Nitrogen Supersaturation Research Articles

Large‐Scale Synthesis of Nanostructured Nitride Layer on Ti Plate Using Mechanical Shot Peening and Low‐Temperature Nitriding

A nanostructured nitride layer is produced on a large titanium plate using mechanical shot peening (MSP) followed by a low‐temperature gaseous nitriding method. The combined effect of the MSP and low‐temperature nitriding on the microstructural evolution and mechanical properties is investigated using X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, as well as hardness, wear resistance, and toughness tests. The results of the characterization are compared to a coarse‐grained specimen produced by a standard nitriding process. The results show that a nitride layer with a thickness of 10–15 µm is produced on the MSP‐treated Ti plate after nitriding at 550 °C for 5 h. The nitride layer is composed of nanostructured ϵ‐TiN and γ‐Ti2N phases with a high supersaturation of nitrogen. The nitriding kinetics is significantly enhanced by the nanocrystalline structure. The surface hardness, thickness of the hardened layer, and wear resistance of the nitrided MSP Ti plate are all enhanced relative to the coarse‐grained nitrided sample. The toughness of the nanostructured nitrides is greatly improved compared with the conventional nitrided specimen.

Enhanced toughness of nitrided layers formed on Ti-6Al-4V alloy via surface mechanical attrition pre-treatment

This study focused on developing a beneficial nitriding process for use on Ti-6Al-4V alloy surfaces, involving surface mechanical attrition treatment (SMAT), which does not contaminate the existing nanocrystalline layer. The average grain size of this layer was refined to 10 nm with random crystallographic orientation. Differential scanning calorimetry confirmed that the recrystallization temperature of the Ti-6Al-4V nanocrystalline layer was 550 °C. The phase compositions, microstructure, hardness and toughness of the nitrided layer were studied by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and hardness testing. The nitrided layer of the SMAT sample was composed of nanoscale TiN, Ti2N and Ti with high nitrogen supersaturation. The nitriding kinetics was enhanced by the nanocrystalline layer at relatively low temperatures (600 °C). Compared to coarse-grained nitrided samples, the SMAT nitrided sample exhibited similar surface hardness but improved surface toughness, which may be attributed to the formation of a nanostructured nitrided layer.

Corrosion-resistant characteristics of nitrided Ni-free stainless steel for bipolar plate of polymer electrolyte fuel cell

In this study, the effect of a high temperature nitriding treatment on the characteristics of the Ni-free stainless steel (SUS445) used as the metallic bipolar plate of a polymer electrolyte fuel cell (PEFC) was investigated. The phase structure, composition, electrical conductivity and electrochemical corrosion resistance of the stainless steel after the nitriding treatment (SUS445-N) were analyzed. The supersaturated nitrogen phase γN, nitrides CrN and Cr2N in the nitride layer with a good electrical conductivity were observed and confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the nitrided SUS445-N stainless steel was significantly improved due to the formation of the chromium-containing nitrides. The conducted fuel cell durability test revealed that when using the nitrided SUS445-N stainless steel as the bipolar plate, the fuel cell voltage performance was almost the same as that using the graphite carbon as the bipolar plate for 2500 h.

Tribological properties and wear mechanisms of DC pulse plasma nitrided austenitic stainless steel in dry reciprocating sliding tests

Expanded austenite (γN), or S-phase, is a special phase of low-temperature nitrided austenite containing highly super-saturated nitrogen in the form of heterogeneous Cr-N nano-clusters. A nitrided layer of singe phase γN is known to provide austenitic stainless steel with combined high hardness, good wear resistance and superior corrosion resistance. This paper reports recent experiments on a comparative study of the sliding wear properties and wear mechanisms of nitrided austenite stainless steel AISI 316, with a special attention paid on worn surface structural evolutions induced by frictional heating and sliding deformation. The samples were prepared by DC pulsed plasma nitriding treatments of various time at a fixed power. Knoop micro-indentation has revealed hardening behaviour of the nitrided samples. The reciprocating ball-on-disc sliding wear and friction properties were investigated at ambient environment conditions using an alumina counterpart ball. The worn surfaces have been analysed by XRD, FEG-SEM and EDX to show wear induced changes in the crystalline characteristics and the wear mechanisms of tribo-oxidation, cracking, abrasive wear and ploughing deformation. Moreover, longitudinal cross-sectional foils of the worn samples have been prepared and analysed using TEM, to investigate the wear induced structural changes, including tribofilm formation, plastic deformation and delamination in depths of nano-scale.

