To explain that diagram for those new to the metallurgy involved:
The diagram is called a continuous cooling curve. The most important word is continuous. It should have as few delays as possible.
A is the austenitization point. In most stainless knife steels it is around 1060C/1950F.
Q is the quench. The rate of drop varies from almost straight down for 1095 to very slow for some high alloy stainless steels. What is required is that the drop take if past the pearlite nose at 540C/1000F while the structure is still austenite.
At that point the structure becomes supercooled austenite. The term supercooled does not mean it is cold ... it is still 480C/900F ... but that the structure would have changed to pearlite if cooled slower. Basically, we tricked it into staying austenite. The structure will remain supercooled if the rate of cooling stays at a medium fast speed. The exact speeds can be found on the metallurgical charts for each steel.
When it hits the Martensite start point ( the white line crossing about two thirds down the Q line) it will suddenly start to convert to martensite - called the Ms point.
The cooling from there converts more and more austenite into martensite. At room temp, 20C/70F, all the austenite has converter for carbon steels, but only about 80-90% for stainless steels. The small rest is to fully allow the stel to equalize at room temperature. On some very delicate parts, there is a short snap temper done at 150C/300F at this point, but for most all stainless knife steels, this isn't necessary.
DF is deep freeze. The cooling curve then continues down into the sub-zero range. At -20C/-5F most of the austenite has converted to martensite. There may be 5% to 8% left, called retained austenite ... RA. The knife blade would work OK, and the hardness is about one Rockwell point higher than it was at room temp.
If you continue the cooling by taking it down to the martensite finish point ... Mf, at -70C/-95F, At this point, all the austenite that will convert has. It will have gained another Rockwell point of hardness, too.
( The Mf for carbon steels is around 100C/200F and well above room temp, which is why a deep freeze treatment does nothing to harden the blade more. There are a few carbon steels that have a lower Mf, but they are still in the room temp range.)
T is the temper. The blade is warmed to room temperature and then placed in an oven at a point near the Ms. This is usually a bit above the Ms, between 200C/400F and 230C/450F. The blade is baked for an hour or more, cooled to room temperature, and re-tempered again. During these tempers, the small amount of retained austenite is converted into martensite, which makes for a harder blade. On some high RA steels, it is given a third temper. During each temper, some of the RA converts into new brittle martensite as it cools to room temp. The second temper will temper this new martensite. Usually, there is a minute amount of retained austenite that will not convert, called stable austenite. This is not actually a bad thing, as it adds a tiny amount of extra toughness.
CRYO, is not shown n the graph, but it is done by taking the blade past the Mf during the sub-zero cooling, and into the the super cold region around -250C/-400F. This creates structures that will become very tiny eta-carbides during the temper cycle. This will increase the blade hardness by one more Rockell point.You only have one shot at making these structures, and a subsequent cryo will do little to increase the hardness.
