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Marcel A.j. Somers - One of the best experts on this subject based on the ideXlab platform.
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experimental and numerical analysis of residual stress in carbon stabilized Expanded Austenite
Scripta Materialia, 2018Co-Authors: Yawei Peng, Jianming Gong, Yong Jiang, Bo Wang, Marcel A.j. SomersAbstract:Abstract 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.
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residual stress in Expanded Austenite on stainless steel origin measurement and prediction
Materials Performance and Characterization, 2018Co-Authors: Marcel A.j. Somers, Thomas Lundin Christiansen, Omer C Kucukyildiz, Casper Alexander Ormstrup, Hossein Alimadadi, Jesper Henri Hattel, Grethe WintherAbstract: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.
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measurement and tailoring of residual stress in Expanded Austenite on austenitic stainless steel
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: Frederico Augusto Pires Fernandes, Grethe Winther, Thomas Lundin Christiansen, Marcel A.j. SomersAbstract:Abstract Expanded Austenite on stainless steel with a high interstitial nitrogen content is characterized by elasto-plastic accommodation of the large composition-induced lattice expansion leading to huge compressive residual stress. The elasto-plastic accommodation as well as the (steep) concentration profile has implications for the measurement strategy to determine lattice strains and associated residual stresses with X-ray diffraction. Lattice strain measurements were performed on nitrided as well as subsequently de-nitrided Expanded Austenite on AISI 316L stainless steel, for various grazing incidence angles. It is demonstrated that keeping the information depth constant by choosing appropriate combinations of grazing incidence and tilt angle leads to reliable results for the 111 reflection, while the 200 reflection should be avoided. Further, it is shown for the first time that the residual stresses in Expanded Austenite can be tailored by de-nitriding after nitriding, such that a condition of virtually zero stress at the surface is obtained.
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on the elusive crystal structure of Expanded Austenite
Scripta Materialia, 2017Co-Authors: Bastian Brink, Kenny Stahl, Thomas Lundin Christiansen, Jette Oddershede, Grethe Winther, Marcel A.j. SomersAbstract:Abstract No consistent structural description exists for Expanded Austenite that accurately accounts for the hkl -dependent peak shifts and broadening observed in diffraction experiments. The best available description for homogeneous samples is a face-centered cubic lattice with stacking faults. Here Debye simulations of stacking fault effects were compared to experimental data for macro-stress free homogeneous Expanded Austenite to show that a faulted structure cannot explain the observed peak displacement anomalies. Instead it is argued that the shifts are the combined result of elastic and plastic anisotropy leading to (strongly) non-linear hkl -dependent elastic behavior during composition-induced plastic deformation on synthesis of Expanded Austenite.
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on the carbon solubility in Expanded Austenite and formation of hagg carbide in aisi 316 stainless steel
Steel Research International, 2016Co-Authors: Thomas Lundin Christiansen, Kenny Stahl, Bastian Brink, Marcel A.j. SomersAbstract:The carbon solubility in Expanded Austenite is investigated by controlled low temperature gaseous through-carburizing of AISI 316 stainless steel thin foils with thermogravimetry and synchrotron powder diffraction analysis. Carburizing is carried out in C2H2–H2–N2 and CO–H2–N2 atmospheres at 380–420 °C and 465–470 °C, respectively. Hagg carbide (χ-M5C2) develops when the carbon content in the Expanded Austenite exceeds the metastable solubility limit; the transformation of carbon Expanded Austenite into Hagg carbide occurs irrespective of carburizing temperature in the investigated temperature range (380–470 °C). The maximum solubility of carbon in Expanded Austenite (380 °C) is found to correspond to an occupancy (yC) of 0.220 of the interstitial octahedral sites of the Austenite lattice (i.e., 4.74 wt% C). Decomposition of Hagg carbide into M7C3 occurs upon prolonged carburizing treatment or thermal exposure in inert atmosphere (in situ synchrotron experiments).
Thomas Lundin Christiansen - One of the best experts on this subject based on the ideXlab platform.
