Super Duplex Stainless Steel UNS Research Articles

Effect of Secondary Phase on Passivation Layer of Super Duplex Stainless Steel UNS S 32750: Advanced Safety of Li-Ion Battery Case Materials.

Aluminum, traditionally the primary material for battery casings, is increasingly being replaced by UNS S 30400 for enhanced safety. UNS S 30400 offers superior strength and corrosion resistance compared to aluminum; however, it undergoes a phase transformation owing to stress during processing and a lower high-temperature strength. Duplex stainless steel UNS S 32750, consisting of both austenite and ferrite phases, exhibits excellent strength and corrosion resistance. However, it also precipitates secondary phases at high temperatures, which are known to form through the segregation of Cr and Mo. Various studies have investigated the corrosion resistance of UNS S 32750; however, discrepancies exist regarding the formation and thickness of the passivation layer. This study analyzed the oxygen layer on the surface of UNS S 32750 after secondary-phase precipitation. The microstructure, volume fraction, chemical composition, and depth of O after the precipitation of the secondary phases in UNS S 32750 was examined using FE-SEM, EDS, EPMA and XRD, and the surface chemical composition and passivation layer thickness were analyzed using electron probe microanalysis and glow-discharge spectroscopy. This study demonstrated the segregation of alloy elements and a reduction in the passivation-layer thickness after precipitation from 25 μm to 20 μm. The findings of the analysis aid in elucidating the impact of secondary-phase precipitation on the passivation layer.

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.

Influence of machining parameters on tool wear, residual stresses, and corrosion resistance after milling super duplex stainless steel UNS S32750

Influence of machining parameters on tool wear, residual stresses, and corrosion resistance after milling super duplex stainless steel UNS S32750

The effect of surface roughness on re-passivation and pitting corrosion of super duplex stainless steel UNS S 32760

Stainless steel is used as a battery case material because of its excellent strength and corrosion resistance. Among the stainless steels used in fields requiring superior corrosion resistance, duplex stainless steels, composed of austenite and ferrite, exhibit improved strength and corrosion resistance. In particular, super duplex stainless steel (SDSS), with a pitting resistance equivalent number (PREN) between 40 and 50, offers excellent strength and corrosion resistance, making it suitable for structural components in various applications. However, the actual corrosion resistance of a material is often lower than its theoretical value, which has led to extensive research efforts to enhance practical corrosion resistance. Recent studies reported atomic-scale measurements of corrosion resistance and identified the influence of atomic layers on corrosion. Corrosion properties were studied by Trinh et al. (2020) and Nilsson (1992) [26]; however, they did not examine the re-passivation of the SDSS. SDSS requires re-passivation because of real corrosion. Although structural stainless steels are processed to achieve surface roughness in the micrometer range and undergo passivation, research on this aspect is still lacking. To enhance the performance of SDSS, research on re-passivation before and after pitting is required. Therefore, this study analyzed the effect of surface roughness variation from 100 to 1000 nm on the corrosion resistance and re-passivation with or without pitting of SDSS. The surface conditions corresponding to different roughness levels were analyzed using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM), while corrosion resistance was evaluated through open circuit potential (OCP) measurements, potentiodynamic polarization tests, and cyclic polarization tests at 0.6 V (before broken passivation layer) and 1.3 V (after broken passivation layer), as well as critical pitting temperature (CPT) tests. Surface roughness reduces the corrosion potential (Ecorr) and increases the corrosion current density (Icorr) for activation polarization. With passivation, as the surface roughness increased, uniform corrosion decreased, resulting in a stronger passive layer. The surface roughness lowered the OCP and increased the corrosion rate, thereby reducing the corrosion resistance. Therefore, when SDSS is used in an environment prone to corrosion, a surface roughness of less than 100 nm exhibits superior corrosion resistance.

A multi-objective robust evolutionary optimization approach applied to the multivariate helical milling process of super duplex steel

This work presents an approach for multivariate modeling and multi-objective robust evolutionary optimization for processes presenting as inputs control and noise variables and as outputs a set of correlated responses regarding process performance and product quality. Factor analysis is applied to obtain a transformed output set that is uncorrelated and retains most part of the information of the original outputs. Models that are robust regarding noise variables relating this transformed output set with the control variables are obtained through robust parameter design. Response models in the function of process and noise variables are obtained through the least squares method. Mean and variance models as a function of process variables are obtained through error propagation. Robust mean square error models are obtained to model the bias and variance of each latent variable. Finally, multi-objective evolutionary optimization is performed. Three evolutionary algorithms were applied and compared through hypervolume. The approach is applied in the helical milling of super duplex stainless steel UNS S32760. Tool overhang length, hole depth, and lubri-cooling flow rate were set as noise factors. Cutting forces were monitored, while roughness and roundness were measured to evaluate process performance and product quality. The three factors, obtained to represent the six outputs, guaranteed dimensionality reduction and minimized the redundancy in output space. The mean square error models obtained to these three factors as a function of control variables minimize both bias and variance regarding noise factors. The AGE-MOEA multi-objective evolutionary algorithm obtained the best performance considering the hypervolume. Some solutions with high trade-off were selected through pseudo-weights to aid the decision-making.

