Duplex Austenitic-ferritic Steel Research Articles

ANALYSIS OF THE MICROSTRUCTURE AND MECHANICALPROPERTIES OF DUPLEX STEEL JOINTS

The article presents a study, the aim of which was to analyze the microstructure and mechanical propertiesof welded joints of austenitic-ferritic duplex steel X2CrMnNiN21-5-1. This steel is characterized by goodresistance to various types of corrosion, good strength properties and good weldability. Due to all theseadvantages, it is used in many industry sectors, and the main joining techniques are welding technologies. Inthis study, two joining techniques were used, SMAW (Shielded Metal Arc Welding) and GTA (Gas TungstenArc). The obtained welded joints were subjected to: macroscopic and microscopic metallographic tests,mechanical tests (static bending test and microhardness measurements), diffraction tests, and wear resistancetests. The results showed that the microhardness of the welds is similar and does not depend on the weldingmethod used. In the microstructure of the analyzed joints there are two phases: austenite (γ) and ferrite (δ),with different morphologies depending on the welding conditions, which affect the phase transformations.Material wear within the weld is greater than in the base material.

Sigma-phase in duplex-stainless steels

Abstract The ferritic-austenitic duplex-steels have a very complex precipitation and transformation behaviour, which requires professional treatment. Especially the precipitation of the σ-phase causes considerable changes with regard to the mechanical as well as the corrosive properties, which are to be considered during the treatment of the duplex-steels.

Investigation of the Influence of Different Hard Coatings on Chip Formation and Process Forces When Machining Duplex Steel 1.4462

When machining duplex steel 1.4462 the choice of the hard coating of the cutting tool plays a major role due to the high process temperatures, caused by relatively low thermal conductivity of this material. Hard coatings can have different thermo-physical properties, e. g. thermal conductivity and specific heat capacity. Thus, they influence the process temperatures in the shear zones and have an impact on the chip formation. For a mechanisms-oriented tool development it is necessary to understand these influences. Experimental cutting tests based on a linearorthogonal kinematic are a suitable method for an experimental approach. A challenge with experimental methods is to vary the thermo-physical properties of the coatings without changing the friction effects in the contact areas of the tool and workpiece at the same time. An unwanted change of the friction leads to superimposed effects and difficulties during the interpretation of the influence of the thermo-physical properties. This work discusses the influence of two different wear protection layers on the behavior of an austenitic-ferritic duplex steel during linearorthogonal cutting experiments. In the investigations, one monolayer coating (TiAlN) and one multilayer coating (TiN-TiCN-Al2O3) were examined. The experiments were carried out under different cutting conditions and wear conditions of the tools to generate various friction conditions. In the results analysis, focus is mainly given to the cutting and feed forces as well as the chip thickness values. The experimental results are supplemented with analytical analyses to understand the influence of thermo-physical properties of coatings and interfacial effects caused by friction.

The Assessment of the Microstructure and Selected Mechanical Properties of Butt Joints in Ferritic-Austenitic Duplex Steel

The Assessment of the Microstructure and Selected Mechanical Properties of Butt Joints in Ferritic-Austenitic Duplex Steel

Ultrasonic fatigue testing in the scanning electron microscope

Abstract The continuing trend to increase the performance and durability of machines requires the use of materials, whose structural integrity must be ensured far beyond the classical fatigue limit. In this so-called „very high cycle fatigue“ regime (VHCF), the materials show a strong scatter in fatigue life, which makes life service assessment difficult. Present studies on austenitic-ferritic duplex steel 1.4462 (X2CrNiMoN22-5-3) have shown that the reason of this scatter in fatigue life can be found in the microstructural length scale mainly due to the barrier effect of grain and phase boundaries. The integration of an ultrasonic fatigue testing system in a high-resolution scanning electron microscope allows observing and evaluating the active microstructural mechanisms of fatigue crack initiation and early crack propagation, such as the formation and propagation of slip bands, during the majority of VHCF life. Additional micro texture measurements by electron backscatter diffraction (EBSD) provide data for the development of a mechanism-oriented numerical short crack simulation. In the context of crack initiation and propagation in the VHCF regime in duplex stainless steel, the focus of the present paper is put on the technical realization of the in-situ ultrasonic fatigue testing in the scanning electron microscope.

