Austeno-ferritic Stainless Steel Research Articles

Laves phase precipitation and sigma phase transformation in a duplex stainless steel microalloyed with niobium.

Duplex stainless steels are used to replace austenitic steels in industrial applications, especially in the chemical industry where corrosion resistance in chloride-containing media is an important requirement. According to literature, niobium has a great influence on the transformation of the phases of these steels (ferrite, austenite and sigma). Given the interest in evaluating this property in terms of wear resistance and corrosion of the material, the aim of this study was to evaluate the effect of niobium on the formation in austenitic-ferritic stainless steel SEW 410 Nr 1.4517 after heat treatment at 850ºC for 15, 30 and 60 minutes. The niobium contents studied were 0.2; 0.5 and 1.5 wt%. Measurements of microhardness, electrochemical tests in saline solution containing 3.5% sodium chloride and wear resistance were carried out. The results show that an increase in niobium content and sigma phase leads to an increase in hardness and wear resistance, associated with a decrease in corrosion potential. Therefore, the use of niobium in austeno-ferritic stainless steel is recommended when wear resistance is an important factor to be considered.

Ultrasound Array Probe: Signal Processing in Case of Structural Noise

Abstract This work focuses on non-destructive examinations using array probe ultrasonic waves on complex materials generating a high structural noise on the examined area. During an ultrasonic examination, multiple scattering of the ultrasonic waves at the grain boundaries makes the distinction between this structurally induced noise and a potential defect challenging. The difficulty of the interpretation can moreover be increased in the near surface area because of the subsurface wave. In order to ease the analysis of these acquisitions, some numerical processing methods are proposed. Statistical properties of the imaging results (for instance, total focusing method or plane wave imaging) are first calculated on several sensor positions. These statistical properties are then used to post-process the imaging results and enhance any signal values that do not belong to the structural noise expected statistics. The method, called “CORUS,” has been successfully tested on cast austenoferritic stainless steel coarse-grained mock-ups, with several dB gain compared to the classical total focusing method. It is now integrated in a civa software plugin and in a prototype version of the real-time PANTHER-phased-array acquisition system from Eddyfi Technologies.

Detrimental Cr-rich Phases Precipitation on SAF 2205 Duplex Stainless Steels Welds After Heat Treatment

Abstract The austeno-ferritic Stainless Steels are commonly employed in various applications requiring structural performances with enhanced corrosion resistance. Their characteristics can be worsened if the material is exposed to thermal cycles, since the high-temperature decomposition of ferrite causes the formation of detrimental secondary phases. The Submerged Arc Welding (SAW) process is currently adopted for joining DSS owing to its relatively simple execution, cost savings, and using molten slag and granular flux from protecting the seam of atmospheric gases. However, since it produces high contents of δ-ferrite in the heat affected zone and low content of γ-austenite in the weld, high-Ni filler materials must be employed, to avoid excessive ferritization of the joint. The present work is aimed to study the effect of 3 and 6 hours isothermal heat treatments at 850°C and 900°C in a SAF 2205 DSS welded joint in terms of phases precipitation. The results showed the presence of σ-phase at any time-temperature combination, precipitating at the δ/γ interphases and often accompanied by the presence of χ-phase. However, certain differences in secondary phases amounts were revealed among the different zones constituting the joint, ascribable both to peculiar elements partitioning and to the different morphology pertaining to each microstructure.

Effect of sintering atmosphere on structure and properties of austeno-ferritic stainless steels

Prealloyed 316L and 430L stainless steel powders along with Cu, Cr, Mo and Ni have been used to produce different austeno-ferritic stainless steels through powder metallurgy route. Cylindrical green compacts of 30 mm diameter and 12 mm height have been made at three different compaction pressures and sintered in nitrogen and argon atmospheres at 1350 °C for 4 h, to study the effect of sintering on densification behaviour and mechanical properties. Stainless steels sintered in argon atmosphere exhibited better densification rate than the nitrogen atmosphere sintering. The microstructure of stainless steel sintered in nitrogen atmosphere revealed lamellar constituents with grain boundary Cr 2N in ferritic matrix. Composition A (50 wt% 316L + 50 wt% 430L) of austeno-ferritic stainless steel sintered in argon atmosphere showed bi-phase structure with high strength and better elongation. The XRD patterns are in line with the microstructure. The ferrite content was measured by ferrite scope and is coherent with the strength and structures.

Effect of temperature on sintered austeno-ferritic stainless steel microstructure

The influence of temperature on microstructural changes of sintered austeno-ferritic steels has been investigated. PM stainless steels have been obtained by sintering mixtures of austenitic and ferritic stainless steel powders. Only temperature-induced phase transformation was observed in austenite, as a result of elements interdiffusion between both phases. Microstructural characterization was completed with atomic force microscopy (AFM) and micro- and nano-indentation test, it is revealed an increase in the hardness with respect to the solutionized materials.

