Low Temperature Gaseous Nitriding Research Articles

Low temperature nitriding behavior in annealed 2205 duplex stainless steel

ABSTRACT Low-temperature gaseous nitriding response of annealed 2205 stainless steel was investigated through a series of nitriding treatments at different temperatures in the range 380–430°C for 25 h. Reflected light microscopy, X-ray diffraction and hardness indentation were utilized to investigate the dependence of the formation of nitrides and cracks on pre annealing temperatures and subsequent nitriding parameters. For most annealing and nitriding conditions applied, the surface adjacent region of the steels was transformed into expanded austenite, or a mixture of expanded austenite and expanded ferrite, supplemented by the presence of iron-based nitrides. Nitriding at a relatively low temperature at a gentle nitriding potential was found to be acceptable as it resulted in a sound nitrided case, where neither chromium nitrides nor cracks developed. Annealing at a relatively low temperature can improve the threshold of nitriding temperature up to 400°C without cracking, while annealing at a high temperature promotes the formation of nitrides.

Improvement in Fatigue Strength of Chromium–Nickel Austenitic Stainless Steels via Diamond Burnishing and Subsequent Low-Temperature Gas Nitriding

Chromium–nickel austenitic stainless steels are widely used due to their high corrosion resistance, good weldability and deformability. To some extent, their application is limited by their mechanical characteristics. As a result of their austenitic structure, increasing the static and dynamic strength of the components can be achieved by surface cold work. Due to the tendency of these steels to undergo intercrystalline corrosion, another approach to improving their mechanical characteristics is the use of low-temperature thermo-chemical diffusion processes. This article proposes a new combined process based on sequentially applied diamond burnishing (DB) and low-temperature gas nitriding (LTGN) to optimally improve the fatigue strength of 304 steel. The essence of the proposed approach is to combine the advantages of the two processes (DB and LTGN) to create a zone of residual compressive stresses in the surface and subsurface layers—the enormous surface residual stresses (axial and hoop) introduced by LTGN, with the significant depth of the compressive zone characteristic of static surface cold working processes. DB (both smoothing and single-pass hardening), in combination with LTGN, achieves a fatigue limit of 600 MPa, an improvement of 36.4% compared to untreated specimens. Individually, smoothing DB, single-pass DB and LTGN achieve 540 MPa, 580 MPa and 580 MPa, respectively. It was found that as the degree of plastic deformation of the surface layer introduced by DB increases, the content of the S-phase in the nitrogen-rich layer formed by LTGN decreases, with a resultant increased content of the ε-phase and a new (also hard) phase: stabilized nitrogen-bearing martensite.

Improved Surface Morphology and Corrosion Resistance Performance of 2205 Duplex Stainless Steel by Low Temperature Gas Nitriding

Improved Surface Morphology and Corrosion Resistance Performance of 2205 Duplex Stainless Steel by Low Temperature Gas Nitriding

Effect of tribo-layer developed during turning of Ti–6Al–4V ELI alloy on its low-temperature gas nitriding

A tribo-layer, produced on parts made of titanium alloys fabricated using various machining techniques, is a natural outcome of these processes. Its removal, improving surface quality before nitriding, may seem not always justified as the latter treatment again causes surface roughening increasing with temperature. Therefore, the aim of the present paper was to investigate the surface integrity aspects of Ti–6Al–4V ELI alloy subjected to turning, i.e. presence of grooving and tribo-layer on the following gas nitriding process but performed at the low temperature of only 540 °C. Roughness and hardness measurements were supplemented by complementary microstructural investigations using SEM, TEM and XPS. These established that the coating of tribo-material on the as-machined alloy consists of oxidized amorphous layers with numerous thicker clumps. Nitriding at 540 °C, for 8 and 16 h, causes a gradual (from top to bottom) transformation of the tribo-layers into mostly crystalline TiO2 rutile phase. Completion of this stage makes the thinner part of tribo-layers permeable to nitrogen, starting nucleation and growth of TiN in the substrate. This process is accompanied by a release of nitrogen from the nitrided region and formation of a thin layer of α-Al2O3 in-between TiO2 rutile and TiN. This shows that efficient nitriding of machined titanium alloy parts, which produces an uniform TiN layer, should be achieved only after a full removal of the tribo-material from their surfaces.

