Expanded Austenite Research Articles

Identification of Expanded Austenite in Nitrogen-Implanted Ferritic Steel through In Situ Synchrotron X-ray Diffraction Analyses

The existence and formation of expanded austenite in ferritic stainless steels remains a subject of debate. This research article aims to provide comprehensive insights into the formation and decomposition of expanded austenite through in situ structure analyses during thermal treatments of ferritic steels. To achieve this objective, we employed the Plasma Immersion Ion Implantation (PIII) technique for nitriding in conjunction with in situ synchrotron X-ray diffraction (ISS-XRD) for microstructural analyses during the thermal treatment of the samples. The PIII was carried out at a low temperature (300–400 °C) to promote the formation of metastable phases. The ISS-XRD analyses were carried out at 450 °C, which is in the working temperature range of the ferritic steel UNS S44400, which has applications, for instance, in the coating of petroleum distillation towers. Nitrogen-expanded ferrite (αN) and nitrogen-expanded austenite (γN) metastable phases were formed by nitriding in the modified layers. The production of the αN or γN phase in a ferritic matrix during nitriding has a direct relationship with the nitrogen concentration attained on the treated surfaces, which depends on the ion fluence imposed during the PIII treatment. During the thermal evolution of crystallographic phase analyses by ISS-XRD, after nitriding, structure evolution occurs mainly by nitrogen diffusion. In the nitrided samples prepared under the highest ion fluences—longer treatment times and frequencies (PIII 300 °C 6 h and PIII 400 °C 3 h) containing a significant amount of γN—a transition from the γN phase to the α and CrN phases and the formation of oxides occurred.

Nitrogen isotope marker experiments in austenitic stainless steel for identification of trapping/detrapping processes at different temperatures

Expanded austenite – austenitic stainless steel containing a colossal amount of nitrogen in solid solution – is still some kind of conundrum with the nitrogen atom transport from the surface into the bulk being enhanced for higher nitrogen content. At the same time, atomic ordering due to an enhanced affinity between chromium and nitrogen is observed.Here, nitrogen transport experiments are performed at different temperatures using 14N and 15N as isotopic tracers. The depth distribution of each of these isotopes as well as the total nitrogen content is investigated with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and glow discharge optical emission spectroscopy (GDOES), respectively. In parallel, phase information is obtained using in situ X-ray diffraction (XRD) during nitriding (for time-resolved data) and during sputter depth profiling (for depth-resolved data).During nitriding, no delayed transport of nitrogen, i.e. trapping, is observed for the expanded austenite. However, as soon as CrN precipitates are formed, nitrogen trapping is a dominant factor with an almost perfect correlation between precipitation and trapping. Cluster ion analysis in SIMS confirms this effect with the abundance of Cr2+ ions significantly increased compared to FeCr+ ions in the presence of precipitation. As nitrogen transport and CrN formation occur in parallel at higher temperatures, further experiments to separate the CrN formation at higher temperatures from the nitrogen transport at lower temperatures have been performed.

Co-existence of γ'N phase and γN phase on nitrided austenitic Fe-Cr-Ni alloys - III. An investigation of the evolution of long-range ordered domains

Experimentally and theoretically, the basic structures of disordered expanded austenite, γN, and ordered expanded austenite, γ'N, have been confirmed to consist of Cr-N short-range ordering (SRO) and Fe4N-like long-range ordering (LRO) in addition to Cr-N SRO, respectively. So far, the transition from γN to γ'N during low temperature nitriding has not been elucidated. In present work, four f.c.c. Fe-Cr-Ni alloys with 0, 6, 12 and about 18 wt.% Cr in low-temperature nitrided condition were investigated with transmission electron microscopy (TEM) to explore the evolution from γN to γ'N. The Fe4N-like LRO is present as small ordered domains separated by antiphase boundaries. Ordering of the interstitial nitrogen involves nucleation, growth and, after impingement, coarsening of the ordered domains. The domain size decreases with Cr content and increases with N content in the investigated alloys. A 3-dimensional Cellular Automaton (CA) was developed to simulate nucleation and growth until impingement into full LRO was reached; also the subsequent coarsening stage was simulated. The simulated distributions of ordered domains for the range of Cr contents are consistent with the TEM results. Cr-N SRO plays both a role of promoting nucleation and limiting growth of the ordered domains. Diffraction simulation by Fourier transforming the CA-simulated nitrogen distribution is consistent with the diffraction data and clarifies the different roles of high-density antiphase boundaries and Cr-N SRO on the superlattice reflections. The simulation is further validated by comparison of the predicted distributions of occupied interstices to literature data for Mössbauer spectroscopy and Extended X-ray Absorption Fine Structure.

