
"Subzero treatment will always cause an increase in size. Machine parts that are subjected to repeated and perhaps drastic changes in temperature, as in aircraft, may eventually cause trouble due to growth or warpage as the austenite gradually changes to martensite. In some instances, the sizes of close-fiting moving parts have increased sufficiently to cause sizure."
it continues on to say:
"A practical remedy is to apply the subzero treatment before the final grinding or other machining operation"
further info on cryo tratment can be found on this site:
http://www.supramania.com/forums/showthread.php?t=9826&highlight=martensite+austenite
3rd post down.
now Jonathan, please dont think that i am trying to flame you or anything like that, because that is not my intention at all. i mearly have incountered information that is different than what you posted. i dont think that this post is the final word on cryo treated pistons, in fact i would still like an answer to my other post. i will even go as far as to point out a hole in my own post, that is that the machinery's handbook is talking about cryo treating STEEL, and the 7m pistons are aluminum. so if anyone could present evidence that aluminum contains no austenite, then that would prove that cryo treated pistons do not nessisaraly grow in size after treatment. so if anyone would like to explain it would be greatly appreciated.
