Improvement of erosion and erosion-corrosion resistance of AISI420 stainless steel by low temperature plasma nitriding

Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at low temperature (350 °C) and normal temperature (550 °C) for 15 h. The composition, microstructure, microhardness profiles, residual stress profiles and electrochemical impedance spectrum analyses of the nitrided samples were examined. The influence of plasma nitriding on the erosion and erosion-corrosion resistance of AISI 420 martensitic stainless steel was investigated using a jet solid particle erosion tester and a slurry erosion-corrosion tester.Results showed that the 350 °C nitriding layer was dominated by ?-Fe₃N and α_N phase, a supersaturated nitrogen solid solution. However, nitrogen would react with Cr in the steel to form CrN precipitates directly during 550 °C nitriding, which would lead to the depletion of Cr in the solid solution phase of the nitrided layer. Both 350 and 550 °C plasma nitriding could improve the erosion resistance of AISI420 stainless steel under dry erosion, but the former showed better results. In both neutral and acid environment, while the erosion-corrosion resistance of AISI 420 was improved by means of 350 °C nitriding, it was decreased through 550 °C nitriding.     

Structural properties of austenitic stainless steel in the initial state and after rf plasma nitriding

Mössbauer spectroscopy, TEM, electrical resistivity and magnetic measurements are used for investigation of structural changes of X5CrNi189 austenitic stainless steel sheet induced by rf plasma nitriding carried out at 400°C for 24 h. The initial structure formed by small grains of austenite and α′-martensite changes after nitriding into expanded austenite close to the surface and austenite with minor contents of ferrite/CrN in the bulk.     

Effect of nitriding time on secondary recrystallization behaviors and magnetic properties of grain-oriented electrical steel

The effect on secondary recrystallization behaviors and magnetic properties of grain-oriented electrical steel of nitriding time from 0 to 240 s in the acquired-inhibitor method has been studied. It was found that the volume fraction of nitride precipitates increased with increasing nitriding time. However, the average diameter of the nitride precipitates decreased with increasing nitriding time. Two kinds of nitride precipitates were found to have formed after primary recrystallization annealing. A fine rod-shaped precipitate was found to be Si₃N₄ and and a coarse, lozenge-shaped precipitate was MnSiN₂. Moreover, primary grain size decreased with increasing nitriding time due to retarding of the grain growth by precipitates. After secondary recrystallization annealing, the specimen that was nitrided for 30 s obtained the largest volume fraction of abnormal growth grains and largest area percentage of Goss grains. Conversely, specimens that were nitrided more or less than 30 s demonstrated poor secondary recrystallization and obtained low area percentage of Goss grains. Furthermore, the optimum nitriding time to obtain the best magnetic properties was 30 s. In addition, the optimum nitrogen content was 150 ppm.     

On the hydrogen etching mechanism in plasma nitriding of metals

Iron alloys and aluminum were nitrogen implanted in a controlled oxygen atmosphere and the role of hydrogen on the surface etching mechanisms studied. The surface composition was analyzed by in situ photoemission electron spectroscopy (XPS). In iron alloys, hydrogen strongly etches oxygen, improving nitrogen retention on the surface. On the other hand, hydrogen removes nitrogen from aluminum surfaces, with a deleterious effect on the nitriding effectiveness. The oxygen removal in iron alloys is associated with the catalytic effect of electrons in d-orbitals and the nitrogen removal in aluminum is associated with a steric effect.     

Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe–4.75 at.% Al alloy

The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe₄N_{1? x} ) was investigated upon nitriding in NH₃/H₂ gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe₂N_{1? z} ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.     

Incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys

A model was developed to predict quantitatively the influence of alloying element (Me) dissolved in the ferrite (α) matrix on the incubation time for iron-nitride layer formation upon gaseous nitriding of iron-based alloys. The model incorporates the coupled, concurrent processes of inward diffusion of nitrogen and the depth dependency of the time dependency of the precipitation of alloying-element nitride particles in the α matrix. Experimental results were obtained by gaseous nitriding of an Fe-2.23 at.% V alloy. The incubation time for iron-nitride formation on Fe–Me alloy is generally much larger than that for iron-nitride formation on pure iron due to a pronouncedly lesser rate of increase of dissolved N content at the surface of Fe–Me alloy. The extent of segregation of N at the MeN/α-Fe interfaces has distinct influence on the incubation time.     

