Duplex Stainless Steel Research Articles

Influence of Ni and PWHT on microstructure evolution and mechanical properties of GTA-welded duplex stainless steel and super duplex stainless steel joints: A comparative investigation

In the present investigation, a new endeavor has been made to analyze the impact of the secondary process (post-weld heat treatment (PWHT)) as well as Ni addition on the microstructure and mechanical characteristics of welds. Welds between similar joining materials from duplex stainless steels (DSS), as well as super DSS (SDSS), were made by the gas tungsten arc welding (GTAW) process. For DSS, after PWHT with the addition of Ni element, the amount of austenite rises and becomes more uniformly distributed with curved boundaries, while for SDSS, the microstructure mainly consists of intergranular and Widmanstätten austenite. According to the comparison of phase percentages obtained based on both ASTM E1245 and ASTM E562, a good balance between ferrite and austenite phases of joint materials was achieved. X-ray diffraction analyses on both joints revealed that phases are mainly ferrite and austenite with different lattice parameters without evidence of unwanted intermetallic phases. The addition of Ni increases the hardness values of DSS weld metal compared to the base metal due to an increased amount of Widmanstätten austenite and ferrite in the microstructure. The lowest ductility was obtained for the SDSS weld sample with the addition of Ni and PWHT because of the existence of secondary austenite in the weld metal and the Widmanstätten morphology of the austenite. The effect of PWHT on the development of microstructure and mechanical behaviors was more than the effect of nickel powder on weld samples.

Study of Electroless Nickel Plating on Super Duplex Stainless Steel for Lithium-Ion Battery Cases: Electrochemical Behaviour and Effects of Plating Time

With increasing demand for Li-ion batteries, studies are focusing on enhancing battery performance and safety. However, studies on battery cases remain scarce. Herein, we propose the use of super duplex stainless steel SAF2507, which is a two-phase (austenite + ferrite) steel, for battery casings. Unlike conventional AISI304, SAF2507 maintains its corrosion resistance and strength at high temperatures and precipitates a secondary phase at approximately 975 °C. However, the effects of Ni plating on this secondary phase are not well documented. Therefore, the electroless Ni plating of SAF2507 after secondary-phase precipitation was studied. Briefly, heat treatment at 1000 °C was used to induce precipitation, and the electroless Ni plating behaviour over varying plating periods was analysed using open-circuit potential, potentiodynamic polarisation, and electrochemical impedance spectroscopy measurements. The plating state and corrosion behaviour were examined using scanning electron microscopy. Heat-treated SAF2507 steel with a secondary phase exhibited excellent electroless Ni plating behaviour, which enhances the safety and durability of Li-ion batteries. Furthermore, uniform plating and electrochemical behaviour were achieved after 180 s, suggesting that SAF2507 is superior to AISI304. These findings contribute to the development of safer and more efficient batteries and address the growing demand for Li-ion battery case materials.

Mechanical, metallurgical and corrosion analysis of forced cooling assisted laser bending of duplex-2205

Laser bending with a small bend angle poses challenges in manufacturing applications. Despite various proposed strategies, the achievable bend angle typically ranges from 0.1 to 2° per scan. This study focuses on enhancing the bend angle of duplex stainless steel by introducing forced cooling (FC). The application of FC aims to reduce the number of scans required, thereby saving time, cost, energy, and controlling material degradation due to excessive heating. The investigation compares the effects of process parameters under natural cooling (NC) and FC conditions. Metallurgical, mechanical, and corrosion properties of the bent specimens are investigated. Results indicate a significant enhancement in the bend angle with FC, showing an improvement of up to 1592 % for specific parameters and sheet geometries. For a 2 mm sheet thickness, the maximum bend angles achieved in five scans are 14.02° and 22.34° under NC and FC conditions, respectively. The influence of process parameters on the bend angle is significantly affected by FC compared to the NC condition. Additionally, the trend of bend angle increment in each scan is almost opposite in FC condition compared to NC. FC results in increased hardness and tensile strength of the bent specimens at the expense of ductility. Microstructural analysis reveals phase rearrangement and grain refinement. Corrosion resistance remains mostly unaffected, although a slight reduction in pitting potential is observed under FC, which is compensated by the formation of a passivation layer. Interestingly, the bend angle achieved in one scan was up to three times higher than the published literature, which could be beneficial for various industries.

