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Mössbauer Spectra and Hyperfine Parameters of Iron-Chromium Carbides in Ferritic Stainless Steel
Abstract
Isolated iron–chromium carbides, as well as several mixtures of them extracted from 29% chromium ferritic stainless steels are studied at room temperature by using Mössbauer spectroscopy. The spectra of the hexagonal M7C3 carbide and the M2X carbonitride (M = Fe, Cr; X = C, N) consist both of a quadrupole doublet, while the spectra of the cubic M23C6 carbide consists of two independent peaks. The spectra of the M7C3 and M23C6 carbides may be interpreted by assuming that chromium atoms enter preferentially the sites that are surrounded by the greatest number of carbon atoms. Using the hyperfine parameters determined from the isolated carbides, it is possible to analyse the spectra of carbide mixtures extracted from stainless steels and thereafter to identify the carbides present in the mixture and to estimate their relative abundance.Des carbures de fer–chrome isolés et en mélange, extraits d'aciers ferritiques inoxydables à 29% de chrome, sont étudiés à la température ambiante par spectrométrie Mössbauer. Les spectres du carbure M7C3 et du nitocarbure M2X hexagonaux (M = Fe, Cr et X = C, N) sont constitués tous les deux d'un doublet quadrupolaire, tandis que le spectre du carbure cubique M23C6 est constituté de deux raies indépendantes. Les spectres des carbures M7C3 et M23C6 peuvent être interprétés en admettant que les atomes de chrome occupent de préférence les sites avec l'environnement le plus riche en carbone. En utilisant les paramètres hyperfins determinés pour les carbures isolés, il est possible d'analyser des spectres de mélanges de carbures extraits d'aciers inoxydables, et par la suite d'identifier les carbures présents et d'estimer leur pourcentage.
... Étant donné que nous n'aurons ni martensite, ni phase σ, ni phase χ dans les nuances non stabilisées qui seront élaborés, nous nous concentrons sur les publications et ouvrages qui traitent de la précipitation des carbures et nitrures de chrome. [Vardavoulias and Papadimitriou, 1992] étudient deux aciers inoxydables ferritiques avec 28 et 29% de chrome. Le premier présente une forte teneur en carbone (0,35%) : les carbures formés sont des M 7 C 3 et des M 23 C 6 avec M désignant le chrome ou le fer, la proportion respective de ces deux carbures dépendent du traitement thermique appliqué. ...
Parmi les nombreuses nuances d’aciers inoxydables, les nuances ferritiques sont utilisées dans de nombreux secteurs grâce à leur résistance à la corrosion et à des propriétés spécifiques comme leur conductibilité thermique ou leurs propriétés ferromagnétiques. Leur teneur résiduelle en nickel en font des aciers économiques en comparaison des aciers austénitiques, plus massivement produits. Toutefois, l’utilisation de cette famille d’aciers en dessous de la température ambiante est limitée par sa fragilité mécanique, la moindre mobilité des dislocations dans le réseau cubique centré rendant l’écoulement plastique plus difficile aux basses températures. La précipitation et la taille des grains sont deux des paramètres microstructuraux qui influent sur la température de transition ductile/fragile des aciers ferritiques. L’objectif de cette thèse est d’étudier et de comprendre l’effet de ces paramètres sur cette température de transition et sur la rupture par clivage des aciers à matrice 100% ferritique contenant 18% de chrome et 2% de molybdène.Pour ce faire, trois coulées ont été élaborées avec la même teneur en carbone et en azote à laquelle s’ajoute, pour deux d’entre elles, du titane ou du niobium. La gamme de traitements thermomécaniques a été choisie pour fabriquer six microstructures distinctes pour lesquelles varient la taille des grains, la nature et la localisation de la précipitation et la teneur en carbone et en azote en solution solide. Ces microstructures sont caractérisées par microscopies optique et électroniques et par diffusion centrale des neutrons aux petits angles. La mise en place d’un essai de traction avec des éprouvettes axisymétriques entaillées permet de déterminer la contrainte de rupture par clivage de chaque microstructure à 20°C et à -40°C, puis de discuter des mécanismes qui régissent la rupture fragile.Les microstructures élaborées sont finalement classées en deux catégories. Pour les microstructures Ti, Nb et NbG, pour lesquelles l’ajout de titane ou de niobium permet d’éviter la précipitation de carbures et nitrures de chrome, la contrainte de rupture par clivage est dépendante de la taille et de la morphologie des particules intragranulaires. Les Ti(N,C), de 4 à 5 µm au maximum, dont la forme s’apparente à celle d’un cube, rendent la microstructure Ti plus fragile que la microstructure Nb caractérisée par des Nb(C,N), certes plus nombreux mais de plus petites dimensions. La taille des grains est un facteur de deuxième ordre sur la contrainte de rupture par clivage de ces microstructures comme