hey guys quick question when you do a cryo treatment i liquid nitrogen is there any pre cooling or anything like that or just dunk the blade in the liquid nitrogen, Alos i have heard people say to leave it in for 2 hours and some say over night????different steel different time? or just some people do it differently than others/  thanks guys!

in addition to the normal hardening procedures, after the quench, allow the blade to get to room temperature. then clean blade and then immerse in the cryo liquid. Allow temperature to equalised then, remove and allow to reach room temperature them temper.  Remember that blade and anything else in contact will be extra cold and freeze/kill any body parts that come in contact with it.  Cryo has its own PPE requirements.  Neither the liquid Nitrogen nor the dry ice and acetone substitute would last 2 hours, over night is out of the question

What you are trying to do is to "shock" the retained austenite into converting to martensite  and so time is not a factor as much as cycles; but the law of diminishing returns starts to play a major factor pretty fast.  For most of the simple steels retained austenite isn't much of a factor and so cryo doesn't buy you much but "bragging rights" and hype for customers who don't know any better.

For some of the high alloy steels it makes a difference and not using it is a warning sign that the maker may not be getting the most out of an expensive alloy.

ok thanks guys, thats good to know that the time isnt a extended period and more of a "quick bath" some of what i read says the same thing but alot of other things say the knife is left in over nite....maybe it only needs a quick soak but some people leave it in over nite either THINKING it will help or maybe just lazy?? 

A long soak won't hurt; it just won't do as much as cycling it twice down to temp.  (The swordmaker I worked under had a father that was a research metallurgist for Battelle and so we had long discussions over such things...)

Cold treatment acetone/dry ice will sometimes increase the conversion of austenite to martensite (for simple high carbon steels). You just need to get the blade to temperature as this conversion is almost instantaneous. Certain alloys (D2 in particular) benefit from cryogenic treatment. The growth of carbides and eta-carbides takes a while and is dependent upon the alloy. Some papers indicate 24-48 hours at temperature. It depends on the alloy. Lots of papers available on the net. At least one good website.