“Colossal” interstitial supersaturation in delta ferrite in stainless steels: (II) low-temperature nitridation of the 17-7 PH alloy

Low-temperature gas-phase nitridation has been studied in interstitially-hardened 17-7 precipitation-hardening stainless steel. After nitridation, conventional transmission electron microscopy reveals that delta ferrite grains in such alloys show a uniform weak diffraction contrast. Chemical analysis reveals that the weak-contrast ferrite grains contain an enormous interstitial nitrogen supersaturation (>20 at.%). Surprisingly, there is no significant tetragonality in these weak-contrast ferrite grains. Such weak diffraction contrast is attributed to a nanometer-scale nitridation-induced spinodal decomposition of delta ferrite grains. In addition, nitride nanoparticles are observed in the steel, and were identified as rocksalt-structured MN nitride (M: Fe, Cr, Ni, and Al). Further, platelets of hexagonal M2N1-x are observed at low nitriding temperatures (623 and 653 K), but were absent during nitridation at a higher temperature (713 K).

Microstructural characterization of low temperature plasma-nitrided 316L stainless steel surface with prior severe shot peening

Surface nanocrystallization by severe deformation has proven beneficial as pre-treatment to plasma nitriding. It aids in achieving thicker nitride layers at lower temperatures thus making the process more economical. In austenitic stainless steels, severe deformation leads to formation of strain induced martensite on the surface while plasma nitriding alone forms expanded austenite. However, structural characteristics of surface layer of pre-deformed steel after plasma nitriding is still a matter of debate. In present study, 316L stainless steel was subjected to severe shot peening: followed by plasma nitriding at 400°C for 4h. Characteristics of sample surface before and after treatment were analyzed by scanning electron microscopy, X-ray diffractometry and transmission electron microscopy techniques. Results showed that, this duplex treatment leads to formation of about 45μm thick nitride layer; without CrN precipitation. This is significantly high compared to reported data considering the temperature and duration of nitriding treatment employed. Selected area electron diffraction pattern from topmost surface confirmed the co-existence of austenite and martensite while subsurface layer was predominantly consisting of lath martensite. This indicates that major phase in the nitrided layer is martensitic in nature and nitrogen supersaturation leads to transformation of small fraction of martensite to expanded austenite.

Enhancement of mechanical property and corrosion resistance of 316L stainless steels by low temperature arc plasma nitriding

Abstract AISI 316 L austenitic stainless steels (316 L SS) were nitrided to enhance their mechanical property and corrosion resistance by a high efficiency low-pressure arc plasma nitriding process at low temperature (~ 400 °C). Arc currents ranging from 55 A to 95 A were applied to generate the arc plasma. The effect of arc current on the microstructure, mechanical property of the nitrided layers was investigated by using XRD, SEM, TEM, Vickers microhardness tester and ball-on-disk tribometer, respectively. The corrosion resistance was also evaluated by potentiodynamic polarization in 3.5% NaCl solution. The results showed that the nitrided layers consisted primarily of the expanded austenite (γN) phase with a trace of a mixture of CrN and Fe3N. The thickness of nitrided layer could reach 15 μm in 1 h at 400 °C in virtue of the high-density and high-energy nitrogen plasma generated from this process, which showed an extremely fast nitriding kinetics. Due to the solution strengthening of the nitrogen super-saturation γN-phase formed on the surfaces of the samples, the surface microhardness and the wear resistance of the nitrided 316 L SS was significantly improved. The worn surfaces revealed that worn behavior of the low current (below 85 A) nitrided samples was remarkably dominated by micro-abrasion and spalling holes, while the 95 A sample was worn in a mode featured by cracks and spalling blocks. Besides, because chromium nitride precipitation was eliminated, the nitrided samples still showed excellent corrosion resistance, which was comparable to untreated 316 L SS.

SPECIFICS OF GAS BUBBLE FORMATION AND GROWTH IN TISSUES DURING DECOMPRESSION AFTER SATURATION DIVES.

It is shown that the decompression schedule after saturation dives to a depth of 30 m designed to hold the nitrogen supersaturation for the most <<slow>> tissues at the acceptable levels is significantly shorter than the decompression schedules with zero supersaturation. of these tissues with nitrogen and all dissolved gases. Equality of the risk of decompression sickness (DCS) onset under this decompression schedule to the risk of DCS onset under the rapid ascent to the surface after saturation dives to a depth of 6.1 m indicates that the effect of the high ambient pressure decreases the concentration of seeds of gas bubbles in tissues and their subsequent growth rate. The DCS symptoms in experienced divers under dangerous decompression profiles not appear due to the lower concentration of gas bubble seeds in their tissues relatively to the average level inherent to the many of humans.