Initial phase formation during nitriding of austenitic stainless steel

Expanded austenite formed by inserting nitrogen into austenitic stainless steel is a puzzling phase since its discovery about 30 years ago: variable nitrogen concentration correlated with a variable lattice constant together with an unusual depth profile normally indicating a compound layer instead of a diffusion layer. While expanded austenite has been described for nitriding experiment durations as low as 5 min and >48 h, the initial phase transition has not been investigated in detail. Here, very low ion current density experiments prolonging the initial phase together with in-situ X-ray diffraction (XRD) experiments during nitriding are presented. Additionally, the formed layers are investigated by depth-resolved in-situ XRD measurements during successive removal of the surface layer. It is shown that two phases, a low expansion phase and a high expansion phase, are formed sequentially over time with the growth rate of the latter phase being much higher, thus obliterating any trace of the initial phase. It is surmised that the driving force is a threshold in the nitrogen concentration.

Nanosized Cr-N clustering in expanded austenite layer of low temperature plasma-nitrided Fe-35Ni-10Cr alloy

Expanded austenite (γN) formed by low temperature nitriding of austenitic stainless steel is a N-enriched fcc containing Cr-N short range ordering, but no direct observation of Cr-N clustering has been reported. In the present study, the nanostructure of an Fe-35Ni-10Cr (at%) alloy plasma-nitrided at 673 K was investigated using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) technique in this study. Nanosized Cr-N clusters were directly observed in the γN accompanied with strong streaks in selected area diffraction, obvious modulated structure in TEM and Cr-N rich regions observed by 3DAP. Thermodynamic calculations suggested that Cr-N clusters were formed by spinodal decomposition due to a strong Cr-N attractive interaction. The hardness of the γN layer was much higher than that of Fe-N austenite steel, which indicates that the hardness is not only due to nitrogen solid-solution hardening but also to a synergetic effect of the coexistence of Cr and N.

Low Temperature Carburizing of Stainless Steels and the Development of Carbon Expanded Austenite*

Abstract Low-temperature carburizing dramatically enhances the inherently low wear resistance of austenitic stainless steels due to the formation of a carbon-supersaturated solid solution, i.e. expanded austenite. The formation of expanded austenite from low-temperature carburizing has been intensively investigated. However, the influence of chemical composition of the stainless steel on the carburizing response has not received the same interest. This contribution addresses the effect of the chemical composition on low-temperature carburizing in terms of carbon solubility, decomposition of expanded austenite upon exceeding the solubility limit and the elasto-plastic accommodation of the carbon-induced lattice expansion. The results demonstrate that the carbon solubility increases with an increasing Cr-equivalent and that higher Cr- and Ni-equivalents favor the formation of Cr-based M7C3 over Fe-based Hägg (M5C2) carbide.

The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review

Low-temperature treatments have become a valuable method for improving the surface hardness of stainless steels, and thus their tribological properties, without impairing their corrosion resistance. By using treatment temperatures lower than those usually employed for nitriding or carburizing of low alloy steels or tool steels, it is possible to obtain a fairly fast (interstitial) diffusion of nitrogen and/or carbon atoms; on the contrary, the diffusion of substitutional atoms, as chromium atoms, has significantly slowed down, therefore the formation of chromium compounds is hindered, and corrosion resistance can be maintained. As a consequence, nitrogen and carbon atoms can be retained in solid solutions in an iron lattice well beyond their maximum solubility, and supersaturated solid solutions are produced. Depending on the iron lattice structure present in the stainless steel, the so-called “expanded austenite” or “S-phase”, “expanded ferrite”, and “expanded martensite” have been reported to be formed. This review summarizes the main studies on the characteristics and properties of these “expanded” phases and of the modified surface layers in which these phases form by using low-temperature treatments. A particular focus is on expanded martensite and expanded ferrite. Expanded austenite–S-phase is also discussed, with particular reference to the most recent studies.

Decomposition kinetics of expanded austenite with high nitrogen contents

Abstract This paper addresses the decomposition kinetics of synthesized homogeneous expanded austenite formed by gaseous nitriding of stainless steel AISI 304L and AISI 316L with nitrogen contents up to 38 at.% nitrogen. Isochronal annealing experiments were carried out in both inert (N2) and reducing (H2) atmospheres. Differential thermal analysis (DTA) and thermogravimetry were applied for identification of the decomposition reactions and X-ray diffraction analysis was applied for phase analysis. CrN precipitated upon annealing; the activation energies are 187 kJ/mol and 128 kJ/mol for AISI 316L and AISI 304L, respectively. Isothermal stability plots for expanded austenite developed from AISI 304L and AISI 316 were obtained.