Immiscibility regions in iron based ferritic solid solutions and their relevance to thermodynamics and kinetics of nitriding

In the process of equilibration of a ferritic iron-based alloy with gaseous nitriding atmosphere, the inwardly diffusing N into the ferritic solid-solution from the gas atmosphere may bring the evolving solid-solution chemistry into immiscible region of the corresponding phase diagram. Distinct kinetic mechanisms of nitriding are operative in different alloy systems, depending on whether the alloy system has a region of immiscibility or not and whether the applied chemical potential of N in the nitriding atmosphere allows the evolving chemistry of the alloy to sample the immiscibility region or not. With this new kind of thermodynamic interpretation, it is now possible to precisely understand the experimental results reported in the literature pertaining to nitrided iron-based binary alloys. Slow kinetics of nitride precipitation in nitrided ferritic Fe-Si and Fe-Al alloys has been attributed to the absence of immiscibility region in ferritic Fe-Si-N and Fe-Al-N systems whereas the slower precipitation of nitrides in ferritic Fe-Mo alloys has been attributed to the implausibility of sampling the immiscibility region of Fe-Mo-N system for typically applied nitriding conditions and Mo contents. Faster kinetics of nitride precipitation in Fe-Ti, Fe-V, Fe-Cr alloys has been attributed to the presence of immiscibility in Fe-Ti-N, Fe-V-N and Fe-Cr-N systems and the applied nitriding conditions allowing sampling of this immiscibility region. Recognising the role of ‘miscibility gap’ in alloy systems to realise rapid kinetics during nitridation treatments opens up a new, unexplored alloy design strategy for the development of steels with favourable nitriding response.     

Nitriding of Stainless Steel in Electron-Beam Plasma in the Pulsed and DC Generation Modes

The influence of electron-beam parameters on the thickness and phase composition of a hardened layer formed upon the nitriding of austenitic stainless steel 12Cr18Ni10Ti in plasma produced by a beam in a low-pressure (3 Pa) nitrogen-argon mixture is studied. The results obtained in the DC and pulse-periodic modes of beam generation with the same mean current and electron energy are compared. In this case the negative bias voltage applied to the samples is 100 V. The nitriding temperature of 400°C is maintained at a mean beam current of 2.6 A and various combinations of frequency (100–500 Hz) and current pulse durations (0.1–0.3 ms) with an amplitude of 80 A. The mean ion-plasma current densities in the DC and pulsed modes are close in magnitude (2–3 mA/cm² at 400°C). The high pulsed ion-current density (35–70 mA/cm²) creates conditions under which the surface sputtering rate during the pulse exceeds the growth rate of the nitrided layer. The nitriding of steel in the pulsed and DC modes over four hours gives the same result. Hardened layers with a thickness of 7–8 μm and a microhardness of the surface component of 15 ± 1 GPa in which the main phase is a supersaturated nitrogen solid solution (expanded austenite) are formed. A possible explanation is that nitriding in an electron-beam plasma proceeds mainly under the action of long-lived active neutral nitrogen particles rather than as a result of ion bombardment.     

Mechanomaking of nanostructure in nitrided Fe–Cr alloys by cyclic “dissolution–precipitation” deformation-induced transformations

Structural-phase transformations in surface layers of iron and Fe? Cr? (Ni) alloys subjected to ion-plasma nitriding and subsequent cold plastic compression shear deformation in Bridgman anvils have been investigated by the methods of Mössbauer spectroscopy, transmission electron microscopy and X-ray analysis. It has been shown that the deformation-induced cyclic phase “dissolution–precipitation” transformations of nitrides in alloys lead to the formation of nitrogen oversaturated solid solutions, precipitation of secondary nitrides and nanostructurization of the metal matrix.     

