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.     

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.     

Microstructure and corrosion resistance of the layers formed on the surface of precipitation hardenable plastic mold steel by plasma-nitriding

Plasma-nitriding is used to improve the wear resistance and corrosion resistance of plastic mold steels by modifying the surface layers of these steels. In this study, a precipitation hardenable plastic mold steel (NAK80) was plasma-nitrided at 470, 500, and 530 °C for 4, 8, and 12 h under 25% N₂ + 75% H₂ atmosphere in an industrial nitriding facility. The microstructures of the base material and nitrided layers as well as the core hardness were examined, and various phases present were determined by X-ray diffraction. The corrosion behaviors were evaluated using anodic polarization tests and salt fog spray tests in 3.5% NaCl solution.The results had shown that plasma-nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma-nitriding of NAK80 mold steel produced a nitrided layer composed of an outer compound layer constituting a mixture of ?-nitride and γ′-nitride and an adjacent nitrogen diffusion layer on the steel surface. The amount of ?-nitride and total nitrides increased with an increase in nitriding temperature and nitriding time. Corrosion study revealed that plasma-nitriding significantly improved the corrosion resistance in terms of corrosion potential, corrosion and pitting current density, and corrosion rate. This improvement was found to be directly related to the increase in the amount of ?-nitride at the surface, indicating the amount of ?-nitride controlling the corrosion resistance.     

Plasticity and ab initio characterizations on Fe4N produced on the surface of nanocrystallized 18Ni-maraging steel plasma nitrided at lower temperature

18Ni-maraging steel has been entirely nanocrystallized by a series of processes including solution treatment, hot-rolling deformation, cold-drawn deformation and direct electric heating. The plasma nitriding of nanocrystallized 18Ni-maraging steel was carried out at 410 °C for 3 h and 6 h in a mixture gas of 20% N₂ + 80% H₂ with a pressure of 400 Pa. The surface phase constructions and nitrogen concentration profile in surface layer were analyzed using an X-ray diffractometer (XRD) and the glow discharge spectrometry (GDS), respectively. The results show that an about 2 μm thick compound layer (mono-phase γ′-Fe₄N) can be produced on the top of the surface layer of nanocrystallized 18Ni-maraging steel plasma nitrided at 410 °C for 6 h. The measured hardness value of the nitrided surface is 11.6 GPa. More importantly, the γ′-Fe₄N phase has better plasticity, i.e., its plastic deformation energy calculated from the load-displacement curve obtained by nano-indentation tester is close to that of nanocrystallized 18Ni-maraging steel. Additionally, the mechanical properties of γ′-Fe₄N phase were also characterized by first-principles calculations. The calculated results indicate that the hardness value and the ratio of bulk to shear modulus (B/G) of the γ′-Fe₄N phase are 10.15 GPa and 3.12 (>1.75), respectively. This demonstrates that the γ′-Fe₄N phase has higher hardness and better ductility.     

Surface engineering of biomedical metallic materials by plasma-based low-energy ion implantation

Plasma-based low-energy ion implantation, including plasma source ion nitriding/carburizing and plasma source low-energy ion enhanced deposition of thin films, for surface engineering of metallic materials was emerged as low-temperature, low-pressure surface modification technique. Plasma source ion nitriding onto AISI 316L austenitic stainless steel produced a high nitrogen face-centered-cubic phase (γ_N) layer about 10 μm thick at the temperature of 380 °C during 4 h with the high microhardness of HK_0.1 N 22.0 GPa. The microhardness of the nitrided surface from the titanium nitride phase [(Ti, Al, V)N] layer on Ti6Al4V alloy at 750 °C during 4 h achieved up to about HK_0.1 N 15.5 GPa. No pitting corrosion in the Ringer’s solution at 37 °C was detected by electrochemical polarization measurement for the nitrided AISI 316L stainless steel and Ti6Al4V alloy, respectively. Plasma source ion nitriding of the metallic materials provided the engineering surfaces with combined improvement in hardness and corrosion resistance.     