Electrochemical characterization of sigma phase in superduplex stainless steels: LSV-KOH as a promising methodology over DL-EPR

This work analyzed the volume fraction of the sigma phase in a superduplex stainless steel UNS S32760. Two electrochemical testing methodologies were applied for this purpose. Firstly the DL-EPR test was performed in three different electrolytic solutions. The LSV-KOH test was also used, a recent methodology proposal for analyzing these alloys. Seven specimens were solution annealed at 1100 °C for 60 min and heat-treated at 850 and 950 °C for 5, 20, and 60 min to promote the formation of different fractions of the sigma phase. The sigma phase volume fraction present in these specimens was determined by quantitative metallography. The DL-EPR analyses in all three conditions proved to be limited in quantifying the sigma phase, especially in specimens heat-treated at 950 °C. On the other hand, from the analyses with the LSV-KOH technique, it was possible to construct a reference curve between the sigma volume fractions and the electrochemical response of the LSV-KOH test (charge density), obtaining a linear regression coefficient of 0.97. This curve can then be applied to quantify the sigma phase. For comparison, the best regression coefficient obtained for DL-EPR was 0.53. Accompanied by the quantitative analysis, a qualitative analysis of the sigma phase can be made from a deeper analysis of the LSV-KOH voltammograms. In these analyses, it was possible to infer whether the sigma phase morphology is lamellar or in blocks. From the results obtained, it was possible to show that the LSV-KOH test is more precise in quantifying the sigma phase and allows a morphological analysis.

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

Localized Corrosion of High-Grade Stainless Steels: Grade Selection in Chlorinated Seawater

Chlorination is widely used in seawater systems to avoid fouling and associated microbial-induced corrosion. Free chlorine is a strong oxidizing agent that prevents biofilm formation on immersed surfaces when used above a certain content. However, the presence of residual chlorine associated with the relatively high chloride content in seawater significantly increases the risk of localized corrosion for most stainless steels. In the present study, a module initially developed to quantify the formation of electroactive biofilms on stainless steels has been used to assess the corrosiveness of chlorinated seawater. Both the electrochemical potential and the cathodic current were measured on super-duplex stainless steel as a function of residual chlorine levels and seawater temperatures. In parallel, long-term localized corrosion tests have been performed in simulated environments to assess the environmental limits for the safe use of high-grade stainless steels in chlorinated seawater. It includes crevice corrosion exposure tests using adapted ISO 18070:2015 crevice formers and internal tube pitting corrosion exposure tests in model tube heat exchangers simulating heat flux from 35°C to 170°C. The synergetic effect of residual chlorine content and temperature on the risk of localized corrosion has been quantified. Corrosion resistance properties are correlated to the electrochemical monitoring data, and the environmental limits of selected base materials stainless steels have been established for duplex stainless steel UNS S32205, super-duplex stainless steel UNS S32750, hyper-duplex stainless steels UNS S32707 and UNS S33207, and the high-grade austenitic stainless steel UNS S31266.

Effects of the Volume Fraction of the Secondary Phase after Solution Annealing on Electrochemical Properties of Super Duplex Stainless Steel UNS S32750

Super duplex stainless steel (SDSS) is used for manufacturing large valves and pipes in offshore plants because of its excellent strength and corrosion resistance. Large valves and pipes are manufactured by forging after casting, and the outside and inside microstructures are different owing to the difference in the cooling rate caused by the thermal conductivity. This microstructural variation causes cracks during solution annealing, which breaks the materials. To study the corrosion resistance of the SDSS forged material, the influence of the microstructure according to the difference between the inside and outside cooling rates of the cast SDSS was evaluated. To analyze the effects of the secondary phase fraction before solution annealing on the solution and corrosion resistance, the corrosion resistance with and without solution annealing was measured using the potentiodynamic polarization test and critical temperature test after the precipitation of the secondary phase. In the potentiodynamic polarization test, the secondary phase decreased the activation polarization and increased the corrosion rate. The critical pitting temperature exhibited the effect of the secondary phase.

