Ordering and phase separation in the Fe-Cr-N system

The structures of Fe-Cr-N alloys are studied, and the types of chemical interactions in the Fe-Cr and Fe-N diffusion couples are determined at various temperatures. At 1200°C, austenite is shown to form in the alloys as a result of a tendency toward phase separation in the Fe-Cr and Cr-N diffusion couples. Below 1150°C, chromium nitrides form in the alloys due to ordering in the Cr-N diffusion couple.     

Application of the diffusion couple to study phase equilibria in the Fe-Cr-S ternary system at 600 °C

The sulfidation of Fe-Cr alloys has a large financial significance for industries that use fossil fuels, such as the electric utility industry. Therefore, the sulfidation of a series of Fe-Cr alloys was studied at 600 °C using a solid-state diffusion couple technique. The diffusion couple technique combined Fe_0.95S powder and FeCr binary alloys together in a configuration that allowed for post-heat-treatment microanalysis using an electron probe microanalyzer (EPMA). The results showed that only two different diffusion couple microstructures formed in samples spanning the entire Fe-Cr binary range. The Fe-rich alloy diffusion couples contained a surface αFeCr layer and an internal sulfide precipitate layer that contained three different sulfide phases. The Cr-rich alloy diffusion couples also possessed an internal precipitate layer, as well as a thick, triplex interfacial scale. The ternary elemental diffusion was described using diffusion paths plotted on the 600 °C isothermal section of the Fe-Cr-S phase diagram. The results also showed that samples with less than 51 wt Pct Cr were more sulfidation resistant. The accuracy of the existing Fe-Cr-S 600 °C isothermal section was assessed, and it was determined that the τ phase field had a larger composition than previously published.     

Structural Characterization of Phase Separation in Fe-Cr: A Current Comparison of Experimental Methods

Self-assembly due to phase separation within a miscibility gap is important in numerous material systems and applications. A system of particular interest is the binary alloy system Fe-Cr, since it is both a suitable model material and the base system for the stainless steel alloy category, suffering from low-temperature embrittlement due to phase separation. Structural characterization of the minute nano-scale concentration fluctuations during early phase separation has for a long time been considered a major challenge within material characterization. However, recent developments present new opportunities in this field. Here, we present an overview of the current capabilities and limitations of different techniques. A set of Fe-Cr alloys were investigated using small-angle neutron scattering (SANS), atom probe tomography, and analytical transmission electron microscopy. The complementarity of the characterization techniques is clear, and combinatorial studies can provide complete quantitative structure information during phase separation in Fe-Cr alloys. Furthermore, we argue that SANS provides a unique in-situ access to the nanostructure, and that direct comparisons between SANS and phase-field modeling, solving the non-linear Cahn Hilliard equation with proper physical input, should be pursued.     

Thermodynamics of α′(Fe-Rich bcc) + α″(Cr-Rich bcc) → α(bcc) and α para → α ferro Transformations in Fe-20 wt pct Cr Alloy: Drop Calorimetry Study and Elucidation of Magnetic Contribution to Phase Stability

There is considerable uncertainty among diverse assessments of phase equilibrium in Fe-Cr alloys, especially around (α′ + α″)/α miscibility gap region. This is largely due to the difficulty associated with the rigorous incorporation of the interplay between magnetic and chemical contribution to phase stability, in particular its composition and temperature dependencies through theory, in the absence of reliable experimental data. Toward this cause, accurate enthalpy measurements have been made on homogenized Fe-20 wt pct Cr alloy using inverse drop calorimetry, in the temperature range 298 K to 1473 K (25 °C to 1200 °C). The experiments revealed two distinct phase transformations: (i) at 720 ± 10 K (447 ± 10 °C), the Fe-20Cr alloy transformed from α′(Fe-rich) + α″(Cr-rich) two-phase microstructure to α single phase and (ii) at 925 ± 10 K (652 ± 10 °C), the ferromagnetic single-phase α transformed to paramagnetic state. Both these transformations are clearly attested by the measured enthalpy increment variation with temperature. The enthalpy data obtained in this study have been combined with available literature information to forge an integrated theoretical assessment of the energetic aspects of α′ + α″ → α, and α _ferro → α _para transformations. In addition, a comprehensive evaluation of enthalpy and heat capacity data for Fe-20Cr alloy in the temperature range 0 K to 1473 K (?273 °C to 1200 °C), with explicit incorporation of magnetic contribution has also been made.     

