Austenitic CrNi Research Articles

Numerical modelling of stress‐strain response and deformation‐induced martensite in metastable austenitic stainless steels under monotonic tensile loading

AbstractA constitutive model describing the deformation‐induced martensite transformation in metastable austenitic CrNi steels is presented. In line with the previous work of Stringfellow et al. [1], the material is considered to have a composite response of the underlying austenite and the evolving martensite phases. The stresses and strains in the individual phases are described using viscoplastic models. The effective properties of the material are then computed by a homogenization method. Strain hardening in the individual phases is represented using separate flow rules. Furthermore, based on the plastic strain in the austenitic phase, a transformation kinetics model describes the evolution of the martensite volume, in the two‐phase composite.A numerical implementation of the model is done in the finite element program FEAP [2]. The parameters in model are identified by experimental data using the least‐squares optimization. Finally, some results are presented using an illustrative boundary value problem of a structured surface.

Influence of the grain orientation and δ-ferrite on the cyclic deformation behavior of an austenitic CrNi steel manufactured by wire and arc additive manufacturing

In contrast to most additive manufacturing processes, Wire and Arc Additive Manufacturing enables the production of big metallic components, as it offers sufficient building rates and dimension limits. As the larger dimensions and higher building rates lead to other thermal conditions compared to other, more frequently used additive manufacturing processes, a sound analysis of the resulting microstructure and the corresponding mechanical properties is required. As shown in the presented work for the austenitic stainless steel X1CrNi19-9 (AISI 308L; DIN EN-ISO: 1.4316), the process-induced microstructure features textured austenite grains with intragranular δ-ferrite dendrites, both arranged in dependency with the building direction. Thus, the monotonic and cyclic deformation behavior was analyzed for three different orientations of the loading direction to the building direction, i.e., parallel (vertical), 45° and perpendicular (horizontal). The presented results reveal a strong anisotropy in elastic and plastic deformation behavior. Due to the preferential orientation of the austenite grains, the 45° orientation led to a higher stiffness, a more pronounced plastic deformability and a higher cyclic hardening potential, when compared to the other orientations. Because of this, at lower stress amplitudes the 45° orientation shows a higher fatigue life than the vertical and horizontal orientation. However, besides the texture also the grain elongation and the orientation of the δ-ferrite dendrites towards the loading direction influence the plastic deformation processes. This results in higher monotonic strength for the horizontal orientation in relation to the vertically oriented specimens.

Effect of Cu addition on solidification, precipitation behavior and mechanical properties in austenitic CrNi–N stainless steel

Solidification and segregation behavior of a novel X3CrNiCuN17-6-4 (concentrations in wt.%) cast steel was studied by thermodynamic calculations and corresponding microstructure investigations. In addition, resulting mechanical properties were examined by tensile tests in a temperature range of −60 to 200 °C and compression tests were performed with varying strain rates ranging from 1 s−1 to 10−4 s−1 at room temperature. The investigated steel showed an anomalous temperature dependence of tensile elongation, which is a typical characteristic of high alloy TRIP/TWIP steels. In the as-cast state before heat treatment, a few micrometer-sized copper precipitates were visible, which were exclusively located in the interdendritic regions. A solution annealing was performed at 1050 °C for 30 min to dissolve nitrides and carbides likely to be present in the as-cast microstructure. However, in areas with a high copper content further nanometer-sized copper precipitates were formed after heat treatment. EDS point measurements and EPMA were used to determine the chemical composition of dendrites, interdendritic regions and copper precipitates before and after solution annealing. While the enrichment of chromium and copper in interdendritic regions could be verified by the thermodynamic calculations, the enrichment of nickel in interdendritic regions contradicted the predictions according to Scheil calculation.

Failure analysis of stainless and heat resisting steels, documented on selected examples

Abstract Failures of stainless and heat-resistant austenitic CrNi steels frequently occur in practice due to incorrect material selection, improper processing or unforeseeable factors under operating conditions. Selected examples will be used to illustrate various of these factors influencing the development of failure.