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residual stress in Expanded Austenite on stainless steel origin measurement and prediction
Materials Performance and Characterization, 2018Co-Authors: Marcel A.j. Somers, Thomas Lundin Christiansen, Omer C Kucukyildiz, Casper Alexander Ormstrup, Hossein Alimadadi, Jesper Henri Hattel, Grethe WintherAbstract: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.
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measurement and tailoring of residual stress in Expanded Austenite on austenitic stainless steel
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: Frederico Augusto Pires Fernandes, Grethe Winther, Thomas Lundin Christiansen, Marcel A.j. SomersAbstract:Abstract Expanded Austenite on stainless steel with a high interstitial nitrogen content is characterized by elasto-plastic accommodation of the large composition-induced lattice expansion leading to huge compressive residual stress. The elasto-plastic accommodation as well as the (steep) concentration profile has implications for the measurement strategy to determine lattice strains and associated residual stresses with X-ray diffraction. Lattice strain measurements were performed on nitrided as well as subsequently de-nitrided Expanded Austenite on AISI 316L stainless steel, for various grazing incidence angles. It is demonstrated that keeping the information depth constant by choosing appropriate combinations of grazing incidence and tilt angle leads to reliable results for the 111 reflection, while the 200 reflection should be avoided. Further, it is shown for the first time that the residual stresses in Expanded Austenite can be tailored by de-nitriding after nitriding, such that a condition of virtually zero stress at the surface is obtained.
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on the elusive crystal structure of Expanded Austenite
Scripta Materialia, 2017Co-Authors: Bastian Brink, Kenny Stahl, Thomas Lundin Christiansen, Jette Oddershede, Grethe Winther, Marcel A.j. SomersAbstract:Abstract No consistent structural description exists for Expanded Austenite that accurately accounts for the hkl -dependent peak shifts and broadening observed in diffraction experiments. The best available description for homogeneous samples is a face-centered cubic lattice with stacking faults. Here Debye simulations of stacking fault effects were compared to experimental data for macro-stress free homogeneous Expanded Austenite to show that a faulted structure cannot explain the observed peak displacement anomalies. Instead it is argued that the shifts are the combined result of elastic and plastic anisotropy leading to (strongly) non-linear hkl -dependent elastic behavior during composition-induced plastic deformation on synthesis of Expanded Austenite.
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on the carbon solubility in Expanded Austenite and formation of hagg carbide in aisi 316 stainless steel
Steel Research International, 2016Co-Authors: Thomas Lundin Christiansen, Kenny Stahl, Bastian Brink, Marcel A.j. SomersAbstract:The carbon solubility in Expanded Austenite is investigated by controlled low temperature gaseous through-carburizing of AISI 316 stainless steel thin foils with thermogravimetry and synchrotron powder diffraction analysis. Carburizing is carried out in C2H2–H2–N2 and CO–H2–N2 atmospheres at 380–420 °C and 465–470 °C, respectively. Hagg carbide (χ-M5C2) develops when the carbon content in the Expanded Austenite exceeds the metastable solubility limit; the transformation of carbon Expanded Austenite into Hagg carbide occurs irrespective of carburizing temperature in the investigated temperature range (380–470 °C). The maximum solubility of carbon in Expanded Austenite (380 °C) is found to correspond to an occupancy (yC) of 0.220 of the interstitial octahedral sites of the Austenite lattice (i.e., 4.74 wt% C). Decomposition of Hagg carbide into M7C3 occurs upon prolonged carburizing treatment or thermal exposure in inert atmosphere (in situ synchrotron experiments).
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composition dependent variation of magnetic properties and interstitial ordering in homogeneous Expanded Austenite
Acta Materialia, 2016Co-Authors: Bastian Brink, Kenny Stahl, Thomas Lundin Christiansen, Cathrine Frandsen, Mikkel Fougt Hansen, Marcel A.j. SomersAbstract:Abstract The crystal structure and magnetic properties of austenitic stainless steel with a colossal interstitial content, so-called Expanded Austenite, are currently not completely understood. In the present work, the magnetic properties of homogeneous samples of Expanded Austenite, as prepared by low-temperature nitriding of thin foils, were investigated with magnetometry and Mossbauer spectroscopy. At room temperature, Expanded Austenite is paramagnetic for relatively low and for relatively high nitrogen contents (yN = 0.13 and 0.55, respectively, where yN is the interstitial nitrogen occupancy), while ferromagnetism is observed for intermediate nitrogen loads. Spontaneous volume magnetostriction was observed in the ferromagnetic state and the Curie temperature was found to depend strongly on the nitrogen content. For the first time, X-ray diffraction evidence for the occurrence of long-range interstitial order of nitrogen atoms in Expanded Austenite was observed for high nitrogen contents.
