Search Results (15)

One of the most critical aspects of turning, and machining in general, is the surface roughness of the finished product, which directly influences the performance, functionality, and longevity of machined components. The accurate prediction of surface roughness is vital for enhancing component quality and machining efficiency. This study presents a machine learning-driven framework for modeling mean roughness depth (Rz) during the dry machining of super duplex stainless steel (SDSS 2507). SDSS 2507 is known for its exceptional mechanical strength and corrosion resistance, but it poses significant challenges in machinability. To address this, this study employs flank-face textured cutting tools to enhance machining performance. Experiments were designed using the L₂₇ orthogonal array with three continuous factors, cutting speed, feed rate, and depth of cut, and one categorical factor, tool texture type (dimple, groove, and wave), along with surface roughness as an output parameter. Gaussian Data Augmentation (GDA) was employed to enrich data variability and strengthen model generalization, resulting in the improved predictive performance of the machine learning models. MATLAB R2021a was employed for preprocessing, the normalization of datasets, and model development. Two models, Least-Squares Support Vector Machine (LSSVM) and Multi-Gene Genetic Programming (MGGP), were trained and evaluated on various statistical metrics. The results showed that both LSSVM and MGGP models learned well from the training data and accurately predicted Rz on the testing data, demonstrating their reliability and strong performance. Of the two models, LSSVM demonstrated superior performance, achieving a training accuracy of 98.14%, a coefficient of determination (R²) of 0.9959, and a root mean squared error (RMSE) of 0.1528. It also maintained strong generalization on the testing data, with 94.36% accuracy and 0.9391 R² and 0.6730 RMSE values. The high predictive accuracy of the LSSVM model highlights its potential for identifying optimal machining parameters and integrating into intelligent process control systems to enhance surface quality and efficiency in the complex machining of materials like SDSS. Full article

Regulating the phase ratio between austenite and ferrite in welded joints is crucial for welding super duplex stainless steel. Nitrogen plays a significant role in maintaining an optimal phase ratio. In this study, the focusing gas channel of gas-focused plasma arc welding was utilized to introduce nitrogen into the arc plasma, which was then transferred to the weld pool. Experiments with and without nitrogen addition were designed and conducted to examine the effects of nitrogen on the microstructure and properties of SDSS 2507 weld joints. The results show that nitrogen addition increased the austenite content in the weld metal from 22.2% to 40.2%. Nitrogen also altered the microstructure of the austenite, changing it from thin grain boundary austenite and small intragranular austenite to a large volume of coarse, side-plate Widmanstätten austenite. The ferrite in the weld metal exhibited a preferred orientation during growth, while the austenite showed a disordered orientation. Additionally, the maximum texture intensity of the ferrite decreased with nitrogen addition. Nitrogen addition led to an increase in the microhardness of the austenite in the weld metal, attributed to the solid solution strengthening effect of nitrogen and increased dislocation tangling, while it decreased the microhardness of the ferrite. This study enhances the welding theory of 2507 super duplex stainless steel and guides the practical application of gas-focused plasma arc welding for 2507 super duplex stainless steel. Full article

The influence of laser beam welding parameters (power, welding rate, focusing, head oscillation, shielding gas) on the microstructure, mechanical properties and corrosion resistance of the super-duplex stainless steel SAF 2507 was studied in this paper. The presented results clearly report the effects of welding parameter changes on the character of the steel’s microstructure. The presence of secondary phase M₂N in weld metals has an important influence on their mechanical properties. Optimal mechanical properties, an acceptable ferrite/austenite ratio, and the minimum content of M₂N nitride required in the weld metal were acquired in the case the following application: 1100 W power, welding speed of 10 mm/s, focusing of 4 mm, and pure nitrogen shielding gas (20 L/min). Full article

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. Full article

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. Full article

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. Full article

Laser powder bed fusion is an attractive technology for producing high-strength stainless steel alloys. Among the stainless steels, 2507 super duplex stainless steel (2507 SDSS) is known for its excellent combination of corrosion resistance and high strength. Although there are some studies that aimed at optimizing the laser powder bed fusion (LPBF) printing parameters to print highly dense 2507 SDSS parts; However, a full optimization study is not reported yet. This study aims at optimizing the printing parameters for 2507 SDSS, namely: laser power, scan speed, and hatch distance. The response surface methodology was used in generating a detailed design of experiment to investigate the different pore formation types over a wide energy density range (22.22–428.87 J/mm³), examine the effects of each process parameter and their interactions on the resulting porosity, and identify an optimized parameter set for producing highly dense parts. Different process parameters showed different pore formation mechanisms, with lack-of-fusion, metallurgical or gas, and keyhole regimes being the most prevalent pore types identified. The lack-of-fusion pores are observed to decrease significantly with increasing the energy density at low values. However, a gradual increase in the keyhole pores was observed at higher energy densities. An optimal energy density process window from 68.24 to 126.67 J/mm³ is identified for manufacturing highly dense (≥99.6%) 2507 SDSS parts. Furthermore, an optimized printing parameter set at a laser power of 217.4 W, a scan speed of 1735.7 mm/s, and a hatch distance of 51.3 µm was identified, which was able to produce samples with 99.961% relative density. Using the optimized parameter set, the as-built 2507 SDSS sample had a ferrite phase fraction of 89.3% with a yield and ultimate tensile strength of 1115.4 ± 120.7 MPa and 1256.7 ± 181.9 MPa, respectively. Full article