A comparative study of the tribocorrosion behaviour of low temperature nitrided austenitic and duplex stainless steels in NaCl solution

This work studied and compared the tribocorrosion behaviour of low temperature nitridedaustenitic (316L) and duplex (2205) stainless steels. After gaseous nitriding at 450 °C for10 h, the S-phase layer formed on the duplex stainless steel is thicker and harder than that onthe austenitic stainless steel. The formation of S-phase layers reduces both mechanical wearand chemical wear of stainless steels but the duplex type exhibits better tribocorrosion performance. Wearing through of the S-phase layer during tribocorrosion leads to asignificant drop in open circuit potential, friction instability, and total material loss from the nitrided specimens. The work concludes that low temperature gaseous nitriding is effective in improving the tribocorrosion behaviour of these materials to avoiding catastrophic surface failures.

Anisotropy effects on gaseous nitriding of austenitic stainless steel single crystals

Low temperature nitrided austenitic stainless steel exhibits strong orientation dependent case growth kinetics. Anisotropy effects during low temperature gaseous nitriding of single crystals of austenitic stainless steel with surface-normal orientations 〈100〉, 〈111〉 and 〈10 7 2〉 were investigated with thermogravimetry, X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GD-OES), nano-indentation and electron back-scatter diffraction (EBSD). Case growth kinetics were fastest for 〈100〉 and slowest for 〈111〉. Depth-profiling over the case depth was accomplished by successive layer removal combined with X-ray diffraction and nano-indentation. The results indicate that anisotropic case-growth kinetics is related to early nucleation of long-range ordered (LRO) over-stoichiometric γ’N−Me4N1+x (Me=metal atoms) in 〈100〉, which is inseparable from short range ordered (SRO) expanded austenite, γN. Depth ranges of elasto-plastic and purely plastic accommodation of the nitrogen induced lattice expansion were distinguished. In the elasto-plastic region stress-gradient enhanced diffusion of N is observed.

TEM analysis of surface layer of Ti-6Al-4V ELI alloy after slide burnishing and low-temperature gas nitriding

Gas nitriding is one of the more effective ways of improving wear and corrosion resistance of titanium alloys, but this treatment performed at a high temperature degrades their high specific strengths. Therefore, we have subjected a Ti-6Al-4V alloy to a hybrid multi-stage treatment including turning with slide burnishing followed by gas nitriding at temperature reduced down to 540 °C. The mechanical treatment introduces a large number of defects in the form of dislocations and grain boundaries—opening additional paths for nitrogen diffusion. High resolution electron microscopy shows that after such mechanical treatment the surface is covered with an amorphous tribo-film. Below the surface, to a depth of some 100 nm, a layer of elongated dislocation cells is seen. The latter was backed by a 500 nm thick layer of strongly defected laths. Gas nitriding at 540 °C caused at first a crystallization of the tribo-layer (after 8 h), producing a mixture of nitrides and oxides. Next, below a tribo-layer, a dense layer of the δ-TiN phase is formed, with a thickness of 5 nm and 50 nm after 16 h and 24 h, respectively. Application of slide burnishing and low-temperature gas nitriding causes an increase in surface hardness by 5–10 % without compromising the strength of the core material.

