Duplex Stainless Steel Research Articles

Microstructural evolution and coarsening behavior of the precipitates in 2205 duplex stainless steel aged at 850 °C

The formation of the secondary phases in the 2205 duplex stainless steel (DSS) influences greatly its mechanical properties. Studies on the microstructural evolution and coarsening behavior of the precipitates in the 2205 DSS have both scientific and technological significance. In the present work, the evolution of the composition and morphology of the precipitates in the 2205 DSS aged at 850 °C for periods up to 200 h are systematically investigated by SEM/EDS (Scanning Electron Microscope/Energy Dispersive Spectrometer) and TEM (Transmission Electron Microscope). In addition, the size statistics for intermetallic precipitates, especially for the σ phase, are carried out based on the SEM images by using deep learning and digital image processing technique. Based on the above, the average interphase energy between the σ and γ phases is reasonably estimated according to the Ostwald coarsening mechanism. This work provides a comprehensive understanding of the microstructural evolution and coarsening behavior of the precipitates in 2205 DSS.

Study on Microstructure and Mechanical Properties of a New Type of Low Ni Duplex Stainless Steel Resistance Spot Welding

Study on Microstructure and Mechanical Properties of a New Type of Low Ni Duplex Stainless Steel Resistance Spot Welding

Electrochemical Determination of Ascorbic Acid by Mechanically Alloyed Super Duplex Stainless Steel Powders

SAF-2507 super duplex stainless steel powders (SDSS) were prepared using a high-energy planetary ball milling process. The X-ray diffraction (XRD) shows peak broadening after 20 h of ball milling and revealed a phase transformation resulting in a two-phase alloy mixture containing nearly equal amounts of ferrite (α) and austenite (γ). After 20 h of ball milling the particle size was reduced to ~201 nm. Scanning electron microscope (SEM) micrographs showed small-size irregular grains with an average particle size ranging from 5–7 µm. The high-resolution transmission microscope (HRTEM) analysis confirmed the presence of nanocrystalline particles with sizes ranging from 10 to 50 nm. The presence of ferrite phase is visible in the corresponding diffraction pattern as well. In this paper, we have discussed the electrochemical sensor application of mechanically alloyed nano-structured duplex stainless steel powders. The fabricated 4 mg duplex stainless steel modified carbon paste electrode (SDSS-MCPE) has shown excellent current sensitivity in comparison with 2, 6, 8, and 10 mg SDSS-MCPEs during the detection of ascorbic acid (AA) in a phosphate buffer solution with a pH of 6.8. The calculated electrode active surface area of SDSS-MCPE was found to be almost two times larger than the surface area of the bare carbon paste electrode (BCPE). The limit of detection (LD) and limit of quantification (LQ) were found to be 0.206 × 10−8 M and 0.688 × 10−8 M, respectively, for the fabricated 4 mg SDSS-MCPE.

Study on the effect of solution treatment on mechanical and corrosion properties of SAF 2507DSS produced by LPBF

In recent years, the application of Laser powder bed fusion (LPBF) technology in the manufacturing industry has become increasingly widespread. Compared to traditional casting processes, LPBF technology allows for the production of complex-shaped and structurally unique components made of 2507 duplex stainless steel (2507DSS), thus improving production efficiency and product quality. This article focuses on determining the LPBF parameters required for printing 2507DSS, successfully fabricating samples with a density of 98.71% and subjecting them to solution treatment followed by tensile testing. The results show that the LPBF-prepared samples exhibit a strength of 1173 MPa and an elongation of 18%. The presence of island-like structures enables excellent mechanical performance in the horizontal direction. The samples subjected to solution treatment at 1100 °C exhibit a strength of 860 MPa and an elongation of 45%. It is observed that when 2507DSS undergoes solution treatment from a single-phase state, the formation of large-angle grain boundaries and residual stresses within the austenite phase occurs, and a reasonable explanation is provided. Furthermore, the article conducts electrochemical tests on the samples. The results indicate that the LPBF-prepared samples possess the same electrochemical properties as the samples treated at 1100 °C, and both outperform the cast samples. Thus, this study reveals that the proportion of phases is not the sole determining factor for the corrosion resistance of DSS, providing valuable insights for the future development of more corrosion-resistant 2507DSS materials.