Additive friction stir deposition of super duplex stainless steel: Microstructure and mechanical properties

Additive Friction Stir Deposition (AFSD) is an emerging solid-state metal additive manufacturing (AM) process that offers several key benefits, including high deposition rates and wrought-equivalent mechanical properties even in the as-deposited condition. The work presented is the first study to report on the development of microstructure and mechanical properties of AFSD-processed duplex stainless steel (DSS2507). The banded microstructure of the starting material was remarkably affected by AFSD processing; the austenite grains exhibited a refined and equiaxed morphology, while the ferrite grains appeared slightly larger and elongated. Microstructural observations revealed that the potential mechanism of microstructure evolution in austenite was discontinuous dynamic recrystallization (DDRX), while in ferrite, it was continuous dynamic recrystallization (CDRX). The occurrence of multiple thermal cycles during the AFSD process resulted in σ phase precipitation, which in turn led to considerable variation in mechanical properties with respect to the build direction. The top region of the as-built part with an insignificant σ phase fraction showed improved tensile strength and ductility combination compared to the as-received DSS2507 as well as other AM-processed DSS2507 alloys.

Flexural Strength of RHS Perforated Lean Duplex Stainless Steel Beam at Temperature 24-900ºC

The investigation of stainless steel structures at elevated temperatures is still limited, especially to those focused on the behaviour of perforated beams. Therefore, a numerical study was conducted to investigate the behaviour and strength of cold-formed lean duplex stainless steel (CFLDSS) beams having a single web perforation that failed due to pure bending at temperatures between 24-900oC. In total, 200 square and rectangular hollow sections (RHSs), which had various cross-section sizes, hole diameters, and temperature simulations, were involved in the parametric study. The numerical study was based on the ABAQUS simulation results of the 200 specimens. The numerical model was developed based on the validated existing studies. Numerical evaluations show that the existing codified strength predictions are conservative, but it has inconsistent safety. Hence, this study suggests modifications to the existing strength prediction, which is more conservative and reliable.

Study on corrosion law of coiled tubing in high CO2 gas field

The corrosion rates of CT80 grade coiled tubing steel (CT80) and 2205 duplex stainless steel (2205DSS), as well as other attachments, were determined using the weight loss method in a high-temperature and high-pressure autoclave to simulate operational corrosion conditions. The rates were compared with the predicted rates based on BP (backpropagation) neural network optimized by genetic algorithm, in response to the severe corrosion issue encountered during coiled tubing operations in high CO2 gas fields in Sichuan, China. In the gas phase, the results revealed that with the increase of the temperature and the CO2 partial pressure in the high CO2 gas field, the corrosion rate of CT80 gradually increases, while the rate of 2205DSS remains relatively stable and lower than that of CT80. In the liquid phase, the corrosion rate of CT80 exhibits an initial increase followed by a subsequent decrease with increasing temperature and partial pressure, while the 2205DSS corrosion rate remains slightly low and fairly stable. Furthermore, the XRD results of the corrosion product in the liquid phase revealed a higher corrosion rate on CT80 due to its loose and porous film structure, with FeCO3 and Fe2O3 being the main corrosion products. In contrast, 2205DSS exhibited only slight corrosion, characterized by a denser corrosion product film primarily composed of Cr2O3. The average error between the measured values and model calculated values is ∼15%, indicating that 2205DSS exhibited better wear resistance, making it suitable for application in the high CO2 gas field.

Role of Cl/F Ions Concentration, pH and Temperature on Pitting Corrosion Behavior of 2507 Duplex Stainless Steel

In the chemical industry, 2507 duplex stainless steel is usually used for the pipes of absorption tower. The residual F− and Cl− in the tail gas could aggregate in the clean solution of ammonium hydroxide, which causes the risk of pitting corrosion. In this study, the role of Cl/F ions concentration, pH and temperature on the pitting corrosion of 2507 stainless steel is investigated by exposing to actual industrial ammonia desulfurization solution. The experimental results reveal that the 2507 stainless steel exposed the serious corrosion solution appears pitting corrosion problem. However, the 2507 stainless steel is free of pitting corrosion at the tested condition of pH value 3.59/5, temperature 80 °C/100 °C, and ion concentration 46375 ppm/80000 ppm. This study provides a fundamental data for the safety service of stainless steel used in chemical industry.

