Cast Duplex Stainless Steel Research Articles

Microstructure and corrosion resistance of Z3CN20.09M stainless steels after different thermo-mechanical processing

Thermo-mechanical process was used to control the microstructure of a nuclear grade cast duplex stainless steel Z3CN20.09M. It is revealed that heat treatment alone cannot change the casting microstructure of coarse columnar grains. After a low-strain deformation (7% tension) and annealing at 1180 °C, a high fraction of low-∑ CSL boundaries, about 76%, were formed due to the strain induced boundary migration. After higher deformation (>10%) and annealing, recrystallization microstructure with fine equiaxed grains could be formed. The electrochemical tests in lithium borate aqueous solution showed that a high fraction of low-∑ CSL boundaries and fine grains after the thermo-mechanical processing improved the corrosion resistance of the Z3CN20.09M.

Effect of Isothermal Holding at 750 °C and 900 °C on Microstructure and Properties of Cast Duplex Stainless Steel Containing 24% Cr-5% Ni-2.5% Mo-2.5% Cu

Changes in the microstructure and selected mechanical properties of two-phase ferritic-austenitic cast steel containing 24% Cr-5% Ni-2.5% Mo-2.5% Cu after isothermal holding at 750 °C and 900 °C are presented. The choice of the two temperatures of isothermal holding was dictated by the precipitation of brittle phases within a range of 600 °C-950 °C, while the holding time depended on the casting cooling time in the mould. Changes in the microstructure were studied by the SEM-EDS and XRD techniques. As a result of the decomposition of the eutectoid ferrite, a σ phase that was rich in Cr, Mo, and Ni and a secondary γ2 austenite with Widmannstätten morphology were formed. Compared to the austenite, the chemical composition of the secondary γ2 austenite showed depletion of Cr and Mo. In the ferrite, the presence of Cr2N nitrides was also detected. After a holding time of 3 h at 900 °C, these phases increased the hardness of the tested cast steel to approximately 275 HV10. At the same time, the UTS value was recorded to decrease with the increasing temperature based on the tensile test results. At 750 °C, the value of UTS was 250 MPa for 1 h of holding and 345 MPa for 3 h of holding. These values decreased after increasing the temperature to 900 °C and amounted to 139 for 1 h holding and 127 MPa for 3 h holding. It was also found that the elongation values at 750 °C ranged from 7-10%, while they amounted to 35-37% at 900 °C. A fracture analysis of the tested cast steel showed that in the prevailing part, the fractures were made of ductile nature with an arrangement of dimples that is typical for this type of fracture. Non-metallic inclusions that are typical for cast steel (i.e., oxides and nitrides) were also found in the area of the fractures.

Corrosion Resistance of Cast Duplex Stainless Steels Grades ASTM A890 1B and 3A in Electrolytes Containing Chloride, Carbon Dioxide and Thiosulfate

Corrosion Resistance of Cast Duplex Stainless Steels Grades ASTM A890 1B and 3A in Electrolytes Containing Chloride, Carbon Dioxide and Thiosulfate

