Super Duplex Stainless Steel Research Articles

Surface roughness reduction in electron beam powder bed fusion additive manufacturing of super duplex stainless steel 2507: investigating optimisation techniques and face orientation-dependent irregularities

AbstractThe use of additive manufacturing in metals by powder bed fusion via electron beam (PBF-EB) is increasing for fabricating high-quality parts meeting industrial standards. However, high surface roughness poses a consistent challenge in PBF-EB. This study investigates two novel approaches to optimise surface roughness for a given machine and powder combination. Using machine control software’s recently introduced research mode functionality, we develop customised beam control code to effectively explore a vast parameter space. Additionally, we explored the impact of beam travel direction and spot morphology on surface roughness. Line-melt-based contours were explored by specimen manufacturing with layer-wise parameter change, whilst spot-melting-based samples were built using a full factorial design of experiments with four factors at three levels. Initial sample characterisation was done using a stylus-based contact profilometer, followed by detailed evaluation using focus variation microscopy. Results reveal that increasing beam power and spot energy exacerbate surface roughness. We also find that a well-defined energy distribution at the spot's edge contributes to smoother surfaces. Whilst the influence of beam travel direction on surface roughness remains uncertain, our findings underscore the importance of parameter selection in achieving optimal results. By adjusting contouring parameters, we achieve a vertical roughness of Ra17.7 ± 0.9 (Sa 21.6), significantly lower than in the current literature. These findings advance our understanding of surface roughness optimisation in PBF-EB and offer practical insights for improving part quality in industrial applications. By harnessing tailored beam control strategies, manufacturers can enhance the capabilities of additive manufacturing technologies in producing metal components.

High-speed cutting performance characteristics of Ti(C, N)–W cermet tools against S32750 super-duplex stainless steel round bars

In this study, TiC0.5N0.5–60 wt% W and TiC0.5N0.5–70 wt% W cermet cutting tool specimens consisting of Ti(C, N) core, W-rich Ti(C, N) rim, and W-based alloy phases with much higher strengths than conventional heat-resistant tool materials at temperatures above 1673 K were fabricated by spark plasma sintering, electric discharging, and polishing. The cutting performance characteristics of the cermet cutting tool specimens were then examined and compared with those of commercially available NX2525, JX1, and HTi10 cutting tool specimens when cutting S32750 super-duplex stainless steel round bars under dry condition at a cutting speed of 800 m/min. The NX2525, JX1, and HTi10 specimens were TiC–Ni-based cermet containing Mo, Nb, and Co, WC–Al2O3-based composites, and WC–Co cemented carbide, respectively. The TiC0.5N0.5–70 wt% W and TiC0.5N0.5–60 wt% W cermet cutting tool specimens exhibited the first and second longest lifetimes among all the noncoated cutting tool specimens. The lifetime of the TiC0.5N0.5–70 wt% W cermet cutting tool specimen was extended by coating it with TiAlN film by physical vapor deposition. In contrast, the TiC0.5N0.5–60 wt% W and TiC0.5N0.5–70 wt% W cermet specimens exhibited compressive strengths much higher than those of the NX2525, JX1, and HTi10 specimens at 1973 K, whereas all the specimens exhibited similar compressive strengths at room temperature.

Study of Effects of Post-Weld Heat Treatment Time on Corrosion Behavior and Manufacturing Processes of Super Duplex Stainless SAF 2507 for Advanced Li-Ion Battery Cases.

Lithium-ion batteries are superior energy storage devices that are widely utilized in various fields, from electric cars to small portable electric devices. However, their susceptibility to thermal runaway necessitates improvements in battery case materials to improve their safety. This study used electrochemical analyses, including open-circuit potential (OCP), potentiodynamic polarization, and critical pitting temperature (CPT) analyses, to investigate the corrosion resistance of super duplex stainless steel (SAF 2507) applied to battery cases in relation to post-weld heat treatment (PWHT) time. The microstructure during the manufacture, laser welding, and PWHT was analyzed using field-emission scanning electron microscopy, X-ray diffraction, and electron backscatter diffraction, and the chemical composition was analyzed using dispersive X-ray spectroscopy and electron probe micro-analysis. The PWHT increased the volume fraction of austenite from 5% to 50% over 3 min at 1200 °C; this increased the OCP from -0.21 V to +0.03 V, and increased the CPT from 56 °C to 73 °C. The PWHT effectively improved the corrosion resistance, laying the groundwork for utilizing SAF 2507 in battery case materials. But the alloy segregation and heterogeneous grain morphology after PWHT needs improvement.