In order to investigate the effect of phase separation (PS) on the super duplex stainless steel SAF 2507, the evolution of the nanostructure, mechanical properties, and corrosion resistance of the alloy was studied after aging at 500 °C for 1, 10, 100, and 1000 h. The nanostructure of PS was quantitatively characterized by small-angle neutron scattering. The hardness, impact toughness, and pitting corrosion resistance were measured for different conditions. The results show that the early stage of PS had a more significant impact on the nanostructure and properties of SAF 2507. The fracture behavior of the alloy was likely determined by the mechanical properties of ferrite for aged conditions. The pitting corrosion resistance of SAF 2507 aged at 500 °C was closely related to the Cr depletion caused by PS, and the resistance became weaker with the progression of PS. The evolution of the passivation region with aging time correlated well with that of mechanical properties and characteristic parameters of PS, indicating that it is possible to develop a new nondestructive electrochemical method to quantify the evolution of PS in SAF 2507. Full article

Next-generation concentrated solar power (CSP) plants are required to operate at temperatures as high as possible to reach a better energy efficiency. This means significant challenges for the construction materials in terms of corrosion resistance, among others. In the present work, the corrosion behavior in a molten eutectic ternary Li₂CO₃-Na₂CO₃-K₂CO₃ mixture at 600 °C was studied for three stainless steels: an austenitic grade AISI 301LN (SS301) and two duplex grades, namely 2205 (DS2205) and 2507 (DS2507). Corrosion tests combined with complementary microscopy, microanalysis and mechanical characterization techniques were employed to determine the corrosion kinetics of the steels and the oxide scales formed on the surface. The results showed that all three materials exhibited a corrosion kinetics close to a parabolic law, and their corrosion rates increased in the following order: DS2507 < SS301 < DS2205. The analyses of the oxide scales evidenced an arranged multilayer system with LiFeO₂, LiCrO₂, FeCr₂O₄ and NiO as the main compounds. While the Ni-rich inner layer of the scales presented a good adhesion to the metallic substrate, the outer layer formed by LiFeO₂ exhibited a higher concentration of porosity and voids. Both the Cr and Ni contents at the inner layer and the defects at the outer layer were crucial for the corrosion resistance for each steel. Among the studied materials, super duplex stainless steel 2507 is found to be the most promising alternative for thermal energy storage of those structural components for CSP plants. Full article

In the present study, microstructural evolution and hardness of the friction stir processed (FSPed) SAF 2507 super duplex stainless steel fabricated at a rotational speed of 650 rpm and a traverse speed of 60 mm/min were investigated. A scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) detector was used to study the microstructure of the stir zone. The grain sizes of austenite and ferrite in the FSPed 2507 were found to be smaller (0.75 and 0.96 μm) than those of the substrate (6.6 and 5.6 μm) attributed to the occurrence of continuous dynamic recrystallization (CDRX) in both phases. Higher degree of grain refinement and DRX were obtained at the advancing side of the FSPed specimens due to higher strain and temperature. A non-uniform hardness distribution was observed along the longitudinal direction of the SZ. The maximum hardness was obtained at the bottom (407 HV₁). Full article

Thermally induced deterioration processes were studied in cold worked (up to 60% deformation) SAF 2507 type super-duplex stainless steel (SDSS) by differential thermal analysis (DTA). DTA results revealed two transformations. Parent and inherited phases of these transformations were examined by other methods too, such as micro-hardness tests, optical metallography and X-ray diffraction (XRD). Finally, these transformations were identified as the formation of α’- and σ-phases. Formation of strain-induced martensite (SIM) and recrystallization were not experienced until 1000 °C, despite high degree of cold working. Activation energies of the σ-phase precipitation and α’-phase formation were determined from the Kissinger plot, through DTA measurements—they are 275 and 220 kJ/mol, respectively—in good agreement with the values found in the literature. Full article

