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Phase-field modeling is used to simulate the formation of sigma phase in a model alloy mimicking a commercial super duplex stainless steel (SDSS) alloy, in order to study precipitation and growth of sigma phase under linear continuous cooling. The so-called Warren–Boettinger–McFadden (WBM) model is used to build the basis of the multiphase and multicomponent phase-field model. The thermodynamic inconsistency at the multiple junctions associated with the multiphase formulation of the WBM model is resolved by means of a numerical Cut-off algorithm. To make realistic simulations, all the kinetic and the thermodynamic quantities are derived from the CALPHAD databases at each numerical time step, using Thermo-Calc and TQ-Interface. The credibility of the phase-field model is verified by comparing the results from the phase-field simulations with the corresponding DICTRA simulations and also with the empirical data. 2D phase-field simulations are performed for three different cooling rates in two different initial microstructures. A simple model for the nucleation of sigma phase is also implemented in the first case. Simulation results show that the precipitation of sigma phase is characterized by the accumulation of Cr and Mo at the austenite-ferrite and the ferrite-ferrite boundaries. Moreover, it is observed that a slow cooling rate promotes the growth of sigma phase, while a higher cooling rate restricts it, eventually preserving the duplex structure in the SDSS alloy. Results from the phase-field simulations are also compared quantitatively with the experiments, performed on a commercial 2507 SDSS alloy. It is found that overall, the predicted morphological features of the transformation and the composition profiles show good conformity with the empirical data.

Duplex stainless steels are designed to solidify with ferrite as the parent phase, with subsequent austenite formation occurring in the solid state, implying that, thermodynamically, a fully ferrit ...

First-principles calculations have been performed to study the effect of C on the stacking fault energy (SFE) of paramagnetic γ -Fe and Fe Cr Ni austenitic steel. In these systems, the local magnetic structure is very sensitive to the volume in both fcc and hcp structures, which emphasizes the importance of the magnetovolume coupling effect on the SFE. The presence of C atom suppresses the local magnetic moments of Fe atoms in the first coordination shell of C. Compared to the hypothetical nonmagnetic case, paramagnetism significantly reduces the effect of C on the SFE. In the scenario of C being depleted from the stacking fault structure or twin boundaries, e.g. , due to elevated temperature, where the chemical effect of C is dissipated, we calculate the C-induced volume expansion effect on the SFE. The volume induced change in the SFE corresponds to more than ∼ 50% of the total C effect on the SFE obtained assuming uniform C distribution.

In this study, three austenitic stainless steels with different compositions are compared in terms of their deformation behaviour. Two of the investigated steels have considerable high nitrogen content which affects their deformation behaviour. The deformation properties and microstructure of the materials were studied by tensile testing and electron backscatter diffraction. We observe that the strain hardening rate curve of the alloy with low nitrogen content shows a continuously decreasing slope, whereas those of the high‑nitrogen steels exhibit a clear plateau. Since no twinning or e-phase formation is observed at the corresponding strain levels, we suggest that the addition of a large amount of nitrogen suppresses cross-slip and promotes dislocation planarisation. The microstructural evolution of the materials during deformation supports the above scenario. Based on the results of the ab initio calculations, the deformation behaviour of high‑nitrogen alloys cannot be explained in terms of the stacking fault energy.

The nanostructure evolution and the corresponding changes in mechanical properties of a super duplex stainless steel 2507 (UNS S32750) during aging at 300. °C up to 12,000. h have been investigated. Microstructural studies using transmission electron microscopy and atom probe tomography show that subtle Cr concentration fluctuations develop during aging. The amplitude of the concentration fluctuations is proportional to the hardness of the ferrite phase, and it is also proportional to the decrease in room temperature impact toughness during aging. The fracture behaviour of the alloy changes gradually from ductile to cleavage fracture, upon aging. The cracks were found to propagate through the ferrite phase, partly along deformation twin interfaces, and delamination between the austenite and ferrite phases was observed.

Segregation of alloying elements and nitrogen loss may have a negative effect on the corrosion resistance of autogenously welded duplex stainless steel. The lean duplex LDX 2101® (EN 1.4162, UNS S32101) has less metal element segregation and improved austenite formation compared to other duplex grades and addition of filler metal is not always necessary to achieve good corrosion properties. In gas tungsten arc welding, nitrogen additions to the shielding gas can counteract nitrogen loss and thereby contribute to achievement of the required phase balance. The element distribution in S32101 welds performed autogenously bead-on-plate with and without nitrogen additions to the shielding gas has been quantified and illustrated with electron probe microanalysis mapping. Local segregation and depletion along the fusion line have been elucidated and explained in terms of epitaxial growth and dendritic solidification behavior. The resulting variations in the corrosion resistance are demonstrated by the pitting resistance equivalent distribution and compared to previously known corrosion data.