Microstructure and mechanical properties of Nb-alloyed austenitic CrNi steel fabricated by wire-feed electron beam additive manufacturing

We investigated the phase composition, microstructure, mechanical properties, and fracture mechanisms of Nb-alloyed austenitic stainless steel, produced by electron beam additive manufacturing (EBAM), after different regimes of the post-EBAM solid-solution treatments. As-build EBAM-produced steel has a predominantly austenitic structure, contains vermicular δ-ferrite (Vδ = 5–7%) and Nb-enriched precipitates (Laves phase Fe2Nb, FeNb and NbC). Niobium-alloying assists the stabilization of austenitic phase but does not suppress the columnar grain growth during EBAM-fabrication of austenitic stainless steels. Solid-solution treatments provide a change in morphology of δ-ferrite, a decrease in the ferrite volume fraction and the dissolution of intermetallic phases (Fe2Nb, FeNb), which all affect the tensile properties. Post-EBAM treatments contribute a decrease in the yield strength of the steel and improve the elongation-to-failure. All EBAM-produced specimens have good mechanical properties, which are comparable with those for steels produced by conventional methods. Irrespective of treatment regime, EBAM-fabricated specimens exhibit ductile transgranular fracture. For the EBAM-produced Nb-alloyed steel, the phase composition is described from the point of view solidification mode, and the effects of the precipitate hardening and solid-solution strengthening on the mechanical properties and fracture mechanisms were discussed.

Constitutive modelling of the deformation‐induced martensite transformation observed in metastable austenitic CrNi steels

AbstractMetastable austenitic CrNi steels undergo phase transformation when loaded or deformed plastically. In the current work a macroscopic and phenomenological constitutive model is presented to model the strain induced transformation of austenite to martensite. The approach is based on the previous works of Olsen and Cohen [1] & Stringfellow et al. [2]. The kinetics of the phase transformation is modelled based on the assumption that the intersections of the shear bands in the austenitic phase, act as potential martensite nucleation locations. Evolution of the shear band density and their intersections are modelled using the plastic strain in the austenitic phase. The probability of the intersection creating martensite is given by a Gaussian cumulative distribution, which in turn depends on the temperature and stress triaxiality. The resulting stress‐ strain behavior considers the volume fraction, plastic strains and the strain hardening parameters of the individual phases as internal variables. An explicit formulation of the material model is implemented as a user subroutine in a bi‐linear element formulation of FEM. Some of the required material parameters are estimated by fitting experimental stress‐strain and martensite volume evolution curves. For the purpose of illustrating the model's behavior, boundary value problems of components with structured surfaces are presented.

Stacking Fault Energy in Relation to Hydrogen Environment Embrittlement of Metastable Austenitic Stainless CrNi-Steels

Metastable austenitic steels react to plastic deformation with a thermally and/or mechanically induced martensitic phase transformation. The martensitic transformation to α’-martensite can take place directly or indirectly via the intermediate stage of ε-martensite from the single-phase austenite. This effect is influenced by the stacking fault energy (SFE) of austenitic steels. An SFE < 20 mJ/m2 is known to promote indirect conversion, while an SFE > 20 mJ/m2 promotes the direct conversion of austenite into α’-martensite. This relationship has thus far not been considered in relation to the hydrogen environment embrittlement (HEE) of metastable austenitic CrNi steels. To gain new insights into HEE under consideration of the SFE and martensite formation of metastable CrNi steels, tensile tests were carried out in this study at room temperature in an air environment and in a hydrogen gas atmosphere with a pressure of p = 10 MPa. These tests were conducted on a conventionally produced alloy AISI 304L and a laboratory-scale modification of this alloy. In terms of metal physics, the steels under consideration differed in the value of the experimentally determined SFE. The SFE of the AISI 304L was 22.7 ± 0.8 mJ/m2 and the SFE of the 304 mod alloy was 18.7 ± 0.4 mJ/m2. The tensile specimens tested in air revealed a direct γ → α’ conversion for AISI 304L and an indirect γ → ε → α’ conversion for 304 mod. From the results it could be deduced that the indirect phase transformation is responsible for a significant increase in the content of deformation-induced α’-martensite due to a reduction of the SFE value below 20 mJ/m2 in hydrogen gas atmosphere.

The Corrosion Propagation Stage of Stainless Steel Reinforced Concrete: A Review

Stainless steel (SS) reinforcement is increasingly used to control corrosion of reinforced concrete in aggressive marine and deicing salt service. It is well established that the chloride threshold of SS is greater than that of plain steel (PS) rebar, yielding substantially increased duration of the corrosion initiation stage (ti). Much less known, however, is if there is a similar benefit to the duration of the corrosion propagation stage (tP). Thus, credit for increased tP in durability forecasts for SS use tends to be conservatively limited. To reduce that uncertainty, the literature was gleaned for the few instances where SS reinforcement had reached, and preferably completed, the corrosion propagation stage. Particular attention was given to actual structural service experience, outdoor tests, and realistic laboratory conditions. Only a single case of actual service in a structure was found for which tP could be estimated, albeit indirectly. The result suggests a tP of several decades for the case of austenitic Cr-Ni rebar in marine service. Outdoor tests without unnatural acceleration showed a few cases where tP was reached, but only for straight Cr ferritic alloys, which showed some limited improvement over tP for PS. With the additional insight from laboratory tests, it was concluded that SS rebar made with high pitting-resistant grades, and thoroughly descaled, had a positive outlook for propagation stage durations that substantially exceed those of PS rebar. Quantification of that improvement is much in need of further field and laboratory assessment.

