Thermodynamic investigation of the austenite and the delta ferrite in the system Fe-Cr-Mn-N

The nitrogen solubility was measured in a wide range of temperatures including both the austenitic and the ferritic phase of iron-chromium-manganese alloys. The mass contents of the samples were 6.0 to 20.5 % Cr and 5.7 to 16.6 % Mn. Parameters describing the austenite and the delta ferrite in the system Fe-Cr-Mn-N were calculated analyzing all available experimental data. This evaluation yielded an improvement of the known parameters describing the austenitic phase by means of the regular solution model. At the first time, accurate thermodynamic parameters describing the delta phase both in the systems Fe-Mn-N and Fe-Cr-Mn-N could be deduced. By means of the phase diagram program package PD-pp the austenite-delta ferrite phase boundaries of the investigated alloys were calculated. They were compared with experimentally determined temperatures and nitrogen concentrations in the two-phase region. There is a good correspondence between theory and experiment. Using published data describing the melt a calculation of phase diagrams in the high temperature region can be performed.     

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. Formerly with Royal Institute of Technology, Stockholm.     

Control of δ‐γ Transformation during Solidification of Stainless Steel Slabs in the Mould

In order to obtain satisfactory workability properties required for defect free slab and strip production, the parameters of the casting process, e.g. cooling rate at the initial solidification for the alloy in question must, on the one hand, be carefully adjusted. On the other hand, controlling the characteristics of the solidification structure by chemical composition then takes on particular significance. The main aims of the work were to find out the influence of the phase transformation δ‐γ during the initial solidification of different variants of AISI 304 slabs cast in the industrial process on the ferrite distribution on slab surface, and how this relationship could favour the improvements of the surface slab quality. This report contains the joint contributions of the collaborative ECSC project among Centro Sviluppo Materiali (CSM), Krupp Thyssen Nirosta (KTN) and the Department of Ferrous Metallurgy of Rheinisch Westfälische Technische Hochschule Aachen (RWTH).     

The isothermal δ-γ transformation of ferritic-austenitic iron-chromium-nickel alloys

Quenching from ferritizing temperatures of 1350 and 1250 °C the isothermal δ-γ transformation of two iron-chromium-nickel alloys with 26.0% Cr, 6.9% Ni resp. 30.0% Cr, 7.05% Ni at temperatures between 700 and 1200 °C is studied. Three different transformation stages are revealed indicating grain boundary diffusion controlled growth in the first and last stage while volume diffusion seems to act at least partially in the second stage. Isothermal TTT diagrams are established and activation energies calculated.     

STT and DTT Diagrams of Austenitic Cr–Mn–Ni As‐Cast Steels and Crucial Thermodynamic Aspects of γ → α′ Transformation

The mechanical behavior and microstructure evolution during deformation of novel austenitic Cr–Mn–Ni as‐cast steels with varied Ni content were investigated at various temperatures using static tensile tests, optical microscopy, and the magnetic scale for the detection of ferromagnetic phase fraction. To summarize all knowledge about the deformation‐induced processes, the STT and DTT diagrams were developed for Cr–Mn–Ni steels. The diagrams illustrate the different deformation mechanisms depending on temperature and tension load, and quantify the elongation of the deformation mechanisms. The deformation‐induced ε‐ and α' martensite formation and twinning – the TRIP and TWIP effects – occur in the Cr–Mn–Ni steels depending on the chemical composition and temperature. The differences of deformation‐induced processes depend on thermodynamics and are confirmed by thermodynamic calculations. The nucleation threshold of γ → α′ transformation was determined for the investigated Cr–Mn–Ni steels.     

Fließspannung im Bereich der (γ → α)-Umwandlung von kohlenstoffarmen Stählen

Auswertung von Fließkurven aus den Warmflachstauchversuchen und quantitativer Gefügeanalyse. Anomalie auf der Fließspannung-Temperatur-Kurve im (γ → α)-Bereich bei Umformung während der anisothermischen Umwandlung. Bilanz der Einflußfaktoren.     