cela est mis en évidence avec l’étude de la rupture fragile de la microstructure NbG, pour laquelle la taille des grains a été multipliée par 10 par rapport à celle de la microstructure Nb.Dans les microstructures CrP, Cr et Cr0, la présence ou non de carbone et d’azote en solution solide sursaturée et les précipitations intra et intergranulaires sont les conséquences des traitements thermiques choisis. Les précipités dans les joints de grains, même s’ils sont de petites dimensions (quelques dizaines à quelques centaines de nm), modifient significativement l’amorçage de la rupture par clivage. La température de transition ductile/fragile est, au premier ordre, dépendante de la limite d’élasticité des microstructures. La contrainte de rupture par clivage n’est plus dépendante de la taille des particules intragranulaires comme c’est le cas pour les microstructures Ti, Nb et NbG mais de la précipitation aux joints de grains et de la dépendance de la contrainte de friction de réseau à la solution solide (modèle de Smith).Au vu de ces résultats, des pistes pour limiter la fragilité des nuances ferritiques sont proposées.
The authors studied radiation-induced structural-phase transformations that had occurred in EP823 steel of industrial application under conditions of its high-dose neutron irradiation in a fast fission reactor. The method of transmission Mössbauer spectroscopy with resonance detection and transmission electron microscopy was utilized. It has been observed that in ferritic-martensitic steel, under the neutron irradiation with doses above 50 dpa at temperatures of 570–660°C, the decomposition of a BCC solid solution together with the release of the alloying elements chromium, molybdenum, and carbon from the metal matrix occurs. It is accompanied by the formation of an intermetallic χ phase and carbides of the Me23C6 type. The authors revealed the formation of the vacancy-clustered and supposedly gas-filled pores in the structure after irradiation.
X-ray diffraction (XRD), transmission Mössbauer spectroscopy, and vibrating sample magnetometry were used to study the influence of precipitates on the magnetic properties of aged Fe-Cr-Mo alloys. XRD patterns and Mössbauer spectra showed that the alloys have body-centered cubic structure similar to that of α-Fe. Small amounts of precipitates were also identified with relative fraction found to be related to Mo content, aging times, and temperatures. Magnetic measurements showed that the increase in the density of precipitates contributes significantly to the increase in the magnetic hardness of the material. This mechanism is related to a process of pinning of magnetic domain walls. It was also found that saturation magnetization is affected by the Mo content in the alloy. The techniques used in this work were shown to be very useful to understand the mechanisms through which the formation of precipitates affects the magnetic properties of the alloys and may be used as complement to the usual microscopy-based techniques for this purpose.
The site preference, stability, crystal structure and mechanical properties of Cr23−xTxC6 and Fe21T2C6 (T = Mo, W) carbides have been investigated using inverted pair potentials. Among the four different kinds of metal sites in these compounds, the most preferential sites for Mo atoms or W atoms are 8c sites. The result of stability indicates that each of the Mo atoms and W atoms significantly increases the stability of Cr23C6 and Fe23C6. The stability of Fe21T2C6 is lower than that of Cr21T2C6 and the carbides containing W atoms are more stable than those containing Mo atoms. The calculated structural properties are in good agreement with experimental data. Furthermore, the mechanical properties have also been evaluated and the results indicate that Mo atoms or W atoms can improve the hardenability of the systems.
The effect of M23C6 carbides on the formation of grain boundary serrations (GBSs) has been systematically investigated in a solid solution strengthened Haynes 230 alloy. It is found that GBS occur in this alloy during the slow cooling process and are accompanied by the precipitation of intergranular planar M23C6 carbides. The amplitude and proportion of GBS increase with the rise of the solution temperature and time. If the specimens are cooled directly without any solution treatments, the grain boundaries remain planar and granular M23C6 carbides precipitate at them. The sequential evolutions of GBS and M23C6 carbides are investigated by scanning electron microscopy (SEM) examination. High-resolution transmission electron microscope (HRTEM) investigations reveal the coherent interfacial plane of M23C6 carbides formed at grain boundaries to be (1 1 1¯). These facts indicate that the nucleation and oriented growth of M23C6 carbides at grain boundaries play an important role in the formation of GBS. Based on the interfacial energy calculations and the tensions balance relation, a semi-quantitative model about the GBS formation is proposed.
