Super Duplex Stainless Steel Alloy Research Articles

Analysis of the Wear-Assisted Corrosion of a Super-Duplex Stainless-Steel Using an Interface Film Growth Model

The wear-assisted corrosion response of a super-duplex stainless-steel alloy from a single asperity contact was measured using the contact mode of an atomic force microscope (AFM). The AFM tip, as illustrated in the figure below, applied increasing loads to an isolated surface region that was polarized to two different static potentials in a 0.6 M NaCl solution. The AFM tip was swept across the isolated, exposed, surface region of the alloy while the alloy response was measured by a potentiostat. The wear-assisted corrosion response was modeled using an interfacial film growth model (IFM) that was used to estimate kinetics parameters for the response of the alloy as a function of the applied loads and polarization potentials.The average response values from the IFM were then approximated using an Arrhenius expression that incorporated the peak applied stress and activation volume for the reaction as modifiers to the energy barrier for oxide film formation. This Arrhenius model suggested that the stress fields from the single asperity contact induced damage in the protective oxide and also disrupted other oxide-formation processes at the oxide-electrolyte interface. The induced damage and the disruption of the oxide-formation processes, as a function of the applied stress, slowed repassivation and allowed increasingly high corrosion currents as the stress fields increased.AcknowledgmentsThe U.S. Naval Research Laboratory sponsored this work under work unit 1E19 as part of its Basic Research Program. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, the Department of Defense, or the U.S. government.Figure CaptionSchematic diagram of experimental setup of the AFM tip loading the isolated region on the super-duplex stainless-steel alloy surface while the potentiostat is used to both polarize the alloy and measure its response to the load. Figure 1

Microstructure Evolution during Hot Deformation of UNS S32750 Super-Duplex Stainless Steel Alloy.

The present paper analyzes UNS S32750 Super-Duplex Stainless Steel hot deformation behavior during processing by upsetting. The objective of this paper is to determine the optimum range of deformation temperatures, considering that both austenite and ferrite have different deformation behaviors due to their different morphology, physical, and mechanical properties. Because the capability of plastic deformation accommodation of ferrite is reduced when compared to austenite, side cracks and fissures can form during the hot deformation process. Consequently, it is important to find the optimum conditions of deformation of this type of stainless steel to establish the best processing parameters without deteriorating the material. The experimental program involved the application of hot deformation by the upsetting method on a series of samples between 1000 °C and 1275 °C, with a total degree of deformation of 30%. The resultant samples were examined by SEM-EBSD to establish and analyze the evolution of the phases present in the structure from several points of view: nature, distribution, morphology (size and shape), and their structural homogeneity. The GROD (Grain Reference Orientation Deviation) distribution map was also determined while taking into account the possible precipitation of the secondary austenite phase (γ2-phase) and the analysis of the dynamic recrystallization process according to the applied deformation temperature. The main conclusion was that UNS S32750 SDSS steel can be safely deformed by upsetting between 1050–1275 °C, with an experimented total degree of deformation of 30%.

Effect of Post-Weld Heat Treatment on Hardness in Superduplex Stainless Steel ASTM A890/A890M - Grade 6A

Superduplex stainless steel alloy exhibit high mechanical and corrosion resistance, which main industrial application is in the petrochemical industry. The manufacture and maintenance of such equipment usually involve welding processes, followed by post-welded heat treatment and it often becomes impossible to apply heat treatments. Thereby, the purpose of this work is to verify the effect of a post-welded heat treatment on shielded metal arc welding in steel grade ASTM A890/A890M - grade 6A. The microstructure in the as-welded condition consisted of austenite, secondary austenite, and ferrite phases and, the post-welded heat treatment condition exhibited only austenite and ferrite. The hardness in the melt zone reached values of 300 HV after welding and, the value was reduced to 260 HV in the post-welded heat treatment condition.

Influence of ageing treatment on microstructural and mechanical properties of a solution treated UNS S32750/EN 1.4410/F53 Super Duplex Stainless Steel (SDSS) alloy

In this present study, the influence of isothermal ageing temperature and ageing duration on microstructural and mechanical properties of a solution treated UNS S32750 Super Duplex Stainless Steel (SDSS) alloy, was investigated by SEM-EBSD (Scanning Electron Microscopy-Electron Backscatter Diffraction) microscopy, tensile and, impact testing techniques. Isothermal ageing treatments, performed at temperatures between 400°C and 600°C and, treatment durations of 3h and, 120h, were applied to a commercial UNS S32750 SDSS alloy. Microstructural characteristics, such as: distribution and morphology of constituent phases, distribution maps of grain's modal orientation (MO) and, obtained mechanical properties were analyzed. The applied isothermal ageing treatments did not induce any significant changes in relation to phase's morphology, grain-size and, MO distribution. Obtained results show that the microstructure consists of elongated γ-phase grains within δ-phase matrix, showing a grain-size close to (45–48) μm in the case of δ-phase, (22–24) μm in the case of γ-phase and, (1.6–2.1) μm in the case of γ2-phase. Significant changes were noticed in the case of mechanical properties (ultimate tensile strength, 0.2 yield strength and, elongation to fracture) due to precipitation of secondary phases. The most influential secondary phase is the R-phase, observed in the case of isothermal ageing treatment performed at 600°C with a treatment duration of 120h (A3.2 state). The A3.2 state, in comparison with solution treated (ST) state, showed a reduction in elongation to fracture of approx. 62% and, an increase in ultimate tensile strength and, respectively, 0.2 yield strength of approx. 24% and, respectively, approx. 16%.

