Staffan Hertzman

A Thermodynamic analysis of the Fe-Cr-N system

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.

An experimental and thermodynamic study of the Fe-Cr-C-N system at 1273 K

The thermodynamic properties of the Fe-Cr-C-N system at 1273 K (1000 °C) have been evaluated using old and new information. The binary systems are well established. The Fe-Cr-C system is fairly well established, but some experiments were performed in this study in order to establish theα/γ/M23C6 equilibrium. The Fe-Cr-N system was evaluated in a parallel study. In the Fe-C-N system the properties of theα andγ phases are well established. No direct information from the Cr-C-N system was used. In order to establish the properties of the quaternary system some experiments were made by equilibrating a set of Fe-Cr-C-N alloys at 1273 K, using a sealed capsule technique. After quenching, the carbon and nitrogen activities were evaluated by analyzing the specimens, which were completely austenitic. Phase equilibria in other specimens were studied by microprobe measurements and X-ray phase identification. When the quaternary system was evaluated thermodynamically, it was found that all the experimental information could be reasonably well accounted for without introducing new parameters for the quaternary system. However, it was necessary to evaluate the properties of the metastableε-Cr2C phase in order to fit the quaternary experimental information. The phase diagram was calculated from the evaluated thermodynamic properties. A number of sections are presented for direct comparison with the experimental data.

Element distribution in lean duplex stainless steel welds

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.

On the effect of nickel substitution in duplex stainless steel

Abstract 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.

A Study of equilibria in the Fe-Cr-Ni-Mo-C-N system at 1273 K

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.

Weldability aspects of a newly developed duplex stainless steel LDX 2101

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 ...

Observations of copper clustering in a 25Cr-7Ni super duplex stainless steel during low-temperature aging under load

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°C for 5800 h. The degree of phase separation into α (Fe-rich) and α′ (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.

First-Principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys

First-principles alloy theory is used to establish the γ-surface of Fe-Cr-Ni alloys as function of chemical composition and temperature. The theoretical stacking fault energy (SFE) versus chemistry and temperature trends agree well with experiments. Combining our results with the recent plasticity theory based on the γ-surface, the stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective stacking fault energy below ∼18 mJ m−2. Alloys with SFE above this critical value show both twinning and full slip at room temperature. Interestingly, twinning remains a possible deformation mode in addition to full slip even at elevated temperatures, in line with observations.

Ab initio prediction of the mechanical properties of alloys : The case of Ni/Mn-doped ferromagnetic Fe

First-principles alloy theory, formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation, is used to study the mechanical properties of ferromagnetic body-centered cubic (bcc) Fe1-xMx alloys (M = Mn or Ni, 0 slip system, respectively. Nickel is found to produce larger effect on the planar fault energies than Mn. The semiempirical 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} is proposed.

Elastic anomalies in Fe-Cr alloys

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.