Effect of Nitrogen Concentration Profile on the Wear Rate of Expanded Austenite Phase

Effect of Nitrogen Concentration Profile on the Wear Rate of Expanded Austenite Phase

Effects of Plasma-Chemical Composition on AISI 316L Surface Modification by Active Screen Nitrocarburizing Using Gaseous and Solid Carbon Precursors

Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated.

Nano-Sized Cr-N Clustering in Expanded Austenite Layer of Low Temperature Plasma Nitrided Fe-35Ni-10Cr Alloy

Nano-Sized Cr-N Clustering in Expanded Austenite Layer of Low Temperature Plasma Nitrided Fe-35Ni-10Cr Alloy

Diffusional Model of Expanded Austenite Layer Growth on AISI 316L Stainless Steel During Low-Temperature Hybrid Thermochemical Treatment

A simultaneous nitrogen and carbon diffusional model in AISI 316L stainless steel is presented in this numerical study to simulate expanded austenite layer growth developed during hybrid thermochem...

Depth-Resolved Phase Analysis of Expanded Austenite Formed in Austenitic Stainless Steel

Expanded austenite γN formed after nitrogen insertion into austenitic stainless steel and CoCr alloys is known as a hard and very wear resistant phase. Nevertheless, no single composition and lattice expansion can describe this phase with nitrogen in solid solution. Using in situ X-ray diffraction (XRD) during ion beam sputtering of expanded austenite allows a detailed depth-dependent phase analysis, correlated with the nitrogen depth profiles obtained by time-of-flight secondary ion mass spectrometry (ToF-SIMS) or glow discharge optical emission spectroscopy (GDOES). Additionally, in-plane XRD measurements at selected depths were performed for strain analysis. Surprisingly, an anomalous peak splitting for the (200) expanded peak was observed for some samples during nitriding and sputter etching, indicating a layered structure only for {200} oriented grains. The strain analysis as a function of depth and orientation of scattering vector (parallel/perpendicular to the surface) is inconclusive.

Plasma Nitrocarburizing of AISI 316L Austenitic Stainless Steel: a First Step for Treatment of Components with Complex Geometries

Abstract The surfaces of AISI 316L austenitic stainless steel components with complex shapes and geometries can be subjected to extreme loads with intensive wear stress resulting a short lifetime. Low temperature active screen plasma nitrocarburizing (ASPNC) can be applied to form a thin duplex layer known as expanded austenite, improving the hardness and wear resistance with acceptable corrosion resistance. However, ASPNC of shaped components and components with different aspect ratios may yield non-uniform expanded austenite layer and/or reproducibility problems limiting their applications for specific cases. In this study, first, ASPNC of AISI 316L treated at different temperatures was investigated using active screen made of carbon-fibre reinforced carbon (CFC) to determine a reliable treatment temperature without CrN precipitation. In addition, several non-uniformity effects related to geometry and shape of structured samples were investigated during ASPNC treatment at different biased conditions. It was shown that in case of structured samples, a weak bias power at the samples was an essential process parameter to guarantee the formation of thick and homogenous expanded austenite layer while in non-biased conditions, thin and inhomogeneous expanded austenite is formed.

Plasma Nitrocarburizing of AISI 316L Austenitic Stainless Steel: a First Step for Treatment of Components with Complex Geometries

Abstract The surfaces of AISI 316L austenitic stainless steel components with complex shapes and geometries can be subjected to extreme loads with intensive wear stress resulting a short lifetime. Low temperature active screen plasma nitrocarburizing (ASPNC) can be applied to form a thin duplex layer known as expanded austenite, improving the hardness and wear resistance with acceptable corrosion resistance. However, ASPNC of shaped components and components with different aspect ratios may yield non-uniform expanded austenite layer and/or reproducibility problems limiting their applications for specific cases. In this study, first, ASPNC of AISI 316L treated at different temperatures was investigated using active screen made of carbon-fibre reinforced carbon (CFC) to determine a reliable treatment temperature without CrN precipitation. In addition, several non-uniformity effects related to geometry and shape of structured samples were investigated during ASPNC treatment at different biased conditions. It was shown that in case of structured samples, a weak bias power at the samples was an essential process parameter to guarantee the formation of thick and homogenous expanded austenite layer while in non-biased conditions, thin and inhomogeneous expanded austenite is formed.