The coarsening of duplex stainless steel microstructures at temperatures above 1000°C was investigated experimentally and theoretically. Hot-rolled duplex 2205 and superduplex 2507 samples were heat treated isothermally at 1100°C up to 700 h, and also between 1000 and 1250°C for 24 h. The ferrite content and the microstructure coarseness, quantified by the measured austenite spacing, AS, were evaluated. It was found that the observed coarsening could be described using Ostwald ripening theory expressed as , where AS0 is austenite spacing of the as-delivered material, k the rate constant, n the exponent, and t the heat treatment time. Using regression analysis, n and k were evaluated and the resulting predictions compared with literature theories and results. The results were also calculated according to the original Lifshitz, Slyozov, Wagner (LSW) theory with parameters taken from literature, as well as with the Dictra software for diffusion and growth in multicomponent systems. The calculations indicated that while the classical LSW theory failed to give a good description of the experimental results, the Dictra simulations proved very accurate, even though systems of equal ferrite and austenite content are considerably outside the intended range of applicability of the software.

The concurrent phase separation and clustering of alloying elements in the ferrite phase of duplex stainless steel weldments after stress aging at 325 degrees C have been investigated by atom probe tomography analysis. Both phase separation, into Fe-rich and Cr-rich ferrite, and solute clustering were observed. Phase separation in the heat-affected zone (HAZ) is most pronounced in the high alloyed SAF 2507, followed by SAF 2205 and SAF 2304. Moreover Cu clustering was observed in the HAZ of SAF 2507. However, decomposition in the weld bead (25.10.4L) was more pronounced than in the HAZs, with both phase separation and clustering of Ni-Mn-Si-Cu. The observed differences in the decomposition behaviors in the HAZ and weld bead can be attributed to the high Ni content and the characteristic microstructure of the weld bead with high internal strains. In addition, an applied tensile stress during aging of weldments has been found to further promote the kinetics of phase separation and clustering.

Atom-probe tomography was used to investigate phase separation and copper (Cu) clustering in the ferrite phase of a 25Cr-7Ni super duplex stainless steel. The steel was subjected to a tensile load during aging at 325 degrees C for 5800 h. The degree of phase separation into alpha (Fe-rich) and alpha' (Cr-rich) was small, but still, it was the highest in the steel subjected to the highest load. Cu was found to cluster, and the number density of clusters increased with increasing load. In the material subjected to the highest load, Cu was enriched in regions that were neither Fe-rich nor Cr-rich. These regions also had the highest number density of Cu clusters.

One of the key issues in the development of the modern duplex stainless steels was the introduction of nitrogen as an alloying element. This was made possible by e.g. the AOD process and has meant that the problems with earlier grades regarding weldability could be solved. The prime reason for the lower weldability, namely a coarse-grained heat affected zone with inferior properties was then removed. Further beneficial effects of nitrogen increases in the new generation of duplex grades were pointed out: localized corrosion resistance particularly in chloride containing solutions, enhancement of fatigue properties, improved creep properties particularly when considering heat treatment...The paper will outline the influence of nitrogen additions in duplex grades by means of thermodynamic calculations including phase diagrams, TTT phase precipitation diagrams from metallographic investigations, mechanical and corrosion properties. Metallographic investigations, corrosion resistance properties and mechanical properties will be presented. The specific links between welded joints microstructures and properties with different nitrogen contents will be discussed.

The equilibrium conditions of four duplex stainless steels; Fe-23Cr-4.5Ni-0.1N, Fe-22Cr-5.5Ni-3Mo-0.17N, Fe-25Cr-7Ni-4Mo-0.27N and Fe-25Cr-7Ni-4Mo-1W-1.5Cu-0.27N were studied in the temperature reg ...