Skłonność do tworzenia pęknięć zimnych złączy ze stali ferrytyczno-austenitycznej typu dupleks 2205 przy spawaniu pod wodą metodą mokrą

Skłonność do tworzenia pęknięć zimnych złączy ze stali ferrytyczno-austenitycznej typu dupleks 2205 przy spawaniu pod wodą metodą mokrą

The Significance of Local Plasticity for the Crack Initiation Process During Very High Cycle Fatigue of High Strength Steels

The present paper deals with experimental results on the fatigue damage of grade SAE 318 LN austenitic-ferritic duplex steel and a modified SAE 4150 tempering steel in the very high cycle fatigue (VHCF) regime. Particular attention was paid to the relation- ship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. Therefore, electrolytically polished bulk specimens have been fatigued under fully reversed loading in an ultrasonic fatigue testing machine while the surface has been observed in-situ by an optical microscope. The specimens were carefully analyzed by means of scanning electron microscopy (SEM) in combination with automated electron back-scatter dif- fraction (EBSD). In case of the austenitic-ferritic duplex steel under VHCF loading conditions, slip band formation is limited to the softer austenite grains. Once being formed, the bands generate high stress concentrations where they impinge the austenite- ferrite (γ−α) phase boundaries, eventually, leading to the crack initiation. In case of the tempering steel crack initiation is caused by slip band formation within the martensitic laths structure. It was found that in both materials microcracks are sensitive to changing crystallographic orientations when crossing a grain or phase boundary.

Mechanisms of short crack propagation in austenitic–ferritic duplex steel

For applications requiring both high strength and high corrosion resistance, austenitic–ferritic duplex steels are often the material of choice. In this study, cyclic deformation experiments were performed on the austenitic–ferritic duplex stainless steel 1.4462. By measuring the crack opening and crack sliding displacement in situ in a scanning electron microscope, the characteristics of the different crack propagation mechanisms in the two phases are determined. In the ferritic phase, two different appearances of short cracks can be observed, one exhibiting a very smooth and the other one a rather rough surface crack path. Electron backscatter diffraction measurements on the crack-containing grains in addition with high resolution imaging of the topography of the crack flanks reveal that contrary to common assumptions in the literature, short cracks in ferrite do not propagate via single slip. Instead, two different slip systems with an identical slip direction, but different slip planes, are activated. In this context, the specific appearance of different crack paths can be explained with the orientation of the respective grains. Furthermore, a model for discontinuous crack propagation especially of rough cracks in ferrite is developed. Finally, a correlation between the crack propagation rate and the plastic deformation of the crack tip is revealed and the possibility of determining the barrier effect of grain and phase boundaries via the measurement of the plastic deformation of the crack tip is investigated.

Short fatigue crack propagation during low-cycle, high cycle and very-high-cycle fatigue of duplex steel – An unified approach

The present paper reviews experimental results on the fatigue damage of austenitic–ferritic duplex steel under various load levels ranging from LCF to VHCF, placing the focus towards the relationship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. A combination between fatigue testing of electropolished specimens and analytical electron microscopy (SEM/EBSD, TEM) revealed that under LCF loading conditions almost all the ferrite and the austenite grains showed plasticity, while under HCF and VHCF loading conditions, slip band formation was limited to the softer austenite grains and a low plastic activity is observed in the ferrite. Once being formed, the bands generate high stress concentrations, where they impinge the α–γ phase boundaries, eventually, leading to the crack initiation. This is discussed by applying a numerical simulation approach based on the finite-element (FEM) and the boundary-element (BEM) method.

Initiation and propagation of short cracks in austenitic–ferritic duplex steel

Austenitic–ferritic duplex stainless steels are widely used for applications, where both excellent strength and high corrosion resistance are required. In the current study, high cycle fatigue experiments were performed on the austenitic–ferritic duplex stainless steel 1.4462 in order to investigate short crack initiation and propagation. It is shown that the most frequent crack initiation sites are grain boundaries between ferritic grains followed by slip bands in austenite and phase boundaries. Taking into account the elastic anisotropy of the austenitic phase, the surprisingly high fraction of cracks formed along slip bands in austenite can be explained with the texture and microstructure of the material. Analyzing the correlation between crack propagation rate and crack length, it is found that in both phases, the transition crack length between the regimes of microstructurally and mechanically short cracks correlates with the mean grain elongation under the average angle between crack path and loading axis. Additionally, the dependence of the crack propagation rate on the distance between crack tip and opposing grain or phase boundary, i.e. the barrier effect of these boundaries for short crack propagation, could be measured. The results obtained expand the knowledge about short crack behavior in the investigated material, which is a key parameter for the lifetime in the high cycle fatigue regime.

