Previous heat treatment inducing different plasma nitriding behaviors in martensitic stainless steels

Abstract

In this work we report a study of the induced changes in structure and corrosion behavior of martensitic stainless steels nitrided by plasma immersion ion implantation (PI3) at different previous heat treatments. The samples were characterized by x-ray diffraction and glancing angle x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy, and potentiodynamic measurements. Depending on the proportion of retained austenite in the unimplanted material, different phase transformations are obtained at lower and intermediate temperatures of nitrogen implantation. At higher temperatures, the great mobility of the chromium yields CrN segregations like spots in random distribution, and the a' -martensite is degraded to -Fe (ferrite). The nitrided layer thickness follows a fairly linear relationship with the temperature and a parabolic law with the process time. The corrosion resistance depends strongly on chromium segregation from the martensitic matrix, as a result of the formation of CrN during the nitrogen implantation process and the formation of CrxC during the heat treatment process. Briefly speaking, the best results are obtained using low tempering temperature and low implantation temperature (below 375 degrees) due to the increment of the corrosion resistance and nitrogen dissolution in the structure with not too high diffusion depths (about 5-10 um). In this work we report a study of the induced changes in structure and corrosion behavior of martensitic stainless steels nitrided by plasma immersion ion implantation ͑PI 3 ͒ at different previous heat treatments. The samples were characterized by x-ray diffraction and glancing angle x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy, and potentiodynamic measurements. Depending on the proportion of retained austenite in the unimplanted material, different phase transformations are obtained at lower and intermediate temperatures of nitrogen implantation. At higher temperatures, the great mobility of the chromium yields CrN segregations like spots in random distribution, and the ␣Ј-martensite is degraded to ␣-Fe ͑ferrite͒. The nitrided layer thickness follows a fairly linear relationship with the temperature and a parabolic law with the process time. The corrosion resistance depends strongly on chromium segregation from the martensitic matrix, as a result of the formation of CrN during the nitrogen implantation process and the formation of Cr x C during the heat treatment process. Briefly speaking, the best results are obtained using low tempering temperature and low implantation temperature ͑below 375°͒ due to the increment of the corrosion resistance and nitrogen dissolution in the structure with not too high diffusion depths ͑about 5 -10 m͒.

The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review

“…The effect of previous heat treatment was reported by Figueroa et al [86]. AISI 420 martensitic stainless samples were quenched and tempered at two different temperatures, 310 and 530 • C, for 1 h, and then they were PIII nitrided in the range of 340-500 • C. It was observed that the tempering temperature influenced the volume fraction of retained austenite, and that this phase, in turn, influenced the phases which formed during nitriding.…”

A study of anode area physical parameters of asymmetric combined gas discharge

“…Thus, in [ 18 ], the induced changes in the structure and corrosion resistance of martensitic stainless steels nitrided by plasma-immersion ion implantation during various previous heat treatments were investigated. In [ 19 ], the results of experimental and analytical studies of ignition modes of a low-pressure discharge in nitrogen with the simultaneous application of constant and high-frequency (HF) electric fields are presented.…”

Corrosion Resistance of Plasma Nitrided Aisi 420 Martensitic Stainless Steel: Influence of the Pretreatment and Treatment Temperature

“…However, for severe surface working conditions, its hardness and wear resistance must be improved [10][11][12]15]. To overcome this limitation, plasma assisted nitriding has been successfully applied [7][8][9][10][11][12][13][14][16][17][18][19][20][21][22][23][24][25]. In the last decades, studies were conducted aiming to improve martensitic stainless steels wear and corrosion resistance by applying plasma assisted technology [16][17][18][19][20][21].…”

“…Moreover, martensitic stainless steels present metastable structure and, as such, require careful choice of treatment conditions to avoid undesirable phase transformation during nitriding. On the other hand, if the process is carried out at lowtemperatures, typically below 400˚C, the chromium nitride precipitation is avoided, and wear and corrosion resistance of the treated stainless steel surface is enhanced [18][19][20][21][22][23]. For low-temperature nitrided martensitic stainless steels, iron nitrides and a supersaturated phase, termed as nitrogen-expanded martensite, have been regularly obtained [8][9][10][11][12]18,22].…”

“…This dependence is more pronounced for low-temperature treatments since highdiffusion paths present lower diffusion activation energy compared to volume diffusion mechanisms. In this context, Figueroa et al [23] have already demonstrated that the previous heat treatment induces different phase formation after nitriding the AISI 420 steel by plasma immersion ion implantation (PIII). Kochmanski and Nowacki [24] studied the influence of initial heat treatment on gas nitriding kinetics for 17-4 PH precipitation-hardening martensitic stainless steel.…”