The Effect of Hydrogen-Charging on Mechanical Properties of Austenitic CrNi Steel Fabricated by Wire-Feed Electron Beam Additive Manufacturing

A comparative study of the mechanical properties, fracture mechanisms and hydrogen embrittlement peculiarities was carried out using the specimens of austenitic CrNi steel produced by two different methods: wire-feed electron beam additive manufacturing and conventional casting followed by solid-solution treatment. Hydrogen-induced reduction of ductility and the increase in the yield strength are observed in steel specimens produced by both methods. Despite hydrogen embrittlement index is comparable in them, the increase in the yield strength after hydrogen-charging is different: 25 MPa for cast steel and 175 MPa for additively manufactured steel. This difference is associated with the peculiarities of phase composition and phase distribution in steels produced by different methods.

The Influence of Deformation and Short-Term Hightemperature Annealing on the Microstructure and Mechanical Properties of Austenitic Steel 17Cr-14Ni-3Mo (316 Type)

The deformation and thermal action on the microstructure and mechanical properties of a stable austenitic Cr-Ni stainless steel is investigated. It is shown that under deformation conditions packets of microtwins and bands of localized deformation with the inner fragmented structure are formed, where the fragments are of the submicro- and nanocrystaalline scales with low- and high-angle misorientation boundaries. These features of microstructure ensure high yield-point values ≈1100 MPa at a relative elongation of ≈6–7%. Short-term (to 150 s) high-temperature (850°С) annealing of the deformed structure gives rise to a local development of the processes of polygonization and recrystallization in the regions of strain localization bands. As a result of such annealing, packets of microtwins, strain localization bands, polygonized subgrains and recrystallized grains of primarily submicron dimensions are observed in the steel structure. The resulting structural states ensure the yield strength values up to 740 MPa at the relative elongations ≈20–28%. The physical processes taking place in the steel under the experimental deformation and annealing conditions are discussed.

PROSPECTS OF APPLICATION OF TUBES MADE OF DUPLEX STEELS OF A NEW GENERATION IN THE HEAT EXCHANGING EQUIPMENT OF NUCLEAR POWER PLANTS

Prospects of using tubes produced by Centravis Production Ukraine JSC of ferritic-austenitic (duplex) steels of a new generation according to the developed innovative technology in the heat exchanging equipment of nuclear power plants (NPP) were scientifically substantiated based on analysis of scientific and technical literature and the results obtained in comprehensive studies. Comparative corrosion studies of the tubes made of austenitic Cr-Ni and Cr-Ni-Mo steels and 02Cr22Ni5NМo3 (UNS S 31803) duplex steel have shown a substantially improved resistance of the latter to intercrystalline, pit and crevice corrosion, corrosion cracking and erosion. Presence of special low-energy γ-γ, α-α grain boundaries and α-γ interphase boundaries characterized by an increased corrosion resistance and a contribution to the growth of steel durability as a whole was found for the first time in the structure of ferritic-austenitic steels.

Influence of surface morphology on the very high cycle fatigue behavior of metastable and stable austenitic Cr-Ni steels

The present study investigates conventional and cryogenically turned specimens of metastable austenitic steel AISI 347 and stable austenitic steel AISI 904L in the VHCF regime. The cryogenic turning process includes cooling by CO2 snow and generates a surface layer on the specimens of metastable austenitic steel, which is characterized by a phase transformation from paramagnetic fcc - austenite to ferromagnetic bcc - martensite and grain refinement. The stable austenitic steel retains its purely austenitic structure after cryogenic turning, but also shows grain refinement in the surface layer. The specimens with different surface morphology were cyclically loaded at ambient temperature using an ultrasonic fatigue testing system developed and built at the authors’ institute. The testing machine operates at frequencies of approx. 20 kHz to achieve high numbers of load cycles within a reasonable time. To avoid self heating of the specimen, the tests were performed in pulse-pause mode. All specimens were tested with a load ratio of R = -1. During cyclic loading, the metastable austenitic steel partially transformed from paramagnetic fcc - austenite to ferromagnetic bcc - martensite, while no phase transformation could be detected in the stable austenitic steel.

