Duplex Alloy Research Articles

Effect of Powder Reuse on Powder Characteristics and Properties of DED Laser Beam Metal Additive Manufacturing Process with Stellite® 21 and UNS S32750

In this work, the influence of powder reuse up to three times on directed energy deposition (DED) with laser processing has been studied. The work was carried out on two different gas atomized powders: a cobalt-based alloy type Stellite® 21, and a super duplex stainless steel type UNS S32750. One of the main findings is the influence of oxygen content of the reused powder particles on the final quality and densification of the deposited material and the powder catch efficiency of the laser deposition process. There is a direct relationship between a higher surface oxidation of the particles and the presence of oxygen content in the particles and in the as-built materials, as well as oxides, balance of phases (in the case of the super duplex alloy), pores and defects at the micro level in the laser-deposited material, as well as a decrease in the amount of material that actually melts, reducing powder catch efficiency (more than 12% in the worst case scenario) and the initial bead geometry (height and width) that was obtained for the same process parameters when the virgin powder was used (without oxidation and with original morphology of the powder particles). This causes some melting faults, oxides and formation of undesired oxide compounds in the microstructure, and un-balance of phases particularly in the super duplex stainless steel material, reducing the amount of ferrite from 50.1% to 37.4%, affecting in turn material soundness and its mechanical properties, particularly the hardness. However, the Stellite® 21 alloy type can be reused up to three times, while the super duplex can be reused only once without any major influence of the particles’ surface oxidation on the deposited material quality and hardness.

Effects of Mo and C addition on the microstructure and corrosion behavior of FeNiCrAl duplex alloy

Effects of Mo and C addition on the microstructural evolution and corrosion resistance in chloride solution of FeNiCrAl duplex alloy were investigated. The results showed that α phase stabilizing element Mo increased the α/γ volume ratio and γ phase stabilizing element C decreased its ratio in FeNiCrAl alloy, and the chemical composition of α and γ phases was changed by Mo and C addition, which affected the corrosion resistance of FeNiCrAl alloy. Pitting corrosion mainly occurred in α phase around NbC precipitated at α/γ interface due to the micro-galvanic corrosion (MGC) effect between α (anode), γ (cathode) and NbC (cathode) phases. The pitting corrosion resistance was improved by Mo and C addition, resulting from that Mo and C enhanced the passivity of passive films and decreased the MGC effect between α, γ and NbC phases.

Creep deformation behavior and microstructure of α-Mg/long-period stacking ordered (LPSO) duplex Mg-Y-Zn alloy prepared by hot extrusion and heat treatment

There exists a great demand for improving the creep resistance of α-Mg/long-period stacking ordered (LPSO) duplex alloys. In this study, microstructure control through hot extrusion and heat treatment was employed on Mg-Y-Zn duplex alloys with the aim of achieving superior creep resistance. The extruded Mg-Y-Zn alloy exhibited an enhancement in the creep resistance with increasing heat treatment temperature. Furthermore, the heat-treated samples demonstrated outstanding creep resistance even under high stress. Their creep behavior is characterized by a low minimum creep rate and a relatively small stress exponent (n ∼ 5) even at high stress. Microstructural observations revealed that heat treatment induces grain coarsening and an increase in solute concentration within the recrystallized grains. Additionally, during creep deformation, a plate-like LPSO phase and kink bands formed within the recrystallized grains. Therefore, it is affirmed that the exceptional creep resistance observed in the Mg-Y-Zn alloys in this study can be attributed to the microstructure evolution induced by heat treatment and creep deformation.