Thermal stability of metastable A15 Fe-Cr phase is investigated through the study of its magnetic and structural properties. This phase presents very interesting mechanical properties suggesting that A15-structured films might be of great interest for tribological applications when considering the hardness H over Young's modulus E ratio i.e., a description in terms of ``elastic strain to failure'' for wear resistance. Indeed, H is multiplied by a factor 2 with respect to the value measured for the bulk cubic centered alpha phase whereas E remains identical. Then, an improvement by a factor 8 of resistance to plastic deformation may be expected since predictive models stand that this quantity is proportional to the H3/E2 ratio. However, heating problems due to sliding during tribological tests may lead to structural transformation in the film and then a loose of mechanical performance. The formation and the stability of the A15 cubic structure (delta phase) in centered-cubic refractory metals are generally attributed to the presence of oxygen atoms in the unit cell. For equiatomic Fe-Cr thin films elaborated by physical vapor deposition techniques, residual oxygen atoms present in the deposition chamber would be absorbed during the deposition process. In this work, the delta-phase transformation has been studied ex situ in the temperature range 400-650 °C structural changes have been accurately investigated thanks to the combination of x-ray diffraction and Mössbauer spectroscopy techniques. Thin films were deposited onto quartz substrates and then annealed ex situ under secondary vacuum. From 400 °C, a ``structural relaxation'' occurring in the delta phase precedes and accompanies the beginning of the phase transformation. Finally, the partially ordered metastable delta-phase transforms into a stable alpha-phase presenting the precipitation phenomenon at temperature above 550 °C and the presence of a tau-carbide phase is clearly visible from 600 °C. The delta-phase transformation is completed at 650 °C. The stability of the A15 type structure up to 550 °C is a promising indication of good tribological performance of such coatings.
Previously published time–temperature–precipitation diagrams showing carbide evolution were modified with the use of additional experimental data. The same low alloy steels as investigated originally were annealed at 793 and 873K for 3000 and 10,000h which, compared to the longest annealing time in the previous investigation of 1000h, brings the system closer to equilibrium. Carbides extracted into carbon replicas were characterised by transmission electron microscopy, including electron diffraction and energy dispersive X-ray spectroscopy. The results of the additional experiment presented in the modified diagrams provide more information about the evolution and stability of M3C, M2C, M7C3, M23C6, M6C and MC in the investigated steels.
X-ray diffraction analysis, energy dispersive X-ray spectroscopy microanalysis and Mössbauer spectroscopy have been carried out to characterize the various carbides in M50 steel. They were electrolytically extracted from the matrix owing to their weak volume fraction. In the annealed state, M2C, MC, M6C, M23C6 have been observed besides an additional carbide M7C3. The carbide populations were evaluated and the hyperfine parameters explained in relation to the alloying element content and the carbide crystalline structure.
The site preference of Fe in Cr23−xFexC6 is investigated based on the interatomic potentials obtained by the lattice inversion method. The calculated results show that Fe atoms preferentially substitute for Cr at 4a sites first and then 8c sites. The structural parameters of Cr23−xFexC6 with content x are calculated and the results are consistent with experimental results. The calculated cohesive energies indicate that the increase in x value is accompanied by the decrease in the stability of Cr23−xFexC6. The thermodynamic properties of Cr23C6, such as the phonon density of states and vibrational entropy, as well as the bulk modulus of Cr23−xFexC6 are evaluated. The calculated results are in good agreement with experimental results. This work provides a simple and promising method for studying the properties of carbides with complex structures.
The presented experiments that were carried out in order to advance the understanding of the transformation of expanded austenite into an amorphous ferromagnetic surface layer during laser carburization of austenitic stainless steel. Therefore laser carburized austenitic stainless steel was investigated by means of X-ray diffraction, Mössbauer spectroscopy, Rutherford backscattering and magneto-optical Kerr-effect. Five to seven sub-spectra attributed to different iron sites were resolved in the Mössbauer spectra for this iron-carbon austenite. The isomer shifts, the quadrupole splittings and in particular the subspectra fractions depend on the carbon content. The formation of an amorphous and soft ferromagnetic phase was found in the laser treated surface.