An Investigation into the Correlation Between Fracture Toughness and Charpy Impact Test for Duplex Stainless Steels

An Investigation into the Correlation Between Fracture Toughness and Charpy Impact Test for Duplex Stainless Steels

Effect of Solution Annealing Time on the Microstructure and Mechanical Properties of Selective-Laser-Melted 2205 Duplex Stainless Steel

The 2205 duplex stainless steel (DSS) produced by selective laser melting (SLM) exhibits high strength (1078.8 MPa) but poor plasticity (15.2%) owing to the high cooling rate during SLM, which inhibits the formation of austenite and creates a nearly entirely ferritic microstructure. The dual-phase nature can be restored through solution annealing, which enables well-matched strength and plasticity, but which has not been extensively studied. We investigate the effects of 5 min, 30 min, and 120 min of solution annealing at 1000 °C on the dual-phase ratio, grain size, texture strength, inclusions, grain boundary characteristics, and mechanical properties of SLM-manufactured 2205 DSS. After 30 min of solution annealing, the elongation increased to 32.2% owing to the restoration of the dual-phase structure, the reduction in dislocation density, the weakening of texture, and the decrease in grain size. Increasing solution annealing time also corresponded to a decrease in the ultimate tensile strength (from 831.7 to 787.5 MPa) and yield strength (from 610.3 to 507.8 MPa) due to grain coarsening and the gradual transformation of ferrite to austenite. Furthermore, the mechanism of the transformation from ferrite to austenite was proposed, and it was observed that the transformation of MnSiO3 to MnCrO4 provided nucleation sites for austenite.

Effects of Passivation with Cu and W on the Corrosion Properties of Super Duplex Stainless Steel PRE 42

Carbon steel is subjected to several pretreatments to enable its use in highly corrosive environments, such as marine structures. However, its surface treatment is problematic owing to various processes, and these problems can be solved by replacing it with super duplex stainless steel (SDSS), which exhibits remarkable strength and corrosion resistance owing to its austenite and ferrite phases. EN 1.4410 and EN 1.4501 are the most extensively used SDSS grades in marine structures, as they exhibit exceptional strength and corrosion resistance in seawater. This study subjected EN 1.4410 and EN 1.4501 samples to specific heat treatment after casting and observed their structural alterations through field emission scanning electron microscopy. Their passivation states, with or without the Cu and W layers, were determined by examining their corrosion properties through open-circuit potential measurements, electrostatic polarisation tests, electrochemical impedance spectroscopy (EIS), and critical pitting temperature (CPT) analysis. The inclusion of Cu significantly improved the uniform corrosion resistance within the passivation layers, whereas the addition of W enhanced the pitting resistance (Epit, CPT). Additionally, the EIS analysis confirmed a double-layer structure in the passivation layer of EN 1.4501. Moreover, Cu did not act as a strengthening element of the passivation layer, whereas W significantly reinforced it.

Optimizing the Phase Distribution in Arc-Based Direct Energy Deposition of Duplex Stainless Steel

Optimizing the Phase Distribution in Arc-Based Direct Energy Deposition of Duplex Stainless Steel

Investigating the Impact of Physical Vapour Deposition (PVD)-Coated Cutting Tools on Stress Corrosion Cracking Susceptibility in Turning Super Duplex Stainless Steel

This work aimed to ascertain the corresponding influences of several PVD-coated cutting tools on the susceptibility of the machined surface of super duplex stainless steel (SDSS) to stress corrosion cracking. Coatings comprised of AlCrN, AlCrN/TiSiN, and AlTiN were applied to cemented carbide cutting tools using the PVD method; these were then used to turn the outer surface of the SDSS tube section. During the cutting process, the material presents the following combination of features: (i) a tendency for strain hardening, reflected in microstructural modifications and residual stresses of the machined surface and (ii) high temperatures in the cutting region, reducing the tool life. The goal of this work was to evaluate the surface integrity (work hardening and corrosion behaviour) of the SDSS obtained after the machining process (finish turning) with cemented carbide tools coated with three different PVD coatings.

Evaluation of hydrogen interactions with defects induced by deformation in the duplex stainless steel microstructure: An internal friction study

Despite their good corrosion resistance and optimal mechanical properties, duplex stainless steels are affected by hydrogen embrittlement. Therefore, understanding the hydrogen-defect interactions in these steels is crucial. This study uses internal friction to evaluate the interactions of hydrogen with microstructural defects. Analysis of the internal friction spectra of the steels subjected to straining and hydrogen charging, together with thorough microstructural characterization, gives new insights in hydrogen interactions with defects present in the different phases, i.e. ferrite (body centered cubic) and austenite (face centered cubic).While no significant effect of tensile deformation can be observed by thermal desorption spectroscopy, the internal friction spectra show a clear influence of the presence of defects. Detailed analysis of these spectra reveals the interactions in the austenite as dominant, while no indications for hydrogen-dislocation interactions in ferrite are observed. This can be related to limited trapping in ferrite due to the austenite sink action or to limited dislocation formation in ferrite. Indications for hydrogen interactions with dislocations in the austenite are found, possibly suggesting enhanced dislocation mobility when surrounded by hydrogen. Moreover, a pronounced influence of hydrogen charging on the vacancy cluster related peaks is observed, indicating strong interactions between hydrogen and vacancy clusters in austenite. This can be put in contrast to behavior of pure ferritic steels, where dislocations provided the strongest hydrogen interactions. As these specific defects are of primary interest in the hydrogen embrittlement mechanisms, internal friction is concluded to provide important unique insights in hydrogen-defect interactions, even for complicated multiphase microstructures.