Simulation of the γ–α‐transformation using the phase‐field method

Phase transformation is a powerful tool to change the properties of steels. Of the known transformations especially the γ–α‐transformation is utilised. It occurs in a temperature range relevant for heat treating and hot deformation processes. In this paper an approach is presented in which the γ–α‐transformation is simulated with Micress. This software applies the multicomponent multiphase‐field model, which is based on the reduction of total free enthalpy. Two different steels have been selected for the simulations, an ULC and an IF steel. Dilatometric tests serve as a basis for the simulations. These tests have shown that the transformation behaviours of the two steel grades are governed by two different kinetics. The transformation kinetics of the IF grade is influenced by the microalloying concept applied, resulting in a very slow start of the transformation. This has also been incorporated in the simulations by choosing two different grain boundary mobilities, one main parameter of the simulation. The simulation results of the ULC grade show the huge influence of nucleation undercooling as another one of the main parameters. Both simulation results are satisfying. They show that the phase‐field method offers a strong simulation tool in the area of phase transformation.     

The continuous δ-γ transformation during cooling of ferritic-austenitic iron-chromium-nickel alloys

Following previously published isothermal TTT diagrams, continuous TTT diagrams for the δ-γ transformation of a series of different iron-chromium-nickel-alloys containing 23.0–35.0 Cr and 6.45–15.6 Ni are presented. The microstructures and transformation behaviors indicate the same nucleation and growth process as in the isothermal reactions. The critical upper cooling rate for complete undercooling of the δ-γ transformation decreases with increasing chromium- and decreasing nickel contents.     

Der Einfluß der Keimbildung auf die irreversiblen Dimensionsänderungen von Reinsteisen bei zyklischen α/γ/α-Umwandlungen

Versuchsanordnung mit symmetrischem Temperaturprofil. Messung der relativen differentiellen Längenänderung nach α/γ-, γ/α- und α/γ/α-Umwandlungen. Einfluβ der Aufheiz- und Abkühlungsgeschwindigkeit und der Vorverformung der Probe auf die irreversible Längenänderung. Beobachtung der Struktur durch thermische Ätzung. Untersuchung der Substruktur durch chemische Ätzung, Laue-Aufnahmen und elekironenoptische Aufnahmen. Zusammenhang zwischen irreversibler Längenänderung und Keimbildung im flachen Teil des Temperaturprofils.     

Verformung von Manganstahl mit 16% Mn während der γ/ε-Umwandlung

An Stahl mit 0,09% C, 1,0% Si und 16% Mn Untersuchung des Einflusses von Zugspannungen unterhalb der Fließgrenze auf die Verformung und bleibende Dehnung beim Durchlaufen der γ/ε-Umwandlung. Erörterung der Ergebnisse.     

Untersuchungen zur Ermittlung des δ-Ferritgehaltes in austenitischen Stählen

Thermomagnetische Analyse und ihre Verwendbarkeit als Meßverfahren zur Ermittlung des δ-Ferritanteils in austenitischen Stählen. Die auf die Masseneinheit bezogene Sättigungsmagnetisierung des δ-Ferrits in Abhängigkeit vom Chrom- und Nickelgehalt. Ermittlung des δ-Ferritanteils aus der spezifischen Sättigungsmagnetisierung bei Raumtemperatur und dem Gehalt an Chrom und Nickel im Stahl. Genauigkeit des Verfahrens.     

Untersuchungen zum Einfluß des Kohlenstoffs und des Stickstoffs auf die δ-γ-Umwandlung ferritisch-austenitischer Chrom-Nickel-Stähle

Einflußsteigender Kohlenstoff- und Stickstoffgehalte (bis O, 13 % C bzw. 0,27 % N) ferritisch-austenitischer FeCrNi-Legierungen mit rd. 24,5% Cr und 7% Ni auf die Phasengrenzen und Konodenrichtung bei 1200°C. Glühtemperaturabhängigkeit des δ-Ferrits und M₂₃C₆-Carbids. Zeitlicher Verlauf der Ferritisierung und Kornvergröberung. Darstellung der δ-γ-Umwandlung in kontinuierlichen ZTU-Diagrammen.     