Cast structure and properties of duplex stainless steels

Currently, duplex stainless steels are increasingly used in industry. Austenite and ferrite in these steels are in approximately equal proportions. During manufacture of cast products from these steels, a chemical and structural heterogeneity is formed in the castings, for the elimination of which heat treatment is carried out. In practice, within the framework of one class or even one steel grade, the chemical composition and, as a consequence, the phase ratio can vary over a wide range without reaching their optimal values. In this paper, the authors investigated the influence of chemical composition and solidification conditions on the structure and properties of cast duplex stainless steels and developed thermodynamic criteria for the selection of casting alloys, taking into account the temperature of beginning of the polymorphic transformation of δ-ferrite into austenite and the average equilibrium rate of this transformation. It was found that in the studied steels with 21 – 26 % of chromium, crystallization proceeds with the formation of δ-ferrite dendrites, and austenite is formed in the solid metal at the places of the former interdendrite spaces. It is shown that at cooling rates, which are realized in practice when obtaining, for example, casings of centrifugal pumps or other products of a similar size, the transformation of δ-ferrite into austenite is practically suppressed when the temperature reaches 1180 – 1200 °C. On the basis of this, a criterion for the development of compositions with the required phase ratio without heat treatment was proposed. The evolution of the structure during heat treatment at temperatures of 1050 – 1250 °C was studied and it is shown how by choosing the optimal temperature of annealing and quenching, depending on the actual chemical composition of steel, it is possible to achieve an acceptable level of pitting potential in steel with a lower alloying, and vice versa, non-optimal heat treatment of a high-doped alloy leads to a catastrophic decrease in corrosion resistance. It is shown that in the steels under consideration optimal properties are achieved at 70 % of δ-ferrite.

A duplex stainless steel (DSS) with striking tensile strength and corrosion resistance produced through wire arc-additive manufacturing (WAAM) using a newly developed flux-cored wire

The austenite ratio in the WAAM DSSs using commercial welding wire is generally higher than the acceptable range. In this work, a new flux-cored wire was especially developed for the WAAM of DSS. The chromium equivalent (Creq) and pitting resistance equivalent number (PREN) of the newly developed flux-cored wire arc additively manufactured duplex stainless steel (FCWA-AM DSS) were significantly high. Its austenite ratio was successfully modified to the acceptable range and massive scattered intragranular austenite was found in the columnar ferrite grains. Such chemical composition and microstructure promised it with good strength and corrosion resistance. Its tensile strength was far beyond the requirements of the rolled and cast DSSs and its corrosion resistance was comparable to the hot-rolled 2205 DSS.

The microstructure and mechanical properties of duplex stainless steel components fabricated via flux-cored wire arc-additive manufacturing

In this research, the wire arc additive manufacturing (WAAM) of duplex stainless steel (DSS) was firstly conducted using flux-cored wires as the feedstock material. Homogeneous microstructure was obtained in the WAAM DSS component fabricated using the commercial E2209 welding wire as the filler material. However, the average austenite content reached up to 74%, which exceeded the standard limitation. Electron backscatter diffraction (EBSD) measurements indicated that the austenite phase exhibited strong 〈101〉//Z texture, and the ferrite phase exhibited strong 〈001〉//Z texture. Evaluations of the mechanical properties revealed that the strength of the WAAM DSS component fulfilled the standard requirements of the cast DSS components, and its impact toughness at −40 °C exceeded that of commercial hot rolled LDX 2101 DSS plate. To modify the austenite content in the as-deposited DSS, two novel flux-cored wires were designed and produced for the flux-cored wire arc-additive manufacturing (FCWA-AM) process. These materials allowed the reduction of the austenite ratios to 37% and 45% in the as-deposited DSS components. The results of this work demonstrated that the FCWA-AM technique is an applicable WAAM method for producing DSS components with desired microstructures and good mechanical properties.

On the Corrosion behavior of 4A and 5A cast duplex stainless steel under different heat treatment conditions

In this analysis, corrosion efficiency and mechanical properties of duplex stainless steel castings (DSS-4A and DSS-5A) were compared to their wrought counterparts. Cast duplex stainless steel DSS-4A and DSS-5A are characterized in two separate solution treated settings, while their wrought counterparts are characterized in both solutions treated and forged form. Optical microscopy and X-ray diffractometry were employed to perform the metallurgical characterization. Electrochemical techniques such as Tafel extrapolation and electrochemical impedance spectroscopy were used to investigate corrosion activity. An electrochemical workstation was used to perform the corrosion studies. To compare the stability and changes in corrosion properties of both grades, an ASTM standard B117 salt spray test was performed in a 3.5 percent NaCl medium. Without any intermetallic phases, the XRD showed austenite and ferrite phases. The elements were partitioned further into the phases that they promoted, according to the elemental study of the phases. The ultimate tensile strength and hardness values of DSS-5A are higher than those of DSS-4A. DSS-5A had a higher corrosion resistance than DSS-4A. When comparing the two media used (H2SO4 and NaCl), DSS-5A demonstrated superior corrosion resistance.

