Phase transformations in Fe–Ni–Cr heat-resistant alloys for reformer tube applications

Fe–35Ni–25Cr–0.4C alloys with different compositions are aged between 750 and 1150°C up to ～10,000?h. As-cast microstructure contains interdendritic carbides of type M₇C₃ (‘Cr₇C₃’) and MC (‘NbC’). At service temperatures, M₇C₃ transform into M₂₃C₆ (‘Cr₂₃C₆’) within hours. Then, a hardening precipitation of secondary intragranular M₂₃C₆ occurs over hundreds of hours, the nose of the ‘temperature-time-hardening’ curve being around 1000°C. G phase forms after long aging; its solvus temperature and formation kinetics depend on silicon content. Z phase is observed after long aging at 950°C or above. G and Z phases form at the expense of MC. Very long aging causes nitridation under air, with first a transformation of M₂₃C₆ into chromium-rich M₂X carbonitrides (X?=?C,N), then of MC into chromium-rich MX carbonitrides.     

Solidification microstructures selection of Fe–Cr–Ni and Fe–Ni alloys

The transition of solidified phases in Fe–Cr–Ni and Fe–Ni alloys was investigated from low to high growth rate ranges using a Bridgman type furnace, laser resolidification and casting into a substrate from superheated or undercooled melt. The ferrite–austenite regular eutectic growth, which is difficult to find in typical production conditions of stainless steels, was confirmed under low growth rate conditions. The transition velocity between eutectic and ferrite cell growth had a good agreement predicted by the phase selection criterion. Which of either ferrite or austenite is easier to form in the high growth range was discussed from the point of nucleation and growth. Metastable austenite formation in stable primary ferrite composition was mainly a result of growth competition between ferrite and austenite. For a binary Fe–Ni system, a planar metastable austenite in the steady state, simultaneous growth such as eutectic and banded growth between ferrite and austenite in an initial transient region are confirmed.     

Density and thermal expansion of Cr–Fe, Fe–Ni, and Cr–Ni binary liquid alloys

Densities and their temperature coefficients of liquid Cr–Fe, Fe–Ni, and Cr–Ni binary alloys have been measured containerless using the technique of electromagnetic levitation. Data have been obtained in a wide temperature range including the supercooled region. The density measurements indicate that these binary systems have a small and positive excess volume, whereas the excess free energies are negative. The temperature coefficients of these alloys can be estimated from those of the pure components. Hence, possible contributions from the temperature dependence of the excess volume can be ignored to calculate the temperature coefficient of density.     

Density and viscosity of ternary Cr–Fe–Ni liquid alloys

The density and viscosity of ternary Cr–Fe–Ni liquid alloys have been investigated over a wide temperature range. The density was measured using electromagnetic levitation as a container-less technique, while viscosity was measured by means of a high-temperature oscillating cup viscometer. Although, the concentration dependence of density shows the influence of the second order (binary) interaction parameter in excess volume, the influence of a third order (ternary) interaction parameter in excess volume can be neglected. The temperature dependences of the viscosities are well described by the Arrhenius law. The viscosity increases monotonically as Fe or Cr concentration increases. For constant temperature, the viscosity as a function of iron molar faction can be described by a thermodynamic model using the enthalpy of mixing as input parameter.     

Role of strain heterogeneity on recrystallisation of oxide dispersion strengthened Fe–Cr–Al alloys for high-temperature applications

Mechanically alloyed iron-based oxide dispersion-strengthened alloys have the potential for application in heat exchangers for biomass power plants, with operating temperature and pressure at entry of the gas turbine working fluid being around 1,100 °C and 15–30 bar, respectively. The yttria dispersion in such alloys improves the high-temperature creep and stress rupture life. The strength is further enhanced by the development of a coarse-grained microstructure during recrystallization. Factors controlling the evolution of this desirable microstructure are explored in this study, focusing specifically on PM 2000. The results can be interpreted if it is assumed that anything which makes the microstructure heterogeneous stimulates recrystallization. The combination of these results with finite element modelling is used to interpret the role of strain heterogeneity on the development of recrystallized grain structure. In this sense, larger strain gradients lead to more refined and more isotropic grain structures.     

