Which steels can be nitrogen quenched? I have been making knives as a hobby mostly for myself and friends for some time now. I have been using mostly carbon steels such as UHB20C/1095, O1, 5160. I want to step up my game with more exotic steels such as AEB-L, RWL-34, ELMAX, D2 and N690Co.

Due to lack of dedicated equipment I found a Steel Heat Treatment company that will help me with quenching, cryo, and tempering. The only problem is that they only offer 'pressurised nitrogen quenching'.

Could you please tell me which of the stainless steels above can be quenched this way?
I could not find any information on Bohler and Uddeholm websites regarding this method.
Any martensitic stainless ( all cutlery grades ) can be cooled / quenched in nitrogen gas. I presume it is gas and has significant circulation to speed cooling . The cutlery grades are all have very good hardenability and do not need rapid cooling. In my experience , nitrogen gas cooling would be very unusual ( not needed ) for martensitic stainless. It would be useful for quenching titanium ( type 5 , 6 Al , 4 V ). If you mean liquid nitrogen , no problem , probably . The very rapid cooling can cause cracking of complicated shapes. Various claims are made of the advantages of cryogenic cooling, some are true . But the possible advantages would not show up in a personal knife; they would show up in an industrial application cutting many tons of material per day.    HOWEVER , heating / austneitizing in a nitrogen atmosphere can be dangerous. Nitrogen can be absorbed into the metal and  stabilize the austenite phase producing a soft surface.