The lubricating lifetime of thin films of a perfluoropolyether (PFPE) based on liexafluoropropene oxide in the presence of ion-implanted 440C stainless steel is presented. Stainless-steel discs, either unimplanted or implanted with N2, C, Ti, Ti + N2, or Ti + C, had a thin film of PFPE (60–400 Å) applied to them reproducibly (± 20%) and uniformly (± 15%) using a device developed for this study. The lifetimes of these films were quantified by measuring the number of sliding-wear cycles required to induce an increase in the friction coefficient from an initial value characteristic of the lubricated wear couple to a final, or failure value, characteristic of an unlubricated, unimplanted couple. The tests were performed in a dry nitrogen atmosphere (< 1 % RH) at room temperature using a 3 N normal load with a relative sliding speed of 0.05 ms−1. The lubricating lifetime of the 440C couple was increased by an order of magnitude by implanting the disc with Ti. Ranked from most to least effective, the implanted species were Ti, Ti + C, unimplanted, N2, C Ti + N2. The mechanism postulated to explain these results involves the formation of a passivating or reactive layer which inhibits or facilitates the production of active sites. The corresponding surface microstructures induced by ion implantation, obtained using X-ray diffraction and conversion electron Mössbauer spectroscopy, ranked from most to least effective in enhancing lubricant lifetime were amorphous Fe-Cr-Ti, amorphous Fe-Cr-Ti-C + TiC, unimplanted, ε-(Fe,Cr)xN, x = 2 or 3, and amorphous Fe-Cr-C amorphous Fe-Cr-Ti-N.
Stainless steel films were reactively magnetron sputtered in argon/methane gas flow onto oxidized silicon wafers using austenitic
stainless-steel targets. The deposited films of about 200nm thickness were characterized by conversion electron Mössbauer
spectroscopy, magneto–optical Kerr-effect, X-ray diffraction, scanning electron microscopy, Rutherford backscattering spectrometry,
atomic force microscopy, corrosion resistance tests, and Raman spectroscopy. These complementary methods were used for a detailed
examination of the carburization effects in the sputtered stainless-steel films. The formation of an amorphous and soft ferromagnetic
phase in a wide range of the processing parameters was found. Further, the influence of the substrate temperature and of post
vacuum-annealing were examined to achieve a comprehensive understanding of the carburization process and phase formation.
Conversion electron Mössbauer spectra and X-ray diffractograms for samples of ball-bearing steel were measured. After heat treatment, samples were subjected to contact fatigue with the stress either lower or higher than the contact fatigue strength. The relative contributions of the retained austenite, martensite and carbide phases after consecutive fatigue tests were determined. It was found that the fraction of retained austenite decreases sharply with the number of fatigue cycles and that the effect is more pronounced for penetration depths of up to 100 nm into the sample as observed using conversion electron Mössbauer spectroscopy than that for penetration depths of up to 10 μm into the sample as determined using X-ray analysis.
On a étudié par spectroscopie Mössbauer les positions possibles de l'emplacement du chrome dans le carbure de type M3C, qui se forment au cours de la métallurgie d'acier. L'analyse de phase a été exécutée en connexion de quelques carbures d'autre type (M7C3, M6C, M23C6) aussi. The possible positions of chromium incorporation into M3C-type chromium containing carbides formed in course of steel production were studied. The phase analysis of this and of some other types (M7C3, M6C, M23C6) of carbides was performed by means of Mössbauer spectroscopy.
Centrifugally cast 0. 4C-25Cr-20Ni (wt%) steel (HK 40) has been subjected to detailed microscopic examination both in the as-cast condition and after long-term creep. The as-cast alloy contained a eutectic of M//7C//3 and austenite, which was subjected to energy-dispersive X-ray analysis. Observations by transmission electron microscopy revealed high dislocation densities near cell boundaries. After creep in the range 750-1000 degree C, the eutectic carbides coarsened and transformed from M//7C//3 to M//2//3C//6; moreover, the M//2//3C//6 carbide precipitated predominantly on dislocations as a finer dispersion within the grains. During coarsening, the M//2//3C//6 particles generated further dislocations in the austenite. The higher creep strength of the cast HK 40 compared to wrought alloys is discussed in terms of the detailed microstructural observations, in particular the M//2//3C//6 dispersion.
