Study on the active screen plasma nitriding and its nitriding mechanism

The active screen plasma and DC plasma nitriding of the low alloy steel 722M24 are investigated. Experimental results showed that the metallurgical characteristics and hardening effect on 722M24 steel nitrided by AS plasma nitriding at both floating potential and grounded potential were similar to those nitrided by DC plasma nitriding. Particles sputtered from the active screen and deposited on the specimen surface play the role of the nitrogen carrier in AS plasma nitriding. XRD and high-resolution SEM analysis indicated that the particles with sizes in sub-micron scale were Fe_xN (x > 2). Based on metallurgical analysis and Optical Emission Spectrometer (OES) experimental results, an AS plasma nitriding model has been proposed considering that AS plasma nitriding is a multi-stage process, involving sputtering, physical adsorption, desorption, diffusion and deposition.