Production of Nb-doped super duplex stainless steel based on recycled material: A study of the microstructural characterization, corrosion, and mechanical behavior

The corrosion behavior of a new Nb-doped stainless steel (SDSS-Nb) designed by the CALPHAD method, produced using an open atmosphere process based on recycled materials, was investigated to improve the circular economy. Three heat-treatment conditions were evaluated to assess the sensitization effects of the precipitates and inclusions. XRD and SEM-EDS were used for phase identification, and sensitization was analyzed by cyclic polarization and Scanning Kelvin Probe Force Microscopy (SKPFM). The thermodynamic stability predicted by Thermo-Calc agrees with that observed by SEM-EDS. It was observed by cyclic polarization that the corrosion sensitization was mainly provided by the σ phase, which was deduced from the results obtained by SEM-EDS, XRD, and Thermo-Calc simulations. Furthermore, it was obtained that the sensitization due to Cr2N precipitates and nonmetallic inclusions was low, and the mechanical response is comparable to commercial UNS32750 super duplex stainless steel, which allows a good performance in severe environments and an efficient industrial application. Additionally, it has been obtained by SKPFM that the shear potential between the σ phase and the austenite is between 210 mV and 241 mV and that its value depends on the stability and equilibrium reached by the σ phase during thermal cycling.

Realizing strength–ductility synergy in a lean duplex stainless steel through enhanced TRIP effect via pulsed electric current treatment

Optimization of the strength–ductility compromise in lean duplex stainless steels (LDSSs) often involves controlling the stability of austenite by chemically adjusting it to be metastable and enabling phase transformation through a transformation-induced plasticity (TRIP) effect during plastic deformation. This study investigated the effects of pulsed electric current (PEC) treatment on the spatial distribution of alloying elements within the microstructure and enhancement of the mechanical properties of a new type of LDSS material, NSSC2120. The results showed that PEC treatment had a significant positive impact on the mechanical properties of NSSC2120. Specifically, after applying 20 pulses of PEC treatment at a current density of 700 A/mm2 for 8 ms, the material exhibited a 27% increase in ductility and 8% increase in strength. The microstructural evolution induced by the PEC treatments was investigated using quasi-in-situ electron backscatter diffraction and energy-dispersive X-ray spectroscopy. The PEC treatment facilitated the diffusion of alloying elements, which played a crucial role in triggering the TRIP effect during plastic deformation owing to the decreased stability of austenite with a lower content of austenite-stabilizing elements, thereby resulting in simultaneous improvement in both strength and ductility. Significantly, the rapid thermal process employed in the PEC treatment did not give rise to detrimental precipitates within the microstructure of the material, rendering PEC treatment a promising alternative to conventional heat treatment. The simplicity, speed, and low energy consumption associated with the PEC treatment make it an appealing choice for processing similar materials and achieving both microstructural modification and optimization of mechanical properties.

Effect of solubilization heat treatment on microstructure and corrosion resistance of joints welded with the autogenous TIG process duplex stainless steel

Duplex stainless steels (DSSs), are used commonly under severe working conditions, subject to high stresses associated with corrosion. A typical DSS is composed of the combination involving two types of microstructures, ferritic and austenitic, which characterizes it as a material with corrosion resistance in aggressive media, due to its ability to passivate. The use of DSS has been increasingly exploited in the oil industry, and its use has occurred in numerous areas, standing out in the production of flexible pipelines, which in most applications, is associated with a welding process. They require special care when welded by the tungsten inert gas (TIG) welding process. Temperature variations in this process, along with heat treatment, promote the formation of precipitates in the matrix and change the phase balance. This significantly influences the mechanical and corrosion properties of DSSs. The objective of this work is to study the influence of two different parameters of solubilization heat treatment after welding on the microstructure and corrosion resistance of a DSSs welded joints. After the autogenous TIG welding process, the welds were subjected to thermal solubilization treatments for 10 and 25 s at 1050 °C. The conditions subjected to the solubilization heat treatment were compared with the as TIG welded control condition. The microstructure was analyzed by confocal microscopy and the corrosion resistance was evaluated by the "A" method of the ASTM G48 standard.