Influence of the tempering temperature on the mechanical properties and the phase composition of thin sheet TRIP steel

The strength and the plasticity properties of sheet high-strength austenitic–martensitic VNS9-Sh TRIP steel (23Kh15N5AM3-Sh) are studied as functions of the tempering temperature in the range 125–600°C. A nonmonotonic decease in the strength and the plasticity properties of the steel has been detected when the tempering temperature increases, and they increase in the range 300–450°C. The influence of aging processes, the precipitation of carbide, and the phase transformations in tempering on the mechanical properties of austenitic–martensitic corrosion-resistant steel is discussed.     

Chromium Extraction <Emphasis Type="Italic">via</Emphasis> Chemical Processing of Fe-Cr Alloys Fine Powder with High Carbon Content

Ferrous alloys are important raw materials for special steel production. In this context, alloys from the Fe-Cr system, with typical Cr weight fraction ranging from 0.45 to 0.95, are prominent, particularly for the stainless steel industry. During the process in which these alloys are obtained, there is considerable production of fine powder, which could be reused after suitable chemical treatment, for example, through coupling pyrometallurgical and hydrometallurgical processes. In the present study, the extraction of chromium from fine powder generated during the production of a Fe-Cr alloy with high C content was investigated. Roasting reactions were performed at 1073 K, 1173 K, and 1273 K (800 °C, 900 °C, and 1000 °C) with 300 pct (w/w) excess NaOH in an oxidizing atmosphere (air), followed by solubilization in deionized water, selective precipitation, and subsequent calcination at 1173 K (900 °C) in order to convert the obtained chromium hydroxide to Cr₂O₃. The maximum achieved Cr recovery was around 86 pct, suggesting that the proposed chemical route was satisfactory regarding the extraction of the chromium initially present. Moreover, after X-ray diffraction analysis, the final produced oxide has proven to be pure Cr₂O₃ with a mean crystallite size of 200 nm.     

Application of MIVM for Pb-Sn System in Vacuum Distillation

The activity coefficients of components of the Pb-Sn binary alloy system were calculated based on the molecular interaction volume model (MIVM). A significant advantage of this model lies in its ability to predict the thermodynamic properties of liquid alloys using only two binary infinite activity coefficients. Based on the MIVM, the vapor-liquid phase equilibrium of the Pb-Sn alloy system in vacuum distillation has been predicted using the activity coefficients of Pb and Sn. The results showed that the content of tin in the vapor phase was 0.008?wt?pct, while in the liquid phase, it was 83?wt?pct at 1173?K (900?°C); it reached 0.022?wt?pct in the vapor phase, while in the liquid phase, it was 92?wt?pct at 1223?K (950?°C); and it was 0.052?wt?pct in the vapor phase, while in the liquid phase, it was 97.88?wt?pct at 1273?K (1000?°C). The content of tin in the vapor phase increased with the distillation temperature increasing. Experimental investigations into the separation of Pb and Sn from the Pb-Sn alloy by vacuum distillation were carried out for the proper interpretation of the results of the model. The influence of the distillation time (20 to 80?minutes) and the distillation temperatures of 1173?K, 1223?K, and 1273?K (900?°C, 950?°C, and 1000?°C) on the separating effect was also studied. The experimental results showed that the content of tin in the vapor phase was 0.085?wt?pct, while in liquid phase, it was 83?wt?pct under the operational conditions of distillation temperature of 1173?K (900?°C), evaporation time of 20?minutes, and chamber pressure of 20?Pa; it reached 0.18?wt?pct in the vapor phase, while in the liquid phase, it was 92?wt?pct at 1223?K (950?°C), 20?minutes, and 20?Pa; and it was 0.35?wt?pct in the vapor phase, while in the liquid phase, it was 97.88?wt?pct at 1273?K (1000?°C), 20?minutes, and 20?Pa. In all these experiments, it was observed that the content of tin in the vapor phase increased as the distillation time and temperatures were increased. The experimental results are in good agreement with the predicted values of the MIVM for the Pb-Sn binary system.     