The development of aligned precipitates during internal carbonitridation of Fe–Ni–Cr alloys

Abstract

An austenitic Fe–25Cr–19Ni stainless steel alloy was carbonitrided at 1,000°C in an atmosphere with a_c=1 and P(N₂)=0.9atm to form two discrete precipitation zones. Local equilibrium between the precipitates and the austenite matrix carbon activity was achieved throughout the reaction zone. Small, globular Cr₇C₃ particles were formed immediately beneath the surface. High aspect ratio Cr₂₃C₆ lamellar plates were formed deeper in the precipitation zone and were found to have a cube–cube orientation relationship with the austenite matrix. The inward growth of these carbides was facilitated by the formation of an austenite/depleted austenite grain boundary at the precipitation front, which transformed the austenite to a more appropriate orientation and accelerated the segregation of chromium to the carbide tips.     

Tensile properties of the Cr–Fe–Ni–Mn non-equiatomic multicomponent alloys with different Cr contents

A series of Fe₄₀Mn₂₈Ni_32 − xCr_x (x = 4, 12, 18, 24 (at.%)) multicomponent alloys was prepared by vacuum arc melting. The Fe₄₀Mn₂₈Ni₂₈Cr₄, Fe₄₀Mn₂₈Ni₂₀Cr₁₂ and Fe₄₀Mn₂₈Ni₁₄Cr₁₈ alloys were ductile single phase fcc solid solutions. The Fe₄₀Mn₂₈Ni₈Cr₂₄ alloy had intermetallic sigma phase matrix and was extremely brittle after homogenization. The tensile properties of the Fe₄₀Mn₂₈Ni₂₈Cr₄, Fe₄₀Mn₂₈Ni₂₀Cr₁₂ and Fe₄₀Mn₂₈Ni₁₄Cr₁₈ solid solution alloys were examined in recrystallized condition with average grain size of ~ 10 μm. The yield strength increased from 210 MPa of the Fe₄₀Mn₂₈Ni₂₈Cr₄ alloy to 310 MPa of the Fe₄₀Mn₂₈Ni₁₄Cr₁₈ alloy. The elongation to fracture of the alloys decreased from 71% to 54%, respectively. Solid solution strengthening by the constitutive elements of the alloys was calculated using Labush approach. Strong solid solution strengthening by Cr was predicted. Gypen and Deruyttere approach was used to estimate solid solution strengthening of the Fe₄₀Mn₂₈Ni_32 − xCr_x alloys. Good correlation between predicted solid solution strengthening and the experimental yield strength values was found.     

Surface tension and density data for Fe–Cr–Mo,Fe–Cr–Ni,and Fe–Cr–Mn–Ni steels

The temperature dependence of surface tension and density for Fe–Cr–Mo (AISI 4142), Fe–Cr–Ni (AISI 304), and Fe–Cr–Mn–Ni TRIP/TWIP high-manganese (16 wt% Cr, 7 wt% Mn, and 3–9 wt% Ni) liquid alloys are investigated using the conventional maximum bubble pressure (MBP) and sessile drop (SD) methods. In addition, the surface tension of liquid steel is measured using the oscillating droplet method on electromagnetically levitated (EML) liquid droplets at the German Aerospace Centre (DLR, Cologne). The data of thermophysical properties for Fe–Cr–Mn–Ni is of major importance for modeling of infiltration and gas atomization processes in the prototyping of a “TRIP-Matrix-Composite.” The surface tension of TRIP/TWIP steel increased with an increase in temperature in MBP as well as in SD measurement. The manganese evaporation with the conventional measurement methods is not significantly high within the experiments (?_Mn < 0.5 %). The temperature coefficient of surface tension (dσ/dT) is positive for liquid steel samples, which can be explained by the concentration of surface active elements. A slight influence of nickel on the surface tension of Fe–Cr–Mn–Ni steel was experimentally observed where σ is decreased with increasing nickel content. EML measurement of high-manganese steel, however, is limited to the undercooling state of the liquid steel. The manganese evaporation strongly increased in excess of the liquidus temperature in levitation measurements and a mass loss of droplet of 5 % was observed.     