Effect of thermal aging on the mechanical, intergranular corrosion and corrosion fatigue properties of Z3CN20.09M cast duplex stainless steel

The effect of thermal aging at 475 °C and 750 °C of Z3CN20.09M cast duplex stainless steel (CDSS) on microstructure, mechanical and intergranular corrosion properties were investigated by transmission electron microscope (TEM), nano indenter, scanning electron microscope (SEM) and corrosion fatigue test system. The result indicated that the spinodal decomposition and G precipitated were occurred after aged at 475 °C, as well as sigma precipitated at 750 °C. The microstructure degeneration of ferrite was saturated after aged for 2000h and 200 h at 475 °C and 750 °C respectively. The mechanical properties, intergranular corrosion resistance and corrosion fatigue lives were continuing deteriorated with increasing the aging time at both temperatures. The difference of the degeneration mechanisms of Z3CN20.09M CDSS aged at 475 °C and 750 °C was analyzed.

Fatigue failure mechanism and life prediction of a cast duplex stainless steel after thermal aging

Pressurized piping systems in nuclear power plants made of cast duplex stainless steels (CDSSs) are subjected to thermal aging and fatigue. Low cycle fatigue tests on a nuclear grade CDSS revealed prolonged fatigue lives after thermal aging at 400 °C up to 10000 h. The underlying mechanism is mainly associated with the reduced plastic strain amplitude, which is presumably a result of thermal aging induced ferrite embrittlement and strengthening of austenite. A life prediction model was proposed with austenite microhardness incorporated into the Basquin-Coffin-Manson equation as a variable to reflect the effect of thermal aging.

Effect of heat treatment on the microstructure and corrosion properties of cast duplex stainless steels

The effect of the quenching temperature on the pitting corrosion resistance of lean duplex stainless steel (DSS) were examined. Using thermodynamic modeling of phase formation processes in steel during solidification and subsequent cooling was shown that the equal amount of austenite and ferrite is achieved at 1210°С for examined composition. Experimental steel samples were quenched from 1100 and 1200°С. It was found that as the temperature rises, the pitting potential increases significantly due to the achievement of a favorable phase ratio. The results of this experimental study made it possible to develop a heat treatment that provides high corrosion properties of lean DSS.

Microstructural Evolution and Mechanical Behavior of Thermally Aged Cast Duplex Stainless Steel.

The microstructural evolution and mechanical behavior of cast duplex stainless steels (CDSSs) at 400 °C for different thermal aging times were investigated by transmission electron microscope (TEM) and small punch test (SPT). The results showed that the spinodal decomposition in ferrite was the main reason for the decrease in toughness, and G-phase did not play an important role in the embrittlement process. The change of membrane stretching zone (Wm) played an important role in the SPT load-displacement curve before and after thermal aging. During the deformation process of Wm in the SPT, for thermal aging for 10,000 h, some completely curved slip bands were generated inside the ferrite phase, which had no contact with the δ/γ phase interface and belonged to the slip bands produced by the independent deformation of ferrite. The combined effect of the curved slip bands and stress concentration led to the initiation of obvious micro-cracks at the δ/γ phase interface. The micro-cracks propagated along the ferrite phase curved slip bands, and eventually penetrated the entire hardened ferrite phase.