Crafting high-strength and ductile powder metallurgy Ti6Al4V alloy with a multi-scale microstructure

In this work, a novel multi-scale microstructure of Ti6Al4V duplex alloy with nano-sized β particles was designed based on powder metallurgy. The common lamellar structure was carefully adjusted to reveal the complex effects of microstructure on mechanical properties. The ideal microstructure transforms into a composition dominated by sparse equiaxed primary α, lots of β-nanoprecipitates, and traces of secondary acicular α′ by heat treatment. The uniform nucleation of nano-sized β particles, driven by intragranular element concentration difference and local distortions, is a pivotal reason for high yield strength by dispersion strengthening. More phase boundaries also act as sustainable sources for geometrically necessary dislocations. An uncoordinated phase interface relationship from variant selection improves strain-hardening ability. The intragranular misorientation causes the value inhomogeneity of the Schmid factor in primary α, resulting in an auxiliary slip effect that benefits ductility. Therefore, the fabricated Ti6Al4V alloy displays an ultrahigh ultimate tensile strength of 1329 MPa, yield strength of 1223 MPa, and reasonably large elongation of 8.5 %, respectively. This work underscores the intricate interplay of microstructure evolution and deformation mechanisms in powder metallurgy duplex titanium alloys, offering valuable insights into enhancing the mechanical properties.

A Cu-Containing High-Entropy Alloy with High Corrosion Resistance and Low Surface Electrical Resistance

Cu-base alloys with high corrosion resistance and low surface electrical resistance in corrosive environments were studied for application to electrical connectors. To achieve both high corrosion resistance and low surface electrical resistance, duplex alloys were fabricated.Non-equiatomic CuSnZnAlNi high-entropy alloys were prepared by induction melting. As raw materials, Cu-Zn alloy and pure metals of Sn, Al, and Ni were used. After casting, heat treatment was performed at 500 ℃ for 5 h (water-quenching). After that, the specimen surfaces were polished with a diamond paste down to 1 µm. The microstructure of the specimens was observed using an optical microscope and a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy system (EDS). Electrochemical measurements were performed in synthetic seawater (pH 8.2) at 25 ℃. Dip-dry corrosion test was conducted. The specimens were dipped in synthetic seawater (pH 8.2) for 3.5 h and dried at 25 ℃ and 30 %RH for 0.5 h. Before and after the dip-dry corrosion test, surface electrical resistance was measured.Cu36Sn16Zn16Al16Ni16 was prepared, and the phases were identified by SEM/EDS. As a result, the alloys were separated into four phases. The corrosion resistance of each phase was compared by SEM/EDS between the specimens before polarization and the specimens after polarization and removal of corrosion products. 20 cycles of dip-dry corrosion test was conducted, with one cycle being the combination of immersion of the specimen in the solution and drying.

Novel Powder Feedstock towards Microstructure Engineering in Laser Powder Bed Fusion: A Case Study on Duplex/Super Duplex and Austenitic Stainless-Steel Alloys

Additive manufacturing of Duplex Stainless Steels (DSS) and Super Duplex Stainless Steels (SDSS) has been successfully demonstrated using LPBF in recent years, however, both alloys feature an almost fully ferritic microstructure in the as-built condition due to the fast cooling rates associated with the Laser Powder Bed Fusion (LPBF) process. Blends of DSS and SDSS powders were formulated with austenitic stainless-steel 316L powder, aiming to achieve increased austenite formation during in the LPBF as-built condition to potentially minimize the post heat treatments (solution annealing and quenching). Powder characteristics were investigated and process parameters were optimized to produce near fully dense parts. Nanoindentation (NI) tests were conducted to measure, not only the local mechanical properties and correlate them with the as-built microstructure, but also to gain a deeper understanding in the deformation behavior of individual phases that cannot be studied directly by macroscopic tensile tests. Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) were employed for microstructural analysis and phase quantification. The microstructural analysis and EBSD phase maps revealed an increase in austenite in the as-built microstructures. Blend 1 resulted in a duplex microstructure consisting of 10% austenite at the XY plane and 20% austenite at the XZ plane. The austenite content increased with increasing proportion of 316L stainless steel in the powder blends. The DSS blend required a much higher volumetric energy density for the fabrication of near fully dense parts. This imposed a slower solidification and a higher melt pool homogeneity, allowing for adequate diffusion of the austenite stabilizing elements. The presented workflow and findings from this study provide valuable insights into powder mixing for the development of custom alloys for rapid material screening in LPBF.