Thermomechanical behavior of Fe–Mn–Si–Cr–Ni shape memory alloys modified with samarium

The deformation and training behavior of Fe–14Mn–3Si–10Cr–5Ni (wt.%) shape memory alloys containing samarium addition has been studied in the iron-based shape memory alloys. It is noticed that thermomechanical treatment (training) has significant influence on proof stress, critical stress and shape memory behavior of the alloys. The improvement in shape memory behavior can be attributed to the decrease in the proof stress and critical stress which facilitates the formation of ? (hcp martensite). It is also observed that alloy 2 containing samarium undergoes less softening as compared to alloy 1 with training which inhibits the formation of ? (bcc martensite) and thus enhances the shape memory behavior. The excessive thermomechanical treatment with increase in the training cycle has led to the formation of ? (bcc martensite) along with ? (hcp martensite) in the alloy 1 which appeared to have decline in the shape memory effect. This has been demonstrated by the examination of microstructure and identification of ? (bcc martensite) martensite in the alloy 1 as compared to alloy 2.     

Role of nickel in the oxidation of Fe–Cr–Ni alloys in air–water vapour atmospheres

Abstract

A series of experimental austenitic alloys has been produced in which the nickel content ranges from 14 to 43%, with constant levels of 20%Cr, 1%Mn and 0.5%Si. A combination of isothermal, discontinuous and cyclic oxidation testing has been used to elucidate the performance in dry air and in air with 10%, 45% or 62% water vapour at 700°C and 1000°C. Evaluation was by means of thermogravimetry, surface analysis with glow discharge optical emission spectroscopy and scanning electron microscopy.

Nickel is shown to have several roles: it accelerates the kinetics of chromia formation yet suppresses chromia spallation at 700°C. At 1000°C, it strongly decreases the breakaway oxidation and spalling associated with iron oxide formation. This effect is particularly marked in environments containing water vapour, where the material loss may be decreased 10-fold by an increase in the nickel content. Results correlate to thermodynamic and kinetic data which show nickel to increase the chromium activity and diffusivity in the alloy.     

Characterization of Co–Cr–Mo alloys after a thermal treatment for high wear resistance

The cobalt–chromium–molybdenum alloys are characterized by a high resistance to wear and corrosion, as well as good mechanical properties, allowing their use in the substitution of hip and knee joints.Five alloys were used as substrates for a coating deposition by a thermal treatment in molten salts, as reported elsewhere, in order to form a tantalum‐rich coating on the sample surface, able to improve the biocompatibility and wear resistance of the materials. However, the temperature (970 °C), reached during this process, is considered critical for the phase transformation of the Co-based alloys.The aim of this work is the evaluation of the temperature effects on the structure, microstructure, mechanical and tribological properties of the considered substrates, after the removal of the coating by polishing. The substrates are characterized through X-ray diffraction (XRD), scanning electron microscopy with energy dispersion spectrometry (SEM-EDS) and profilometry. The mechanical behavior is evaluated by the macro- and micro-hardness and bending tests, whereas the tribological properties are analyzed through a ball on disc test. A comparison between the as-received alloys and thermal treated substrates is reported. The biocompatibility feature is not reported in this work.The substrate crystalline structure changed during the heat treatment, inducing the formation of the hexagonal cobalt phase and the decrement of the cubic one. This crystallographic modification does not seem to influence the tribological behavior of the substrates. On the contrary, it affects the strength and ductility of the substrates.     

Electrochemical characteristics of nano and microcrystalline Fe–Cr alloys

The electrochemical behavior of nano and microcrystalline Fe–10Cr and Fe–20Cr alloys was determined using potentiodynamic polarization in 0.5 M H₂SO₄. Disks of the alloys were prepared by high-energy ball milling followed by compaction and sintering. In the current study, nanocrystalline Fe–Cr alloys reveal significantly different electrochemical characteristics, typified by lower anodic current densities and more negative passivation potentials, compared with their microcrystalline counterparts. In addition to the differences in grain boundary density, compositional characterization of corrosion films carried out by X-ray photoelectron spectroscopy indicates a higher Cr content in the film developed upon nanocrystalline Fe–Cr alloys. Mechanisms for observed enhancement in the corrosion performance of the nanocrystalline Fe–Cr alloys are discussed.     