Orientation dependence of nitrogen supersaturation in austenitic stainless steel during low-temperature gas-phase nitriding

A hardened surface layer was produced on 316L austenitic stainless steel via low-temperature gas-phase nitriding. The effects of nitriding temperature, nitrogen activity and processing time were systematically studied. Concentration–depth profiling by Auger electron spectroscopy revealed very high levels (up to 25at.%) of interstitial nitrogen in solid solution. This causes a lattice parameter expansion of up to 10% at the alloy surface. X-ray diffractometry revealed that the expansion – and thus the level of nitrogen supersaturation – strongly depends on the crystallographic orientation. We attribute this effect to the elastic anisotropy of austenite, exacerbated by a nitrogen-induced paramagnetic-to-ferromagnetic transition.

Use of combination of accelerator-based ion-beam analysis techniques to the investigation of the corrosion behavior of CoCrMo alloy

Nuclear Reaction Analysis – NRA in combination with d-RBS (Ed: 1.35MeV) was applied in order to investigate the corrosion behavior of CoCrMo alloy. The corrosion resistance of the alloy was compared to that of modified CoCrMo samples by several techniques as plasma nitriding and oxidizing at moderate temperature (∼400°C). Electrochemical techniques in simulated body fluid 0.9% NaCl (37°C) were applied in order to accelerate the corrosion process. The nitrogen depth distribution before and after the corrosion was determined using the 14N(d,α)12C and the 14N(d,p)15N nuclear reactions whereas the oxygen by the 16O(d,p)17O. The surface morphology and microstructure was investigated using microscopy techniques. It was found that surface treatments produce thick nitrided layers (5–6μm) consisting of a supersaturated nitrogen solution (nitrogen concentration is ∼30at.%) in the matrix (expanded phase γN) and a thin oxygen solution (0.3μm). The samples subjected to plasma nitridation and oxidation exhibited the lowest deterioration and better resistance to corrosion compared to the single nitrided or single oxidized and the untreated material. This could be attributed to the modified surface region with the high nitrogen content and the presence of oxygen.

Abnormal Nitride Morphologies upon Nitriding Iron-Based Substrates

Nitriding of iron-based components is a very well-known surface engineering method for bringing about great improvement of the mechanical and chemical properties. An overview is presented of the strikingly different nitride morphologies developing upon nitriding iron-based alloy substrates. Observed abnormal morphologies are the result of intricate interplay of the thermodynamic and kinetic constraints for the nucleation and growth of both alloying element nitride particles in the matrix and iron nitrides at the surface of the substrate. Alloying elements having strong Me-N interaction, such as Cr, V, and Ti, precipitate instantaneously as internal Me-nitrides, thus allowing the subsequent nucleation and growth of “normal” layer-type iron nitride. Alloying elements having weak Me-N interaction, such as Al, Si, and Mo, and simultaneously having low solubility in iron nitride, obstruct/delay the nucleation and growth of iron nitrides at the surface, thus leading to very high nitrogen supersaturation over an extended depth range from the surface. Eventually, the nucleation and growth of “abnormal” plate-type iron nitride occurs across the depth range of high nitrogen supersaturation. On this basis, strategies can be devised for tuned development of specific nitride morphologies at the surface of nitrided components.

Effect of Pre-Shot Peening on Plasma Nitriding Kinetics of Austenitic Stainless Steel

The fine grains and strain-induced martensite were fabricated in the surface layer of AISI 304 austenitic stainless steel by shot peening treatment. The shot peening effects on the microstructure evolution and nitrogen diffusion kinetics in the plasma nitriding process were investigated by optical microscopy and X-ray diffraction. The results indicated that when nitriding treatments carried out at 450°C for times ranging from 0 to 36h, the strain-induced martensite transformed to supersaturated nitrogen solid solution (expanded austenite), and slip bands and grain boundaries induced by shot peening in the surface layer lowered the activation energy for nitrogen diffusion and evidently enhanced the nitriding efficiency of austenitic stainless steel.