Numerical Prediction for the Effects of Welding Interpass Temperature on the Thermal History and Microstructure of Duplex Stainless Steels

Numerical simulation was used to predict the thermal behavior and the resulting microstructure at the heat-affected zone (HAZ) of a 170 mm diameter and 3.5 mm thickness super duplex stainless steel (SDSS) UNS S32750 tube. In order to evaluate the thermal response from the model, a usual welding situation involving interpass temperature (IT) and its influence on the HAZ microstructure was exploited. Thus, two superimposed autogenous welding passes were simulated, the first clockwise with the tube in the room temperature and the second, counterclockwise, with the tube at the temperature of 250oC. Even subjected to successive thermal cycles and high interpass temperature, the proportion and morphology of the phases at the HAZ and Fusion Zone (FZ) did not present significant differences when comparing the two welding passes. Meanwhile, nitrogen losses should be avoided during welding in order to obtain a balanced microstructure in DSS welds, contributing to guarantee satisfactory toughness in addition to resistance to pitting corrosion. The predictions from the simulation were validated by using experimental results obtained from the autogenous TIG (Tungsten Inert Gas) process.

Effect of the Plating Time on Nickel Electroless Coating Properties Deposited on the Super Duplex Stainless Steel UNS S 32750

Effect of the Plating Time on Nickel Electroless Coating Properties Deposited on the Super Duplex Stainless Steel UNS S 32750

Effect of Heat Energy Input on Electrochemical Properties of Solution-Annealed Super-Duplex Stainless Steel UNS S 32750 Laser Welding

Effect of Heat Energy Input on Electrochemical Properties of Solution-Annealed Super-Duplex Stainless Steel UNS S 32750 Laser Welding

Failure analysis of a flange of superduplex stainless steel by preferential corrosion of ferrite phase

The failure of a flange for seawater service in the top side of an oil platform was investigated in this work. The flange was made of superduplex stainless steel UNS S32760, and the chemical composition and the microstructure were found to be adequate, according to ASTM A-182 standard. Deleterious phases such as sigma and chi phases were not found by optical and scanning electron microscopy. The material's mechanical properties and corrosion resistance were also typical of superduplex steel without intermetallic phases. However, the flange has undergone severe crevice corrosion in the gasket region. In the microstructural scale, the ferrite phase was preferentially attacked. EDS analyzes revealed elements typically found in corrosion processes caused by microbiological bacteria. The work shows that the superduplex steel is susceptible to crevice microbiological induced corrosion (MIC), even when the microstructure and fabrication process used are correct.

Effect of Solution Annealing on Austenite Morphology and Pitting Corrosion of Super Duplex Stainless Steel UNS S 32750

Effect of Solution Annealing on Austenite Morphology and Pitting Corrosion of Super Duplex Stainless Steel UNS S 32750

Strength Enhancement of Superduplex Stainless Steel Using Thermomechanical Processing

The impact of microstructure evolution on mechanical properties in superduplex stainless steel UNS S32750 (EN 1.4410) was investigated. To this end, different thermomechanical treatments were carried out in order to obtain clearly distinct duplex microstructures. Optical microscopy and scanning electron microscopy, together with texture measurements, were used to characterize the morphology and the preferred orientations of ferrite and austenite in all microstructures. Additionally, the mechanical properties were assessed by tensile tests with digital image correlation. Phase morphology was not found to significantly affect the mechanical properties and neither were phase volume fractions within 13% of the 50/50 ratio. Austenite texture was the same combined Goss/Brass texture regardless of thermomechanical processing, while ferrite texture was mainly described by α-fiber orientations. Ferrite texture and average phase spacing were found to have a notable effect on mechanical properties. One of the original microstructures of superduplex stainless steel obtained here shows a strength improvement by the order of 120 MPa over the industrial material.

Effect of Ultrasonic Shot Peening on the Microstructure of Austenitic Stainless Steel 316 and Super Duplex Stainless Steel UNS S32750.

Shot peening for nanocrystallization of the surface region is a good way of improving corrosion-, fatigue-, and wear-resistance, etc. of metallic parts (Lu, K., 2014. Making strong nanomaterials ductile with gradients. Science, 345(6203), pp.1455-1456). The technique has been widely used for various materials as a method of surface modification. SUS316 has excellent corrosion and oxidation resistance with good formability However, its application is limited by the low mechanical strength and hardness. S32750 (duplex stainless steel) is one of most used tubing materials for oil/gas delivery systems in a corrosive environment under high pressure (Nilsson, J.O., 1992. Super duplex stainless steels. Materials Science and Technology, 8(8), pp.685-700). Thus, improving corrosion resistance is a key for the wider application and better maintenance of S32750. In our study, the alloy S32750 was heat-treated at 1070°C to obtain a precipitation-free microstructure (γ and δ dual-phase structure). It was then ultrasonic shot peened and microstructures were analyzed for: (1) surface nanocrystallization, (2) effect of the treatment processing parameters, and (3) the determination of microstructural evolution and the effect of the shot peening process.