Sigma Phase Effect on Pitting Corrosion of Super Duplex Stainless Steel UNS S32750

Sigma Phase Effect on Pitting Corrosion of Super Duplex Stainless Steel UNS S32750

Structural property of stainless steel minor-axis joints of interior columns with box panel

With the component method, a design approach was derived for predicting the structural properties of stainless steel end-plate joints with box panel, where box panel was formed using the skin-plates welded between the column flanges along the minor-axis. Four end-plate minor-axis joints with box panel were tested under static and cyclic loading, involving beam-to-interior column joints made of austenitic and duplex stainless steels. The monotonic and hysteretic properties of the experimental joints were discussed, including the failure characteristics, experimental curves (F-Δ), and degradation and energy dissipation behaviour. Meanwhile, elaborate finite element (FE) models were developed for further analysis of the structural behaviour of the tested joints, and a parametric study was conducted afterwards. The proposed design approach provided reasonable estimations for the experimental and FE results of stainless steel minor-axis joints with box panel, demonstrating the applicability of this proposal.

Evolution of the sigma phase and its effect on the corrosion resistance of ASTM A890 3A duplex stainless steel

The sigma (σ) phase is a detrimental precipitation that affects the performance and application of duplex stainless steel (DSS). In this study, the evolution of the σ phase and its influence on the corrosion resistance of the ASTM A890 3 A DSS were analyzed quantitatively by aging at 750 °C for 3–96 h for the first time. With increasing aging time, the amount and rate of precipitation of the σ phase significantly changed. The fraction of the σ phase increased from 0.65% to 23.51%, accompanied by a decrease in ferrite content and an increase in austenite content. During the evolution of the σ phase, a combined process of nucleation and growth controlled by the eutectoid reactions in the first 3–24 h and a growth process controlled by diffusion in the subsequent 24–96 h were observed. The deteriorated corrosion resistance after aging was closely related to the fraction and distribution morphology of the σ phase. Under a precipitation fraction of the σ phase between 0% and 0.65%, the passive current density of the DSS increased, the pitting potential decreased. The width of the passivation region between 0.65% and 19.07% of the σ phase narrowed, and instability formed. The critical aging time for the transformation from passive to active DSS was between 48 h, corresponding to a content of the σ phase of 19.07%. The passivation region almost completely disappeared. In addition, the uniformity in distribution of the σ phase also influenced the corrosion resistance.

Friction welding process with SFPC consumable pin applied on stainless steel duplex SAF 2205

The objective of this work is to investigate the use of Friction Hydro Pillar Processing (FHPP), in the Duplex Stainless Steel- Sandvik Austenite Ferrite 2205 (AID SAF 2205), with low Axial Force of application and to qualify it in a similar way to repairs by conventional welding processes, through standard tests foreseen in norms. In addition, it is intended to go beyond conventional tests and bring new information about precipitated phases during processing, their consequences on microstructural evaluation and corrosion performance. Through the FHPP there are the advantages of: being able to repair the structure with the plant and/or the equipment in operation, as it does not involve melting the material, thus allowing a low structural distortion in addition to being a typically surgical type of repair where little material needs to be removed to perform the repair.

Effect of variable polarity during cold metal transfer on microstructure and mechanical properties of directed energy deposition-arc built 2209 duplex stainless steel

Effect of variable polarity during cold metal transfer on microstructure and mechanical properties of directed energy deposition-arc built 2209 duplex stainless steel

Exploring the Impact of Cooling Rate on Microstructural Features, Mechanical Properties, and Corrosion Resistance of a Novel Nb-Stabilized Super Duplex Stainless Steel in Shielded Metal Arc Welding

The corrosion and mechanical response produced by quenching in the welded joint of a new Nb-doped stainless steel designed by the CALPHAD method and produced by open-atmosphere casting with recycled materials were investigated to contribute to the circular economy and to establish disruptive manufacturing criteria based on metallurgical principles. The steel was initially subjected to solubilization heat treatment and partial solubilization treatment at 1090 °C to obtain an appropriate α/γ balance and carbide solubilization. It was then welded by the SMAW process, quenched, and tempered at three different cooling rates. As a result, a good fit between the phases predicted by the CALPHAD method and those observed by X-ray diffraction and scanning electron microscopy were obtained, with minor differences attributable to the precipitation and diffusion kinetics required for dissolution or nucleation and growth of the phases in the system. The forced air quenching mechanism was identified as providing an α/γ phase equilibrium equivalent to 62/38 as the most effective quenching method for achieving the optimum mechanical and corrosion response, even with the post-weld σ phase and showing superior results to those of the base metal. The outstanding mechanical and corrosion responses resulted from a proper balance of the primary phases in the duplex steel with a precipitation-strengthening mechanism. The damage tolerance obtained by forced air quenching was superior to that obtained by water and air quenching, with a PSE of 24.71 GPa% post-welding.