Frederico Augusto Pires Fernandes - One of the best experts on this subject based on the ideXlab platform.
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measurement and tailoring of residual stress in Expanded Austenite on austenitic stainless steel
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: Frederico Augusto Pires Fernandes, Grethe Winther, Thomas Lundin Christiansen, Marcel A.j. SomersAbstract:Abstract Expanded Austenite on stainless steel with a high interstitial nitrogen content is characterized by elasto-plastic accommodation of the large composition-induced lattice expansion leading to huge compressive residual stress. The elasto-plastic accommodation as well as the (steep) concentration profile has implications for the measurement strategy to determine lattice strains and associated residual stresses with X-ray diffraction. Lattice strain measurements were performed on nitrided as well as subsequently de-nitrided Expanded Austenite on AISI 316L stainless steel, for various grazing incidence angles. It is demonstrated that keeping the information depth constant by choosing appropriate combinations of grazing incidence and tilt angle leads to reliable results for the 111 reflection, while the 200 reflection should be avoided. Further, it is shown for the first time that the residual stresses in Expanded Austenite can be tailored by de-nitriding after nitriding, such that a condition of virtually zero stress at the surface is obtained.
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Low Temperature Plasma Nitriding and Nitrocarburising of a Superaustenitic Stainless Steel
Materials Performance and Characterization, 2016Co-Authors: Frederico Augusto Pires Fernandes, Luiz Carlos Casteletti, Juno GallegoAbstract:Low temperature surface engineering of stainless steels by dissolving large quantities of nitrogen and carbon has become a favorable technological process. Such treatments usually yield the so-called Expanded Austenite, which is a super-saturated diffusion zone and can be produced by a salt, gas, or plasma-based processes. The present manuscript addressed the production of Expanded Austenite on a superaustenitic stainless steel at low temperature. Gas mixtures with nitrogen or both nitrogen and carbon bearing gases were applied in a plasma atmosphere at 400°C for 5 h. Microstructural characterization was conducted applying light optical microscopy, X-ray diffraction, and transmission electron microscopy. In addition, microhardness measurements were performed at the surface of the samples. Plasma nitriding and nitrocarburising at 400°C resulted in a homogeneous case composed solely by Expanded Austenite as detected by X-ray diffraction. Similar microhardness was observed for both processes, whereas nitrocarburising provided a thicker diffusion zone when compared to nitriding. Fine iron-chromium nitride precipitation was only identified by transmission electron microscopy (TEM). Selected area electron diffraction yields similar lattice parameters for both processes.
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on the determination of stress profiles in Expanded Austenite by grazing incidence x ray diffraction and successive layer removal
Acta Materialia, 2015Co-Authors: Frederico Augusto Pires Fernandes, Grethe Winther, Thomas Lundin Christiansen, Marcel A.j. SomersAbstract:Abstract Surface layers of Expanded Austenite resulting from nitriding typically exhibit large gradients in residual stress and composition. Evaluation of residual-stress profiles is explored by means of grazing incidence X-ray diffraction (GI-XRD), probing shallow depths, combined with successive layer removal. Several factors complicating the stress determination are analysed and discussed: (1) ghost stresses arising from a small variation in the shallow information depths probed with GI-XRD, (2) selection of the grain interaction model used to calculate the X-ray elastic constants for conversion of lattice strains into residual stress and (3) the composition dependence of these elastic constants.