Corrosion behaviour of new stainless steels reinforcing bars embedded in concrete

The activities concerned with the evaluation, repair and restoration of structures are estimated to amount to 35% of the total volume of the work in the building sector and this continues to increase. The corrosion of rebars in the reinforced concrete structures (RCS) is the main reason for their degradation, so the use of reinforcing stainless steels seems to be one of the possible solutions with most probabilities of solving this problem. In this work, in order to demonstrate the advantages of using reinforcing stainless steels, the corrosion behaviour of AISI 304 and 316 reinforcements embedded in concrete slabs (C35/45 and C60/70 concrete) with two chloride contents are compared with three low-cost and low-Ni austenoferritic stainless steels and with the conventional carbon steel. The lower chloride contamination selected in this research, was enough to cause the corrosion in the active state of the carbon steel reinforcements, whereas the highest one exceeded the expected contamination in the natural environments, including sea media. The metallic materials remaining in the passive state can be considered, from the point of view of corrosion resistance, adequate as reinforcements in the RCS.

Influence of chemical composition and residual stresses on mechanical properties of duplex stainless steel studied by X-ray and neutron diffraction

The parameters characterizing the elastoplastic mechanical behaviour of each phase in textured austeno-ferritic duplex stainless steels were determined using X-ray and neutron diffraction during uniaxial tensile test. A self consistent model was used to predict the evolution of the internal stresses during loading, and to identify critical resolved shear stresses \and strain hardening parameters of the material. Calculations based on this model were compared with the results of mechanical tests and diffraction data. The effect of residual stresses and the chemical composition on the elastoplastic phase behaviour was determined.

Magnetic properties of austeno-ferritic stainless steel after cathodic hydrogen charging

The influence of cathodic hydrogen charging on the magnetic properties of austeno-ferritic stainless steel was investigated. The measurements using atomic force microscope working in magnetic force microscopy mode were performed. The changes of the magnetic properties of the steel were observed. The magnetic phase in the form of laths has been revealed.

AFM study of austeno-ferritic stainless steel microstructure after cathodic hydrogen charging

Microstructural changes occurring in austenitic–ferritic stainless steel after two variants of thermo-mechanical treatment and subjected to cathodic hydrogen charging were studied using atomic force microscopy operated in the magnetic force imaging mode. Hydrogen-induced phase transformations were observed to occur in both steel variants. In austenite, they were manifested by the formation of α ′ martensite magnetic laths. In ferrite, the microstructural changes resulted in a pronounced surface relief being formed.

Finite-element method for the determination of the surface stress field from the real microstructure of anisotropic materials

A method is described for determining the surface stress distribution from the real microstructure of materials. The major advantages of the analyses, including the determination of anisotropic mechanical and crystallographic properties, are identified. The calculation involves the development of image analysis processing and the implementation in the finite-element code Cast3M of a new procedure for meshing the real microstructure and material models capable of describing the complex behaviour of real materials. The method is applied to the case of austenoferritic stainless steels.

Quantification of Damage Progression in a Thermally Aged Duplex Stainless Steel

Ferrite of austeno-ferritic stainless steels maintained for a long time at temperatures in the range of 270 °C to 400 °C is embrittled like the known 475 °C embrittlement of ferritic stainless steels. Deformation and damage micromechanisms of a material must be known in order to apply the local approach to fracture (LAF) methodology. In this work we test a previous model of damage nucleation and evolution, extending its validity to low temperature - long term aging. We have determined cracking damage evolution by taking replicas of planar tensile specimens during uniaxial traction tests. Voronoi (Dirichlet) tessellation quantitative metallography was applied to characterize and quantify non-uniform damaging. Clustering criteria allowed the determination of the size, density and internal damaging rate of damage clusters.

Sensitization Phenomena on Aged SAF 2205 Duplex Stainless Steel and Their Control Using the Electrochemical Potentiokinetic Reactivation Test

Abstract Microstructural changes and resulting properties were studied for SAF 2205 (UNS S31803) austeno-ferritic stainless steel (SS) aged between 700°C and 900°C for up to 2 weeks and then water-...

A thermal outgassing method (t.o.m.) To measure the hydrogen diffusion coefficients in austenitic, austeno-ferritic and ferritic–perlitic steels

A thermal outgassing method (T.O.M.) and a baking method have been applied to measure the hydrogen diffusion coefficient at high temperature (150–600°C) in an austenitic stainless steel (AISI A286), an austeno-ferritic stainless steel (Z2CND2205) and a ferritic–perlitic steel (FM35). The results obtained by the above mentioned methods are in good agreement with those measured by the permeation technique at high temperature. Furthermore, in the austeno-ferritic steel Z2CND2205 the inverse of the total hydrogen diffusion coefficient 1\\ D H( α+γ) is equal to f α \\ D H α + f γ \\ D H γ , where f α and f γ are, respectively, the volume fraction of α and γ phases and D H α and D H γ are the hydrogen diffusion coefficients for each phase.

Novel nanoindentation method for characterising multiphase materials

We present the nano-hardness tester (NHT), a new depth-sensing instrument with a differential capacitive sensor providing nanometer depth resolution and allowing partial elimination of the frame compliance. With micron lateral positioning of the sample and topographical measurement of the residual indent, using scanning force microscopy, it is possible to gain a better understanding of the indentation method and its effects on the material being tested. Furthermore, parallel imaging allows the material response to be directly investigated. Experimental results are presented for a selection of aged austeno-ferritic stainless steels; these confirm the ability of the NHT to become a common test instrument in quality assurance and process development of multiphase materials.

