Effect of Pulse Repetition Time on Surface Properties of Pulsed Plasma Nitrided AISI 4340 Steel

Abstract

Background/Objectives: Pulsed Plasma Nitriding (PPN) process is carried out on AISI 4340 steel at constant temperature and time. The key parameters which influence the nitrided compound layer properties in PPN are Pulse Duration (PD), Pulse Repetition (PR) time and Gas ratios of Hydrogen and Nitrogen. To obtain tailored properties of nitrided compound layer by varying the PR time and study the Microstructure, Crystalline phase formation and Wear properties and optimize the process parameters. Methods: In PPN process, PR time was varied from 100 μs to 500 μs and PD duration was fixed to 50μs. The nitrided samples were characterized by Optical Microscope, X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Pin on Disc test. Findings: The decrease of PR time increases the formation of Iron Nitrides (Gamma nitride: γ-Fe4N and Epsilon Nitride: e-Fe3N) and it decreases when PR time is increased. Samples treated at PR-300 μs and PR-0 μs are observed with strong and weak intensities of Chromium Nitrides (Cr2N) respectively. The surface hardness is increased from 250±20 HV1 to 650±50 HV1, and the wear properties have increased by three times higher than base material. The nitrided properties are strongly influenced by PR time and the formation of Iron nitrides is inversely proportional to PR time. Novelty/Improvements: This investigation suggests that the PR time is a critical parameter for optimization of nitrided compound layer properties and the surface properties of AISI 4340 steel is improved by PPN process.