Effect of Adjusted Gas Nitriding Parameters on Microstructure and Wear Resistance of HVOF-Sprayed AISI 316L Coatings

Gas nitriding is known as a convenient process to improve the wear resistance of steel components. A precipitation-free hardening by low-temperature processes is established to retain the good corrosion resistance of stainless steel. In cases of thermal spray coatings, the interstitial solvation is achieved without an additional surface activation step. The open porosity permits the penetration of the donator media and leads to a structural diffusion. An inhomogeneous diffusion enrichment occurs at the single spray particle edges within the coating’s microstructure. A decreasing diffusion depth is found with increasing surface distance. The present study investigates an adjusted process management for low-temperature gas nitriding of high velocity oxy-fuel-sprayed AISI 316L coatings. To maintain a homogeneous diffusion depth within the coating, a pressure modulation during the process is studied. Additionally, the use of cracked gas as donator is examined. The process management is designed without an additional surface activation step. Regardless of surface distance, microstructural investigations reveal a homogeneous diffusion depth by a reduced processing time. The constant hardening depth allows a reliable prediction of the coatings’ properties. An enhanced hardness and improved wear resistance is found in comparison with the as-sprayed coating condition.

Abrasive resistance and corrosion properties of AISI 316 sieve via low-temperature gaseous nitriding

AISI 316 austenitic stainless steel samples were treated via gaseous nitriding at low temperature (430 °C) to obtain a single expanded austenite layer (S-phase). The structural phases were characterized via both optical microscopy and X-ray diffraction. Microhardness and dry-sliding wear behavior were investigated via scanning electron microscopy, electron probe of microanalysis, Vickers indentation tester, and ball-on-flat wear tester, respectively. The wear of the substrate AISI 316 steel was severe and characterized by strong adhesion, abrasion, and spallation, while the wear of the S-phase layer was mild and dominated by slight abrasion. Furthermore, the corrosion properties were investigated via potentiodynamic polarization testing in 3.5% NaCl solution. The results show that the S-phase layers was produced at low nitrided temperature with improved wear resistance as well as improved corrosion resistance.

Low-temperature gas nitriding of AISI 4140 steel accelerated by LaFeO3 perovskite oxide

Rare earth nitriding was considered as an effective method to accelerate the nitriding efficiency. However the catalytic mechanism was not clear due to the uncertainty of rare earth (RE) elements’ existence during nitriding. In the present work, the LaFeO3 perovskite oxide was attempted to use as a catalyst in the gas nitriding for the first time. The effect of nitriding temperature on the microstructure and properties of the RE nitrided layer was investigated and the catalytic mechanism of the RE was discussed. The microstructure, phase composition and microhardness profiles of nitrided layers were characterized by the optical microscope, X-ray diffraction and micro-hardness tester, respectively. The corrosion resistance of nitrided layers was tested by the potentiodynamic polarization technique. Pin-on-disc tribometer were applied to evaluate the wear property. The X-ray photoelectron spectroscopy and scanning electron microscope equipped with an energy dispersive X-ray spectroscopy were used to identify the catalytic mechanism of the RE by analyzing the chemical state and elemental distribution on the nitrided surface. The results show that a perovskite-type oxide, LaFeO3, would form after the gas nitriding, which presents an obvious catalytic activity and increases the thickness of nitrided layers. In addition, the mechanical and corrosion properties could also be improved after nitriding with RE addition. The LaFeO3 perovskite oxide could accelerate the nitriding rate by adjusting the ratio of the adsorbed nitrogen (N) and dissolved N. On the other hand, the sufficient oxygen vacancies in the LaFeO3 oxide could provide more diffusion routes for N atoms.