Evolution of magnetic phase at low aging temperature in a heavily cold-drawn stainless steel fiber

The evolution of the magnetic phase upon aging at 300–520 °C in a heavily cold-drawn AISI 316L austenitic stainless steel fiber was studied using thermomagnetic analysis (TMA) and magnetic force microscopy with a heating stage. An increasing trend of magnetization from 50 °C to around 470 °C in the heating curves of TMA in austenitic stainless steels after a cold-drawing process was observed. No significant M_s temperature signal in the TMA curve at cooling indicated an increase in magnetization upon cooling period without significant phase transformation. A series of in situ magnetic force microscopy observations reveal a growth of the magnetic domain structure after aging at 300 °C for 2.5 h. Results show that the ferromagnetic increase during aging at lower annealing temperature resulted from the growth of martensite.     

Effect of silver on antibacterial properties of stainless steel

The microstructural variation and antibacterial properties of the AISI 304 stainless steel containing silver (Ag) element have been investigated by means of optical microscopy (OM), grazing incidence X-ray diffractometry (GIXRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDS). Furthermore, the antibacterial testing was performed according to JIS Z2801:2000 specification. As the alloy contained Ag elements, the microstructure of the alloys was a mixture of (α + γ + Ag-rich compound)-phases. The amounts of α phase and Ag-rich compound increased as Ag contents increased. The Ag-rich compound has FCC structure with the lattice parameter a = 0.251 nm. No precipitates were found within the matrix and grain boundaries in the present alloys after SHT. Moreover, when the alloy is added to Ag element, antibacterial property was seen obvious against E. coli. It has an AR nearly of 100%.     

Improvement of corrosion resistance of nitrided low alloy steel by plasma post-oxidation

Post-oxidizing treatments can be performed to improve the corrosion resistance of nitrided steel samples. In this paper, plasma nitriding treatments were performed at 540 °C for 4 h using ammonia as the working gas, and plasma post-oxidizing treatments were carried out at temperatures ranging from 350 °C to 500 °C for 2 h in oxygen gas. The treated samples were characterized by using optical microscopy, SEM, XRD, and electrochemical polarization. The X-ray analysis revealed the formation of iron-nitride phases of ?-Fe_2-3N and γ′-Fe₄N during plasma nitriding and iron oxide phases of hematite (Fe₂O₃) and magnetite (Fe₃O₄) through the post-oxidizing treatment. In particular, it was found that the very thin magnetite layer 0.8-1.5 μm in thickness on top of the compound layer was obtained by plasma post-oxidized at 400 °C and 450 °C. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment.     

Development of nitride-layer of AISI 304 austenitic stainless steel during high-temperature ammonia gas-nitriding

Ammonia-gas nitriding of AISI 304 austenitic stainless steel was studied at temperatures higher than 800 °C using SEM and X-ray diffraction. The result showed that S-phase, an expanded austenite, was formed even at such high temperatures due to a high nitriding potential of ammonia gas. The equilibrium phase, CrN was formed through a decomposition of S-layer in two different modes; the one was through continuous precipitation of particles at the surface-side of S-layer due to a higher nitriding potential; the other through a discontinuous(-like) precipitation at the austenite interface-side, producing a fine lamellar structure of austenite and CrN. The γ-phase in the surface-side resulting from the precipitation of CrN particles subsequently transformed into Fe₄N because of a fast enrichment of N atoms and a limited mobility of Cr atoms at the surface-side. A coarse lamellar structure made of austenite and Cr₂N was developed in front of fine lamellae composed of austenite and CrN by the decomposition of supersaturated austenite through a discontinuous precipitation via grain boundary movement.     

Surface improvement and biocompatibility of TiAl24Nb10 intermetallic alloy using rf plasma nitriding

The present work describes the surface improvement and biocompatibility of TiAl₂₄Nb₁₀ intermetallic alloy using rf plasma nitriding. The nitriding process was carried out at different plasma power from 400 W to 650 W where the other plasma conditions were fixed. Grazing incidence X-ray diffractometry (GIXRD), Auger electron spectroscopy (AES), tribometer and a nanohardness tester were employed to characterize the nitrided layer. Further potentiodynamic polarization method was used to describe the corrosion behavior of the un-nitrided and nitrided alloy. It has been found that the Vickers hardness (HV) and corrosion resistance values of the nitrided layers increase with increasing plasma power while the wear rates of the nitrided layers reduce by two orders of magnitude as compared to those of the un-nitrided layer. This improvement in surface properties of the intermetallic alloy is due to formation of a thin modified layer which is composed of titanium nitride in the alloy surface. Moreover, all modified layers were tested for their sustainability as a biocompatible material. Concerning the application area of biocompatibility, the present treated alloy show good surface properties especially for the nitrided alloy at low plasma power of 400 W.     