Weld oxides have a strong influence on corrosion resistance, but have hitherto only been studied to a limited extent for duplex stainless steels. X-ray photoelectron spectroscopy (XPS) has here been used to study heat tint formed on gas tungsten arc (GTA) welds on the commercial duplex grades LDX 2101 (EN 1.4162/UNS S32101) and 2304 (EN 1.4362/UNS S32304) welded with and without nitrogen additions to the shielding gas. The process of heat tint formation is discussed in terms of transport phenomena to explain the effect of atmosphere, temperature and composition. The oxides formed were found to be enriched in manganese and corrosion testing shows that nitrogen has a strong influence on the weld oxide. A mechanism is proposed including evaporation from the weld pool and subsequent redeposition.

Duplex grades have, due to balanced chemical compositions of both filler and base metals, a weldability that allows for successful welding using a majority of the technically relevant techniques of ...

The present experimental and theoretical study investigates the effect of nickel on the phase balance and resulting properties of a 22Cr duplex stainless steel. The decrease in nickel was balanced by nitrogen and manganese additions. It was found that a minimum nickel content was required to maintain mechanical and corrosion properties at technically relevant levels. The influence of increasing nitrogen content on resulting phase composition and properties is discussed.

First-principles alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to study the mechanical properties of ferromagnetic body-centered cubic (bcc) Fe$_{1-x}$M$_x$ alloys (M=Mn or Ni, $0\le x \le 0.1$). We consider several physical parameters accessible from \emph{ab initio} calculations and their combinations in various phenomenological models to compare the effect of Mn and Ni on the properties of Fe. Alloying is found to slightly alter the lattice parameters and produce noticeable influence on elastic moduli. Both Mn and Ni decrease the surface energy and the unstable stacking fault energy associated with the $\{110\}$ surface facet and the $\{110\}\langle111\rangle$ slip system, respectively. Nickel is found to produce larger effect on the planar fault energies than Mn. The semi-empirical ductility criteria by Rice and Pugh consistently predict that Ni enhances the ductility of Fe but give contradictory results in the case of Mn doping. The origin of the discrepancy between the two criteria is discussed and an alternative measure of the ductile-brittle behavior based on the theoretical cleavage strength and single-crystal shear modulus $G\{110\}\langle111\rangle$ is proposed., Comment: 14 pages, 11 figures

The influence of nitrogen on the microstructure and properties of austenitic and duplex stainless steel welds is reviewed. Three aspects are covered: intermetallic phase formation, nucleation and g ...

The microstructure and resulting properties of manual metal arc super duplex weld metals deposited using three different filler metals and two heat inputs were investigated using various techniques. Analytical transmission electron microscopy, electron probe microanalysis, and scanning electron microscopy were used to study the microstructure and Charpy V notch impact energy and different corrosion test methods were used to characterise the weld metal properties. The fillers were of approximate composition (wt-%) Fe-25.5Cr-(3–4)Mo-9.5Ni-0.25N; one filler had additions of tungsten and one had additions of tungsten plus copper. The effect of composition and heat input on secondary austenite γ2 formation was evaluated experimentally and the results interpreted in terms of a theoretical model for diffusion controlled austenite growth. The equilibrium conditions were calculated using the Thermo-Calc database. The formation of γ2 in the as welded condition appears to be promoted by a low nitrogen and hi...

Three duplex grades, one molybdenum-free, one 22Cr type, and one super duplex grade, have been subjected to weld simulation treatments, and the resulting microstructures have been quantified by automatic image analysis techniques. Substantial differences between the duplex grades were observed with an increased ability to reform austenite with increased alloying content. A theoretical model has been applied, based upon the paraequilibrium concept elaborated by Hillert, and the paraequilibrium compositions of individual phases were calculated as a function of temperature using the THERMOCALC database. A model based on Cahns theory of grain boundary nucleated reactions has also been utilized to calculate the kinetics of the reaction. By using this model, the grain size effects could be included in the treatment. The results of the calculations were compared with experimental data, and the experimental results were reproduced using the same parameter set for the three materials, with the exception of the diffusion coefficient values which had to be adjusted. This adjustment has in a later study been verified experimentally. The results validate the model used and the physical relevance of using the paraequilibrium model. The appropriateness of a paraequilibrium approach is also supported by experimental evidence from weld metal compositions. It is shown that the nitrogen content of the alloys plays an important role, and a higher nitrogen content results in more efficient austenite reformation. This implies that the alloy nitrogen compositions should lie close to the upper specification limits for these materials and nitrogen losses should be avoided on welding since the material properties, both mechanical and corrosive, are strongly related to the austenite-ferrite phase ratio.

