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 µm 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 µm down to 0.15 µm.