Low-temperature plasma nitriding of titanium layer on Ti/Al clad sheet

A nanostructured surface layer was fabricated on titanium layer of a Ti/Al clad sheet composite by means of the surface mechanical attrition treatment (SMAT). The low-temperature plasma nitriding treatment of the SMAT sample was investigated in comparison with the coarse-grained sample by using structural analysis (X-ray diffraction, scanning electron microscopy, and transmission electron microscopy) as well as mechanical and corrosion property measurements. Results showed that the nitriding kinetics of the SMAT sample with the nanostructured surface layer was greatly enhanced, so that the nitriding temperature could be as low as 550°C, which is significantly lower than the melting point of the aluminum layer on Ti/Al clad sheet (conventional nitriding can lead to melting of the aluminum layer due to higher temperature about 800°C). The nitrided layer of the SMAT sample was composed of nanostructured ε-TiN and γ-Ti2N phase with high supersaturation of nitrogen. The surface hardness and the hardened surface layer thickness as well as the wear and corrosion resistances of the nitrided SMAT sample were all substantially enhanced relative to the nitrided coarse-grained sample.

Unusual precipitation of amorphous silicon nitride upon nitriding Fe–2at.%Si alloy

Silicon-nitride precipitation in ferritic Fe–2at.%Si alloy was investigated upon nitriding in NH3/H2 gas mixtures, using light microscopy, hardness measurements, scanning and transmission electron microscopy, X-ray diffraction and electron-probe microanalysis for microstructural characterisation. Surprisingly, ideally weak nitriding behaviour occurred upon nitriding thick (1 mm) recrystallised Fe–2at.%Si alloy specimens. This phenomenon can be attributed to the onset of silicon-nitride precipitation only after a certain degree of nitrogen supersaturation has been established at all depths in the specimen. Silicon-nitride precipitates formed inside the ferrite grains and also along the ferrite grain boundaries. The precipitates were amorphous and had a stoichiometry of Si3N4. The amorphous nature of the tiny precipitates has a thermodynamic origin. Nitride precipitation occurred very slowly due to the very large volume misfit of the nitride with the matrix. An anomalous non-monotonous hardness change occurred with increasing nitriding time, which was ascribed to the initially fully elastic accommodation of precipitate/matrix misfit. The nitrogen uptake rate increased upon continued nitriding as a result of “self-catalysis”. The possible, favourable application of amorphous silicon-nitride precipitates as grain-growth inhibitors in the production of grain-oriented electrical steel is discussed.

Persistence of critical flicker fusion frequency impairment after a 33 mfw SCUBA dive: evidence of prolonged nitrogen narcosis?

One of the possible risks incurred while diving is inert gas narcosis (IGN), yet its mechanism of action remains a matter of controversy. Although providing insights in the basic mechanisms of IGN, research has been primarily limited to animal studies. A human study, in real diving conditions, was needed. Twenty volunteers within strict biometrical criteria (male, age 30-40years, BMI 20-23, non smoker) were selected. They performed a no-decompression dive to a depth of 33mfw for 20min and were assessed by the means of critical flicker fusion frequency (CFFF) measurement before the dive, during the dive upon arriving at the bottom, 5min before the ascent, and 30min after surfacing. After this late measurement, divers breathed oxygen for 15min and were assessed a final time. Compared to the pre-dive value the mean value of each measurement was significantly different (p<0.001). An increase of CFFF to 104±5.1% upon arriving to the bottom is followed by a decrease to 93.5±4.3%. This impairment of CFFF persisted 30min after surfacing, still decreased to 96.3±8.2% compared to pre-dive CFFF. Post-dive measures made after 15min of oxygen were not different from control (without nitrogen supersaturation), 124.4±10.8 versus 124.2±3.9%. This simple study suggests that IGN (at least partially) depends on gas-protein interactions and that the cerebral impairment persists for at least 30min after surfacing. This could be an important consideration in situations where precise and accurate judgment or actions are essential.

Neuroprotective role of the TREK-1 channel in decompression sickness

Nitrogen supersaturation and bubble formation can occur in the vascular system after diving, leading to death and nervous disorders from decompression sickness (DCS). Bubbles alter the vascular endothelium, activate platelets, and lead to focal ischemia with neurological damage mediated by the mechanosensitive TREK-1 neuronal potassium ion channel that sets pre- and postsynaptic resting membrane potentials. We report a neuroprotective effect associated with TREK-1. C57Bl6 mice were subjected to decompression from a simulated 90 msw dive. Of 143 mice that were wild type (WT) for TREK-1, 51.7% showed no DCS, 27.3% failed a grip test, and 21.0% died. Of 88 TREK-1 knockouts (KO), 26.1% showed no DCS, 42.0% failed a grip test, and 31.8% died. Mice that did not express TREK-1 had lower DCS resistance and were more likely to develop neurological symptoms. We conclude that the TREK-1 potassium channel was neuroprotective for DCS.