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determination of stress profiles in Expanded Austenite by combining successive layer removal and gi xrd
Advanced Materials Research, 2014Co-Authors: Frederico Augusto Pires Fernandes, Thomas Lundin Christiansen, Marcel A.j. SomersAbstract:The present work deals with the evaluation of the residual-stress profile in Expanded- Austenite by successive removal steps using GI-XRD. Preliminary results indicate stresses of several GPa's from 111 and 200 diffraction lines. These stresses appear largest for the 200 reflection. The strain-free lattice parameter decayed smoothly with depth, while for the compressive stress a maximum value is observed at some depth below the surface. Additionally a good agreement was found between the nitrogen profile determined with GDOES analysis and the strain-free lattice parameter from XRD.
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Microstructure of nitrided and nitrocarburized layers produced on a superaustenitic stainless steel
Journal of materials research and technology, 2013Co-Authors: Frederico Augusto Pires Fernandes, Luiz Carlos Casteletti, Juno GallegoAbstract:The Expanded Austenite N can be produced in austenitic stainless steels by plasma nitriding, carburizing or nitrocarburizing at low temperatures. This metastable phase presents higher hardness and toughness if compared with traditional nitride layers whilst also maintaining the corrosion resistance. However, the application of plasmas composed by both nitrogen and carbon is technologically recent and the effect of such process on the microstructure and properties of the nitrocarburized layers is still under investigation. In this study, samples of UNS S31254 superaustenitic stainless steel were produced by plasma nitriding and nitrocarburizing at 400 ◦ C, 450 ◦ C and 500 ◦ C for 5 h. The plasma treated samples were observed by optical and transmission electron microscopy and also analyzed by X-ray diffraction. The thickness of the layers increased with temperature and the nitrocarburized layers were thicker than nitrided at a given temperature. The presence of Expanded Austenite was confirmed by X-ray diffraction through its characteristic anomalous shift on the diffracted peaks related to the Austenite. Nitride formation on samples produced at 400 ◦ C was only identified by transmission electron microscopy where fine rounded particles with 10–15 nm size revealed reflections consistent with the CrN cubic chromium nitride. The estimated lattice parameter from the Expanded Austenite ranged from 0.38 to 0.41 nm depending on the employed { hkl } reflection which was found to be 6–11% larger than the untreated Austenite lattice parameter.
Kenny Stahl - One of the best experts on this subject based on the ideXlab platform.
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on the elusive crystal structure of Expanded Austenite
Scripta Materialia, 2017Co-Authors: Bastian Brink, Kenny Stahl, Thomas Lundin Christiansen, Jette Oddershede, Grethe Winther, Marcel A.j. SomersAbstract:Abstract No consistent structural description exists for Expanded Austenite that accurately accounts for the hkl -dependent peak shifts and broadening observed in diffraction experiments. The best available description for homogeneous samples is a face-centered cubic lattice with stacking faults. Here Debye simulations of stacking fault effects were compared to experimental data for macro-stress free homogeneous Expanded Austenite to show that a faulted structure cannot explain the observed peak displacement anomalies. Instead it is argued that the shifts are the combined result of elastic and plastic anisotropy leading to (strongly) non-linear hkl -dependent elastic behavior during composition-induced plastic deformation on synthesis of Expanded Austenite.
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on the carbon solubility in Expanded Austenite and formation of hagg carbide in aisi 316 stainless steel
Steel Research International, 2016Co-Authors: Thomas Lundin Christiansen, Kenny Stahl, Bastian Brink, Marcel A.j. SomersAbstract:The carbon solubility in Expanded Austenite is investigated by controlled low temperature gaseous through-carburizing of AISI 316 stainless steel thin foils with thermogravimetry and synchrotron powder diffraction analysis. Carburizing is carried out in C2H2–H2–N2 and CO–H2–N2 atmospheres at 380–420 °C and 465–470 °C, respectively. Hagg carbide (χ-M5C2) develops when the carbon content in the Expanded Austenite exceeds the metastable solubility limit; the transformation of carbon Expanded Austenite into Hagg carbide occurs irrespective of carburizing temperature in the investigated temperature range (380–470 °C). The maximum solubility of carbon in Expanded Austenite (380 °C) is found to correspond to an occupancy (yC) of 0.220 of the interstitial octahedral sites of the Austenite lattice (i.e., 4.74 wt% C). Decomposition of Hagg carbide into M7C3 occurs upon prolonged carburizing treatment or thermal exposure in inert atmosphere (in situ synchrotron experiments).