Achieving enhanced strength-ductility and wear resistance in directed energy deposited super duplex stainless steel via inoculating NbN particles

In wire-based directed energy deposition (DED), super duplex stainless steel (SDSS) typically encounters issues including coarse grains, a contradictory relationship between strength and ductility, and inferior wear resistance properties. Herein, we report the introduction of micron-sized NbN particles into DEDed SDSS to achieve an optimized combination of fine microstructure, high strength, impressively larger ductility and better wear resistance. During the DED process, NbN inoculant dissolves and re-precipitates, forming fine NbN and σ particles around 100 nm in size. These NbN particles act as nucleation sites for equiaxed grains, refining both ferrite and austenite. In addition, Nb and N elements left from NbN dissolution enhance the solution strengthening effect. These factors collectively boost hardness and strength in DEDed SDSS. Furthermore, the high Nb level in DEDed SDSS facilitates a dense Cr2O3 layer formation on the specimen surface during wear tests, enhancing wear resistance. Increased Nb and N also lower stacking fault energy (SFE) of SDSS austenite, promoting the occurrence of deformation twinning, and the improvement of work hardening and ductility during plastic deformation. This work shows the great potential of wire-arc DED technology to fabricate SDSS with unique microstructures, excellent wear resistance, and an exceptional combination of strength and ductility for practical applications.

Drilling performance analysis on super duplex stainless steel (AISI 2507) using scalable pulse power plasma TiSiN- and AlTiN-coated tools and optimizing machining parameters through Taguchi–Fuzzy approach

Drilling performance analysis on super duplex stainless steel (AISI 2507) using scalable pulse power plasma TiSiN- and AlTiN-coated tools and optimizing machining parameters through Taguchi–Fuzzy approach

Friction stir welding of 2507 super-duplex stainless steel: feasibility of butt joint groove filling at different process parameters

ABSTRACT The present work explores the feasibility of groove-filling welding of 6 mm-thick 2507 super-duplex stainless steel (SDSS) in a similar butt-joint configuration using friction stir welding (FSW). A 60° V-groove with a 3 mm root face was designed and machined. The FSW trials were performed at varying rotation speeds (400–600 rpm), downward forces (15–25 kN), and a constant travel speed of 25 mm/min using two WC tools with two different cobalt (Co) contents of 8 and 10 wt.%. The groove filling achievements of the welded joints were monitored using three non-destructive tests: visual, penetrant, and radiographic. All the welded joints were subjected to transverse tensile testing in addition to hardness measurements across the joint area. Microstructure and fracture surface characterisation were also investigated to gain insights into the impact of heat input on joint performance through a comparison with the as-received SDSS. Among the trial FSW processes, the results indicate that the FSW parameters of 25 mm/min, 15 kN, and 400 rpm using a WC-8 wt.% Co tool were successfully used to completely fill the groove and produce a defect-free SDSS butt-joint. This joint has the highest average stir zone (SZ) hardness (342 ± 5 HV) and tensile strength (1256 ± 21 MPa) compared to partially jointed specimens.

On the correlation between fatigue behaviour and the multiphase microstructure of a super duplex stainless steel

This work investigates the relationship between the multiphase microstructure and the macroscale fatigue behaviour of a cast super duplex stainless steel (SDSS). The microstructure of a cast SDSS is typically composed of a FCC γ-phase and a BCC δ-phase. However, deleterious secondary phases (SP), such as sigma and chi, may precipitate when SDSS is exposed to high-temperature ranges, jeopardising its mechanical properties, namely fatigue performance. The purpose of this investigation was to quantify the correlation between SP amounts and fatigue life of a cast SDSS (CD3MWCuN, ASTM A890). Fatigue tests were conducted using miniature specimens taken from a cast C-Ring with different SP amounts. A numerical model was proposed to calibrate the fatigue test parameters. After the tests, SEM and EBSD techniques were used to quantify the SP and analyse the fatigue-crack propagation. The microstructural analysis was complemented with hardness and tensile tests. Regression models were developed to correlate the percentage of SP, the fatigue resistance and the number of cycles to failure, and to predict the microstructural stress concentration from the amount of SP. In addition, a methodology is proposed for predicting S-N curves as a function of the amount of SP, showing a high correspondence in the increase of the percentage of SP with the decrease in fatigue life. Furthermore, the fatigue strength, for a given number of cycles to failure, tends to decrease linearly with increasing the SP, while the microstructural stress concentration factor tends to increase with an increase in SP. These findings contribute to further knowledge on predicting the behaviour of cast SDSS under different cyclic conditions, which can be valuable for optimizing design and performance in various applications.