Influence of edge-deposited layers on mechanical and corrosion properties of laser beam welds of 15 mm thick AISI 2205 duplex stainless steel

AISI 2205 duplex stainless steel is used in a variety of industries, including the chemical and petrochemical industries. This is due to its high tensile strength combined with good ductility and corrosion resistance. However, in laser beam welding, these properties are negatively affected by the high cooling rates typical of the welding process. The resulting higher ferrite content in the weld metal than in the base material leads to a reduction in the ductility and corrosion resistance of the welded joint. To overcome this problem, in this study, thick plates were coated by direct energy deposition (DED) prior to laser beam welding, whereas a duplex powder mixture containing a higher nickel concentration was used as a coating material. To improve the weld quality for the proposed two-step process, a method of additional material deposition instead of conventional tack weld was investigated. The resulting welded joints showed a well-balanced austenite to ferrite ratio and their properties and microstructure were verified by metallographic analysis, electron backscatter diffraction and Charpy impact testing. Using the standard ASTM G48 test method, it was found that the corrosion resistance of the welds was improved by a factor of four in average compared to the conventionally welded joints. The resulting properties, such as good ductility and corrosion resistance, of the welds with pre-coated edges showed good agreement with those of the base metal and confirmed the proposed two-step process as a promising alternative to the conventional approaches for welding thick duplex stainless steel plates.

Time-Lapse Observation of Crevice Corrosion in Grade 2205 Duplex Stainless Steel.

The objective of this study was to investigate and visualize the initiation and propagation of crevice corrosion in grade 2205 duplex stainless steel by means of time-lapse imaging. Transparent Poly-Methyl-Meth-Acrylate washer and disk were coupled with duplex stainless steel to create an artificial crevice, with electrochemical monitoring applied to obtain information about the nucleation and propagation characteristics. All nucleation sites and corroding areas inside crevices were recorded in situ using a digital microscope set-up. Localized corrosion initiated close to the edge of the washer, where the crevice gap was very tight, with active corrosion sites then propagating underneath the disk into areas with wider gaps, towards the crevice mouth. The growth was associated with a rise in anodic current interlaced with sudden current drops, with parallel hydrogen gas evolution also observed within the crevice. The current drops were associated with a sudden change in growth direction, and once corrosion reached the crevice mouth, the propagation continued circumferentially and in depth. This allowed different corrosion regions to develop, showing selective dissolution of austenite, a region with dissolution of both phases, followed by a region where only ferrite dissolved. The effect of applied electrochemical potential, combined with time-lapse imaging, provides a powerful tool for in situ corrosion studies.

Additive manufacturing multi-material components of SAF 2507 duplex steel and 15-5 PH martensitic stainless steel

The study reports the successful deposition of 15–5 precipitation-hardened (PH) martensitic steel on SAF 2507 duplex stainless steel (and vice versa) to form functionally graded materials (FGMs) using powder-based, directed-energy-deposition (DED) technology. The evolution of the microstructure and the mechanical properties of the 15–5 PH/SAF 2507 functionally graded material in the as-built state were investigated systematically. The results proved that the microstructural transition zone (MTZ) is formed above the fusion line due to the dilution effect of both steels. The phase composition of the MTZ consists of ferrite and, in comparison to the base materials, an increased amount of austenite. The interface has the lowest hardness owing to the formation of a significant proportion of the austenitic phase. However, the tensile mechanical properties were not affected by the interface as failure occurred in both SAF 2507 and at the interface regions. The research presents a promising application of FGMs in a horizontal configuration to form a high - quality, metallurgical joint between heterogeneous materials. This study introduces a novel approach by additive manufacturing (AM) of heterogeneous multi-materials, merging the favorable properties of duplex SAF 2507 and martensitic 15-5 PH stainless steels through superior metallurgical bonding. The combination of SAF 2507 and 15-5 PH in a functionally graded material has not been previously explored, as the existing studies deal with the duplex stainless steels SAF 2507 and either martensitic stainless-steel 15-5 PH deposited separately by laser powder-bed-fusion (L-PBF) or DED methods. This research pioneers the investigation of these materials in tandem, paving the way for the development of novel FGMs with optimized properties for specific applications. The resulting FGM exhibits exceptional corrosion resistance and high strength, making it highly versatile for applications in diverse industries such as offshore oil and gas production, aerospace, aviation, and chemical processing equipment.