Passivation effects on corrosion and cavitation erosion resistance of UNS S32205 duplex alloy in 3.5% NaCl

ABSTRACT The electrochemical and electronic properties of UNS S32205 duplex alloy passive film were studied with EIS and Mott–Schottky techniques, respectively. The role of Fe and Cr oxides in the passive film formation was checked using XPS. The cavitation behaviour was evaluated using an optical microscope OM. The studies were carried out in a 3.5% NaCl solution saturated with CO2 on various types of passive films: the air-formed (AF), the long-term aged (LTA), and the passivated (P1 and P2). The results showed that the electrochemical passivation treatment at passive region 1 improved the corrosion and cavitation behaviour compared to AF, while the long-term aging process made it worse. Passivation at passive region 2 was harmful and recorded the worst results under corrosion and cavitation erosion conditions. Thus, exposing the equipment made of this alloy to the passivation treatment at special conditions makes it more resistant to corrosion and cavitation erosion.

Duplex and graded austenitic-to-martensitic steels by powder metallurgy: Interface diffusion and strength composite effect

Versatility of powder metallurgy was used to design new duplex and compositionally graded steels. Rapid consolidation by Spark Plasma Sintering (SPS) and longer consolidation by Hot Isostatic Pressing (HIP) were applied on two austenitic 316L and martensitic Fe-9Cr powders, either homogeneously blended or uniaxially graded before sintering.The microstructural investigations by micro-hardness, SEM, EDX and EBSD showed a continuous crystallographic structure at the austenite–martensite interface and a similar grain size in SPS and HIP samples, whereas a larger interdiffusion length of Cr and Ni was observed in the HIP sample, as confirmed by diffusion calculations. The tensile behaviour of materials could not be described by a simple law of mixtures. To better understand this phenomenon, the obtained materials were described as a composite and iso-strain and iso-stress models were used and discussed. The microstructural characterizations show that during the consolidation, diffusion of the chemical species modifies the nature and the respective fractions of phases, which explains the discrepancy between the models and the experimental mechanical behaviour of the duplex alloys.

Integrated effect of aging and heavy ion radiation on FeNiCrAl duplex alloy for accident-tolerant fuel cladding

Alumina-forming duplex FeNiCrAl alloy with superior mechanical properties and high-temperature oxidation resistance has been regarded as an accident-tolerant fuel cladding material. This work investigated the effect of aging and the radiation resistance of FeNiCrAl duplex alloy after up to 2000 hours thermal aging and 3.5 MeV Fe13+ ion radiation. The ferrite phase containing B2 phase shows much fewer radiation defects and lower radiation hardening rate than conventional FeCrAl alloy. The coherent B2 phase precipitate is critical in hindering dislocation and inhibiting the formation of radiation induced defects which is proved via experiment and molecular dynamics simulation. Besides, the Al depletion of B2 phase and nano-sized precipitation are characterized by using Time-of-Flight Secondary Ion Mass Spectrometry and Atom Probe Tomography. These findings effectively illuminate microstructure evolution of FeNiCrAl duplex alloy after long-term aging and radiation and lead to new insight for designing advanced accident-tolerant fuel material.

Effect of Nb content on the microstructure and corrosion resistance of FeCoCrNiNbx high-entropy alloys in chloride ion environment

In this paper, FeCoCrNiNbx high-entropy alloys were prepared by vacuum arc melting, and their microstructure and corrosion resistance in a chloride ion environment were investigated. The results indicated that FeCoCrNi alloy was a single FCC solid solution and FeCoCrNiNbx alloy was a duplex alloy with FCC phase and Laves phase. FeCoCrNiNb0.15 showed the best corrosion resistance. The corrosion current density of FeCoCrNiNb0.15 decreased by one order compared with 316LSS, and the corrosion rate after the immersion experiment was only one-third of 316LSS. The P-N junction improved the corrosion resistance of the passive film of FeCoCrNiNb0.15, while the passive film of FeCoCrNi mainly exhibited n-type semiconductor characteristics with less obvious p-type semiconductor characteristics.