Tensile Properties and Fracture Analysis of Duplex (2205) and Super Duplex (2507) Stainless Steels, Produced via Laser Powder Bed Fusion Additive Manufacturing

Additive manufacturing of duplex (DSS) and super duplex stainless steel (SDSS) has been successfully demonstrated using laser powder bed fusion (LPBF) in recent years. Owing to the high cooling rates, as-built LPBF-processed DSS and SDSS exhibit close to 100% ferritic microstructures and require heat treatment at 1000–1300 °C to obtain the desired duplex microstructure. In this work, the mechanical properties of DSS and SDSS processed via LPBF were investigated in three building directions (vertical, horizontal, diagonal) and three processing conditions (as-built, stress-relieved, annealed, and quenched) using uniaxial tensile testing. As-built samples exhibited tensile and yield strength greater than 1000 MPa accompanied by less than 20% elongation at break. In comparison, the water-quenched samples and samples annealed at 1100 °C exhibited elongation at break greater than 34% with yield and tensile strength values less than 950 MPa. Stress relief annealing at 300 °C had a negligible impact on the mechanical properties. Austenite formation upon high-temperature annealing restored the reduced ductility of the as-built samples. The as-built and stress-relieved SDSS showed the highest yield and tensile strength values in the horizontal build direction, reaching up to ≈1400 and ≈1500 MPa (for SDSS), respectively, as compared to the vertical and diagonal directions. Fractographic investigation after tensile testing revealed predominantly a quasi-ductile failure mechanism, showing fine size dimple formation and cleavage facets in the as-built state and a fully ductile fracture in the annealed and quenched conditions. The findings in this study demonstrate the mechanical anisotropy of DSS and SDSS along three different build orientations, 0°, 45°, 90°, and three post-processing conditions.

Microstructural Characterization of Aged Welded Joints of a UNS S32750 Superduplex Stainless Steel

Microstructural Characterization of Aged Welded Joints of a UNS S32750 Superduplex Stainless Steel

Ag-Coated Super Duplex Stainless Steel AISI2507 with or without Crystallization of Secondary Phase as Advanced Li-Ion Battery Case Material

Li-ion batteries used in portable electronic devices and electric vehicles require high safety standards, necessitating the use of high-performance structural materials for battery casings. Super duplex stainless steel (SDSS) is a structural material suitable for portable electronic products owing to its excellent strength and corrosion resistance. SDSS AISI2507 was used to construct a Li-ion battery casing, a Ag coating was applied via physical vapor deposition (PVD) after the heat treatment of AISI2507 with or without a secondary phase, and the coating thickness was controlled by varying the PVD time. The thickness of the Ag coating layer increased proportionally with time, thereby enhancing the electrical conductivity. The structure and coating behavior were confirmed using FE-SEM, XRD, and GDS. The secondary phase was crystallized by the segregation of the alloy and formed a BCC structure. The FCC lattice structure exhibited excellent coating behavior on the austenite (FCC structure) of AISI2507. Conversely, the secondary phase exhibited low adhesion owing to differences in composition and crystal structure. However, the Ag coating layer on AISI2507 exhibited excellent electrical conductivity, outperforming conventional Ni-plated Li-ion battery casings comprising AISI304. However, the precipitation of the secondary phase must be controlled, as the formation of the secondary phase acts as a factor that decreases electrical conductivity from 58.8 to 53.6 (ICAS) %. The excellent performance of Ag-coated AISI2507 makes it suitable for the fabrication of enhanced Li-ion battery casings.