Crack propagation behavior of solution annealed austenitic high interstitial steels

Austenitic stainless steels provide a beneficial combination of chemical and mechanical properties and have been used in a wide field of applications for over 100years. Further improvement of the chemical and mechanical properties was achieved by alloying nitrogen. But the solubility of N within the melt is limited and can be increased in substituting Ni by Mn and melting under increased pressure. In order to avoid melting under pressure and decrease production costs, a part of N can also be substituted by C. This leads to austenitic high interstitial steels (AHIS). Within the solution annealed state strength and ductility of AHIS is comparable or even higher of those of AHNS and can be further improved by cold working. Unfortunately the endurance limit does not follow this trend as it is known from cold-worked austenitic CrNi steels. This is due to the differences of the slip behavior which is governed by the stacking fault energy as well as other near field effects. Construction components operating under cyclic loads over long periods of time cannot be considered being free of voids or even cracks. Thus the crack propagation behavior is of strong interest as well. This contribution presents the tensile, fatigue, crack propagation and fracture toughness properties of AHNS and AHIS in comparison to those of CrNi-steels. The differences are discussed in relation to microstructural characteristic as well as their alterations under cyclic loading.

Impact Strength of Austenitic and Ferritic-Austenitic Cr-Ni Stainless Cast Steel in -40 and +20°C Temperature

Abstract Studies described in this paper relate to common grades of cast corrosion resistant Cr-Ni steel with different matrix. The test materials were subjected to heat treatment, which consisted in the solution annealing at 1060°C followed by cooling in water. The conducted investigations, besides the microstructural characteristics of selected cast steel grades, included the evaluation of hardness, toughness (at a temperature of -40 and +20oC) and type of fracture obtained after breaking the specimens on a Charpy impact testing machine. Based on the results of the measured volume fraction of ferrite, it has been found that the content of this phase in cast austenitic steel is 1.9%, while in the two-phase ferritic-austenitic grades it ranges from 50 to 58%. It has been demonstrated that within the scope of conducted studies, the cast steel of an austenitic structure is characterised by higher impact strength than the two-phase ferritic-austenitic (F-A) grade. The changing appearance of the fractures of the specimens reflected the impact strength values obtained in the tested materials. Fractures of the cast austenitic Cr-Ni steel obtained in these studies were of a ductile character, while fractures of the cast ferritic-austenitic grade were mostly of a mixed character with the predominance of brittle phase and well visible cleavage planes.

Resistance Spot Welding of Low Carbon Steel to Austenitic CrNi Stainless Steel

The contribution deals with resistance spot welding of low carbon steel to austenitic CrNi stainless steel. The thickness of welded dissimilar steels was 2 mm. DeltaSpot welding gun with process tape was utilized for welding of the above-mentioned combination of steels. Resistance spot welds were produced under different welding currents. The welding currents used were 7 kA, 7.5 kA and 8 kA, respectively. Optical microscopy, microhardness measurement across the weld joint and EDX analysis across the weld joint interface were used to evaluate the quality of resistance spot welds of dissimilar steels.

Comparison of the Operating Life of Tank Containers, Tank Vehicles and Rail Tank Cars for the Carriage of Dangerous Goods in Practice, Analysis of Causes of Damage