Phase-selective recrystallization makes eutectic high-entropy alloys ultra-ductile

Excellent ductility is crucial not only for shaping but also for strengthening metals and alloys. The ever most widely used eutectic alloys are suffering from the limited ductility and losing competitiveness among advanced structural materials. Here we report a distinctive concept of phase-selective recrystallization to overcome this challenge for eutectic alloys by triggering the strain hardening capacity of the duplex phases completely. We manipulate the strain partitioning behavior of the two phases in a eutectic high-entropy alloy (EHEA) to obtain the phase-selectively recrystallized microstructure with a fully recrystallized soft phase embedded in the skeleton of a hard phase. The resulting microstructure fully releases the strain hardening capacity in EHEA by eliminating the weak boundaries. Our phase-selectively recrystallized EHEA achieves a high ductility of ∼35% uniform elongation with true stress of ∼2 GPa. This concept is universal for various duplex alloys with soft and hard phases and opens new frontiers for traditional eutectic alloys as high-strength metallic materials.

Understanding of the duplex high entropy of FeCoNiCuAl alloys magnetic properties dependence on the CuAl amount and annealing temperature

High entropy alloys (HEA) FeCoNi(CuAl)x=0.8 and FeCoNi(CuAl)x =1.6 were studied in the form of small arc melted samples (x = 0.8) and also in successfully obtained hot forged strips (x = 1.6). High annealing temperature of 1200 °C was applied to the alloys. The magnetic properties as saturation magnetization (Ms), magnetostriction, coercive field (Hc) and remanent induction (Br) as well as electrical resistivity were measured and correlated to the alloy’s microstructure. The thermomechanical process due to hot rolling as well as the annealing at high temperature kept the FCC phase as the majority phase in the duplex alloy and caused changes in the magnetic properties. The unexpected Ms improvement of the alloy with x = 1.6 compared to the one with x = 0.8 was attributed to the Cu segregation originating Cu rich nanoprecipitates, which in turn cause the increase of magnetic elements in the duplex phases. The FeCoNi(CuAl)x=1.6 strip has Hc = 260 A/m, Br = 0.162 T, an initial susceptibility (χin) of 240 and a resistivity of 100 μΩ.cm. The alloy had smaller Hc and higher χin values compared to other FeCoNi(CuAl)x alloys with 0 ≤ x ≤ 1.2.

Corrosion behaviour of lean duplex stainless steel in advanced oxidation process (AOP) based wastewater treatment plants

The corrosion of lean duplex stainless steel alloys is examined when applied as a construction material in advanced oxidation processes. Both electrochemical and immersion experiments have been carried out when subjecting wastewater to ozone or Fenton oxidation. The electrochemical experiments suggest that the presence of dissolved ozone at the levels tested does not result in an increased pitting susceptibility for none of the alloys included in the research. However, the application of Fenton reagents induces pitting corrosion to the studied lean duplex alloys. The immersion experiments highlight that crevice corrosion occurs during wastewater treatment with both ozone and Fenton reagents.

Evaluation of impact characteristics of reduced alloy duplex stainless steel sheet

SUS821L1 has been effectively used on bridges and land rocks. However, it has been anticipated that these structures may experience impact loading from earthquakes and tsunamis. It is required to evaluate the mechanical properties of SUS821L1 by impact testing because impact loading yields different mechanical properties than static loading. In this study, a falling drop impact tester with the split-Hopkinson bar method is used to evaluate the impact parameters of SUS821L1. The impact tensile and the static tensile test have demonstrated that the maximum stress is 100MPa greater than the static tensile test and the maximum strain is 3% greater than the impact tensile test.