Effects of Heating and Cooling Rates on Phase Transformations in 10 Wt Pct Ni Steel and Their Application to Gas Tungsten Arc Welding

10 wt pct Ni steel is a high-strength steel that possesses good ballistic resistance from the deformation induced transformation of austenite to martensite, known as the transformation-induced-plasticity effect. The effects of rapid heating and cooling rates associated with welding thermal cycles on the phase transformations and microstructures, specifically in the heat-affected zone, were determined using dilatometry, microhardness, and microstructural characterization. Heating rate experiments demonstrate that the Ac₃ temperature is dependent on heating rate, varying from 1094 K (821 °C) at a heating rate of 1 °C/s to 1324 K (1051 °C) at a heating rate of 1830 °C/s. A continuous cooling transformation diagram produced for 10 wt pct Ni steel reveals that martensite will form over a wide range of cooling rates, which reflects a very high hardenability of this alloy. These results were applied to a single pass, autogenous, gas tungsten arc weld. The diffusion of nickel from regions of austenite to martensite during the welding thermal cycle manifests itself in a muddled, rod-like lath martensitic microstructure. The results of these studies show that the nickel enrichment of the austenite in 10 wt pct Ni steel plays a critical role in phase transformations during welding.     

Influence of deformation on the structure and mechanical and corrosion properties of high-nitrogen austenitic 07Kh16AG13M3 steel

The correlation has been studied between the structure of a high-nitrogen austenitic Cr-Mn-N steel formed in the process of combined hardening treatment, including cold plastic deformation (CPD), and its mechanical and corrosion properties. The structure and properties of commercial high-nitrogen (0.8% N) 07Kh16AG13M3 steel is analyzed after rolling by CPD and aging at 500 and 800°C. It is shown that CPD of the steel occurs by dislocation slip and deformation twinning. Deformation twinning and also high resistance of austenite to martensitic transformations at true strains of 0.2 and 0.4 determine the high plasticity of the steel. The contribution of the structure imperfection parameters to the broadening of the austenite lines during CPD is estimated by X-ray diffraction. The main hardening factor is stated to be lattice microdistortions. Transmission electron microscopy study shows that heating of the deformed steel to 500°C leads to the formation of the intermediate CrN phase by a homogeneous mechanism, and the intermtallic χ phase forms along the austenite grain boundaries in the case of heating at 800°C. After hardening by all investigated technological schemes, exception for aging at 800°C, the steel does not undergo pitting corrosion and is slightly prone to a stress corrosion cracking during static bending tests, while aging at 800°C causes pitting corrosion at a pitting formation potential E _pf = ?0.25 V.     

Phase transformations in Ti-6Al-4V-xH alloys

Microstructures, phases, and phase transformations in Ti-6Al-4V alloy specimens containing 0, 10, 20, and 30 at. pct hydrogen were investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and microhardness testing. Alloying with hydrogen was achieved by holding the specimens in a pure hydrogen atmosphere of different pressures at 780 °C for 24 hours. The phases present in the temperature range of 20 °C to 1000 °C were determined by microstructural characterization of the specimens quenched from different temperatures. Increasing the hydrogen addition from 0 to 30 at. pct lowered the beta-transus temperature of the alloy from 1005 °C to 815 °C, significantly slowed down the kinetics of the beta-to-alpha transformation, and led to formation of an orthorhombic martensite instead of the hexagonal martensite found in quenched specimens containing 0 pct H. A hydride phase was detected in specimens containing 20 and 30 at. pct hydrogen. The time-temperature-transformation (TTT) diagrams for beta-phase decomposition were determined at different hydrogen concentrations. The nose temperature for the beginning of the transformation decreased from 725 °C to 580 °C, and the nose time increased from 12 seconds to 42 minutes when the hydrogen concentration was increased from 0 to 30 at. pct.     