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composition dependent variation of magnetic properties and interstitial ordering in homogeneous Expanded Austenite
Acta Materialia, 2016Co-Authors: Bastian Brink, Kenny Stahl, Thomas Lundin Christiansen, Cathrine Frandsen, Mikkel Fougt Hansen, Marcel A.j. SomersAbstract:Abstract The crystal structure and magnetic properties of austenitic stainless steel with a colossal interstitial content, so-called Expanded Austenite, are currently not completely understood. In the present work, the magnetic properties of homogeneous samples of Expanded Austenite, as prepared by low-temperature nitriding of thin foils, were investigated with magnetometry and Mossbauer spectroscopy. At room temperature, Expanded Austenite is paramagnetic for relatively low and for relatively high nitrogen contents (yN = 0.13 and 0.55, respectively, where yN is the interstitial nitrogen occupancy), while ferromagnetism is observed for intermediate nitrogen loads. Spontaneous volume magnetostriction was observed in the ferromagnetic state and the Curie temperature was found to depend strongly on the nitrogen content. For the first time, X-ray diffraction evidence for the occurrence of long-range interstitial order of nitrogen atoms in Expanded Austenite was observed for high nitrogen contents.
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thermal expansion and phase transformations of nitrogen Expanded Austenite studied with in situ synchrotron x ray diffraction
Journal of Applied Crystallography, 2014Co-Authors: Bastian Brink, Kenny Stahl, Thomas Lundin Christiansen, Marcel A.j. SomersAbstract:Nitrogen-Expanded Austenite, γN, with high and low nitrogen contents was produced from AISI 316 grade stainless steel powder by gaseous nitriding in ammonia/hydrogen gas mixtures. In situ synchrotron X-ray diffraction was applied to investigate the thermal expansion and thermal stability of Expanded Austenite in the temperature range 385–920 K. Evaluation of the diffractograms of the sample with a high nitrogen content, corresponding to an occupancy of the interstitial lattice of 56%, with Rietveld refinement yielded a best convergence after including the stacking fault probability as a fitting parameter. The stacking fault density is constant for temperatures up to 680 K, whereafter it decreases to nil. Surprisingly, a transition phase with composition M4N (M = Fe, Cr, Ni, Mo) appears for temperatures above 770 K. The linear coefficient of thermal expansion depends on the nitrogen content and is lowest for the sample with a high level of nitrogen.
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extended x ray absorption fine structure investigation of annealed carbon Expanded Austenite
Steel Research International, 2012Co-Authors: Jette Oddershede, Thomas Lundin Christiansen, Kenny Stahl, Marcel A.j. SomersAbstract:Carbon Expanded Austenite synthesized through carburizing of austenitic stainless steel powder at 380°C was annealed at 470°C and investigated with extended X-ray absorption fine structure (EXAFS) and synchrotron powder diffraction (SPD). SPD showed that the samples consisted of carbon Expanded Austenite and Hagg carbide, χ-M5C2. EXAFS showed that the Cr atoms were mainly present in environments similar to the carbides Hagg χ-M5C2 and M23C6. The environments of the Fe and Ni atoms were concluded to be largely metallic Austenite.
J Feugeas - One of the best experts on this subject based on the ideXlab platform.