PLASMA–BASED LOW–ENERGY NITROGEN ION IMPLANTATION OF 2Cr13 MARTENSITIC STAINLESS STEEL USED IN PUMPS AND VALVES

The 2Crl3 martensitic stainless steel used in pumps and valves was modified by plasma-based low-energy nitrogen ion implantation at a processing temperature of 450℃for a treatment time of 4 h.The modified layer on the 2Crl3 stainless steel had a thickness range of 10 12μm.The modified layer consisted of monophase and has a high supersaturated nitrogen concentration up to 35%—40%(atomic fraction).The microhardness of thee-Fe_(2-3)N phase layer was measured to be 15.7 GPa,and the increased wear resistance of the modified layer was obtained on a ball on disc tribometer with a decreased friction coefficient from 1.0 of the original stainless steel to 0.85.A typical course from self-passivation to pitting corrosion of the modified layer in 3.5%NaCl solution was observed with a corrosion potential of—185 mV(vs SCE),a passive current density of 10~(-1)μA/cm~2, and a pitting potential of—134 mV(vs SCE).The pitting corrosion resistance of the modified layer was improved in comparison with that of the original stainless steel with non anodic passivation.It was found that the plasma-based low-energy nitrogen ion implantation of 2Crl3 martensitic stainless steel could meet the wear and corrosion resistance requirement for use in pumps and valves.

In situ transmission electron microscopy investigations of the kinetics of α″-Fe 16N 2 precipitation during the ageing of nitrogen–ferrite

In situ transmission electron microscopy investigations were carried out on a supersaturated nitrogen–ferrite to characterise the precipitation kinetics of α″-Fe16N2 nitrides at 85 °C. The coarsening behaviour consists of two stages. The first stage obeys the LSW theory and the coarsening rate was determined as 11.2 nm3 s−1. The second stage corresponds to the stabilization of coarsening, and was assumed to be related to the coherency loss between the precipitates and the matrix.

Mössbauer spectroscopy study on the corrosion resistance of plasma nitrided ASTM F138 stainless steel in chloride solution

Plasma nitriding of ASTM F138 stainless steel samples has been carried out using dc glow discharge under 80% H 2–20% N 2 gas mixture, at 673 K, and 2, 4, and 7 h time intervals, in order to investigate the influence of treatment time on the microstructure and the corrosion resistance properties. The samples were characterized by scanning electron microscopy, glancing angle X-ray diffraction and conversion electron Mössbauer spectroscopy, besides electrochemical tests in NaCl aerated solution. A modified layer of about 6 μm was observed for all the nitrided samples, independent of nitriding time. The X-ray diffraction analysis shows broad γ N phase peaks, signifying a great degree of nitrogen supersaturation. Besides γ N, the Mössbauer spectroscopy results indicated the occurrence of γ′ and ε phases, as well as some other less important phases. Corrosion measurements demonstrate that the plasma nitriding time affects the corrosion resistance and the best performance is reached at 4 h treatment. It seems that the ε/γ′ fraction ratio plays an important role on the resistance corrosion. Additionally, the Mössbauer spectroscopy was decisive in this study, since it was able to identify and quantify the iron phases that influence the corrosion resistance of plasma nitrided ASTM F138 samples.

Crystal interstitial sites contribution to nitrogen supersaturation in mechanically alloyed Fe–Cr–Mn–N alloys

In the recent years, the addition of nitrogen to iron alloys by mechanical alloying has attracted considerable attention. In this paper, mechanical alloying of Fe–18Cr–8Mn powder mixture under a nitrogen atmosphere is considered from the viewpoints of nitrogen supersaturation and phase transformation. By progression of milling, austenitization and amorphization transformations are detected. The contribution of interstitial sites of the nanocrystalline phases to the nitrogen supersaturation is estimated by X-ray diffraction experiments. It is found that the contribution is progressively decreased by increasing the total nitrogen content. Typically, only about 4% of total incorporated nitrogen is distributed among the crystal interstitial sites of the Fe–18Cr–8Mn–2.5N alloy.