B.J. Gómez

Ion nitriding of stainless steels. Real time surface characterization by synchrotron X-ray diffraction

Abstract Three types of steels with different Cr and Ni contents were ion nitrided by means of 100 Hz glow discharge in a N 2 -H 2 gas mixture using a special reactor installed in the SAXS beamline of the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil. The studied materials were austenitic (AISI 304 and DIN WNr 1.4882) and ferritic (SAE HNV3) steels. The experimental setup allowed the study of the structural evolution in situ and in real time of steel surfaces during the ion nitriding process itself, using the X-ray diffraction technique. The use of a well-collimated and intense X-ray beam of the synchrotron radiation source allowed successive short exposure diffraction patterns to be taken. A very fast ion nitriding process was verified for the three studied steels. The measurements concerning the nitriding kinetics of both austenitic steels indicated the formation of an S phase. Nevertheless, while in the AISI 304 steel, the S phase was later replaced by a compound layer of γ′-Fe 4 N, in the DIN WNr 1.4882 steel, the S phase remained until the end of the nitriding process. In both cases, CrN was formed during the process. Instead, in the ferritic SAE HNV3 steel, the S and CrN phases were not observed.

A study by emission spectroscopy of the active species in pulsed DC discharges

From the optical emission of and radiative states in the negative glow of DC pulsed discharges at pressures 1 - 4 Torr (133 - 532 Pa) and current densities , it has been determined that the vibrational excitation of the state deviates from a Boltzmann distribution and strong intensities of the second positive system (especially from v = 1) were also observed in the afterglow. Such strong emission is interpreted in terms of the growing of the vibrational distribution in the post-discharge region as the gas temperature is reduced. The ) states are mainly produced by electron collisions on ions under discharge conditions and quickly disappear in the afterglow as a result of - electron recombination.

Hydrogen permeation modification of 4140 steel by ion nitriding with pulsed plasmas

Abstract It is widely known that the hydrogen in steel produces embrittlement. This effect may cause the failure of the elements (confining walls, mechanical parts, etc.) whose surfaces are in contact with this gas or with processes in which hydrogen is continuously generated. In this work it is shown that the ion nitriding of the surface of AISI 4140 is a good mechanism to act as a barrier against hydrogen permeation in its bulk. The ion nitriding was performed using a square wave DC glow discharge. The development of a compound layer of iron nitrides was observed as the cause of the hydrogen permeation reduction. For equal duration of treatment, thicker compound layers were developed in higher discharge/post-discharge ratios in the square wave of the applied voltage onto the sample (cathode), with a greater reduction of hydrogen permeation coefficient as a consequence. Nevertheless, the permeation was not reduced to zero in any of the treatment conditions used. The results of the analysis of the permeation tests and the image of the photomicrographs showed that the existence of cracks, fractures, failures, etc. in the compound layer (pre-existing in the AISI 4140 steel) could be the cause of the residual hydrogen permeation. This can be attributed to the movement of the hydrogen through these defects diffusing through the original α-Fe phase of the non-treated steel.

Determination of the electron temperature and density in the negative glow of a nitrogen pulsed discharge using optical emission spectroscopy

A new method for experimentally determining the electron density (ne) and the electron temperature (Te) in the negative glow of a nitrogen pulsed discharge is presented. It is based on optical emission spectroscopy (OES) and consists of a variation and refinement of relatively similar schemes previously reported for different working conditions by other authors. The bottom line is the measurement of the emission intensities of the (0,0) bands of the first negative system at 391.44?nm and of the (0,2) bands of the second positive system at 380.49?nm.The suggested procedure allows the establishment of the absolute values of ne and Te, as long as one calibration point is provided, such as the electron density at one specific discharge condition. If this calibration point is unavailable, the method nonetheless yields a qualitative dependence of Te and ne. Langmuir probe measurements confirm and validate the OES results for ne, thereby legitimizing the diagnostic technique developed. The interpretation of the results for Te is slightly more complex, and in some circumstances an accurate determination of Te may require further analysis.

