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The aim of this paper was to compare duplex (DSS) and super duplex stainless steel processed by laser powder bed fusion (LPBF) based on the process parameters and microstructure–nanomechanical property relationships. Each alloy was investigated with respect to its feedstock powder characteristics. Optimum process parameters including scanning speed, laser power, beam diameter, laser energy density, and layer thickness were defined for each alloy, and near-fully dense parts (>99.9%) were produced. Microstructural analysis was performed via optical (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The samples were subjected to stress relief and high-temperature annealing. EBSD revealed the crystallographic orientation and quantified the phases in the as-built and annealed sample conditions. The as-built samples revealed a fully ferritic microstructure with a small amount of grain boundary austenite in the SDSS microstructure. High-temperature solution annealing resulted in the desired duplex microstructure for both alloys. There were no secondary phases present in the microstructure after both heat treatments. Nanoindentation generated nanomechanical (modulus) mapping grids and quantified the nanomechanical (both hardness and modulus) response; plasticity and stress relief were also assessed in all three conditions (as-built, stress-relieved, and annealed) in both DSS and SDSS. Austenite formation in the annealed condition contributed to lower hardness levels (~4.3–4.8 Gpa) and higher plastic deformation compared to the as-built (~5.7–6.3 Gpa) and stress-relieved conditions (~4.8–5.8 Gpa) for both alloys. SDSS featured a ~60% austenite volume fraction in its annealed and quenched microstructure, attributed to its higher nickel and nitrogen contents compared to DSS, which exhibited a ~30% austenite volume fraction.

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.

Approaches to determining ferrite fraction (%) and ferrite number (FN) were examined for super duplex stainless steel (SDSS) welds. A reference sample was produced by bead-on-plate gas–tungsten arc welding of a type-2507 SDSS plate. By comparing different etchants and measurement practices, it was realized that etching with modified Beraha followed by computerized image analysis (IA) was the most accurate and quickest technique to measure ferrite fraction, which determined the same ferrite fraction (68.0 ± 2.6%) as that measured by electron diffraction backscattered analysis (67.6 ± 2.3%). A Round Robin test was performed on a reference sample at University West, Swerea KIMAB, Outokumpu Stainless, and Sandvik Materials Technology to investigate the repeatability of the technique. The ferrite fraction measurements performed at different laboratories showed very small variations, which were in the range of those seen when changing microscope in the same laboratory. After verification of the technique, the relationship between ferrite fraction and ferrite number (measured with FERITSCOPE®) was determined using 14 single (root) pass welds, including butt, corner, and T-, V-, and double V-joint geometries. The best-fit equation found in this study was ferrite number (FN) = 1.1 × ferrite fraction (%). To conclude, the ferrite fraction technique suggested in the present paper was accurate and repeatable, which made it possible to determine a ferrite fraction–ferrite number formula for SDSS single-pass welds.

In this study, the energy deposited at the welding interface was controlled by changing the stand-off between the flyer and base plates. Pure titanium (TP 270C) and duplex stainless steel (SUS 821L1) were welded under 5- and 15-mm stand-offs, respectively. When the stand-off was 5 mm, the average wavelength and average amplitude of the welding interface were 271 and 61 μm, respectively; at 15 mm stand-off, the average wavelength and average amplitude of the welding interface were 690 and 192 μm, respectively. The differences between the two welding conditions were compared using a tensile test, fracture analysis, a 90° bending test, Vickers hardness, and nanoindentation related to the mechanical properties of materials. The experimental results indicated that the sample with a 5-mm stand-off had better mechanical properties.

The influence of heat input and multiple welding cycles on the microstructure of the heat-affected zone in autogenously TIG-welded 6 mm 2507 type super duplex stainless steel plates was investigated. In order to produce multiple thermal cycles, one to four pass bead-on-plate welds were made with arc energies of 0.47 and 1.08 kJ/mm, corresponding to heat inputs of 0.37 and 0.87 kJ/mm. Several thermocouples were attached to record thermal cycles on the front and back sides of the plates. Finite element modelling was successfully done to map and correlate measured and calculated peak temperatures. Only minor changes were seen in the ferrite content at 1 and 2 mm from the fusion boundary. Nitrides formed in all passes of the low heat input samples in a region next to the fusion boundary, but only after the third and fourth passes of the high heat input samples. Sigma phase precipitated only in a zone heated to a peak temperature in the range of approximately 828 to 1028 °C. Multiple reheating was found to promote precipitation of sigma phase relatively more than slower cooling. A precipitation free zone was observed between the nitride and sigma phase bands. The precipitation behaviour could be understood from equilibrium phase diagrams, evaluation of local thermal cycles and by correlating results from the modelling and measurements of peak temperatures. It is suggested that the peak temperature, the accumulated time in the critical temperature range between approximately 828 and 1028 °C, and the number of thermal cycles are the most relevant criteria when evaluating the risk of sigma phase formation.