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stability of Expanded Austenite generated by ion carburizing and ion nitriding of aisi 316l ss under high temperature and high energy pulsed ion beam irradiation
Surface & Coatings Technology, 2013Co-Authors: Garcia J Molleja, L Nosei, Edoardo Bemporad, M Milanese, Mattia Piccoli, R Moroso, J Niedbalski, J Burgi, J FeugeasAbstract:Abstract Expanded Austenite can be generated on austenitic stainless steels either by ion carburizing or ion nitriding. In both cases the resulting fcc crystal structure, supersaturated with nitrogen or carbon, is strongly hardened with improved wear-resistance, while maintaining the original resistance to corrosion. In this work, we have studied the stability of Expanded Austenite, generated by ion nitriding and ion carburizing on AISI 316L SS with N and C, under: a—high temperature (225 °C – 504 °C), and b—under irradiation with high energy (30 keV – 500 keV), high fluence (~ 10 15 cm − 2 ), short duration (~ 400 ns) light (deuterium and helium) ion beams. It was found that Expanded Austenite is stable below 325 °C. Between 325 °C and 504 °C Expanded Austenite lattice parameter presents gradual reduction with increasing temperature. We observed microstructural changes related only to the temperature treatment. We did not observe any microstructure change due to the duration of the heat treatment. Over 504 °C, the lattice parameter returns to the material's Austenite original parameter. On the other hand, when irradiated with pulsed ion beams, a gradual reduction of the lattice parameter corresponding to the Expanded Austenite with the number of pulses was observed. This behavior can be explained through the thermal shock induced on the surface by each beam, consisting in fast heating followed by fast cooling that induces the gradual exo-diffusion of N (or C). Nevertheless, after 20 ion pulses, a final lattice parameter slightly higher than the corresponding to the original Austenite was found as stable limit. This residual expansion can be attributed to partial amorphization of the first few micrometers that induces stresses on the crystals of Austenite which are closer to the surface layers.
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characterization of Expanded Austenite developed on aisi 316l stainless steel by plasma carburization
Surface & Coatings Technology, 2010Co-Authors: Garcia J Molleja, L Nosei, J Ferron, Edoardo Bemporad, J Lesage, Didier Chicot, J FeugeasAbstract:Abstract Expanded Austenite generation through ion carburizing of AISI 316L using two different reactive gas mixtures (Ar 50%, H 2 45%, CH 4 5% and Ar 80%, H 2 15%, CH 4 5%) has been studied. It was found that an ∼ 14 µm surface layer of Expanded Austenite was developed with 30 min processing for both gas mixtures. Nevertheless, AES analyses have shown that on the ∼ 150 nm surface layer carbon in a concentration of ∼ 12% was diffused and located as carbide. For longer periods of processing, while for the gas mixture with 50% of Ar no significant modifications within those 150 nm surface layer were produced, for the gas mixture with 80% of Ar a gradual increase in the carbon concentration with time was found, with the extra carbon remaining as free carbon. The difference between both situations can be attributed to the different resulting current densities that have been of 7.0 mA cm − 2 and 8.1 mA cm − 2 for 50% and 80% of Ar respectively. Higher current densities result in higher carbon and Ar ions fluxes inducing, from one side surface element concentration modification through sputtering, and from the other the enhancement of carbon diffusion on the first hundred nanometers of the surface layers. This free carbon on top of the surface layers can act as solid lubricant reducing wear rate. Nevertheless, and in spite of the fact that Expanded Austenite was proved to be corrosion resistant, a reduction against NaCl solution corrosion in relation to the base material was observed. This lost to corrosion resistance can be attributed to carbide development on the layers closer to the surface that can work as a trigger for localized corrosion.
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stability under temperature of Expanded Austenite developed on stainless steel aisi 316l by ion nitriding
Thin Solid Films, 2004Co-Authors: L Nosei, M Avalos, B J Gomez, L Nachez, J FeugeasAbstract:Abstract Ion nitriding of steels is a process widely used on steels because it can give the surface important improvements in their tribological and mechanical properties attributable to the iron nitridings formation. On the other hand, on stainless steels, this improvement can normally be obtained by sacrificing their resistance to the corrosion. Nevertheless, several years ago, a new type of phase known as Expanded Austenite on austenitic stainless steels has been developed, a phase that have shown a high micro-hardness conserving at the same time the resistance to corrosion. In this work, we study the ion nitriding conditions to obtain this type of phase on stainless steel AISI 316L. We have found that the Expanded Austenite was completely developed after only 30 min of ion nitriding. We obtain images of the surface treated, showing the austenitic nature of the phase, with slip bans that confirms the stress state originated in the expansion of the original austenitic lattice during the ion nitriding process. Further studies of thermal treatment at different temperatures have shown that this Expanded Austenite developed on AISI 316L is stable for 40 h at temperatures of 300 °C, conserving their structure, X-ray diffractogram and a micro-hardness between 1200 and 1400 HV0.025.
