Four martensitic stainless steels: AISI 410, 420, 422, and 440C, were solution nitrided in a vacuum furnace at 1149^oC (2100^oF), 1121^oC (2050^oF), and 1093^oC (2100^oF).  The nitriding results were evaluated by chemical analysis, microhardness measurement traversals, and optical metallography on specimens that were nitrided at pressures of 150, 300, and 630 torr for a nitriding time of 12 hours.  Surface nitrogen contents were determined.  The values of the surface nitrogen contents and Sievert’s Law were used to calculate the surface nitrogen content for various nitriding conditions.  After nitriding, specimens were given an in-furnace 10 bar nitrogen quench.  The nitrogen quench provided rapid cooling under a protective nitrogen atmosphere.  This facilitated keeping the nitrogen in solution and thereby controlling nitride precipitation.  After quenching the case (surface) structure was martensitic and/or retained austenite.  The core structures observed were either martensite or martensite–austenite.  The resulting case and core structures depended on alloy composition, the amount of dissolved nitrogen, and cooling rate.  The effects of nitriding pressure and temperature on surface nitrogen content and case depth were determined.  In addition, the effects of nitriding conditions on case and core hardness were established.  Processing treatments were developed to produce either the presence or absence of nitrides in the nitrided surface layer.