Magnetic and Mössbauer effect measurements on the nitrides with hcp metal lattice of V, Cr, Mn and Fe are carried out as a function of composition. Hexagonal VNx and hexagonal CrMx are Pauli paramagnetic. Hexagonal MnNx shows antiferromagnetism, while both hexagonal and orthorhombic FeNx show ferromagnetism at low temperatures. In these Phases, nitrogen atoms suppress magnetic ordering in consequence of the reduction of magnetic moments. As a whole, the observed types of magnetism of hexagonal nitrides resemble those of their metals in spite of the difference in crystal structures. Type II superconductivity is found in hexagonal VNx although it is not yet conclusive.
Type 316 stainless steel is aged at 550, 650, 750 and 900 °C for various times to induce grain boundary carbides of different sizes and densities. The impact energy at − 196 °C is shown to be seriously reduced by the grain boundary carbides and the fracture surfaces become intergranular. Fine dense grain boundary carbides are most detrimental to the low temperature impact energy. It is also shown that a thin layer of grain boundary martensite can cause a significant reduction in the room temperature impact energy of this steel.
Magnetization and Mössbauer absorption spectra have been measured on cementite substituted partially with Cr or Ni. The magnetic moment increases slightly with the increase of nickel concentration, while it decreases very abruptly from 1.9 muB in Fe3C to 0.4 muB in (Fe0.799Cr0.201)3C. The internal field and the Curie temperature also decrease abruptly for the Cr-substituted cementite. The observed experimental results strongly indicate that the magnetic properties of the (Fe1-xMex)3C system are closely analogous to those of gamma Fe-Ni Invar alloy.
Centrifugally cast 0·4C–25Cr–20Ni (wt-%) steel (HK 40) has been subjected to detailed microscopic examination both in the as-cast condition and after long-term creep. The as–cast alloy contained a eutectic of M7C3 and austenite, which was subjected to energy-dispersive X-ray analysis. Observations by transmission electron microscopy revealed high dislocation densities near cell boundaries. After creep in the range 750–1000°C, the eutectic carbides coarsened and transformed from M7C3 to M23C6; moreover, the M23C6 carbide precipitated predominantly on dislocations as a finer dispersion within the grains. During coarsening, the M23C6 particles generated further dislocations in the austenite. The higher creep strength of the cast HK 40 compared to wrought alloys is discussed in terms of the detailed microstructural observations, in particular the M23C6 dispersion.MST/135
We have performed Mössbauer experiments on FeMn carbides having the cementite structure (Fe1−xMnx)3C, where x ranges from 0 to 0.6, and on a Hägg carbide with the Pd5B2 structure, (Fe1.1Mn3.9)C2, over the temperature range from 12 to 500 °K. Above the Curie temperature, the spectra exhibit a quadrupole interaction ranging from about 0.20 to 0.28 mm/s and the contributions from Fe atoms in the two types of cementite lattice sites can be identified. It is found that Mn atoms have a slight preference for “general” over “special” lattice sites in cementite. The specimens containing 0 to 20% Mn all become ferromagnetic at low enough temperatures, and the effective field decreases with Mn concentration very rapidly, extrapolating to 0 at about 34% Mn. Below Tc, the quadrupole splitting decreases drastically to less than 0.05 mm/s. Most of the results seem to be consistent with a donor theory in which C donates both of its 2p electrons to the metal d-band, giving the Fe in cementite an average electron configuration of approximately 3d7.66 4 s¹.
A relatively new research tool has been brought to bear on a relatively old problem. Mossbauer effect spectroscopy, in conjunction with X-ray diffraction techniques, were used to study the phenomenon of stainless steel sensitization. A ferromagnetic phase, thought to be pseudomartensite, has been found in sensitized Type 301. Stresses generated in the matrix by morphology changes of the precipitated carbides are deemed responsible for the creation of this phase. No such phase was detected in the Type 304L. It was confirmed that the precipitated carbides were of the complex type (Cr, Fe)2 3C6. Furthermore, it was established that these carbides are paramagnetic, In the presence of austenite, the mossbauer peak for the carbides is overshadowed