Development of Ni–Fe–Al-based alloys precipitating cubic γ′ for fabrication of nanoporous membranes

During this study, new Ni-base alloys precipitating cubic γ′ were developed which shall be used for the production of polycrystalline nanoporous membranes. The polycrystalline nanoporous membranes are produced through a combination of cold rolling and heat treatment in order to get directional coarsening of the γ′-phase which is selectively dissolved afterwards. Conventional Ni-based superalloys have a γ/γ′ -microstructure with cubic γ′-precipitates and show the needed etching behaviour but their high strength and limited ductility at room temperature do not allow to produce the polycrystalline nanoporous membranes by means of the before mentioned method. Thus, the new alloys with simpler composition were developed which have a γ/γ′ microstructure. The alloy Ni–13Fe–8Al–4Ti (composition in atomic percent) which was produced by Schmitz (Cullivier, ISBN 978-3-86955-523-2, 16) served as basis and showed the promising characteristics. To obtain cubic γ′-precipitates, the misfit was estimated to values of at least |0.2| % by a method presented by Mishima (Acta Metall 33:1161–1169, 23). Further, the phase compositions as well as phase volume fractions of γ-matrix and γ′-phase were calculated by means of Thermocalc^® simulations (database: TTNi7). The etching behaviour of the new alloys was adjusted by adding chromium and molybdenum which passivate the γ-matrix so that the γ′-precipitates dissolve during the leaching process. The well-aligned cubic γ′-precipitates were obtained by partially replacing titanium by niobium. Furthermore, the hardness could be significantly lowered compared to conventional superalloys by reduction of alloying elements. Hence, the promising alloys were found to get directional coarsening of the γ′-precipitates in a combined process of cold rolling and heat treatment.     

The role of cementite in the metal dusting of Fe–Cr and Fe–Ni–Cr Alloys

Abstract

Model Fe–25 w/o (weight percent) Cr and Fe–25 Cr–Ni alloys containing 2.5, 5, 10 and 25w/o nickel were exposed to a CO–26H₂–6H₂O (vol. pct) mixture at 680°C under thermal cycling conditions. The supersaturated carbon activity was calculated to be 2.9 (referred to graphite) and M₃C was predicted to form on Fe–25Cr and Fe–25 Cr–2.5 Ni, but not on higher nickel content alloys. Metal dusting occurred on all alloys, accompanied by internal carburisation. Transmission electron microscopy of the dusting deposit showed that much of the carbon consisted of hollow graphite nanotubes. Small, metal-rich particles were found at the carbon filament tips. These were identified as single crystal Fe₃C in the case of Fe–25 Cr, and M₃C, containing low levels of nickel, in the case of Fe–25 Cr–2.5 Ni and Fe–25 Cr–5 Ni. In contrast, the particles found at the filament tips on the higher nickel, two phase, alloys were both M₃C and austenitic Fe–Ni. Strong orientation relationships were determined between the graphite and cementite particles, however, no consistent and clear crystallographic relationship was deduced between the graphite and austenite particles. It is concluded that carbon deposition from the gas is catalysed by both Fe₃C and austenite. Subsequent carbon nanotube growth reflects the orientation relationship between Fe₃C and the graphite.     

Laser-assisted synthesis of Ti–Mo alloys for biomedical applications

This paper presents the results of a laser-based combinatorial investigation of the Ti–Mo system, aiming at finding alloys with promising properties for orthopedic applications. Variable powder feed rate laser cladding was applied to synthesize Ti–xMo alloys with composition continuously varying in the range of 4–19 wt.% Mo. Screening was performed on the basis of the alloys' mechanical properties, in particular hardness and Young's modulus, measured by microindentation tests. Microstructural analysis showed that alloys with Mo content between 4 and 8 wt.% are composed of acicular martensite and retained β-phase, the proportion of the later phase increasing with increasing Mo content. Alloys with Mo content of 10 wt.% and higher consist entirely of β phase. All the alloys present a Mo segregation pattern indicating that solidification occurred with a cellular solid–liquid interface. Though β-phase alloys present lower values of Young's modulus and hardness than α′- or α″- containing alloys, minimum values of Young's modulus (75 GPa) and hardness (240 VHN) were achieved for the Ti–13 wt.% Mo alloy.     