Application of double loop electrochemical potentiodynamic reactivation test to austenitic and duplex stainless steels

Double loop electrochemical potentiodynamic reactivation (DLEPR) tests are performed on an austenitic (317L) and an austeno-ferritic (UNS S31803) stainless steels. In order to optimise results, the standard electrochemical solution has been modified by adding NaCl to the composition and by increasing the concentration of H 2SO 4. Results show a high resistance to intergranular corrosion of the 317L heat treated at 675°C for 30 min to 24 h. On contrast, the duplex stainles steel corrosion resistance decreases with time of sensitisation. Low exposure time (1 h at 675°C) provides low reactivation current density whereas longer exposure time (10 h) shows an increase in the value of this reactivation current, whichis due to the formation of chromium carbides and sigma phase at grain boundaries leading to more chromium depleted areas. Heat treatment at 900°C shows a high reactivation proportional to σ-phase precipitation. The aim of this work is to validate the DLEPR technique for detecting susceptibility to intergranular corrosion of duplex stainless steels. This test is not yet normalised and could be used for on-site measurement.

Hydrogen embrittlement in the duplex stainless steel Z2CND2205 hydrogen-charged at 200°C

Physical and mechanical properties of austeno-ferritic stainless steels depend on the microstructure and phase transformations: many intermetallic phases, carbides and nitrides precipitate at different tempering temperatures. Hydrogen behaviour in steels is affected by the morphology and by the presence of precipitates, both for its diffusional behaviour and for the importance of trapping phenomena. In this paper, the hydrogenation of a 22Cr-5Ni duplex stainless steel has been achieved at 200°C in molten salts bath in potentiostatic conditions, and hydrogen embrittlement has been characterised using low strain rate tensile tests. The influence on hydrogen embrittlement of different intermetallic phases, carbides and nitrides has been considered via tempering heat treatment with tempering temperatures between 200 and 1050°C. The possibility of the recovery of the mechanical properties of charged steel outgassing at room temperature has been considered, investigating also the influence of different intermetallic phases, carbides and nitrides. Moreover, a new thermal technique based on high-temperature outgassing tests has been performed in order to calculate the hydrogen coefficient of diffusion.

ChemInform Abstract: Influence of Nitrogen and Molybdenum on Passive Films Formed on the Austenoferritic Stainless Steel 2205 Studied by AES and XPS.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The influence of nitrogen and molybdenum on passive films formed on the austenoferritic stainless steel 2205 studied by AES and XPS

Passivation of the austeno-ferritic (duplex) stainless steel 2205, and the corresponding austenitic and ferritic single phase alloys, has been studied with Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) with a focus on the lateral and depth distributions of nitrogen and molybdenum in and near the passive layer. The results can be explained by the bipolar model of the passive film; the film is stabilised by the interaction of molybdates and ammonium ions on the surface. The synergism between nitrogen and molybdenum is of particular importance for duplex stainless steels, since they owe part of their corrosion resistance to interaction between the nitrogen-rich austenite and the molybdenum-rich ferrite.

Sulfidation behavior of austeno-ferritic steels

Two austeno-ferritic stainless steels were sulfidized at temperatures of 783, 873, and 963 K under sulfur pressures in the range 4×10−12 to 3×10−5 atm. In all cases a triplex scale developed, consisting of an outermost layer of (Fe, Cr)1−xS, an intermediate layer of FeCr2S4, and an innermost layer of porous (Cr, Fe)1−xS containing particles of Mo2S3. Parabolic kinetics were observed except at the lowest temperature where one of the steels reacted according to irregular kinetics. The Mo2S3 particles in the innermost layer acted as inert markers, imaging the former positions of the steels' ferrite phase in which Mo is enriched. The lamellar microstructure of the steel was thus reproduced in the innermost sulfide layer. The positions of the Mo2S3 particles together with the porosity of the inner layer are taken to imply inward sulfur transport through this layer and outward metal transport through all three layers.

On the behaviour of austeno-ferritic stainless steel strip-claddings with respect to intergranular corrosion

The susceptibility to intergranular corrosion of several austeno-ferritic stainless strip claddings on a pressure vessel steel after stress relief heat treatment has been investigated. With the aid of the normal Strauss test and other test methods it has been shown that even non stabilized layers deposited by the submerged arc welding process can be optimized with respect to their chromium and carbon (and hence ferrite) content such as to ensure sufficient corrosion resistance after annealing at temperatures between 580°C and 620°C. From time-temperature-sensitization (TTS) diagrams it may be concluded that the limiting temperature above which sensitization is no more possible can be essentially lowered by increasing the chromium to carbon ratio without risking sigma phase precipitation. Mechanism leading to the different kinetics of chromium depletion at grain boundaries in wrought austenitic and weld deposited austeno-ferritic stainless steels are discussed.