Nitrogen supersaturation process in the AISI420 martensitic stainless steels by low temperature plasma nitriding

A high-density RF-DC plasma nitriding system was employed on AISI420-J2 martensitic stainless steel at 653 K, 673 K, and 693 K for 14.4 ks. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), x-ray diffraction (XRD), and electron backscattered diffraction (EBSD) were utilized to make analysis and characterization of the nitrided layers. These layers with the thickness of 85 m from the surface was mainly nitrogen super-saturated with formation of nitrides at the vicinity of surface. The nitrogen content depth profile was nearly constant by 10 at% except for the gradual decrease from the maximum content by 30 at% at the surface and for the decay toward the nitriding front end. The lattice expansion by the strain of 1.6 % drove the phase transformation from the original martensite to austenite. High plastic straining following this elastic lattice expansion also caused the grain size refinement from the original size of 10 m down to 0.15 m.