Simulation of corrosion of a dual phase alloy steel based on its constituent phase polarization properties

In this study, the corrosion of dual phase alloy steel namely super duplex stainless steel (SDSS) is simulated by taking into consideration the presence of two distinct phases that are austenite and ferrite. A simulation algorithm is developed by the authors for simulating micro galvanic corrosion by random assignment of anode and cathode zones assuming non-linear polarization curves. The governing equation for electric potential in the electrolyte is Laplace equation owing to charge neutrality implying diffusion limited corrosion rate. The metal-electrolyte interface is allowed to dissolve as a moving boundary problem approach in a computer simulation by assigning ferrite property to anode and austenite property to cathode randomly. To validate this proposition, the true non-linear polarization data and its fluctuations in the form of fitted Butler–Volmer expressions of individual phases of the SDSS alloy are used as the boundary condition on anode and cathode zones corresponding to ferrite and austenite phases. This modeling approach is applied to pure austenite as SS304 and pure ferrite as SS430 and 50–50 austenite-ferrite as SDSS. The obtained simulation results of corrosion rate of the SDSS and its constituent pure phases compared well with the direct polarization prediction of corrosion rate for various concentrations of NaCl in water. The simulation prediction is found to be a good fit than by weighted average method.

Effect of short duration solution treatment at temperatures below 1000 °C on σ‐phase precipitation in a super duplex stainless steel alloy

AbstractSuper duplex stainless steel alloys are characterized by a high content of chromium, nickel, molybdenum and nitrogen. In these steels, the main role of chromium, nickel and molybdenum alloying elements is to increase corrosion resistance. Increased mechanical properties are obtained by adding nitrogen, which promotes structural hardening. The microstructure of super duplex stainless steels consists of a mixture of ferrite and austenite phases, while other phases are also formed, such as: sigma, chi, chromium nitride, carbides and secondary austenite. All secondary phases are considered deleterious phases due to their negative effect on corrosion resistance and mechanical properties. The precipitation formation mechanism of the sigma phase assumes depletion of chromium and molybdenum from the matrix, worsening the alloy properties. The aim of the present work is to show the microstructural changes occurring during a short duration (10 minutes) solution treating of an UNS S32760/1.4501/F55 alloy, three temperatures being considered: 800 °C, 900 °C and 1000 °C. Scanning electron microscopy (energy dispersive spectroscopy and electron backscattered diffraction) was used for quantifying the alloying elements distribution, to distinguish between the observed phases, and to better understand the evolution of each phase.

Phase-Field Modeling of Sigma-Phase Precipitation in 25Cr7Ni4Mo Duplex Stainless Steel

Phase-field modeling is used to simulate the formation of sigma phase in a model alloy mimicking a commercial super duplex stainless steel (SDSS) alloy, in order to study precipitation and growth of sigma phase under linear continuous cooling. The so-called Warren–Boettinger–McFadden (WBM) model is used to build the basis of the multiphase and multicomponent phase-field model. The thermodynamic inconsistency at the multiple junctions associated with the multiphase formulation of the WBM model is resolved by means of a numerical Cut-off algorithm. To make realistic simulations, all the kinetic and the thermodynamic quantities are derived from the CALPHAD databases at each numerical time step, using Thermo-Calc and TQ-Interface. The credibility of the phase-field model is verified by comparing the results from the phase-field simulations with the corresponding DICTRA simulations and also with the empirical data. 2D phase-field simulations are performed for three different cooling rates in two different initial microstructures. A simple model for the nucleation of sigma phase is also implemented in the first case. Simulation results show that the precipitation of sigma phase is characterized by the accumulation of Cr and Mo at the austenite-ferrite and the ferrite-ferrite boundaries. Moreover, it is observed that a slow cooling rate promotes the growth of sigma phase, while a higher cooling rate restricts it, eventually preserving the duplex structure in the SDSS alloy. Results from the phase-field simulations are also compared quantitatively with the experiments, performed on a commercial 2507 SDSS alloy. It is found that overall, the predicted morphological features of the transformation and the composition profiles show good conformity with the empirical data.