Kohärente Ausscheidung in Substitutionsmischkristallen des α-Eisens

Bildungsbedingungen, Merkmale und Formen kohärenter Ausscheidungen in Nichteisen- und Eisenmetallegierungen. Beispiele für die kohärente Ausscheidung vor allem in den Systemen des α-Eisens mit Kupfer, Gold, Aluminium, ferner mit Beryllium, Chrom, Molybdän und Wolfram. Änderung von mechanischen und physikalischen Eigenschaften (vor allem Härte, Kennwerte im Zugversuch; elektrischer Widerstand, Koerzitivfeidstärke) in Abhängigkeit vom Ausscheidungszustand.     

Numerical Modeling of δ‐Ferrite Dissolution Kinetics in Austenitic Stainless Steels

In the present paper a numerical model that describes the diffusion processes of Ni and Cr between ferrite and austenite in Fe‐Cr‐Ni ternary systems has been developed by using the finite difference method (FDM). The model employed the tie‐line relationships given by Kajihara and Kikuchi. The moving ferrite/austenite interface was determined by solving the mass balance equations using the Newton‐Raphson method. The model has been applied to predicting the dissolution kinetics of ferrite in austenitic stainless steels, which were processed by welding, continuous casting and strip casting, respectively. The accuracy of the model has been confirmed to be quite satisfactory as compared with measured values, with advantages over another numerical model developed by Vitek et al. This model was reproduced in the present work.     

δ-γ phase equilibria in iron-rich iron-chromium-nickel alloys at temperatures between 1200 and 1350 °C

The δ-γ phase equilibria of iron-rich iron-chromium-nickel alloys with 23 to 38% Cr, 7 to 22% Ni are established at temperatures between 1200 and 1350 °C. The effect of chemical compositions and annealing temperatures on equilibrium δ ferrite fraction is discussed.     

Einfluß der δ-γ-Umwandlung des Eisens auf den Wärmeübergang zwischen Block und wassergekühlter Kupferkokille

Gießversuche mit Reineisen sowie mit ferritischen Fe-V- und Fe-Si-Legierungen im Vakuuminduktionsofen. Thermische Messungen des Erstarrungsablaufes in einer wassergekühlten Kupferkokille. Messung der Wärmeflußdichte zwischen Block und Kokille bei Gießen und Erstarren unter Vakuum ohne Gaswärmeleitung im Spalt. Ermittlung der Wärmeübergangskoeffizienten. Aussagen zur Spaltbildung mit einem Strahlungsmodell und anhand der dilatometrisch bestimmten Längenänderung bei der Abkühlung. Einfluß der δ-γ-Umwandlung des Eisens auf die Spaltbildung in unterschiedlich zusammengesetzten Stählen und mögliche Folgerungen für die Vergießbarkeit in Stranggießkokillen.     

Cold rolling texture development of α/γ duplex stainless steels

The cold rolling texture development of two α/γ duplex stainless steels (DSS) with similar volume fractions of both phases but with totally different microstructures were investigated. Due to the limited number of available pole figures using X-rays, for the calculation of the ODFs both a direct method and a recent iterative series expansion method were used. The results were checked by neutron diffraction measurements. The austenitic phases of both DSS behave similarly to single phase materials with a low stacking fault energy which develop a brass-type rolling texture. In contrast, the texture development of the ferritic phases strongly differs from those of single phase ferrites. Instead of a fibre type texture the α-phase in both DSS exhibits a peak dominated texture regardless of whether it is the matrix phase or not. These differences, as well as the sharpness of both phases, are explained by the presence of the second phase.