Premature Failure of Grade-316Ti Stainless Steel Tubing in a Boiler Feed-Water Heat Exchanger in a Steel Complex

Premature failure of seamless tubes made of 316Ti stainless steel in a boiler feed-water (BFW) heat exchanger occurred at a local steel complex, which led to about 65–70% service-life reduction. The heat exchanger is a fixed-tube-sheet type with hydrogen gas flowing inside the tubes and water circulating through the shell side. The cooled hydrogen gas is purged to box annealing furnaces to create a reducing atmosphere. Failure analysis was conducted on the tubes to determine the contributing factors to premature failure of the heat exchanger. Four issues including corrosion fatigue, pitting corrosion attack, under-deposit corrosion, and fouling contributed to the failure of the tubes in the heat exchanger. The pitting corrosion attack was caused by chloride in circulating water. Chloride was also detected in accumulated deposits on the tubes causing the under-deposit corrosion issue. Contamination of boiler feed-water with Ca, Si, Mg, and Cl occurred during the water treatment process, a congruent phosphate–pH treatment in an in-house water treatment unit. The quality of the supplied water to the heat exchanger does not meet the recommended specifications. As a major recommendation, the tubes of the heat exchanger must be manufactured of super duplex stainless steel UNS S32750, based on specifications of the ASTM A789 standard.

Evaluation of Hot Deformation Behaviour of UNS S32750 Super Duplex Stainless Steel (SDSS) Alloy

The super-duplex stainless steel UNS S32750 consists of two main phases, austenite and ferrite, which differ not only by their morphology, physical, and mechanical properties, but also by their deformation behaviour. A heterogenous deformation can be obtained during thermomechanical processing, generating internal stresses and sometimes fissures or cracks on sample lateral surfaces, due to ferrite’s phase lower potential of plastic deformation accommodation in comparison with austenite phase. The research objective is to determine the optimum range of the applied deformation degree, during hot deformation processing by upsetting of the super-duplex steel (SDSS) UNS S32750. In the experimental program several samples were hot deformed by upsetting, by applying a deformation degree between 5–50%, at 1050 °C and 1300 °C. The most representative hot-deformed samples were selected and analysed by scanning electron microscope-Electron Backscatter Diffraction (SEM-EBSD), to determine the main microstructural characteristics obtained during thermomechanical processing. When considering the experimental results, the influence of the applied deformation degree on the microstructure has been evaluated. Microstructural features, such as nature, distribution, morphology and relative proportion of constituent phases, Grain Reference Orientation Deviation (GROD), and recrystallization (RX), were analysed, in correlation with the applied deformation degree. Finally, it was concluded that the UNS S32750 alloy can be safely hot deformed, by upsetting, at 1050 °C and 1300 °C, with a maximum applied deformation degree of 20% at 1050 °C and, respectively, by 50% at 1300 °C.

Sigma and Chi Phases Analysis in UNS S32750 Superduplex Stainless Steel by Optimized Linear Sweep Voltammetry in Alkaline Medium

Linear sweep voltammetry (LSV) test parameters in alkaline medium were optimized by Doehlert matrix design in order to quantify the deleterious phases in a superduplex stainless steel UNS S32750. The microstructural analysis was performed, in several heat treated specimens, by Light Optical (LOM) and Scanning Electron (SEM) Microscopies and correlated with the electrochemical tests. In these tests, optimized parameters were obtained for tests in aqueous solutions of KOH. The concentration of 3.55 mol l−1, scan rate of 3.42 mV s−1 and initial potential of −0.818 V, showed a good correlation between the deleterious phases precipitated and charge density values. Differently from LOM characterization, chi and sigma deleterious phases can be distinguished by LSV optimized test. Finally, this test can be a non-destructive powerful tool of quality control to detect embrittlement and corrosion resistance decay that commonly affected this stainless steel as consequence of inadequate fabrication processes.

Effect of heat input on microstructure and mechanical properties of Nd: YAG laser welded super duplex stainless steel-numerical and experimental approach

This study reports the effect of different heat input (126 J/mm to 330 J/mm) on the microstructure and mechanical properties of pulsed Nd: YAG laser welded super duplex stainless steel UNS S32750 (SDSS) butt weld joints. Macrostructure exhibit incomplete, complete and complete penetration with a crater at the top of the weld bead, for low, middle and higher heat input respectively. Experimental macrostructure is in agreement with the numerical simulation performed by SYSWELD - a 3D software package. Maximum temperature is obtained at the mid section of the weld for all attempted heat inputs. Micro-analysis display an enhancement in the ferrite concentration in the weld zone for an increase in the heat input. Sigma phase, a reaction compound, is observed for the highest heat input. Higher micro-hardness is witnessed at the interface for the mid range heat input. The results of tensile test show that the tensile strength tends to reach a peak and then decrease with the increasing heat input.