Influence of Solution Treatment Temperature on Microstructural Properties of an Industrially Forged UNS S32750/1.4410/F53 Super Duplex Stainless Steel (SDSS) Alloy

In this present study, the influence of solution annealing temperature on microstructural properties of a forged Super Duplex Stainless Steel (SDSS) was investigated by SEM-BSE (Scanning Electron Microscopy-Backscattered Electrons) and SEM-EBSD (Scanning Electron Microscopy-Electron Backscatter Diffraction) techniques. A brief solution treatment was applied to the forged super duplex alloy, at different temperatures between 800 °C and 1100 °C, with a constant holding time of 0.6 ks (10 min). Microstructural characteristics such as nature, weight fraction, distribution and morphology of constituent phases, average grain-size and grain misorientation were analysed in relation to the solution annealing temperature. Experimental results have shown that the constituent phases in the SDSS alloy are δ-Fe, γ-Fe and σ (Cr-Fe) and that their properties are influenced by the solution treatment temperature. SEM examinations revealed microstructural modifications induced by the Cr rich precipitates along the δ/γ and δ/δ grain boundaries, which may significantly affect the toughness and the corrosion resistance of the alloy. Solution annealing at 1100 °C led to complete dissolution of σ (Cr-Fe) phase, the microstructure being formed of primary δ-Fe and γ-Fe. The orientation relationship between δ/δ, γ/γ and δ/γ grains was determined by electron back scattering diffraction (EBSD). Both primary constituent phase’s microhardness and global microhardness were determined.

Influence of Solution Treatment Duration on Microstructural Features of an Industrial Forged UNS S32750/1.4410/F53 Super Duplex Stainless Steel (SDSS) Alloy

The microstructural changes induced by solution treatment of an industrial forged F53 Super Duplex Stainless Steel alloy were studied, in order to emphasize how component phases are influenced by heat treatment temperature and duration. The solution treatment was done at a temperature of 1100°C, with variable holding times: 0.6 ks (10 min), 3.6 ks (60 min) and 10.8 ks (180 min). Scanning electron microscopy–electron backscattered diffraction was used as main characterization technique, to obtain and analyse data referring to microstructural features, such as: nature and morphology of constituent phases, average grain-size and grain misorientation. It was shown that in all studied cases the microstructure consisted of a mixture of about 45% δ-Fe (ferrite) and 55% γ-Fe (austenite). Besides δ-Fe and γ-Fe phases, other phases were also identified, such as τ-phase (chromium-iron carbide), σ-phase (chromium-iron) and δ-(Cr–Fe) (ferrite).

ELECTRALLOY ALLOY EC25540

Abstract Electralloy EC25540 is a super duplex stainless steel alloy providing excellent resistance to aqueous corrosion and a restricted chemistry to ensure a minimum PREN of 40. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SS-1179. Producer or source: Electralloy, a Division of G.O. Carlson Inc..

Optimization of activated tungsten inert gas welding of super duplex alloy 2507 based on experimental results

In this work, the effect of activated tungsten inert gas welding process parameters on the depth of penetration of super duplex stainless steel alloy 2507 has been analyzed using response surface methodology. The design matrix for conducting the experiments was generated using the central composite rotatable design of response surface methodology of design of experiments. The input process parameters like current, torch speed and arc gap were varied at five levels. Based on the generated design matrix, bead-on-plate welds were made on 10-mm-hick super duplex stainless steel plate. The depth of penetration, which is the response in the present analysis, was measured on the samples. A second-order response surface model was developed. Then, response optimization was performed for obtaining the maximum depth of penetration using desirability approach. The validation of the model showed good agreement between the predicted and actual values of depth of penetration.

ELECTRALLOY ALLOY EC32750

Abstract Electralloy EC32750 is super duplex stainless steel alloy providing excellent resistance to aqueous corrosion, coupled with good strength and pitting resistance. This datasheet provides information on composition, physical properties, hardness, and tensile properties as well as fracture toughness. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: SS-1075. Producer or source: Electralloy, A Division of G.O. Carlson Inc.

Corrosion Fatigue Crack Growth Inhibition OfDuplex Stainless Steel

The main objective of this study is to examine the inhibition effect of chromate on corrosion fatigue crack growth in a super duplex stainless steel alloy. Corrosion fatigue crack propagation tests were performed for Zeron 100 in air, 3.5% NaCl, and in 3.5% NaCl inhibited with sodium chromate (Na2CrO4) using two chromate concentrations. Specimens were tested in the TL orientation. Results show that crack growth rates are enhanced in 3.5% NaCl solution relative to air over the entire range of ∆K tested. Addition of 0.02 M Na2CrO4 to 3.5% NaCl solution results in notable decreases in crack growth rates up to a specific value of ∆K above which crack growth rates approach values measured in pure 3.5% NaCl solution. When the chromate concentration is increased by ten fold, corrosion fatigue crack growth rates are markedly reduced and approach measurements obtained in air for the entire range of ∆K. Examination of fracture surfaces by the scanning electron microscope reveals the presence of ductile striations in austenite and brittle striations in ferrite. The basic striations characteristics in the austenitic phase are apparently the same in all the environments. However, ferrite striations appear to be ∆K and environment dependent. Scratch tests were carried out to examine the effect of chromate concentration on repassivation kinetics of the duplex alloy. Chromate inhibition effectiveness during corrosion fatigue of duplex stainless steel is proposed to correlate with chromate repassivation kinetics and the crack tip strain rate.