Duplex Stainless Steels—Alloys for the 21st Century

Duplex stainless steels were first manufactured early in the 20th century, but it was the introduction in the 1970s of the argon-oxygen decarburisation (AOD) steel making process and the addition of nitrogen to these steels, that made the alloys stronger, more weldable and more corrosion resistant. Today, duplex stainless steels can be categorised into four main groups, i.e., “lean”, “standard”, “super”, and “hyper” duplex types. These groups cover a range of compositions and properties, but they all have in common a microstructure consisting of roughly equal proportions of austenite and ferrite, high strength, good toughness and good corrosion resistance, especially to stress corrosion cracking (SCC) compared with similar austenitic stainless steels. Moreover, the development of a duplex stainless-steel microstructure requires lower levels of nickel in the composition than for a corresponding austenitic stainless steel with comparable pitting and crevice corrosion resistance, hence they cost less. This makes duplex stainless steels a very versatile and attractive group of alloys both commercially and technically. There are applications where duplex grades can be used as lower cost through-life options, in preference to coated carbon steels, a range of other stainless steels, and in some cases nickel alloys. This cost benefit is further emphasised if the design engineer can use the higher strength of duplex grades to construct vessels and pipework of lower wall thickness than would be the case if an austenitic grade or nickel alloy was being used. Hence, we find duplex stainless steels are widely used in many industries. In this paper their use in three industrial applications is reviewed, namely marine, heat exchangers, and the chemical and process industries. The corrosion resistance in the relevant fluids is discussed and some case histories highlight both successes and potential problems with duplex alloys in these industries. The paper shows how duplex stainless steels can provide cost-effective solutions in corrosive environments, and why they will be a standard corrosion resistant alloy (CRA) for many industries through the 21st century.

Mechanical Response of Stainless Steels at Low Strain Rate

The deformation behavior of the selected stainless steels (UNS S31603, UNS S32205, UNS S32550 and UNS S32760) was evaluated at strain rates ranging between 10-2 and 10-7 s-1. Microstructural evaluation was conducted using optical and scanning electron microscopy, electron backscattered diffraction and x-ray diffraction. The total strain at fracture remained approximately constant as the strain rate increased from 10-7 to 10-3 s-1. Further increase in strain rate above 10-3 s-1 resulted in relatively lower strain. The strain rate sensitivity of each steel decreased with increasing strain. Also, the strain hardening rate and exponent of the alloys increased with decreasing strain rate. Post-deformation microstructure evaluation on UNS S31603 specimens showed slip, twinning and martensitic transformation, while the duplex alloys exhibited slip, twinning with less noticeable martensitic transformation. Fractographic examination after tensile test indicates an increase in dimple diameter with strain rate and subsequent decrease in dimple density.

Multi-heterostructure and mechanical properties of N-doped FeMnCoCr high entropy alloy

High-entropy alloys (HEAs) have been extensively studied in recent years. However, yield strength of HEAs in which austenite is the dominating phase is usually low, far from satisfying the engineering demands. Improving performance-cost ratio of such alloys will help for their practical structural applications. Here we report a novel strategy to produce ultrastrong, tough, and low-cost HEAs, in which heavy nitrogen-doping (2.6 at.%) was applied to an inexpensive metastable FeMnCoCr HEA. Coupled with simple thermomechanical processing, we produced a multi-heterostructure, which consisted of fine α-martensite laths, deformed austenite with dense dislocations, recrystallized ultrafine grains and nano-nitride precipitates. Our novel FeMnCoCrN HEA exhibits a high yield strength of 1310 MPa which is ~5.2 times stronger than its base alloy without nitrogen doping. In particular, the highly dislocated body-centered cubic (bcc) martensite laths formed in the austenitic deformation matrix has an unexpected area fraction up to 24%. The hetero-deformation induced strengthening then reaches 750 MPa at the yield point, leading to a remarkable yield strength elevation of the material. Moreover, the high nitrogen content changes the dominant deformation mechanism from martensitic transformation to twinning, which contributes to a satisfactory uniform elongation of 16.5%, while the material is further strengthened by the dynamically refined microstructure. The high-nitrogen duplex alloy design strategy developed here provides a new paradigm for developing high-performance fcc HEAs.

Recent advances in the kinetics of normal/abnormal grain growth: a review

Recent progress in the kinetics of grain coarsening and abnormal grain growth (AGG) is presented in this overview article. The factors affecting the kinetics of grain growth is reviewed with the emphasis on the recent findings on the solute drag and Zener pinning effects as well as the special case of duplex alloys, where the latter is discussed for the behavior of dual-phase steels during intercritical annealing. The common isothermal kinetics models for grain growth are listed, which is followed by the critical discussion on the simplifications and the commonly used methods for the determination of grain growth exponent (n) and activation energy (Q). The obtained values of n and Q for several classes of important engineering alloys such as microalloyed steels, stainless steels, magnesium alloys, aluminum alloys, titanium alloys, and high-entropy alloys are summarized with the discussion on the obtained values of kinetics parameters and their deviation from the theoretical expectations. Finally, the factors leading to AGG (such as the coarsening and dissolution of pinning particles and the crystallographic texture), the proposed mechanisms (such as the solid-state wetting and the grain boundary faceting/defaceting phenomena), and the kinetics of AGG (based on the empirical power law and the similarity of AGG to primary recrystallization in the form of secondary recrystallization) are reviewed. This overview can shed light on the understanding of grain growth and its effects.

