Q: Which Stainless steel bar stock is the most difficult for processing?

A: 4C13 is much harder.
Processing of stainless steel bar stock is difficult, not depends on how hard it is.
Some low-hardness stainless steel, due to "stick knife", makes them difficult to process.

Turning or milling time, 0Cr18Ni9Ti is difficult to process than 2Cr13.