The behavior of silicon in the solidification of Zn-55Al-1.6Si coating on steel

Silicon is an essential element in the Zn-55Al-1.6Si coating. It is added to promote the formation of an adherent coating and prevent the excessive growth of an intermetallic alloy layer at the steel/coating interface. The addition of silicon also results in the formation of a silicon phase distributed in the interdendritic region of the overlay, having a flowery pattern on the surface, and appearing needlelike when observed inside the overlay. The behavior of silicon during the solidification process of the Zn-55Al-1.6Si coating is examined in the current study. It is found that the coating solidification proceeds in three stages. At stage I, primary α-Al dendrites develop at about 566 °C to 520 °C, forming the framework of the coating structure. This is followed by stage II at about 520 °C to 381 °C, where the binary Al-Si eutectic reaction takes place, with the majority of the silicon phase forming at about 520 °C to 480 °C. At stage III the remaining molten phase undergoes a ternary Al-Zn-Si eutectic reaction forming the interdendritic zinc-rich network. The ternary Al-Zn-Si eutectic reaction is essentially equivalent to the binary Al-Zn eutectic reaction because of the very low level of silicon at the Al-Zn-Si eutectic point.     

Displacive transformations in Au-18 wt pct Cu-6 wt pct Al

A gold alloy with 18 wt pct Cu and 6 wt pct Al undergoes a reversible displacive phase transformation between an incompletely ordered L2₁ parent phase and a tetragonal product. The characteristics of these transformations were studied using acoustic emission, dilatometry, X-ray diffraction, and metallography. The morphology of the transformation products, the structure of the parent phase, and the generation of significant acoustic emission during the transformations indicate that they are at least quasi-martensitic, if not martensitic, and that this system is an example of a β-phase shape-memory alloy (SMA). The onset temperatures of the transformations depend on the prior thermal history of the sample. The martensite start (M _s ) temperature is between 30 °C and 20 °C. The system exhibits hysteresis and will revert to the parent phase when reheated, with an austenite start (A _s ) temperature between 55 °C and 80 °C. However, freshly cast or solution-annealed and quenched samples of the alloy do not transform to the tetragonal phase. Aging of such material at temperatures between 30 °C and 200 °C is required before they will manifest the displacive transformation. The “martensite” phase is considerably more resistant to aging-induced stabilization than that of most other SMAs.     

Creating Weldable High-Strength Structural Steel with Nanostructuring

Exploitation of the Arctic calls for the creation of economical high-strength steel capable of operating at low temperatures. Research shows that, to that end, means of controlling the steel structure must be identified, so as to create quasi-isotropic fragmented nanostructure within the metal. The formation of finegrain structure is possible by a combination of intense plastic deformation with recrystallization and phase transformations. To confirm the theoretical and experimental preconditions for this process, experiments are conducted on the Gleeble-3800 instrument and a Kvarto-800 four-high rolling mill. Experimental steel melts with a carbon equivalent C_equ = 0.44–0.87% are investigated. In simulation on the Gleeble-3800 instrument, compressive deformation is applied in two stages: roughing at 1080–1100°C; and finishing at 950 and 820°C. That simulates the deformation cycle in industrial Kvarto-5000 four-high mills. The grain size in the steel is decreased from 6.5 to 2.2 μm after deformation at 950°C and 1.1 μm after deformation at 810°C. Fragments smaller than 500 nm constitute 20–37% of the steel structure. In the steel with C_equ = 0.44–0.65%, the yield point is 500–700 MPa, which is 40% greater than the standard values. In the steel with C_equ = 0.65–0.87%, the yield point is 700–1150 MPa. These values are obtained with increase in nickel content in the steel to 3%. At higher Ni concentrations, no improvement in yield point is seen. After rolling on the Kvarto-800 mill, with C_equ = 0.60–0.87% in one pass (with 70% reduction) at 1100°C and direct quenching with subsequent tempering at 600°C, the yield point is 1060 MPa. In this case, variation in the Ni content and C_equ has little influence on the yield point. The steel consists of bainite (mean grain size 6.9–8.3 μm), with a large dislocation density (1–2) × 1015 m^–2 and considerable fragmentation within the grain. On the basis of the new technology, a group of low-temperature steels with yield points of 270–690 MPa and C_equ = 0.32–0.65% is created. The thickness of the steel sheet is up to 130 mm; it may operate at temperatures as low as –60°C. Such steel may be used for atomic icebreakers, other Arctic vessels, and fixed and floating drilling platforms for oil and gas extraction from Russia’s Arctic shelf. This research demonstrates the possibility of creating structural steels with relatively little alloying (up to 20–30%) and with standardized chemical composition.     