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.     

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.     

Isothermal decomposition behavior of the high nitrogen concentration γ-Fe[N] prepared from pure iron

The isothermal decomposition characteristic of the homogeneous high nitrogen austenitic samples prepared by a new multi-stage nitriding process was investigated by SEM and TEM in this paper. Lamellar-structure precipitations arranged on the decomposed austenite grain boundaries (GBs) and the flaky γ′ particles and network-structure precipitations appeared inside of the γ matrix. The extra high Vickers hardness more than 800 HV was found in the 5-h aged samples, which was different from those of the bainitic/martensitic structures in Fe-C alloys. The SAED analysis indicates the γ′ has the coherent relation with the parent γ-Fe[N] phase and the interstitial nitrogen atoms are inclined to aggregate on {1 1 0}γ′/γ planes, which also contributes to the hardness of the matrix.     

Influence of process time on microstructure and properties of 17-4PH steel plasma nitrocarburized with rare earths addition at low temperature

17-4PH stainless steel was plasma nitrocarburized at 430 °C for different time with rare earths (RE) addition. Plasma RE nitrocarburized layers were studied by optical microscope, scanning electron microscope equipped with an energy dispersive X-ray analyzer, X-ray diffraction, microhardness tests, pin-on-disc tribometer and anodic polarization tests. The results show that rare earths atoms can diffuse into the surface of 17-4PH steel. The modified layer depths increase with increasing process time and the layer growth conforms approximately to the parabolic law. The phases in the modified layer are mainly of γ′-Fe₄N, nitrogen and carbon expanded martensite (α′_N) as well as some incipient CrN at short time (2 h). With increasing of process time, the phases of CrN and γ′-Fe₄N increase but α′_N decomposes gradually. Interestingly, the peaks of γ′-Fe₄N display a high (2 0 0) plane preferred orientation. The hardness of the modified specimen is more than 1340 HV, which is about 3.7 times higher than that of untreated one. The friction coefficients and wear rates of specimens can be dramatically decreased by plasma RE nitrocarburizing. The surface hardness and the friction coefficients decrease gradually with increasing process time. The corrosion test shows that the 8 h treated specimen has the best corrosion resistance with the characterization of lower corrosion current density, a higher corrosion potential and a large passive region as compared with those of untreated one.     

The corrosion behavior of intermetallic compounds Ni3(Si,Ti) and Ni3(Si,Ti) + 2Mo in acidic solutions

The corrosion behavior of the intermetallic compounds homogenized, Ni₃(Si,Ti) (L1₂: single phase) and Ni₃(Si,Ti) + 2Mo (L1₂ and (L1₂ + Ni_ss) mixture region), has been investigated using an immersion test, electrochemical method and surface analytical method (SEM; scanning electron microscope and EPMA: electron probe microanalysis) in 0.5 kmol/m³ H₂SO₄ and 0.5 kmol/m³ HCl solutions at 303 K. In addition, the corrosion behavior of a solution annealed austenitic stainless steel type 304 was studied under the same experimental conditions as a reference. It was found that the intergranular attack was observed for Ni₃(Si,Ti) at an initial stage of the immersion test, but not Ni₃(Si,Ti) + 2Mo, while Ni₃(Si,Ti) + 2Mo had the preferential dissolution of L1₂ with a lower Mo concentration compared to (L1₂ + Ni_ss) mixture region. From the immersion test and polarization curves, Ni₃(Si,Ti) + 2Mo showed the lowest corrosion resistance in both solutions and Ni₃(Si,Ti) had the highest corrosion resistance in the HCl solution, but not in the H₂SO₄ solution. For instance, it was found that unlike type 304 stainless steel, these intermetallic compounds were difficult to form a stable passive film in the H₂SO₄ solution. The results obtained were explained in terms of boron segregation at grain boundaries, Mo enrichment and film stability (or strength).     