Corrosion Risk and Repassivation of Duplex Stainless Steel UNS S82551 in Treated Seawater Injection Service

In seawater injection wells, the available well tubing materials are generally low alloy steel, glass-reinforced epoxy-lined low alloy steel or corrosion-resistant alloys such as super duplex stainless steel. However, in treated seawater the corrosion risk can be controlled and lower-grade alloys (low alloy steel) can be considered. But for long well lifetime designs (20 y plus), then low alloy steel tubing can be challenged. In this respect, recent efforts have focused attention on better dissolved oxygen control which permits the investigation and on the possible use of more cost-effective materials such as the duplex stainless steels UNS S82551 and UNS S82541 (the latter is a higher strength version, but same PRENw). Full-scale testing of tubes joined together with a proprietary premium threaded connection (pipe-coupling premium connection [PCPC] couplings) was performed in controlled seawater loops simulating service conditions at 30°C. The flow rate and dissolved oxygen were controlled at 5 m/s and <20 ppb, respectively. Weekly dissolved oxygen excursions corresponding to 24 h at 100 ppb followed by 1 h at 300 ppb were performed during the 5 months exposure. Corrosion results of UNS S82551/S82541 tubing were compared to UNS S31803 and UNS S39274. In parallel, laboratory exposures of creviced coupons for parametric study were performed in dissolved oxygen-controlled cells, allowing the measurement of electrochemical potentials as function of dissolved oxygen content and the related corrosion resistance. The results showed that dissolved oxygen content should be properly controlled below critical values to avoid crevice corrosion of the lesser alloyed duplex stainless steels. The ability of UNS S82551/S82541 to recover or repassivate after prolonged exposures to high dissolved oxygen concentrations (DOC) was also determined with both the use of full-sized PCPC test cells and electrochemical testing involving a remote crevice assembly. The repassivation potential was investigated after different active crevice corrosion durations. The results of the study allowed to precisely define the limits of use of UNS S82551/S82541 in treated seawater, i.e., the critical DOC conditions for corrosion initiation and for repassivation of UNS S82551/S82541. For all tested conditions, the UNS S82551/S82541 showed a rather good ability to repassivation when normal service conditions (i.e., low dissolved oxygen) are recovered.

A Review and Analysis of Industrial Valve Material Failures Due to Corrosion and Proposals for Prevention Measures Based on Industrial Experiences in the Offshore Sector of the Oil and Gas Industry

Valve failure is a big risk and a costly phenomenon in the offshore sector of the oil and gas industry. It results in severe negative consequences such as loss of asset; loss of production due to plant shutdown; health, safety and environmental (HSE) issues such as hydrocarbon (oil and gas) spillage; environmental pollution; and sometimes loss of human life. Different types of valve failures have occurred in the Norwegian offshore industry for various reasons such as poor material selection, corrosion, mechanical failure of valve components due to high stresses and loads, lack of coating and lack of inspection. The aim of this paper is to review and critique some of the wrong material selection approaches for industrial valves in the offshore sector of the oil and gas industry that lead to the corrosion and failure of valves, and to provide an appropriate material selection solution to mitigate these failures. Seven cases of material failures due to corrosion, including a valve gear box in coated cast iron, bolting made of low alloy steel with hot dip galvanized coating, a stem key made of a low alloy steel, stem material in 17-4 PH, spring material in Inconel X750, duplex, super duplex and hard nickel alloys in subsea as well as graphite packing in seawater, are reviewed, and preventive measures are proposed to avoid corrosion risk. In general, the use of carbon and low alloy steel for valves in the offshore environment should be limited and the use of corrosion-resistant alloys (CRAs) is proposed.