Experimental and Numerical Evaluation on Deformation and Fracture Mechanism of Cast Duplex Stainless Steel Tubular Specimen

The deformation behavior and fracture mechanism of cast duplex stainless steel tubular specimens under different tensile stages were investigated through experimental and numerical evaluation. The results showed that the axial stress was redistributed due to the necking of the tubular specimen, the axial stress near the internal wall was larger than those near the external wall, and its maximum axial stress was distributed between the internal wall and the center of the wall thickness. Microcracks and voids were initiated under the maximum shear stress along the δ/γ phase interface and propagated to the ferrite interior. The voids were connected and merged into the main crack through the propagation of the microcracks. Moreover, the main crack first propagated to the internal wall and then rapidly propagated to the external wall. The fracture morphology can be divided into three types: shear lip zones that can be found on both the internal and external walls, and shear lip zones that can be found on either only the internal wall or the external wall.

Phase Stress Measurement of Centrifugally Cast Duplex Stainless Steel by Neutron Diffraction

Residual stress can be easily generated during material processing and affect the performance of structural components. Phase stress distribution in austenitic-ferritic duplex stainless steels (DSSs) is complicated due to the different material properties between the two phases. In this study, residual phase stress distribution along the thickness direction of centrifugally cast DSS hollow cylinder was measured by pulsed neutron diffraction with the time-of-flight (TOF) method. The triaxial phase stress distribution along the thickness direction shows that the phase stress of austenitic phase is generally in tension and higher than that of ferrite phase. From the outer surface to the inner surface, the macro-stress distributes from −400 MPa to 200 MPa. The mechanism of macro-stress formation was deduced by taking into consideration the thermal shrinkage behavior during the cooling process of water quench after the solution heat treatment. Furthermore, the lattice strain and phase stress evolution under the uniaxial tensile loading was evaluated by in-situ neutron diffraction measurement. The results indicated that the magnitude of phase stress could be affected by plastic working as well. All these measurements were conducted at Japan Proton Accelerator Research Complex (J-PARC).

Effects of the α/γ-Phase Ratio on the Corrosion Behavior of Cast Duplex Stainless Steel

The effects of the α/γ-phase ratio on pitting corrosion initiation and growth in cast duplex stainless steel were studied, including the preferential dissolution of the two phases inside the pits, using pitting potential measurement and potentiostatic polarization measurement with a high concentration of chloride ions and a low pH. The initiation of pitting was not dependent on the α-phase ratio. The γ phase preferentially dissolves when a high potential in the active dissolution region is applied, and the α phase preferentially dissolves when a low potential is applied. In addition, with an increase in α-phase ratio, the potential range where the α phase preferentially dissolves enlarged toward the higher potential side. The growth rate of stable pitting increased with the α-phase ratio. Dissolution of the α phase increased with an increase in the α-phase ratio. This phenomenon is presumably caused by the decreased amount of Cr in the α phase, resulting from the increased α-phase ratio, as well as by Cr depletion around Cr nitrides.

Monte Carlo simulations of Cu/Ni–Si–Mn co-precipitation in duplex stainless steels

First-Passage Kinetic Monte Carlo (FPKMC) simulations of species migration in duplex stainless steels were performed in order to establish relationships between alloy content, segregation behavior, and the formation of Ni–Si–Mn rich particles in cast duplex stainless steels during thermal aging. The Ni–Si–Mn-rich second phase forms after extended aging at reactor operating temperatures and degrades alloy properties. Simulations of Ni–Si–Mn cluster formation were validated through comparison with experimental results obtained through Atom Probe Tomography (APT) on similar alloy compositions, identifying several trends. First, Cu promotes the formation of Ni–Si–Mn clusters, but only when Ni or Mn prefer segregation to the surface of Cu particles; without the segregation of these species, the critical composition for the clusters to form was not achieved. Second, the width of precipitate-denuded zones near γ/δ interfaces increases with decreasing Cu content. This finding was in strong agreement with APT and Scanning Transmission Electron Microscopy results, further validating our model. Finally, our model predicts that Ni–Si–Mn cluster formation will be the most extensive when the Si:Mn ratio is approximately 1:1 and the least extensive when one of these key elements is less concentrated (Mn ≤ 0.5 at.% or Si ≤ 0.5 at.%). The implications of these results on how to improve alloy properties are discussed.