Oxidation resistance,thermal expansion and area specific resistance of Fe-Cr alloy interconnector for solid oxide fuel cell

It is promising for metal especially ferritic stainless steel(FSS)to be used as interconnector when the solid oxide fuel cell(SOFC)is operated at temperature lower than 800°C.However,there are many challenges for FSS such as anti-oxidant,poisoning to cathode and high area specific resistance(ASR)when using as SOFC interconnector.The effect of Cr content(12-30 mass%)in Fe-Cr alloys on thermal expansion coefficient(TEC),oxidation resistance,microstructure of oxidation scale and ASR was investigated by thermo-gravimetry,scanning electron microscopy,energy dispersive spectroscopy and four-probe DC technique.The TEC of Fe-Cr alloys is(11-13)×10~(-6) K~(-1),which excellently matches with other SOFC components.Alloys have excellent oxidation resistance when Cr content exceeds 22mass% because of the formation of chromium on the surface of alloy.The oxidation rate constants kdand ksdecrease rapidly with increasing the Cr content and then increase slowly when the Cr content is higher than 22mass%.The kinetic results indicate that Cr evaporation must be considered at high temperature for Fe-Cr alloys.After the alloys were oxidized in air at 800°C for500 h,log(ASR/T)(Tis the absolute temperature)presents linear relationship with 1/T and the conduct activation energy is 0.6-0.8eV(Cr16-30).Optimal Cr content is 22-26mass%considering the oxidation resistance and ASR.     

Behavior of steels near the incipient melting temperature

A new method of incipient melting temperature (IMT) detection has been developed in which a constant-strain-rate tensile deformation is applied to a specimen whose temperature is simultaneously increasing. The IMT is determined in a single test, and any phase transformations before the IMT will also be detected by the effects on the stress vs strain behavior in the same experiment. By means of such tests, the incipient melting behavior of a series of steels with carbon levels from 0.031 to 0.45 wt pct was examined. For the steels containing 0.08 to 0.097 pct C and about 1.5 pct Mn, it was found that incipient melting occurs in the two-phase (γ + δ) region in the temperature range from 1470 °C to 1480 °C and is significantly influenced by microalloying elements. In the ultralow-carbon steel (0.031 pct C), the IMT is in the single-phase δ region, and for the medium-carbon steel containing 0.42 pct C (hyperperitectic) it is in the γ single phase.     

Thermodynamic properties of solid alloys of chromium with nickel and iron

The thermodynamic properties of chromium have been determined in the Ni-Cr and Fe-Cr binary systems and in the Fe-corner of the Fe-Ni-Cr system. These properties are based on experimental measurements using solid oxide electrolyte cells of the type: Cr, Cr₂O₃ I ThO₂-Y₂O₃Cr (alloy), Cr₂O₃. In the Ni-Cr system, between 900 and 1300°, the activity of chromium exhibits negative deviation from ideality up to about 25 at. pct chromium. For alloys higher in chromium content, the activity of chromium exhibits positive deviation from ideality. In the Fe-Cr system, between 900 and 1200°, and 0 and 63 at. pct Cr, the chromium activity when referred to solid pure chromium exhibits positive deviation from ideality in both the γ and α phases, approaching ideality with increasing temperature. The nickel and iron activities in these two respective binary systems were calculated by a Gibbs-Duhem integration. The activity of chromium, referred to solid pure chromium, was measured between 900 and 1200° in solid Fe-Ni-Cr alloys with chromium concentrations of 9, 20, and 30 at. pct and Ni concentrations of 8, 18, and 30 at. pct. Additions of nickel to Fe-Cr alloys in the above concentration range are found to increase the chromium activity. The effect of nickel in increasing the chromium activity is greater at both greater chromium contents and lower temperatures. Formerly Graduate Student at The University of Michigan, is Staff Associate, Gulf Energy and Environmental Systems, LaJolla, California. This paper is based on a portion of a thesis submitted by F. N. MAZANDARANY in partial fulfillment of the requirements for the degree Doctor of Philosophy at The University of Michigan.     