Additive manufacturing (AM) is a rapidly growing field of technology. In order to increase the variety of metal alloys applicable for AM, selective laser melting (SLM) of duplex stainless steel 2205 powder and the resulting microstructure, density, mechanical properties, and corrosion resistance were investigated. An optimal set of processing parameters for producing high density (>99.9%) material was established. Various post-processing heat treatments were applied on the as-built predominantly ferritic material to achieve the desired dual-phase microstructure. Effects of annealing at temperatures of 950 °C, 1000 °C, 1050 °C, and 1100 °C on microstructure, crystallographic texture, and phase balance were examined. As a result of annealing, 40–46 vol.% of austenite phase was formed. Annealing decreased the high yield and tensile strength values of the as-built material, but significantly increased the ductility. Annealing also decreased the residual stresses in the material. Mechanical properties of the SLM-processed and heat-treated materials outperformed those of conventionally produced alloy counterparts. Using a scanning strategy with 66° rotation between layers decreased the strength of the crystallographic texture. Electrochemical cyclic potentiodynamic polarization testing in 0.6 M NaCl solution at room temperature showed that the heat treatment improved the pitting corrosion resistance of the as-built SLM-processed material.

Duplex stainless steels (DSSs) used in subsea structures and desalination industries require high corrosion and erosion resistance as well as excellent mechanical properties. The newly introduced cast duplex grade ASTM A890 7A has a unique composition and is expected to have a much better resistance to corrosion and erosion compared with the super-duplex grades 5A and 6A. This work is a comparative study of the mechanical properties, corrosion, and erosion-corrosion resistance of super-duplex grades 5A and 6A and the hyper-duplex grade 7A. The three DSSs exhibited equiaxial austenite islands in the ferrite matrix and balanced phase ratios. The hardness of the grade 7A was nearly 15% higher than those of the super-duplex grades, which is attributed to the effect of the higher contents of W and Mn in 7A. The impact toughness of grade 7A was found to be lower than those of the super-duplex grades due to the carbide precipitation resulting from the partial substitution of Mo with W. The oxide layer strengthening effect of rare earth elements and the higher pitting resistance equivalent number (PREN) of grade 7A resulted in higher corrosion resistance. The harder and more passive grade 7A showed a 35% lower material loss during erosion-corrosion.

Wire arc additive manufacturing (WAAM) direct energy deposition is used to process two different duplex stainless steels (DSS) wire chemistries. Macro- and micromechanical response variables relevant to industrialization are studied using a design of the experiment (DoE) approach. The tested operation window shows that the variation of layer height and over-thickness are highly correlated with travel speed and wire feed speed and positively correlated with heat input. The maximum achieved average instantaneous deposition rate is 3.54 kg/h. The use of wire G2205, which contains 5 wt% nickel content, results in a ferrite-to-austenite ratio that is equally balanced, while wire G2209, with 9 wt% nickel, provides a lower ferrite content. The spatial distribution of Fe% is influenced by part geometry and path planning, and higher heat inputs result in coarser microstructures. The manufacturing weaving strategy generates a heterogeneous microstructure characterized by fluctuations in Fe%. Thus, understanding the effect of complex thermal history, higher-dimensional design spaces, and uncertainty quantification is required to drive metal WAAM toward full industrialization.

Duplex stainless steels consisting of ferrite and austenite are widely used due to their excellent mechanical properties and corrosion resistance. Compared with ferritic stainless steels, duplex stainless steels have better plasticity, toughness, and welding performance. They also possess higher strength and better resistance to stress, pitting, and crevice corrosion than austenitic stainless steels. In addition to the above-mentioned properties, there are cost-saving advantages in duplex stainless steels due to their lower nickel content. Today, the types of duplex stainless steel are mainly divided into four categories: lean duplex stainless steel, standard duplex stainless steel, super duplex stainless steel, and hyper duplex stainless steel. Alloying design of duplex stainless steel is an important strategy to achieve high performance. In the last two decades, significant progress has been made in both theoretical calculations and experiments. By adjusting alloying elements such as chromium, nickel, molybdenum, nitrogen, copper, tungsten and rare earth, etc., the mechanical properties and/or corrosion resistance of the duplex stainless steels can be further improved. Summarizing the comprehensive progress of alloying design of duplex stainless steel is of great significance in providing a data basis for establishing the corresponding relationship between chemical compositions and properties. Therefore, this paper reveals the specific roles of alloying elements in the duplex stainless steels and provides a reference for alloying design with different performance requirements.

The influence of surface grinding and microstructure on chloride induced stress corrosion cracking (SCC) behavior of 2304 duplex stainless steel has been investigated. Grinding operations were performed both parallel and perpendicular to the rolling direction of the material. SCC tests were conducted in boiling magnesium chloride according to ASTM G36; specimens were exposed both without external loading and with varied levels of four-point bend loading. Residual stresses were measured on selected specimens before and after exposure using the X-ray diffraction technique. In addition, in-situ surface stress measurements subjected to four-point bend loading were performed to evaluate the deviation between the actual applied loading and the calculated values according to ASTM G39. Micro-cracks, initiated by grinding induced surface tensile residual stresses, were observed for all the ground specimens but not on the as-delivered surfaces. Loading transverse to the rolling direction of the material increased the susceptibility to chloride induced SCC. Grinding induced tensile residual stresses and micro-notches in the as-ground surface topography were also detrimental.