More than 400 Mt of dangerous materials are transported in Germany every year, of which 150 Mt are by road. Tank containers, tank vehicles and rail tank cars are used for the carriage of dangerous goods in large quantities. Data on the operating life of tanks are only available, in practice, to a minor degree. They are only partly published, mainly after accidents.The BAM-List - Requirements for Tanks for the Carriage of Dangerous Goods, which has compatibility evaluations of metallic and polymeric materials, has been the basis for substance-related prototype approvals for tank containers and portable tanks designed for the carriage of dangerous goods by the BAM since publication of the first edition in 1989. These data are also used for the approval of rail tank cars and road tank cars and are used as a source of knowledge not only in Germany but also worldwide. Tank leakages caused by tanks or rail tank cars being made of tank and sealing materials which are not resistant to the fill goods are avoided by using the material resistance data of the BAM.Corrosion damage is among the main causes of damage. Uniform and non-uniform area corrosion without mechanical stress in aqueous substances is one of the most frequent types of corrosion during the transport of chemicals in tanks. Much damage by pitting corrosion occurs during the transport of substances containing chlorides or substances which separate chloride ions in the presence of moisture. Operational stresses are caused by the effects of both the road and the dangerous goods being transported. Mechanical damage often results from long-term overstressing and occurs after longer operating times.Operational failures cause damage which may appear during the service of tank containers, road tank cars or rail tank cars. The damage results from the inattention of employees when opening and closing the valves. Traffic accidents also cause damage to tanks and frames.Many cases of damage are due to weld area cracks resulting from four basic errors in construction, material, manufacturing and operation. Manufacturing errors can be undetected and the starting point of cracks which only grow under service conditions.Manufacturing errors result from variations in measurement and design, fittings, state of surface (hardness, abrasiveness) and mechanical surface damage (scratches, cracks). Incorrectly welded joints and errors during mechanical deformation are typical mistakes when installing the component parts. Variations in the wall thickness or combinations of different materials lead to manufacturing errors too.A lot of tanks are not used for the transport of dangerous goods before the end of their service life as they do not fulfil the revised technical safety requirements in the Dangerous Goods Regulations.Predictions in the BAM-List based on literature data and corrosion test results are reflected in the service (operating) life. Rail tank cars made of carbon steel, for example, which are mainly used for the transport of petroleum products, can achieve a service life of 40 – 50 y.Rail tank cars produced of austenitic CrNi- or CrNiMo-steel can reach an operating life of at least 30 y, whereby the corrosiveness of the transport substances plays an important role.

Hot-Rolling of Advanced High-Manganese C-Mn-Si-Al Steels

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on a recrystallization behaviour was investigated. Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light microscopy. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25) Mn alloys. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. The obtained microstructure – hot-working relationships can be useful in the determination of powerful parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures.

Resistance Spot Welding of dissimilar Steels

This paper presents an analysis of the properties of resistance spot welds between low carbon steel and austenitic CrNi stainless steel. The thickness of the welded dissimilar materials was 2 mm. A DeltaSpot welding gun with a process tape was used for welding the dissimilar steels. Resistance spot welds were produced with various welding parameters (welding currents ranging from 7 to 8 kA). Light microscopy, microhardness measurements across the welded joints, and EDX analysis were used to evaluate the quality of the resistance spot welds. The results confirm the applicability of DeltaSpot welding for this combination of materials.

Dynamic impact response of high-density square honeycombs made of TRIP steel and TRIP matrix composite material

Two designs of square-celled metallic honeycomb structures fabricated by a modified extrusion technology based on a powder feedstock were investigated. The strength and ductility of these cellular materials are achieved by an austenitic CrNi (AISI 304) steel matrix particle reinforced by an MgO partially-stabilized zirconia building up their cell wall microstructure. Similar to the mechanical behaviour of the bulk materials, the strengthening mechanism and the martensitic phase transformations in the cell walls are affected by the deformation temperature and the nominal strain rate. The microstructure evolution during quasi-static and dynamic impact compression up to high strain rates of 103 1/s influences the buckling and failure behaviour of the honeycomb structures. In contrast to bending-dominated quasi-isotropic networks like open-celled metal foams, axial compressive loading to the honeycomb’s channels causes membrane stretching as well as crushing of the vertical cell node elements and cell walls. The presented honeycomb materials differ geometrically in their cell wall thickness-to-cell size-ratio. Therefore, the failure behaviour is predominantly controlled by global buckling and torsional-flexural buckling, respectively, accompanied by plastic matrix flow and strengthening of the cell wall microstructure.

Comparison of Austenitic High-Mn-Steels with Different Mn- and C-Contents Regarding their Processing Properties

This paper evaluates the difference between three high-manganese austenitic steels (X120Mn12, X80Mn16 and X60Mn22) and a standard Cr-Ni-austenitic steel X5CrNi18-10 regarding some selected aspects of their processing behavior. The hot deformation and dynamic recrystallization behavior between deformation steps as well as the static recrystallization from a cold rolled state were investigated. To evaluate mechanical properties and cold forming behavior, characteristic values were determined by means of Erichsen testing. In addition, structural investigations were conducted by XRD and optical microscopy as well as scanning electron microscopy. Austenitic high-manganese steels require lower deformation forces than austenitic Cr-Ni steels and exhibit a significantly more rapid dynamic recrystallization. Thereby the X60Mn22 steel grade showed significantly higher yield strength than the other high-manganese steels. Static recrystallization takes place after short holding durations at temperatures lower than required for solution treatment. All austenitic high-Mn steels showed a better formability but higher stretch-forming forces during cold forming compared to the austenitic Cr-Ni-steel.