Duplex Steels: Part I: Genesis, Formation, Structure

The ferritic–austenitic duplex steels are equipped with a mechanical–technological combination of properties, which is advantageous compared to the features of stainless completely ferritic or completely austenite steels. The duplex steels crystallize by fully ferritic or ferritic–austenite solidification with the austenite precipitation due to the solid solution reactions during the further cooling. To adjust the ferrite–austenite ratio, the steels must be heat treated by temperatures above the field of precipitation stability, followed by water quenching. The temperature and the time of the heat treatment effect the element distribution according to their higher solubility in the ferritic or austenitic phases. The typical microstructure of the duplex stainless steels can only be realized due to deformation and recrystallisation processing.

Initiation and short crack growth in austenitic–ferritic duplex steel‐effect of positive mean stress

ABSTRACTThe effect of the mean stress on the crack initiation and short crack growth of austenitic–ferritic duplex steel has been studied. High mean stresses and stress amplitudes result in appreciable mean strain relaxation and long‐term hardening. Mean stress produces unidirectional slip bands and slip steps that serve as nuclei for persistent slip bands and persistent slip markings. It leads to the acceleration of the crack initiation and production of a high density of cracks. Crack linkage contributes to the growth of short cracks. The concept of equivalent crack was used to describe the crack growth. The kinetics of short crack growth with positive mean stress is similar to that in symmetric loading, that is, exponential growth is observed. Positive mean stress results in earlier crack initiation and in the acceleration of the crack growth rate. Both factors contribute to the decrease of the fatigue life.

Hydrogen Absorption And Diffusion in different welded Duplex Steels

In this study, hydrogen absorption and diffusion were investigated for various high-alloyed ferritic-austenitic duplex steels. On account of the specific transformation and solidification behaviour, respectively, of duplex steels as compared to single-phase ferritic and austenitic steels, special conditions have to be considered concerning hydrogen absorption which may ultimately lead to microstructure-dependent hydrogen-assisted weld metal cracking. Hydrogen absorption during welding may occur via the shielding gas, moisture from Qthe surroundings or via the welding filler material. As a contribution to the interpretation and prediction of “ hydrogen-induced cracking in welded duplex steels, the actual hydrogen absorption via the arc as well as the weld metal hydrogen diffusion was investigated for the first time in a duplex steel DS (1.4462), a super duplex steel SDS (1.4501) and in a lean duplex steel LDS (1.4162). Isothermal heat treatment using carrier gas hot extraction enabled quantification of the amounts of hydrogen trapped in the respective microstructure areas. The hydrogen diffusion coefficients were determined by analytical and numerical calculation. The total hydrogen concentrations and the diffusion coefficients were found to be nearly identical. Trapped hydrogen was however observed to be dependent on the material and on the microstructure condition. The influence of hydrogen on the mechano-technological properties of the weld metal was characterized with the help of tensile tests. In addition, the hydrogen embrittlement effect was detected in scanning electron microscopic analyses

Cyclic Plasticity and Cyclic Creep in Austenitic-Ferritic Duplex Steel

Smooth specimens made from austenitic-ferritic duplex steel were subjected to constant stress amplitude loading with positive mean stresses. Hysteresis loops were recorded during the fatigue life and plastic strain amplitude and cyclic creep rate were determined. Fatigue hardening/softening curves, cyclic creep curves and cyclic stress-strain curves for different positive mean stresses were evaluated. Typical dislocation structures developed in both phases of the duplex steel were identified using TEM, compared with the saturated plastic strain amplitude and correlated with the decrease of the cyclic creep rate during cycling and the slope of the cyclic stress-strain curve.

Self-consistent modeling of rolling textures in an austenitic–ferritic duplex steel

Rolling textures of the constituent phases in an austenitic–ferritic duplex stainless steel are measured by X-ray diffraction experiments, showing that the brass-type texture, typical of f.c.c. materials with low SFE, is developed in the austenitic phase, and the rotated-cube and brass-R textures are developed in the ferritic phase. On the basis of the experimental texture components and fibers at different reductions, rolling textures of the respective phases in the duplex steel are simulated using a self-consistent model. After considering various micromechanical interactions within the steel, a reliable prediction of the evolution of grain orientation distributions for the phases at small reductions is achieved. An attempt in modeling the brass-type texture for the f.c.c. metallic phase is also performed by incorporating the shear banding mechanism into the presented model.