Improvement in dry-sliding wear properties and corrosion resistance of AISI 304 steel with low temperature gaseous nitriding

Improvement in dry-sliding wear properties and corrosion resistance of AISI 304 steel with low temperature gaseous nitriding

Improvement in dry-sliding wear properties and corrosion resistance of AISI 304 steel with low temperature gaseous nitriding

AISI 304 stainless steel samples were treated by gaseous nitriding at 430°C and 560°C in order to obtain a single S-phase and a multiple phases of CrN as well as Fe4N nitride layer, respectively. The structural phases, micro-hardness and dry-sliding wear behaviour of were investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, electron probe of microanalysis, Vickers indentation tester and ball-on-flat wear tester. Besides, the corrosion properties were tested by means of potentiodynamic polarisation testing in 3.5% NaCl solution. The results show that the S-phase layers exhibited a better wear resistance as well as corrosion resistance. Meanwhile, multiple phases also showed a good wear resistance, except a deteriorated corrosion resistance. Wear of the substrate AISI 304 steel was severe and characterised by strong adhesion, abrasion and peeling off, whilst wear of the nitrided samples was mild and dominated by plastic deformation, slight abrasion.

Low-temperature nitriding of nanocrystalline Inconel 718 alloy

Surface severe plastic deformation was induced in Inconel 718 alloy using ultrasonic nanocrystal surface modification (UNSM). Low-temperature gas nitriding was carried out on non-processed and UNSM-processed Inconel 718 samples. The microstructure and material properties were characterized using X-ray diffraction, scanning electron microscopy, the Vickers hardness test, and wear and corrosion testing. The results demonstrated that gas nitriding efficiency was significantly enhanced by the UNSM treatment. Higher nitrogen concentration was observed in the surface layer of the UNSM samples. The nitride layer in the UNSM+nitriding sample exhibited an increased hardness over that of the nitriding sample and the UNSM treated sample. Additionally, both wear resistance and corrosion resistance were significantly improved in the UNSM+nitriding sample.

Magnetic Properties of Austenitic Stainless Steel 316l and 316lvm after High Temperature Gas Nitriding Treatment

Biometallic materials for implant devices not only have to good corrosion resistance but also stable nonmagnetic properties. Various method have been developed for enhanced the corrosion resistance i.e low temperature gas nitriding treatments. Unfortunatelly, low temperature gas nitriding produce weakly ferromagnetic due the presence of expanded austenitic phases. Another treatments methods which is capable for improvement the mechanical properties is high temperature gas nitriding. However, the evaluation of magnetic properties of austenitic stainless steel 316L and 316LVM not yet investigation. The evaluation of magnetic properties of austenitis stainless steel 316L and 316LVM after high temperature gas nitriding treatments have been succesfully done. The magnetic properties are evaluated by vibrating sample magnetometre (VSM) test. The magnetic properties such as magnetic remenance, magnetic saturation and magnetic permeability are improved. As treated 316L and 316LVM have more stable non-magnetic properties and they more safe and compatible for MRI test

Erratum to: “Synthesis of micron particles with Fe–Fe4N core–shell structure at low-temperature gaseous nitriding of iron powder in a stream of ammonia”

Erratum to: “Synthesis of micron particles with Fe–Fe4N core–shell structure at low-temperature gaseous nitriding of iron powder in a stream of ammonia”

Synthesis of micron particles with Fe–Fe4N core–shell structure at low-temperature gaseous nitriding of iron powder in a stream of ammonia

Synthesis of the single-phase γ′-Fe4N on the surface of the micron-sized particles of iron at low-temperature gaseous nitriding of carbonyl iron powder in a stream of ammonia is studied. It is shown that synthesis of particles with such structure is possible with simultaneous control of the number of process parameters: temperature, degree of dissociation of ammonia, and treatment time. It is found that, at temperature T = 400°C and nitriding potential of the atmosphere rN ≈ 1.3 atm−1/2, the shells with a thickness of about 1 μm are formed on the particles within ~15–20 min and the powder consists of the γ-Fe4N phase within ~60 min of treatment. The mechanisms of formation of microparticles with a core–shell structure are considered. A qualitative model for the thermochemical treatment of the micron iron powder with consideration of the diffusion processes of the transport of ammonia molecules in the pore space of the powder and atomic nitrogen diffusion inside the particles is developed. Geometric and dimensional effects at nitriding of iron powders are discussed.