Duplex and Super Duplex Stainless Steels are very prone to secondary phases formation related to ferrite decomposition at high temperatures. In the present paper the results on secondary phase precipitation in a 2510 Duplex Stainless Steel, heat-treated in the temperature range 850–1050 °C for 3–30 min are presented. The precipitation starts at grain boundaries with a consistent ferrite transformation for very short times. The noses of the Time–Temperature–Precipitation (TTP) curves are at 1000 °C for σ-phase and at 900 °C for χ-phase, respectively. The precipitation sequence involves a partial transformation of χ into σ, as previously evidenced in 2205 and 2507 grades. Furthermore, the experimental data were compared to the results of Thermo-Calc calculations. Understanding and ability to predict phase stability in 2510 duplex stainless steel is a key factor to design optimal welding processes that avoid any secondary phase precipitation in the weld bead as well as in the heat-affected zone. Full article

Duplex stainless steel (DSS) is used for desalination equipment, pressure vessels, marine applications, offshore applications, and in oil/gas plants where a highly corrosive environment is present. Super duplex stainless steel (SDSS) 2507 has excellent mechanical properties, such as high strength, high toughness, high fatigue life, and high corrosion resistance. Friction stir processing (FSP) is used to refine the grain structure of the processed region such that properties like strength, hardness, fracture toughness, fatigue life, and corrosion resistance are enhanced. In this paper, an optimized friction stir process of 2507 SDSS is carried out to refine the microstructure of the material in order to improve its mechanical properties. Microstructure analysis revealed that grains were refined from a size of around 160 µm in the base material to 2–30 µm in the processed zone. This grain size reduction resulted in improved strength, hardness, and fracture toughness of the material by up to 14%, 11%, and 12%, respectively. However, FSP has reduced the fracture strain by about 30%. Full article

Sigma phase is commonly considered to be the most deleterious secondary phase precipitating in duplex stainless steels, as it results in an extreme reduction of corrosion resistance and toughness. Previous studies have mainly focused on the kinetics of sigma phase precipitation and influences on properties and only a few works have studied the morphology of sigma phase and its influences on material properties. Therefore, the influence of sigma phase morphology on the degradation of corrosion resistance and mechanical properties of 2507 super duplex stainless steel (SDSS) was studied after 10 h of arc heat treatment using optical and scanning electron microscopy, electron backscattered diffraction analysis, corrosion testing, and thermodynamic calculations. A stationary arc was applied on the 2507 SDSS disc mounted on a water-cooled chamber, producing a steady-state temperature gradient covering the entire temperature range from room temperature to the melting point. Sigma phase was the major intermetallic precipitating between 630 °C and 1010 °C and its morphology changed from blocky to fine coral-shaped with decreasing aging temperature. At the same time, the average thickness of the precipitates decreased from 2.9 µm to 0.5 µm. The chemical composition of sigma was similar to that predicted by thermodynamic calculations when formed at 800–900 °C, but deviated at higher and lower temperatures. The formation of blocky sigma phase introduced local strain in the bulk of the primary austenite grains. However, the local strain was most pronounced in the secondary austenite grains next to the coral-shaped sigma phase precipitating at lower temperatures. Microstructures with blocky and coral-shaped sigma phase particles were prone to develop microscale cracks and local corrosion, respectively. Local corrosion occurred primarily in ferrite and in secondary austenite, which was predicted by thermodynamic calculations to have a low pitting resistance equivalent. To conclude, the influence of sigma phase morphology on the degradation of properties was summarized in two diagrams as functions of the level of static load and the severity of the corrosive environment. Full article

In this study, the wear mechanisms and tribological performance of uncoated and coated carbide tools were investigated during the turning of super duplex stainless steel (SDSS)—Grade UNS S32750, known commercially as SAF 2507. The tool wear was evaluated throughout the cutting tests and the wear mechanisms were investigated using an Alicona Infinite Focus microscope and a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Tribo-film formation on the worn rake surface of the tool was analyzed using X-ray Photoelectron Spectroscopy (XPS). In addition, tribological performance was evaluated by studying chip characteristics such as thickness, compression ratio, shear angle, and undersurface morphology. Finally, surface integrity of the machined surface was investigated using the Alicona microscope to measure surface roughness and SEM to reveal the surface distortions created during the cutting process, combined with cutting force analyses. The results obtained showed that the predominant wear mechanisms are adhesion and chipping for all tools investigated and that the AlTiN coating system exhibited better performance in all aspects when compared with CVD TiCN + Al₂O₃ coated cutting insert and uncoated carbide insert; in particular, built-up edge formation was significantly reduced. Full article