From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

Austenitic stainless steels are employed in many industrial fields, due to their excellent corrosion resistance, easy formability and weldability. However, their low hardness, poor tribological properties and the possibility of localized corrosion in specific environments may limit their use. Conventional thermochemical surface treatments, such as nitriding or carburizing, are able to enhance surface hardness, but at the expense of corrosion resistance, owing to the formation of chromium-containing precipitates. An effective alternative is the so called low temperature treatments, which are performed with nitrogen- and/or carbon-containing media at temperatures, at which chromium mobility is low and the formation of precipitates is hindered. As a consequence, interstitial atoms are retained in solid solution in austenite, and a metastable supersaturated phase forms, named expanded austenite or S phase. Since the first studies, dating 1980s, the S phase has demonstrated to have high hardness and good corrosion resistance, but also other interesting properties and an elusive structure. In this review the main studies on the formation and characteristics of S phase are summarized and the results of the more recent research are also discussed. Together with mechanical, fatigue, tribological and corrosion resistance properties of this phase, electric and magnetic properties, wettability and biocompatibility are overviewed.

Is “expanded austenite” really a solid solution? Mössbauer observation of an annealed AISI 316L nitrided sample

Expanded austenite, also known as S phase, m phase or γN phase forms during low temperature (<420 °C) nitriding of austenitic materials containing chromium. In this communication, annealing of a nitrided sample is used as a tool to understand the nature of expanded austenite. Starting from an AISI 316LN sample plasma nitrided for 4 h at 410 °C, the decomposition of expanded austenite is studied by conversion electron Mössbauer spectroscopy (CEMS) and X ray diffraction (XRD) for different annealing times at 400 °C. For the as nitrided sample, the CEMS spectra are analysed using two hyperfine field distributions (HFD). HFD A is attributed to a nitrogen enriched expanded austenite, HFD B is attributed to a bcc environment without or with very low nitrogen content. Both the local deformation of the bcc lattice and the presence of other elements as iron neighbouring atoms explain the broad range of hyperfine field in HFD B. The vanishing of HFD A, after 2 h of annealing, is explained by the formation of short-range order (SRO) between chromium and nitrogen and by the redistribution of nitrogen in excess. For the 3 h of annealing, a bcc phase is observed by XRD. We propose to describe this phase as martensite. An interplay between local internal stress and local nitrogen content is proposed to explain the unexpected formation of such a martensitic phase in a strongly gamma stabilizing environment. From the microscopic point of view expanded austenite can be considered to be constituted of at least two different environments: a gamma environment supersaturated in nitrogen and a martensitic environment without nitrogen.

Expanded Austenite Phase Formed by Low-Temperature Plasma Carburizing of Austenite Stainless Steel and Its Sliding Characteristics

Expanded Austenite Phase Formed by Low-Temperature Plasma Carburizing of Austenite Stainless Steel and Its Sliding Characteristics

Residual Stress in Expanded Austenite on Stainless Steel; Origin, Measurement, and Prediction

Abstract Expanded austenite is a supersaturated solid solution of nitrogen/carbon in austenite that forms as a case by the diffusion of nitrogen/carbon into austenitic stainless steel. Expanded austenite has a high level of hardness that provides resistance against galling and wear, superior resistance against localized corrosion, and contributes to improvement of the fatigue performance. This latter characteristic is a consequence of the huge compressive residual stresses in the expanded austenite case. Such stresses are induced by the high interstitial content in the austenite lattice and are accommodated elasto-plastically. The experimental assessment of the elastic lattice strains is complicated by the presence of steep composition-depth and stress-depth profiles, which necessitate special measurement or correction procedures to unravel the influence of composition and stress on the lattice spacing and avoid artifacts arising from (steep) lattice-spacing gradients. In the present work the sin2ψ method was combined with grazing incidence X-ray diffraction to keep the information depth during measurement shallow, independent of the (effective) tilt angle ψ. The plastic strains in the expanded austenite 27 zone were estimated from the lattice rotations, as determined with electron backscatter diffraction. It is demonstrated that the level of elastic lattice strains in expanded austenite can be adjusted by retracting part of the dissolved nitrogen. The experimental results for elastic and plastic strains are compared to those predicted by a comprehensive numerical model that simulates the time-dependent development of composition-depth and stress-depth profiles in expanded austenite. The work described in this manuscript is a combination of a review of previously achieved and published results as well as the newest results of ongoing research activities.

Experimental and numerical analysis of residual stress in carbon-stabilized expanded austenite

Expanded austenite obtained by gaseous carburizing of stainless steel was investigated with X-ray diffraction to determine composition-depth and residual stress-depth distributions. Avoiding ghost stress effects in the analysis of X-ray diffraction data, the obtained composition- and stress-depth profiles are in excellent quantitative agreement with those obtained with other techniques. The residual stress-depth profile was attempted calculated from the composition-depth profile assuming elastic-plastic accommodation of the lattice expansion. In the model, composition-dependence of Young's modulus, yield stress and work hardening exponent were considered. Excellent quantitative agreement was achieved between the experimental and numerical residual stress-depth profiles.