Calorimetric and magnetic study of solid Fe–Cr–Co alloys

Abstract

An adiabatic calorimeter was used to measure the molar heat capacities and heats of transformation of iron alloys containing up to 14 at.-% Cr and 15 at.-% Co over the temperature range 700–1500 K. A Sucksmith magnetic balance was used to measure the magnetic properties of these alloys and of Fe–Co and Fe–Cr alloys previously investigated calorimetrically. In all but one of the ternary alloys the α → γ transformation occurred at a temperature below the Curie temperature (T_c), but the heats of transformation, corrected to the transformation temperature of pure iron (1184 K), correlated well with the difference between the extrapolated Curie temperature and 1184 K, as had been observed previously for alloys with clearly defined values of T_c. The fact that all observations fell on the same curve supported the proposal that a significant amount of magnetic order remains after the α → γ transformation for most iron alloys.

MST/198     

Microstructural changes during annealing of cast Ni–Cr–Hf alloys

Abstract

In order to gain knowledge of the nickel–rich corner of the Ni–Cr–Hf phase diagram, microstructures of cast alloys and their evolution towards equilibrium conditions have been followed by optical and scanning electron microscopy, microprobe analysis, and quantitative metallography. The evolution of as–cast microstructures involves the development of a Ni₅Hf plate–like precipitate in two–phase alloys, and various morphological changes in three–phase alloys. As–cast structures are compared with those obtained by arc melting. Direct processing of the scanning electron microscope image has proved to be a valuable tool for microstructural characterization of the transforming phases.

MST/349     

The effect of Fe on the crystallization behavior of Al–Mm–Ni–Fe amorphous alloys

The crystallization behavior and thermal stability of Al₈₆Mm₄Ni_10–x Fe _x alloys were investigated as a function of Fe content. Alloys, produced by a single roll melt-spinner at a circumferential speed of 52 m/s, revealed fully amorphous structures. The thermal stability of the present amorphous alloys increased with the increase of Fe content. The activation energy for crystallization of -Al increased as the Fe content increased. This increase of activation energy resulted in the simultaneous precipitation of -Al and intermetallic phase observed especially in Al₈₆Mm₄Ni₅Fe₅ and Al₈₆Mm₄Ni₂Fe₈ alloys. The glass transition was observed in DSC thermogram only after proper annealing treatment. The effect of alloy composition on the thermal stability could be explained in terms of the atomic structure of the amorphous alloy.     

Bimodal grain size distribution: an effective approach for improving the mechanical and corrosion properties of Fe–Cr–Ni alloys

The effect of nanocrystalline grain size and bimodal distribution of nano- and microcrystalline grain sizes on the oxidation resistance and mechanical properties of Fe-based alloys has been investigated. Nanocrystalline and bimodal Fe–10Cr–5Ni–2Zr alloy pellets, prepared by mechanical alloying route, have been compared with conventional microcrystalline stainless steel alloys having 10 and 20 wt% Cr. Zr addition has been shown to improve the grain size stability at high temperatures. A significant improvement in the ductility of bimodal alloys with respect to nanocrystalline alloys was seen presumably due to the presence of the microcrystalline grains in the matrix. The high temperature oxidation of nanocrystalline and bimodal alloys at 550 °C shows superior oxidation resistance over microcrystalline alloy of similar composition (Fe–10Cr–5Ni) and comparable to that of microcrystalline alloy having twice as much Cr (Fe–20Cr–5Ni). Secondary Ion Mass Spectroscopy depth profiling confirms the hypothesis that nanostructure facilitates the enrichment of Cr at the oxide metal interface resulting in the formation of a passive oxide layer.     

Morphology and crystallographic phase of V–C particles formed in Fe–Cr–Ni–V–C alloys

Abstract

The morphology and crystallographic phase of V–C carbide particles formed in cast Fe–Cr–Ni–V–C alloys were investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM). The combination of results obtained with these techniques revealed that cuboidal, cruciform and spherical carbide particles were formed, depending on the alloy composition, all having the cubic-VC_1?x structure (Fm-3m). Detailed TEM observations suggested that small carbide particles were initially cubic in shape and became spherical with increasing particle size. All cuboidal and spherical carbides were single crystallites with no grain boundary at any particle sizes, even after growing to 6 μm in diameter.