Time evolution of Cr and N on AISI 304 steel surface during pulsed plasma ion nitriding

Abstract Chromium concentration on steels plays an important role in ion nitriding processes. In this work, the time variation of the concentration of the elements in the near-surface region of the AISI 304 (18% of Cr) stainless steel, during pulsed ion nitriding, was studied. The techniques used were the real time and in situ X-ray diffraction by Synchrotron radiation. Auger spectroscopy, Nuclear Reaction Analysis, and Conversion Electron Mossbauer Spectroscopy. Ion nitriding was performed with a 100 Hz square wave pulsed glow discharge, with different treatment times, in an atmosphere of 80% N2 and 20% H2 mixture, under a total pressure of 5.6 mbar. The high Cr concentration in the near surface layers (∼17 A) and the intermediate ‘S’ phase formation were explained through the nitrogen ion sputtering during the ion nitriding process.

Oscillations of a string with concentrated masses

In this work, the oscillations of a homogeneous string fixed at both ends, and loaded with a finite number of masses, are studied. Through a simple device, the cases with one and two concentrated masses are analysed in detail. The normal modes are observed and the corresponding frequencies are recorded. The experimental results and the solutions of the wave equation that satisfy suitable boundary conditions are compared. The theoretical and experimental results are in very good agreement.

The time variations of N2 active species in pulsed N2-H2 dc discharges

Time-varying intensities of N2 and N2+ bands under discharge and post-discharge conditions of a pulsed dc 2-H2 diode plasma have been analysed. It is shown that, at a constant discharge current, the discharge voltage and the substrate temperature continuously decrease with the introduction of H2 into N2. As a consequence the degree of ionization ne/[N2] keeps a nearly constant value. Under such conditions, a V-V excitation of N2(X,v) by H2(X,v) explains small increases in intensity of the N2 second positive system and N2+ first negative system in the discharge with about 5% H2 in N2. In the afterglow, the N2 and N2+ band intensities decrease with the amount of H2 let into N2 but a vibrational excitation of N2(C,v´) radiative states has been observed in the N2-H2 afterglow, just like in pure N2, indicating that pooling reactions of N2(X,v) vibrational ground-state molecules are always active in the post-discharge regime.

In situ synchrotron radiation diffraction study of low carbon steel during ion nitriding

The volume close to the surface of materials subjected to ion implantation or ion diffusion exhibits structural transformations that are usually studied by x-ray diffraction. In order to characterize the kinetic aspects of structural transformations in low carbon AISI 1010 steel during ion nitriding, an in situ synchrotron radiation diffraction study was performed. An experimental set-up, a specially designed reactor, was constructed for x-ray measurements in real time. The variation in the lattice parameter with time of the initial Fe- phase was attributed to the heating effect produced by the action of an electric discharge during the first stages of the process. The analysis of the x-ray diffraction patterns indicates the formation of several iron nitrides. The Fe3N phase exhibits a progressive decrease in the interplanar distances, which is clearly a different behaviour with respect to the other observed phases, even considering a phase with nearly the same stoichiometry.

Pulsed-plasma-nitrided API 5L X-65 steel : hydrogen permeability and microstructural aspects

Nitriding of an API 5L X-65 steel was performed using a low-frequency dc pulsed-plasma technique. A 6 h treatment was found to produce a compact surface nitride layer composed of and nitrides with a thickness of around . The plasma-nitriding treatment resulted in a significant reduction in hydrogen permeability, from (for the untreated steel) to for electrolytic hydrogen charging via the nitrided surface. When the sample was oriented with the nitrided surface as the hydrogen-exit face, the permeability was reduced even further, to .

Study of the excitation mechanisms of the second positive system in the negative glow of a N2–Ar discharge

In an Ar?N2 discharge, the high excitation transfer from Ar(3P2,0) to N2 produces an overpopulation of the high rotational levels of the bands of the second positive system (SPS), and so the spectra interpretation is not straightforward.This paper presents a fit function for the SPS bands measured in Ar?N2, which allows us to study the excitation process contributions to the N2(C) level.The procedure was tested in the negative glow of a pulsed Ar?N2 discharge at a pressure of 2.5?Torr, for different mixture concentrations. In this discharge, through the fitting, it was possible to calculate the variation of the N2(C) densities produced by different excitation processes as well as the variation of Ar metastable density.