Characterization of the nitrided layers of XC38 carbon steel obtained by R.F. plasma nitriding

XC38 carbon steel was nitrided in a low-pressure R.F. plasma using a mixture of 60% N₂-40% H₂ without cathodic bias on the samples. The experiments were carried out at different temperatures for various time durations. The generated nitride layers were characterized by SEM observations, XRD and GDOS analyses. These analyses indicate that the compound layer was composed of the γ′-Fe₄N phase with a surface content of N close to 6 wt%. An approach was used to study the growth kinetics of the compound layer at 500 °C. Furthermore, it was concluded that its kinetics follows a power law, which deviates from the classical parabolic growth.     

Laser Nitriding of Iron,Stainless Steel,and Plain Carbon Steel Investigated by Mössbauer Spectroscopy

Nitriding is a common method for improving the hardness, mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in air or nitrogen atmospheres with short laser pulses leads to a fast take-up of nitrogen into the irradiated surfaces. This process has been extensively investigated for pure iron, but usually, no tools or functional parts are made of pure iron. Mainly steel or cast iron is used as a base material. Therefore, when looking for technical applicability, also the influence of alloying elements on the laser nitriding process is of great interest. Besides the pure iron various carbon steels and an austenitic stainless steel were studied in laser nitriding experiments in order to investigate the influence of the material itself. Here, the process is investigated via Conversion Electron and X-ray Mössbauer Spectroscopy (CEMS and CXMS), Resonant Nuclear Reaction Analysis (RNRA), and X-Ray Diffraction (XRD). It appears that carbon steels are even better suited for the laser nitriding process than pure iron, and the laser nitriding also works efficiently for the stainless steel which is normally difficult to be nitrided.     

Investigations of meta-stable and post-spinel silicon nitrides

Since the discovery of post-spinel Si₃N₄, its fundamental physical properties are highly required. In this paper, theoretical calculations are performed to investigate the structural and elastic properties of the β-, γ-, wII- and post-spinel Si₃N₄ polymorphs. The calculated ground-state properties compare well with available experiments. The phase transformations of the β-, γ-, wII- and post-spinel phases are investigated by the famous plane-wave pseudo-potential density functional theory. From the elastic constants obtained, we find that β-, γ- and wII-Si₃N₄ are stable at 0 GPa and the post-spinel phase is unstable/stable at 0 GPa/160 GPa. When the high-temperature β→γ transformation is bypassed due to kinetic reasons, β-Si₃N₄ is predicted to undergo a first-order phase transition to a new phase (wII-Si₃N₄). It is found that the transition pressures of β→wII and γ→post-spinel transitions are 20.8 GPa and 152.5 GPa, respectively. The phase boundary of the γ→post-spinel transition can be described as P=152.3631−6.39×10⁻³T+2.01062×10⁻⁵T²−1.93962×10⁻⁹T³. Through the quasi-harmonic approximation, the dependences of heat capacity, entropy, thermal expansion coefficient and the Debye temperature on temperature, are also successfully predicted.     

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.     

Investigation of the Thermal Stability of Laser Nitrided Iron and Stainless Steel by Annealing Treatments

Laser nitriding has revealed to be a very promising and effective treatment to improve the technical properties, like surface hardness and corrosion-wear resistance, of iron and steels. The high nitrogen concentration, the fastness and precision of the treatment and the easy experimental setup make this technique very suitable for applications on industrial scale. Samples of pure iron and austenitic stainless steel have been irradiated with ns laser pulses in the UV radiation range and analyzed by means of Conversion Electron Mössbauer Spectroscopy (CEMS), Resonant Nuclear Reaction Analysis (RNRA), Grazing Incidence X-Ray Diffraction (GXRD) and Microhardness. Mössbauer Spectroscopy, in particular, is capable of detecting the phase composition of the nitrided layer and therefore represents an essential tool for these kind of analysis. The thermal stability of the treated samples have been investigated by subsequent annealings at increasing temperatures in vacuum and in air. For iron samples the annealing treatment at 250°C shows a rather drastic phase transformation from phase (fcc) into (Fe₄N) while a strong depletion of N has been observed for 400°C or higher, regardless of the ambient pressure (atmospheric or vacuum). On the other hand, the stainless steel shows a very good thermal stability up to 500°C, but higher temperatures induce a gradual decrease in the nitrogen concentration which seems to be a common feature for both pure iron and stainless steel. Furthermore, annealing in air leads to the formation of a thin oxide layer on the surface of the iron sample which is easily characterized by Mössbauer spectroscopy.