Structure and Properties of Burnished and Nitrided AISI D2 Tool Steel

D2 belongs to traditional steels, frequently used in metalworking industry. Shot peening and nitriding are known to improve the wear resistance of D2. In this work we focus on processes of slide burnishing and industrial low temperature gas nitriding. The D2 steel specimens were first subjected to heat treatments (HT) prescribed by the manufacturer, turning (T), then burnishing (B) and nitriding (N). The reason for turning was achieving appropriate surface roughness. Deformation induced in slide burnishing can be better controlled then in shot peening because of deterministic nature of this process. Four different paths to prepare surfaces were employed: HT + T, HT + T + B, HT + T + N, HT + T + B + N. D2 steel is very sensitive to the final finishing, wear rates vary up to 300 %. Two of our procedures (HT + T + N and HT + T + B + N) are much superior to the others. Moreover, in the HT + T + N case, apparently the surface fatigue scaling off takes place. DOI: http://dx.doi.org/10.5755/j01.ms.21.4.7224

The Nature and Origin of “Double Expanded Austenite” in Ni-Based Ni-Ti Alloys Developing Upon Low Temperature Gaseous Nitriding

Gaseous nitriding of Ni-4 wt pct Ti alloy plates led to the development of double expanded austenite (γ N1 and γ N2) at the surface of the nitride plates. Grazing-incidence X-ray diffraction analysis demonstrated that the component γ N1 is located close to the surface and the component γ N2 is located at a certain depth below the specimen surface, in correspondence with a layered character of the nitrided zone beneath the surface as revealed by optical microscopy. Electron probe microanalysis, atom probe tomography, and Laue microdiffraction analysis did not reveal a significant difference in nitrogen content of the γ N1 and γ N2 sublayers. By X-ray diffraction stress analysis it was shown that the only significant differences of the two expanded austenite layers is a pronounced difference in compressive stress parallel to the surface: the γ N1 layer is subjected to a huge compressive stress, as large as a few GPa, whereas a relatively modest stress prevails in the γ N2 layer.

Low Temperature Nitriding of a Martensitic Stainless Steel

Nitriding is as an effective technique applied for many years to improve the surface hardness and wear resistance of low carbon and tool steels [1]. In the case of stainless steels, increase of surface hardness and wear resistance accompany by a drop in corrosion resistance due to the precipitation of CrN. In this respect, many attempts have been made to modify the surfaces of austenitic stainless steels to increase their surface hardness and wear resistance without scarifying the corrosion resistance [2-6]. It is finally concluded that, nitriding at temperatures lower than conventional nitriding process (which is generally about 550°C) has potentiality to produce a nitrogen expanded austenite (also known as S-phase), on the surface without formation of CrN. Due to the superb properties of the S-phase, the low temperature nitrided austenitic stainless steels exhibit very high surface hardness, a good wear resistance, and more importantly, an excellent corrosion resistance. Recently some attempts have been made to apply low temperature nitriding to martensitic stainless steels, which are widely used in the industries of medicine, food, mold and other civil areas [7-9]. In these works, where nitriding has been conducted by plasma processes, superior surface hardness, along with excellent wear and corrosion resistances have been reported for AISI 410 and AISI 420 grade martensitic stainless steels. This work focuses on low temperature gas nitriding of AISI 420 grade martensitic stainless steel in a fluidized bed reactor. In this respect the microstructures, phase compositions, hardness, wear and corrosion behaviours of the original and nitrided martensitic stainless steels have been compared.

Low temperature gaseous surface hardening of stainless steel

Abstract The present contribution gives an overview of some of the technological aspects of low temperature thermochemical treatment of stainless steel. Examples of low temperature gaseous nitriding, carburising and nitrocarburising of stainless steel are presented and discussed. In particular, the morphology, microstructure and characteristics of so-called expanded austenite “layers” on stainless steel are addressed.