A comparison of microstructure and mechanical characteristics correlation of the joint specimens for duplex stainless steel UNS S32304 and super-duplex stainless steel UNS S32750: The role of post-weld heat treatment

The present investigation studies the impact of post-weld heat treatment (PWHT) on the microstructure and mechanical characteristics of joints made by gas tungsten arc welding between duplex stainless steel and super-duplex stainless steel specimens. The PWHT process was considered at 1100 °C for 10 minutes followed by water quenching on UNS S32304 and UNS S32750 materials. Scanning electron microscopy, optical microscopy, X-ray diffractometry (XRD), and energy dispersive X-ray analyses have been applied in microstructure evolution, including recognizing local chemical contents and elemental distribution. To analyze the mechanical characteristics of samples, tensile and hardness tests were performed. It was shown that after applying PWHT, the morphology of the austenite varied becoming mostly intergranular/spheroidal in shape, accompanied by an improvement in the percentages of austenite. This investigation indicates the PWHT results in restoring an equal percentage of both phases. The phase percentages, based on both ASTM E1245 and ASTM E562, show a good agreement between the austenite and ferrite phases of weld materials. According to XRD, the phases are mainly ferrite and austenite with different lattice parameters with no evidence of unwanted intermetallic phases. It was concluded that the PWHT process plays a crucial role in the ductility of joint specimens due to the phase balance between austenite and ferrite.

Revealing the mechanism of ductility discrepancy between two 2205 duplex stainless steels containing layered and island austenite through in-situ experiment

A notable discrepancy in tensile ductility was observed between two 2205 duplex stainless steels (DSS) containing layered and island-like austenite microstructures. The deformation behavior of both DSS 2205 was compared through in-situ Electron Backscatter Diffraction (EBSD) and Scanning Electron Microscope (SEM) techniques. The results indicated that the microscopic deformation mechanism for both DSS 2205 was governed by dislocation slip. The observed tensile ductility disparity was attributed to differences in dislocation slip behavior. In DSS 2205 with island austenite, slip transfer and blocking across grain boundaries occurred simultaneously, which was determined by the geometrical compatibility factor (m') calculations of adjacent ferrite and austenite grains. This intensified plastic inhomogeneities and promoted non-Schmid dislocation slip behavior. While in layered austenite of DSS 2205, the activation of slip system strictly adhered to the principle of maximizing the Schmid factor. Multiple slip systems in the ferrite on {110} and {112} planes were activated, inducing cross-slip, which enhanced ductility. The better ductility of DSS 2205 featuring layered austenite compared to the counterpart was further explained by quantitative analysis of geometrically necessary dislocations (GND) distribution in both phases and kernel average misorientation (KAM) values during deformation.

Evaluation of methods used for simulation of heat-affected zones in duplex stainless steels

The weldability of duplex stainless steels partly depends on the ferritization of the high-temperature heat-affected zone (HT-HAZ). This area is rather narrow, and it can be challenging to visualize and determine its actual impact on the properties. To address this, various methods were applied to study the grain growth and austenite reformation in the HT-HAZ of the lean duplex grade UNS S32101. Thermo-mechanical Gleeble® simulations were conducted at 1360 °C with different holding times and cooling rates. Subsequently, the grain size and ferrite content were measured on polished and etched cross-sections. Bead-on-plate welds were performed on the same heat of 6-mm plate thickness using the gas tungsten arc welding (GTAW) process. The shielding gas was Ar + 0–8% N2 to illustrate the effect of nitrogen additions on the HT-HAZ morphology. The arc was either stationary, welding at one spot for 0.5–120 s, or travelling at different speeds to generate varying heat inputs and temperature gradients. The thermo-mechanical simulations approximated the results obtained by travelling arc welding and allowed for a more comprehensive investigation. Stationary arc welding was not suitable for HT-HAZ studies as it quickly caused nitrogen depletion and resulted in significantly higher ferrite contents compared to the travelling arc welds.