Using ab initio alloy theory, we determine the elastic parameters of ferromagnetic and paramagnetic Fe(1-c)Cr(c) (0 ≤ c ≤ 1) alloys in the body centered cubic crystallographic phase. Comparison with the experimental data demonstrates that the employed theoretical approach accurately describes the observed composition dependence of the polycrystalline elastic moduli. The predicted single-crystal elastic constants follow complex anomalous trends, which are shown to originate from the interplay between magnetic and chemical effects. The nonmonotonic composition dependence of the elastic parameters has marked implications on the micro-mechanical properties of ferrite stainless steels.

TIG welding using different heat inputs, arc lengths and shielding gas nitrogen contents was performed. The aim was to evaluate the possibilities to avoid nitrogen losses on welding or even increase the weld metal nitrogen content and thereby improve the corrosion properties and, in the case of duplex grades, also to improve the phase balance. Three different nitrogen-alloyed duplex grades and one superaustenitic grade were investigated. The corrosion resistance in terms of CPT, Critical Pitting Temperatue, of the super duplex material was found to be strongly correlated to the nitrogen content of the weld metal. In the case of the superaustenitic weld metal, the increased nitrogen content was found to be associated with an increased pore formation, leading to a lower corrosion resistance and thereby masking the positive effect of the increased introgen content. In order to illuminate the nitrogen exchange reactions between the arc, weld pool, and shielding gas, stationary weld experiments were also performed. The results from these stationary trials indicated that the net weld pool nitrogen content could be qualitatively understood if the various fluxes involved in the nitrogen transport between the plasma arc, weld pool and weld pool-shielding gas were considered. At short times the weld pool was limited in area and the nitrogen content of the weld pool increased due to high nitrogen activity in the arc. At longer times the nitrogen escaped from the weld pool to the shielding gas. This flux became then the dominating factor due to the increased weld pool area exposed to the shielding gas. The situation then approached the equilibrium conditions that were expected from the gas nitrogen activity and weld pool alloy composition according to thermodynamic calculations using the Thermo-Calc database.

Stainless steels are among the most important engineering materials, finding their principal scope in industry, specifically in cutlery, food production, storage, architecture, medical equipment, etc. Austenitic stainless steels form the largest sub-category of stainless steels having as the main building blocks the paramagnetic substitutional disordered Fe-Cr-Ni-based alloys. Because of that, austenitic steels represent the primary choice for non-magnetic engineering materials. The presence of the chemical and magnetic disorder hindered any previous attempt to calculate the fundamental electronic, structural and mechanical properties of austenitic stainless steels from first-principles theories. Our ability to reach an ab initio atomistic level approach in this exciting field has become possible by the Exact Muffin-Tin Orbitals (EMTO) method. This method, in combination with the coherent potential approximation, has proved an accurate tool in the description of the concentrated random alloys. Using the EMTO method, we presented an insight to the electronic and magnetic structure, and micromechanical properties of austenitic stainless steel alloys. In the present contribution, we will discuss the role of magnetism on the stacking fault energies and elastic properties of paramagnetic Fe-based alloys.

The elastic properties of ferromagnetic Fe1-xMx (M=Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0

The phase equilibria in the Fe-rich corner of the Fe-Cr-Ni-Mo-C-N system have been studied at 1273 K using a sealed capsule technique to measure the C and N activities and the electron microprobe to measure the compositions of the individual phases following identification by X-ray analysis. Some of the new information was combined with previous assessments of the Fe-Cr-N, Fe-Ni-N, and Fe-Cr-Ni systems and a new assessment of the Cr-Ni-N system in order to assess the thermodynamic properties of the Fe-Cr-Ni-N system. A set of parameters is obtained, based mainly upon experimental information from 1273 K (1000 °C) and 1473 K (1200 °C), which can be used for calculations of the Fe-Cr-Ni-N phase diagram in this temperature range. Isothermal sections are presented and show reasonable agreement with experimental data not used in the assessment. The thermodynamic analysis is restricted to the Fe-Cr-Ni-N system, but some experimental data are also presented for alloys containing Mo and C.

New experimental information on the phase relations in the Fe-Cr-N system at 1273 K is presented. Together with previous information it is used in a thermodynamic analysis of the system in the temperature range from 1273 to 1473 K. A set of parameter values is derived which defines the Gibbs energy of the individual phases and allows the phase diagram to be calculated. Satisfactory agreement with experimental information is obtained.