Interfacial microstructure and electrochemical behavior of diffusion welded joints of Zr-Alloy and super duplex stainless steel

Interfacial microstructure and electrochemical behavior of diffusion welded joints of Zr-Alloy and super duplex stainless steel

Avoiding heat source calibration for finite element modeling of the laser powder bed fusion process

The melt pool characteristics govern process stability and microstructure development during Laser Powder Bed Fusion (LBPF). The melt pool is strongly affected by the laser parameters and based on the melt pool depth-to-width ratio, three melting modes can be distinguished: conduction, transition, and keyhole mode. Finite element (FE) simulations have been extensively used to investigate the cyclic thermal history of the LPBF process and the effects of different process parameters on the part quality. However, the heat sources applied in such simulations can only accurately predict the melt pool behavior, and thus the temperature profile, when processing in conduction mode. As for the transition and keyhole regime modeling, heat source parameters or material properties are usually adjusted to make the simulated melt pool dimensions match the real values. This study proposes a novel approach to bypass the conventional heat source calibration strategy to simulate the LBPF process across different melting regimes. The predicted melt pool dimensions are validated for a wide range of process parameters applied to three different alloys: Ti-6Al-4 V, 316 L austenitic stainless steel (316 L), and 2507 super duplex stainless steel (SDSS). The average errors obtained on the melt pool width, depth, length, and aspect ratio are 9.7 %, 12.4 %, 14.5 %, and 14.3 % respectively. These results demonstrate how the proposed approach can capture the melt pool dimensions throughout different materials and melting modes eliminating the time-consuming heat source calibration steps and facilitating the LPBF modeling workflow.

Investigating microstructure, mechanical properties, and pitting corrosion resistance of UNS S32760 super duplex stainless steel after linear friction welding

High-performance alloys such as super duplex stainless steels (SDSS) are of interest to the oil and gas industry, especially in deep and ultra-deep reservoirs where high pressures and highly corrosive environments are prevalent. SDSS have high nitrogen contents and pitting resistant equivalent number (PREN) values greater than 40, however, fusion welds of these alloys can exhibit unsuitable microstructures and defects that may result in failure during usage. Solid-state linear friction welding (LFW) offers an alternative for the effective joining of SDSS with a less detrimental effect on microstructure and properties. In this work, four LFW joints in UNS S32760 SDSS were produced and investigated by evaluating the process parameters required to achieve a high-quality joint with the desirable metallurgical, mechanical, and localized corrosion properties. These properties were assessed through metallography (optical microscopy and scanning electron microscopy), tensile and microhardness tests, and corrosion analysis according to the ASTM G48A standard. Results indicated that one specific combination of LFW parameters led to a defect-free joint, and that the mechanical properties of the weld in this case were comparable to those of the base material. Additionally, an adequate balance between ferrite and austenite phases was achieved throughout the microstructural gradients seen in the weld region. Microhardness values were below 350 HV, thus complying with the DNV–OS–F101 standard. Furthermore, no pitting corrosion was observed in this joint under the testing conditions suggested by ASTM G48A.

A Comparison of Sigma Phase Formation in Solubilized Hyper Duplex Stainless Steel and Super Duplex Stainless Steel Filler Metals

This study focuses on the kinetic analysis of sigma phase formation in filler metal wires on Super Duplex Stainless Steel (SDSS) and Hyper Duplex Stainless Steel (HDSS). Precipitation data reveal that in the solubilized microstructure, sigma phase kinetics are more prominent in SDSS. This increased susceptibility is attributed to the greater number of nucleation sites, which is facilitated by the larger interface area/volume and the higher chromium content in the ferrite. The difference in interface area/volume is significantly more influential in determining kinetics than the composition difference, with nucleation sites playing a central role. The sigma phase transformation in both materials was modeled using the JMAK kinetic law. The JMAK plots exhibit a transition in kinetic mechanisms, evolving from discontinuous precipitation to diffusion-controlled growth. In SDSS, the JMAK values indicate “grain boundary nucleation after saturation,” followed by “thickening of large plates.” In contrast, HDSS values point to “grain edge nucleation after saturation,” followed by “thickening of large needles.” The higher kinetics in SDSS are characterized by a smaller nucleation activation energy of 56.4 kJ/mol, in contrast to HDSS's 490.0 kJ/mol. CALPHAD-based data support the JMAK results, aligning with the maximum kinetics temperature of SDSS (875 °C to 925 °C) and HDSS (900 °C to 925 °C). Therefore, the JMAK sigma phase kinetics effectively describe the experimental data and its dual kinetics behavior, even though CALPHAD-based TTT calculations often overestimate sigma formation.