Deciphering the effect of sea water on corrosion behavior of epoxy/graphite-flakes composite coated LDX 2101 stainless steel

In the present investigation, the electrochemical behaviour of lean duplex stainless steel (LDX 2101 SS) and epoxy (EP) coated with and without containing graphite flakes in a marine or seawater environment (3.5 wt% NaCl) was studied. The epoxy/graphite flakes (EGF) coating was prepared using a spin coater, and then the morphological and physical properties of the coating were analysed by scanning electron microscopy (SEM) and water contact angle (WCA). The performance of EP and EGF composites as protective coatings against corrosion of LDX 2101 was analysed using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The final result shows that EGF composite has excellent corrosion protective ability compared to bare and epoxy-coated steel.

Experimental investigation to optimize machining parameters for super duplex stainless steel in spark EDM using die-sinking and MQL system

The current experimental work discusses the electric discharge machining (EDM) of super duplex stainless steel with conventional die-sinking and minimum quantity lubrication (MQL) approach. The dielectric used in the conventional die-sinking EDM process is kerosene, and sunflower oil with air mixture is used in MQL-type machining conditions. The solid-type alpha brass electrodes of diameter 10 mm were used for machining. The most prominent input variables like pulse time (OFF and ON) and discharge current (varied at 3 levels) were selected, and Taguchi's L9 orthogonal array design was selected for experimentation. The responses like electrode wear rate (EWR), hole circularity (HC), material removal rate (MRR), and surface roughness (SR) were analyzed. The output results obtained from the experimental trials were analyzed using S/N ratios and analysis of variance (ANOVA). The most contributing input parameter affecting the responses like SR, HC, MRR, and EWR outputs were analyzed. The results showed that the optimum machining conditions were obtained at L3 and L9 levels for conventional die-sinking and MQL-type machining conditions, respectively. Out of three varying input parameters, pulse time (OFF) is the most contributing factor in deciding various responses. The same has been confirmed by ANOVA analysis. The theoretical prediction of experimental data was made using the Fuzzy logic approach, and the best optimal level is found out individually for all four responses.

Axial compression behaviour of circular and square UHPC-filled stainless steel tube columns

To improve the durability and mechanical behaviour, an ultra-high performance concrete (UHPC)-filled duplex stainless steel tube stub column is proposed in this study. To explore the axial compression performance of UHPC-filled duplex stainless steel tube (UFSST) stub columns, a total of 12 UFSST, 6 confined concrete-filled duplex stainless steel tube (CFSST) and 6 hollow duplex stainless steel tube (HSST) stub columns were subjected to axial compression. The experimental parameters included the section type, the tube thickness and the core concrete strength. For the UFSST stub columns, when the constraining index (ξ) exceeds 0.82 and 1.52 for circular and square stub columns, respectively, the failure mode changes from shear-type failure to waist-drum failure. The concrete contribution rate (CCR) is positively correlated with the concrete strength and negatively correlated with the tube thickness, whereas the variation law for the strength index (SI) is the opposite. The circular UFSST stub columns have a higher CCR than the square columns. A test database, including UFSST and UHPC-filled steel tube (UFST) stub columns, was established to evaluate the applicability of existing design codes. Furthermore, a modified ultimate load prediction formula with higher accuracy is proposed and verified.

Designing gradient nanograined dual-phase structure in duplex stainless steel for superior strength-ductility synergy

Similar to other metallic materials, duplex stainless steel dramatically loses its advantage of high ductility as they are strengthened. Here, we produce a gradient nanograined dual-phase structure in the 2101 duplex stainless steel, thus facilitating a superior strength-ductility synergy: a yield strength of 1009.5 MPa being two times higher than that of the as-received sample, a total elongation of 23.4% and a uniform elongation of 5.9%. This novel structure is produced through a processing route of ultrasonic severe surface rolling and annealing, which realizes a superposition of gradient nanostructure and lamellar dual-phase structure with austenite and ferrite. During the tension deformation of gradient nanograined dual-phase structured duplex stainless steel, a significant accumulation of geometrically necessary dislocations occurs. These dislocations are formed to accommodate the deformation incompatibility caused by the layer-by-layer difference in strength and hardness of individual phase domains, as well as the inherent difference in properties between the austenite and ferrite domains. This results in a stronger hetero-deformation induced strengthening and hardening significantly contributing to superior mechanical properties. Our study provides a new avenue to develop advanced steels with high strength and ductility.