The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 4140 steel by means of surface mechanical attrition treatment (SMAT). Plasma nitriding of SMAT and un-SMAT AISI 4140 steel was carried out by a low-frequency pulse excited plasma unit. A series of nitriding experiments has been conducted at temperatures ranging from 380 to 500 °C for 8 h in an NH₃ gas. The samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that a much thicker compound layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples after nitriding at the low temperature. In particular, plasma nitriding SMAT AISI 4140 steel at 380 °C for 8 h can produced a compound layer of 2.5 μm thickness with very high hardness on the surface, which is similar to un-SMAT samples were plasma nitrided at approximately 430 °C within the same time.     

Laser diffusion nitriding of Ti-6Al-4V for improving hardness and wear resistance

Owing to poor tribological properties, titanium (Ti) alloys are usually surface-treated to enhance their surface properties. Laser surface nitriding, among others, is a common method employed to increase hardness and wear resistance for Ti alloys. Conventional laser nitriding involves surface melting of Ti alloys in a nitrogen atmosphere. This inevitably results in a roughened surface and post-treatment might be required. The present study aims at laser diffusion nitriding Ti alloys without surface melting via carefully selecting the laser processing parameters. The nitrided surface was characterized by X-ray diffractometry (XRD), optical microscopy (OM), scanning-electron microscopy (SEM), and profilometry. The nitride layer formed was about 1.62 μm upon repeated passes. The change in surface roughness resulting from laser diffusion nitriding was only minimal. Nanoindentation measurements revealed that the hardness of the nitride layer was around 11.3 GPa, being about 2.3 times that of Ti-6Al-4V. Ball-on-slab sliding wear test recorded a reduction in wear volume by about 8 times. The results of the present work thus demonstrate the feasibility of diffusion nitriding of Ti-6Al-4V by laser treatment for enhancing its surface properties and performance.     

On the structure and chemistry of complex oxide nanofeatures in nanostructured ferritic alloy U14YWT

The remarkable radiation damage resistance of nanostructured ferritic alloys (NFAs) is attributed to the large numbers of matrix nanofeatures (NFs) of various types, which can enhance the recombination of displacement defects and trap transmutant helium in fine scale bubbles. Characterizing the chemistry, crystallographic structure and orientation relationships of the NFs is critical to understanding how they enhance the radiation damage resistance of NFAs. Conventional and high-resolution transmission electron microscopy and energy-dispersive spectroscopy were used to characterize the various types of NF and larger oxide phases in a model 14Cr–3?W–0.4Ti–0.25Y₂O₃ NFA (14YWT) hot isostatic pressed (HIP-ed) at 1150°C. Large CrTiO₃ precipitates (50–300?nm) and small diffracting NFs (<5?nm) were found in this alloy. One major new result is the observation of an additional type of nanofeature (10–50?nm), orthorhombic in structure, with a square center cross-section, which constitutes a new kind of Y–Ti-oxide phase with lattice parameters different from those of known Y and Ti complex oxides. The interfaces of these particles seem to be semicoherent, while manifesting a possible orientation relationship with the BCC matrix. The ratio of Y to Ti varies between <1 and 2 for these larger NFs.     

Surface modification of 30CrNiMo8 low-alloy steel by active screen setup and conventional plasma nitriding methods

In this paper, we report on a comparative study of active screen plasma nitriding (ASPN) and conventional dc plasma nitriding (CPN) behavior of 30CrNiMo8 low-alloy steel that has been examined under various process conditions. The process variables included active screen setup parameters (screen and iron plate top lids placed on the screen setup with 8 mm of hole size), treatment temperature (550 and 580 °C), gas mixture (75/25 and 25/75 of N₂/H₂) and treatment time (5 and 10 h) in 500 Pa pressure. The structure and phases composition of the diffusion zone and compound layer were studied by X-ray diffraction (XRD), microhardness tests, light optical microscopy and scanning electron microscopy (SEM). It was observed that treated sample surfaces in both CPN and ASPN methods consist of γ′ and ? phases, and while the nitriding time and/or temperature increases, the intensity of ? phase in the compound layer will increase for ASPN and decrease for CPN method. Results show that the amount of nitrogen transferred from holes of screen toward the sample surface via sputtering and re-condensation mechanism can be affected due to the hardness and thickness of the layer.     

