Martensite is an acicular, needle-like crystal with body centered carbides.
In the "as hardened" condition (prior to tempering) the steels in question are very hard, ranging from HRC 61-68. Obviously, at this level of hardness, it should go without saying that they are also extremely brittle. Even the much favoured ATS-34 and 400 series with their retained austenite (as much as 30%) and to a lesser extent Sandvic 12c27 can suffer. This is why it is essential to properly temper the steel, reducing the gross hardness, while generally rendering greater toughness and shock resistance.
Sandvic 12C27 is yet another excellent stainless steel from Sweden where its base metallurgic content is proof of the extremely clean source ore from which it is smelted at point of manufacture (Carbon 0.6%, Manganese 0.35%, Chromium 14.0%, trace Sulphur and Phosphorus).
The Cryogenic Quench is a modern addition to heat treating tool steels. After the normal heat treat process the steel temperature is lowered further using a controlled liquid nitrogen soak. The duration of soak time and the number of tempering steps undertaken can greatly affect the final balance of hardness to wear and shock resistance.
If you need specifics for particular allows feel free to ask since the above is a very general overview of your question.