The role of TiN inoculant on microstructure and properties in wire arc directed energy deposition of super duplex stainless steels

The challenges in Wire Arc Directed Energy Deposition (Wire-DED) of super duplex stainless steels (SDSS) are the growth of large columnar grains and undesirably high austenite content. This results in a detrimental effect on the material properties. Here we report the additively manufactured SDSS with the addition of TiN inoculant via Wire-DED. And the effectiveness of incorporating TiN inoculant into additively manufactured SDSS was explored. The addition of trace amounts of TiN reduced the content of the austenite phase from 74.0% to 61.5%. Additionally, the introduction of TiN facilitated deoxidation and slag formation, leading to decreased inclusion content and a shift in inclusion type, which acted as nucleation sites for ferrite, promoting grain refinement. This results in weakened anisotropy of tensile properties and enhanced corrosion resistance. The anisotropy of the ultimate tensile strength and yield strength reduced from 9% to 3% and from 5% to 2%, respectively. This study demonstrates the potential of TiN inoculation in fabricating Wire-DEDed SDSS with tailored microstructures and improved performance, making them suitable for a wide range of practical applications.

Post-Processing Effect on the Corrosion Resistance of Super Duplex Stainless Steel Produced by Laser Powder Bed Fusion.

This study examines the microstructural characteristics and corrosion resistance of super duplex stainless steel (SDSS) produced through laser powder bed fusion (LPBF). The analysis shows that the as-printed samples mainly exhibit a ferritic microstructure, which is due to the fast-cooling rates of the LPBF technique. X-ray and microstructure analyses reveal the presence of minor austenite phases in the ferritic matrix. The process of solution annealing led to a more balanced microstructure. Analyses of corrosion resistance, such as potentiodynamic polarization tests and EIS, indicate that heat treatment has a significant impact on the corrosion behavior of SDSS. Solution annealing and stress relieving at 400 °C for 1 h can improve corrosion resistance by increasing polarization resistance and favorable EIS parameters. However, stress relieving at 550 °C for 5 h may reduce the material's corrosion resistance due to the formation of chromium nitride. Therefore, stress relieving at 400 °C for 1 h is a practical method to significantly enhance the corrosion resistance of LPBF-printed SDSS. This method offers a balance between microstructural integrity and material performance.

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.

Structure–property relationship assessment of dissimilar gas tungsten arc welded joint of pipeline steel and super duplex stainless steel for marine applications

Structure–property relationship assessment of dissimilar gas tungsten arc welded joint of pipeline steel and super duplex stainless steel for marine applications

Weld Morphology and Corrosion Characteristics of Flux-Assisted Gas Tungsten Arc-Welded Super Duplex Stainless Steel

Weld Morphology and Corrosion Characteristics of Flux-Assisted Gas Tungsten Arc-Welded Super Duplex Stainless Steel

Optimizing Machining Efficiency in High-Speed Milling of Super Duplex Stainless Steel with SiAlON Ceramic Inserts

Super duplex stainless steels (SDSSs) are widely utilized across industries owing to their remarkable mechanical properties and corrosion resistance. However, machining SDSS presents considerable challenges, particularly at high speeds. This study investigates the machinability of SDSS grade SAF 2507 (UNS S32750) under high-speed milling conditions using SiAlON insert tools. Comprehensive analysis of key machinability indicators, including chip compression ratio, chip analysis, shear angle, tool wear, and friction conditions, reveals that lower cutting speeds optimize machining performance, reducing cutting forces and improving chip formation. Finite element analysis (FEA) corroborates the efficacy of lower speeds and moderate feed rates. Furthermore, insights into friction dynamics at the tool–chip interface are offered, alongside strategies for enhancing SDSS machining. This study revealed the critical impact of cutting speed on cutting forces, with a significant reduction in forces at cutting speeds of 950 and 1350 m/min, but a substantial increase at 1750 m/min, particularly when tool wear is severe. Furthermore, the combination of 950 and 1350 m/min cutting speeds with a 0.2 mm/tooth feed rate led to smoother chip surfaces and decreased friction coefficients, thus enhancing machining efficiency. The presence of stick–slip phenomena at 1750 m/min indicated thermoplastic instability. Optimizing machining parameters for super duplex stainless steel necessitates balancing material removal rate and surface integrity, as the latter plays an important role in ensuring long-term performance and reliability in critical applications.