Continuous and discontinuous precipitation in Fe-1 at.%Cr-1 at.%Mo alloy upon nitriding; crystal structure and composition of ternary nitrides

The internal nitriding response of a ternary Fe–1 at.%Cr–1 at.%Mo alloy, which serves as a model alloy for many CrMo-based steels, was investigated. The nitrides developing upon nitriding were characterised by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and atom probe tomography. The developed nitrides were shown to be (metastable) ternary mixed nitrides, which exhibit complex morphological, compositional and structural transformations as a function of nitriding time. Analogous to nitrided binary Fe–Cr and Fe–Mo alloys, in ternary Fe–Cr–Mo alloys initially continuous precipitation of fine, coherent, cubic, NaCl-type nitride platelets, here with the composition (Cr_½,Mo_½)N_¾, occurs, with the broad faces of the platelets parallel to the {1?0?0}_α-Fe lattice planes. These nitrides undergo a discontinuous precipitation reaction upon prolonged nitriding leading to the development of lamellae of a novel, hexagonal CrMoN₂ nitride along {1?1?0}_α-Fe lattice planes, and of spherical cubic, NaCl-type (Cr,Mo)N _x nitride particles within the ferrite lamellae. The observed structural and compositional changes of the ternary nitrides have been attributed to the thermodynamic and kinetic constraints for the internal precipitation of (misfitting) nitrides in the ferrite matrix.     

Analytical criteria for controlling glow-discharge nitriding

A system of analytical characteristics is proposed to estimate the results of glow-discharge nitriding using the energy concepts of this process. The following parallel subprocesses, which form a modified surface layer, are taken into account: the formation of nitride structures, the diffusion of nitrogen deep into a metal, and surface sputtering.     

CO2 laser gas assisted nitriding of Ti-6Al-4V alloy

Laser gas assisted nitriding of Ti-6Al-4V alloy is carried out and nitride compounds formed and their concentration in the surface vicinity are examined. SEM, XRD and XPS are accommodated to examine the nitride layer characteristics. Microhardness across the nitride layer is measured. Temperature field and nitrogen distribution due to laser irradiation pulse is predicted. It is found that the nitride layer appears like golden color; however, it becomes dark gold color once the laser power irradiation is increased. The δ-TiN and ?-TiN are dominant phases in the surface vicinity. The needle like dendrite structure replace with the feathery like structure in the surface region due to high nitrogen concentration. No porous or microcracks are observed in the nitrided layer, except at high power irradiation, in this case, elongated cracks are observed in the surface region where the nitrogen concentration is considerably high.     

Laser nitriding of iron: Nitrogen profiles and phases

Armco iron samples were surface nitrided by irradiating them with pulses of an excimer laser in a nitrogen atmosphere. The resulting nitrogen depth profiles measured by Resonant Nuclear Reaction Analysis (RNRA) and the phase formation determined by Conversion Electron Mössbauer Spectroscopy (CEMS) were investigated as functions of energy density and the number of pulses. The nitrogen content of the samples was found to be independent of the number of pulses in a layer of 50 nm from the surface and to increase in depths exceeding 150 nm. The phase composition did not change with the number of pulses. The nitrogen content can be related to an enhanced nitrogen solubility based on high temperatures and high pressures due to the laser-induced plasma above the sample. With increasing pulse energy density, the phase composition changes towards phases with higher nitrogen contents. Nitrogen diffusion seems to be the limiting factor for the nitriding process.     

Effect of ion nitriding on the crystal structure of 3 mol% Y2O3-doped ZrO2 thin-films prepared by the sol-gel method

We investigated the effect of ion nitriding on the crystal structure of 3 mol% Y₂O₃-doped ZrO₂ (3YSZ) thin-films prepared by the sol-gel method. For this purpose, we used X-ray diffractometry to determine the crystalline phases, the lattice parameters, the crystal sizes, and the lattice microstrains, and glow discharge-optical emission spectroscopy to obtain the depth profiles of the elemental chemical composition. We found that nitrogen atoms substitute oxygen atoms in the 3YSZ crystal, thus leading to the formation of unsaturated-substitutional solid solutions with reduced lattice parameters and Zr_0.94Y_0.06O_1.72N_0.17 stoichiometric formula. We also found that ion nitriding does not affect the grain size, but does generate lattice microstrains due to the increase in point defects in the crystalline lattice.