Thermodynamic modeling of G-phase and assessment of phase stabilities in reactor pressure vessel steels and cast duplex stainless steels

Thermodynamic modeling of G-phase in iron-based multicomponent systems was revised and updated by means of a careful literature survey. The thermodynamic parameters were optimized based on DFT data. Additionally intermetallic phases with possible competition to G-phase in multicomponent systems were also optimized. The thermodynamic model parameters were added to multicomponent thermodynamic database and used for calculation of technological steels and possible competition between phases are tested and shown.

Microstructural Features and Ductile-Brittle Transition Behavior in Hot-Rolled Lean Duplex Stainless Steels

The characteristics of texture and microstructure of lean duplex stainless steels with low Ni content produced through hot rolling followed by annealing were investigated locally with electron backscatter diffraction and globally with neutron diffraction. Then, the ductile–brittle transition (DBT) behavior was studied by Charpy impact test. It is found that the DBT temperature (DBTT) is strongly affected by the direction of crack propagation, depending on crystallographic texture and microstructural morphology; the DBTT becomes extremely low in the case of fracture accompanying delamination. A high Ni duplex stainless steel examined for comparison, shows a lower DBTT compared with the lean steel in the same crack propagating direction. The obtained results were also discussed through comparing with those of cast duplex stainless steels reported previously (Takahashi et al., Tetsu-to-Hagané, 100(2014), 1150).

Development of (Nb0.75,Ti0.25)C-Reinforced Cast Duplex Stainless Steel Composites

Improvements to the wear performance of CD4MCu (a cast Fe-Cr-Ni-Cu-Mo duplex stainless steel grade) would be of great benefit in pump applications. This study demonstrates that CD4MCu stainless steels reinforced with (Nb0.75,Ti0.25)C particles can be fabricated by simply melting the elements required for the CD4MCu stainless steel together with the amounts of Nb, Ti and C required to achieve a specific volume fraction of (Nb0.75,Ti0.25)C particles in a miniature laboratory argon arc furnace. Their potential as erosion resistant materials was evaluated through microstructural characterization, hardness testing, and sliding wear tests. The formation of (Nb0.75,Ti0.25)C particles was found to improve the bulk hardness of the CD4MCu stainless steels without significantly altering its duplex microstructure, which then led to better wear performance. The results indicate that large-scale casting of the proposed materials should be possible and that they have potential as erosion resistant materials.

Effects of Silicon on the Microstructure and Propertiesof Cast Duplex Stainless Steel with Ultra-HighChromium and High Carbon

Effects of Silicon on the Microstructure and Propertiesof Cast Duplex Stainless Steel with Ultra-HighChromium and High Carbon

Optimization study and wear behavior of 25 Cr- 8 Ni- 4 Mo -N cast 5A duplex stainless steel

Duplex stainless steels (DSS) exhibit excellent corrosion resistance and mechanical properties to the severe environments and hence are used in many engineering applications such as oil and marine industries, chemical storage, nuclear power plants, and many more. These are used as valves, fittings, and pipes to suck the crude oil in oil fields. Austenite (γ) and ferrite (δ) phases constitute the microstructure of DSS. However, one needs to control both the phases towards making it lucrative in desired applications. Investigations on tribological behavior of cast DSS are limited. The current work focuses on optimizing the wear parameters for slurry jet erosion and analyzing their effect on wear of the 5A DSS. Grey relation analysis (GRA) and Taguchi design of experiments have been used for optimization. Slurry jet erosion tester was used investigate the response of the material to various designed experiments. Worn-out surfaces morphologies and wear mechanisms were revealed using Scanning electron microscopy (SEM), whereas white light interferometer was used to understand 3D morphologies the surfaces.