Cathodic cage plasma nitriding of austenitic stainless steel (AISI 316): influence of the working pressure on the nitrided layers properties

Abstract

In this work, the influence of working pressure on the characteristics of the layers produced by the nitriding treatment on the AISI 316 austenitic stainless steel surface using the Cathodic Cage Plasma Nitriding technique (CCPN) is assessed. The treatments were carried out at a temperature of 723 K for 5 hours under working pressures of 120, 250 and 500 Pa. The morphology, microstructure and corrosion resistance were studied through optical microscopy, x-ray diffraction, and electrochemical potential curves. We analyzed the effects of working pressure on the layer thickness, microhardness and corrosion resistance. The E×i curves and the electrochemical impedance of polarized samples were measured in order to investigate the effect of the ion transfer inside the produced film placed into the anodic solution. The electrochemical impedance of the films were measured, and showed an apparent capacitive behavior. For treatment under a pressure of 120 Pa we found the nitrides Fe 3 N, Fe 4 N and CrN. For treatment at a pressure of 250 Pa, the CrN disappears and the observed phases are the nitrides Fe 3 N, Fe 4 N and the S-phase. For a treatment pressure of 500 Pa there is an increase in the corrosion resistance of nitrided layers, confirmed by the predominance of S-Phase in the x-ray diffraction pattern. On the other hand, the sample nitrided under the pressure of 250 Pa showed very unstable impedance behavior when immersed in NaCl solution, indicating that the treatment at this specific working pressure has a negative effect on the corrosion resistance of plasma nitrided AISI 316 stainless steel surfaces.

Study of plasma nitriding and nitrocarburizing for higher corrosion resistance and hardness of 2205 duplex stainless steel

“…At 450 and 500 ºCthe peaks corresponding to the austenite phase disappear and the peaks corresponding to CrN and Fe 3 N phases appear. At these temperatures, the expanded austenite decomposes to form CrN and Fe 3 N phases [29]. Moreover, the extent to which the CrN forms at 500 ºChas been found to be higher than that formed at 450 ºC.…”

The influence of prior deformation on phase composition and strength properties of austenitic stainless steel in ion-plasma treatment

“…The dissolution of a large amount of active atomic nitrogen and carbon into the crystal lattice results in distortion and expansion of the austenite and leads to increase in lattice constant. Supersaturated with nitrogen and carbon austenite (Fe-γ N, C ) decomposes into Fe-α N, C , Cr(N, C) and Fe 4 (N, C) phases when treatment temperature exceeds 450°C [1][2][3][4][5][6][7][8]10,21]. A high density of the defects of the crystal structure contributes to great and deep diffusion processes under surface saturation of metals with interstitials [1,7,12,13].…”

“…Ion-plasma treatment (IPT) allows to reduce treatment temperature (below 600°C) and processing time as compared to diffusion saturation in gases. Surface IPT-assisted properties are strongly dependent on process parameters (treatment temperature [2,3], processing time [4], pressure [5], etc. ), the initial microstructure [7], chemical and phase composition [8] of saturated material.…”

“…IPT-processing is accompanied with diffusion alloying of steel surface with nitrogen and carbon and can facilitate series of phase transformations with different resultant products -precipitates (based on Fe, Cr, V, Mn, etc. ), austenite supersaturated with nitrogen or carbon (so-called expanded austenite γ N, C or S-phase) and ferrite [1][2][3][4][5][6][7][8][9][10][11]. Initial microstructure of the material is one of the key parameters during IPT.…”

Corrosion and Passivation of High Nitrogen Face-Centered-Cubic Phase Formed on AISI 304L Austenitic Stainless Steel in Borate Buffer Solution

“…phase (γ N ) with a peak nitrogen concentration up to 35 at.% on the austenitic stainless steel. 1 It was verified that the γ N phase layer on austenitic stainless steel has superior pitting corrosion resistance and equivalent general corrosion resistance in comparison with that of original austenitic stainless steel in NaCl, [2][3][4][5][6][7][8][9][10][11][12][13][14] and Na 2 SO 4 15 and H 2 SO 4 2,6,10,16,17 aqueous solutions. However, a few reports have described the corrosion resistance of the γ N phase layer on austenitic stainless steel in borate solution, 18,19 although different nitrogenmodified austenitic stainless steel has been made in an effort to increase hardness and wear resistance of austenitic stainless steel for use in nuclear power systems.…”