Cyclic plastic response and fatigue life in symmetric and asymmetric cyclic loading

The response of materials to cyclic loading using different loading conditions is analyzed. Hysteresis loops of austenitic steel cycled in symmetric and asymmetric strain- and stress controlled loading are analyzed to evaluate the contribution of effective stress and probability distribution of internal critical stresses. The austenitic-ferritic duplex steel is cycled with various positive mean stresses and cyclic stress-strain response, cyclic creep rates, resulting dislocation structures and fatigue life are evaluated. The effect of the mean stress on cyclic plasticity and fatigue life is discussed. © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11

The significance of microstructural barriers during fatigue of a duplex steel in the high- and very-high-cycle-fatigue (HCF/VHCF) regime

Austenitic–ferritic duplex steels are used for structural applications when high strength in combination with excellent corrosion resistance is required. Many of these applications imply cyclic loading and hence, fatigue damage needs to be considered for dimensioning. Most dimensioning strategies make use of the fatigue-limit concept. However, while the original fatigue-limit concept is based on the idea that existing slip bands or microcracks are blocked by microstructural barriers like grain or phase boundaries, more recent research work has shown that metallic structures may fail far below the conventional fatigue limit even at very high numbers of cycles to fracture. The present paper deals with the observation of local plasticity and fatigue damage in the VHCF regime by means of high-frequency fatigue testing in combination with scanning electron microscopy (SEM) and with electron back-scattered diffraction (EBSD). The results reveal that fatigue damage in the VHCF regime indeed causes the formation of slip bands followed by initiation and propagation of microstructurally short cracks in a very localized manner, manifesting itself by heat generation. The SEM observations and measurements of slip band geometries were correlated with calculations using a finite-element and a numerical short-crack model, which take the real two-phase microstructure and its elastic/plastic anisotropy into account and allow the prediction of both: (i) the fatigue crack initiation sites and (ii) microcrack propagation rates.

Propagation Behaviour of Microstructurally Short Fatigue Cracks in the High-Cycle- and Very-High-Cycle Fatigue Regimes

In the present paper examples for propagating and non-propagating conditions of slip bands and short fatigue cracks in a ferritic-austenitic duplex steel are given, which were quantified by means of SEM in combination with automated EBSD. To classify the results within the scope of predicting the service life under HCF- and VHCF-loading conditions a numerical model based on the boundary-element method has been developed, where crack propagation is described by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.

Propagation mechanisms of microstructurally short cracks—Factors governing the transition from short- to long-crack behavior

Microstructural short fatigue cracks are known to exhibit an abnormal propagation behavior as compared to long cracks, which grow by a rate that can be described by, e.g., the Klesnil–Lukáš relationship. By means of carefully recording crack length versus number of cycles in the high-cycle fatigue regime in combination with a microtexture analysis using automated electron back-scattered diffraction, the parameters determining the scatter in microcrack propagation rates and the transition from short- to long-crack behavior were identified for an austenitic–ferritic duplex steel. The three-dimensional orientation relationship of the slip planes in grains involved in the crack propagation process turned out to be most significant. This relationship determines the barrier effect of grain and phase boundaries as well as the local crack propagation mechanisms, either operating crystallographically by single slip or perpendicularly to the applied load axis operating by double/multiple slip. To predict the propagation behavior of microstructurally short cracks in a mechanism-based way, a numerical model has been developed that accounts for local interactions between the crack tip and the microstructure as the current driving force for crack advance.

Experimental characterization and two-dimensional simulation of short-crack propagation in an austenitic–ferritic duplex steel

On the basis of a thorough experimental study of the interactions between microstructural features, like the crystallographic orientation distribution and the respective crack propagation rate a numerical micro-mechanical crack-propagation model has been developed, which accounts for the abnormal propagation behaviour of short cracks as compared to long cracks in a two-phase austenitic-ferritic duplex steel. In order to simulate real cracking events, the barrier strengths of phase and grain boundaries, beside the orientation of slip planes as the most important input parameters for the model, were determined by means of a Hall–Petch analysis using cyclic stress–strain data of single-phase austenitic and ferritic steels as well as of the duplex steel.