Effect of variable thickness cross rolling on microstructure gradient of 2205 duple stainless steel

Quantitative description of the complex relationship between rolling process and microstructure is crucial for improving the performance of 2205 duplex stainless steel products. In this work, four distinct rolling processes were used for three passes of hot rolling of 2205 duplex stainless steel. The study delved into the macroscopic morphology, microstructural gradients, and texture evolution of 2205 duplex stainless steel sheets in various rolling states. In addition, a data expansion method was used to expand the original experimental data, and the support vector machine model optimized by genetic algorithm was used to accurately describe the complex relationship between different rolling processes and microstructures. The results indicate that the proposed variable thickness cross-rolling method renders a more uniform microstructure in the rolled piece, with significantly refined grains and the formation of a distinct structural gradient. Among them, when using process 3 rolling, the grain size in the edge area of the plate after three passes of rolling is finer and the degree of recrystallization is higher, which is conducive to improving the problem of poor thermal plasticity at the edges and providing a basis for studying the suppression of edge crack damage in 2205 duplex stainless steel. In addition, the proposed machine learning model accurately predicted the grain size of plates in different rolling states, aligning closely with experimental findings.

Description of the constitutive behaviour of stainless steel reinforcement

This paper presents a comprehensive analysis of the constitutive relationship of stainless steel reinforcement and proposes new material models for both austenitic and duplex stainless steel bars. These are an advancement on existing models which have largely been developed for structural stainless steel plate, rather than for reinforcement bars. Current design guidance for material modelling of reinforcing bars does not include representative stress-strain relationships which capture the unique mechanical properties of stainless steel reinforcement. Codes include idealised elastic-plastic material models which are inappropriate and inefficient for the highly nonlinear and ductile material response of stainless steel. The present study aims to address this issue by first conducting a series of tensile tests to ascertain the stress-strain material responses and then employing this data to examine the validity of existing approaches and propose new material models where required. It is shown that new material models are required and those that are developed are able to accurately capture the stress-strain response of stainless steel reinforcement, and provide a better, more accurate, representation than existing methods.

Influence of the Interaction of Hydrogen with the Phase Boundary of Duplex Stainless Steel on Corrosion

This study simulates the behavior of 2205 duplex stainless steel (DSS) in an actual service environment under hydrogen charging/releasing cycles. Electrochemical and immersion experiments are conducted to compare the pitting susceptibility and corrosion behavior of 2205 DSS under single hydrogen charging and several hydrogen charging/releasing cycles. The results reveal that under single hydrogen charging, pits preferentially initiate and propagate within the austenite phase; this finding is consistent with the results of previous studies. However, under cyclic hydrogen charging/releasing cycles, pits initiate at the phase boundary and propagate along it. Moreover, a hydrogen microprint technique is used to characterize the hydrogen enrichment sites in 2205 DSS. The results demonstrate that pits preferentially initiate at the same hydrogen enrichment sites. The study creates a schematic of hydrogen redistribution and hypothesizes that under actual service conditions, the phase boundary, rather than the austenite phase, is the primary site for pitting corrosion because of its transformation into a hydrogen enrichment site from a hydrogen trap.

Characteristics of high-temperature heat-affected zones in duplex stainless steels

AbstractThe level of ferritization in the high temperature heat-affected zone (HT-HAZ) affects the weldability of duplex stainless steels. The modern grades show different sensitivity and especially the nitrogen content has been proposed to control the grain growth and austenite formation. In this work, 3-, 4- and 5-mm thick UNS S32101, UNS S32304, UNS S32205 and UNS S32750 with a nitrogen range of 0.09–0.28 wt.-% were considered. Gas tungsten arc welding (GTAW) was conducted bead-on-plate with pure argon (Ar) and Ar + 2% N2 as shielding gas. Measurements of the HT-HAZ width and ferrite content were compared with values reported for welds and thermo-mechanical Gleeble® simulations in existing literature. The HT-HAZ width decreased with the material thickness and the nitrogen content in the base metal, while only the nitrogen alloying had a clear effect on the phase balance. Nitrogen additions to the shielding gas efficiently counteracted weld metal nitrogen loss and improved the austenite formation as compared to 100% Ar but had no clear effect on the HT-HAZ width nor ferrite content. A positive influence of the base material nitrogen content on the phase balance could be validated with thermodynamic simulations using Thermo-Calc and DICTRA, but the negative effect of high cooling rates was underestimated.