Optimization of Process Conditions for Ultralightweight Steel with More Than 13 wt% of Al and 5 wt% of Cr

An ultralightweight Fe–30Mn–13.2Al–1.6C–5Cr steel, which contains more than 13 wt% of Al and thereby reduces the density by 20%, is developed. The ultralightweight steel, which is very brittle due to high Al content, is fabricated by optimizing hot rolling and heat treatment conditions. Hot rolling is conducted after soaking at the temperature range of 1100–1200 °C for 2 h. The ultralightweight steel is hot-rolled successfully after soaking at 1100 °C, whereas specimens soaked at 1150 and 1200 °C are intergranularly cracked after hot rolling, resulting from coarse grain and a large fraction and size of ferrite, which is transformed to ordered DO₃ phase during cooling, at the grain boundaries. In homogenization heat treatment, water quenching and air cooling are performed, respectively, after holding at 1050 °C for 2 h. The air-cooled steel has inferior tensile property due to the formation of brittle ordered DO₃ phase at grain boundaries. Meanwhile, the water-quenched steel shows an excellent tensile property, which is attributed to a uniform microstructure comprising austenite, fine κ-carbide in austenite, and a very small fraction of the ordered DO₃ phase.     

The effect of continuous heating on the phase transformations in zinc- iron electrodeposited coatings

The crystal structure and phase transformations of electrodeposited zinc-iron coatings were investigated by several techniques. The phase composition of as-plated zinc-iron coatings was found to contain both 17 phase and8 phase and, according to the equilibrium phase diagram, was in a nonequilibrium condition for the compositions investigated. The transformation toward equilibrium during heating was a two-stage process. The first stage of transformation, at temperatures between 25 °C and 360 °C, is accomplished with very little change in composition, and the phase transformation occurs within the coating itself. It is proposed, through a simplified analysis, that the phase transformation in this stage is supported by short-range diffusion. In the second stage of the transformation, at temperatures above 360 °C, the iron content in the coating increases as a result of interdiffusion between the coating and the base steel, and the phase structure moves to the iron-rich side in accordance with the equilibrium phase diagram. MINGYUAN GU, formerly Visiting Research Scientist, Energy Research Center     

Phase transformations and control of residual stresses in thick spray-formed steel shells

Large, thick steel shells for tooling applications have been produced using a robot manipulated electric arc spraying technique with steady-state temperatures ranging from 170 °C to 450 °C. Critical to these experiments has been the use of a real-time feedback control system for surface temperature based on infrared thermal imaging. There was a reproducible trend in net residual shell distortion as a function of temperature with residual tensile stresses in the shell for temperatures ≤210 °C and ≥390 °C, and net compressive stresses at intermediate temperatures. In-situ linear displacement sensor experiments have been used to investigate the dynamic distortion of sprayed steel shells on steel substrates, over the same range of surface temperatures. Residual and in-situ distortion measurements confirmed two manufacturing temperatures at which stresses in the steel shells were either minimized or eliminated. A numerical model has been developed to relate shell quench and transformations stresses to the shell dynamic distortion behavior. It is proposed that tensile quench stresses are balanced by the time- and temperature-dependent expansive austenite-to-bainite phase transformation.     

Extraction of iron from copper-plant slag

In the production of copper from sulfide ore, slag accumulates at the processing plant. In the present work, the phase transformations that occur in such slag during its reduction by the gasification products of carbon at 1100 and 1200°C are investigated. Experiments show that most of the iron present in such slag may be converted to metallic form by the gasification products of carbon in cupola furnaces at 1100°C. Subsequent increase in temperature to 1200°C improves the extraction of iron. In the indirect reduction of slag at temperatures above its melting point, metallic iron is mainly concentrated at the outer surface of the product, forming large inclusions that are easily extracted by magnetic separation. An expedient approach is to reduce laminar batch consisting of slag layers no thicker than 5 mm and interlayers of ground coal.