S31803 Duplex Stainless Steel Research Articles

Influência da corrosão no aço inoxidável duplex S31803 submetido a íons de cloreto em diferentes temperaturas

Neste trabalho foi avaliada a influência do meio corrosivo e da temperatura nas propriedades de resistência à corrosão do aço inoxidável duplex S31803. O processo corrosivo foi realizado utilizando solução ácido clorídrico 1,5%, à 25, 50 e 80 °C, que é uma solução típica quando se deseja avaliar a corrosão puntiforme, por envolver íons de cloreto. A microestrutura da amostra conforme recebida foi analisada antes e após processo corrosivo utilizando as técnicas de microscopia ótica e confocal. Em seguida, foram realizados os ensaios mecânicos e de resistência à corrosão. A microestrutura do aço inoxidável duplex S31803 é composta de duas fases, ferrita e austenita, orientadas no sentido de laminação, com um balanço aproximado de 50% em volume das frações de fases presentes. O meio corrosivo de ácido clorídrico, nas temperaturas de 25 e 50 °C, proporcionaram o desenvolvimento da corrosão uniforme e da corrosão por pites no material, o que é típico de soluções contendo íons de cloretos. Com o aumento da temperatura até 80 °C, foi observado a deterioração do filme passivo, aumentando expressivamente a corrosão uniforme, onde está se desenvolveu de forma seletiva, tendo a ferrita como anodo e a austenita como catodo. As amostras quando submetidas ao meio corrosivo e as diferentes temperaturas, tendem a diminuir seus valores de propriedades mecânicas quando sofrem a corrosão puntiforme. Em contrapartida quando a corrosão uniforme foi verificada nas amostras, a tendência foi ter um aumento nos valores das propriedades mecânicas quando comparado com o estado inicial; fato que está relacionado à corrosão seletiva do material.

Effect of Cooling Rate on the Precipitation Behavior of a Fe–Cr–Ni Alloy

Ferrite (δ) in two-phase austenite–ferrite Fe–Cr–Ni alloys decomposes into Mo- and Cr-rich phases like sigma (σ) and chi (χ), when aged in the temperature range of 873–1273 K (600–1000 °C). The precipitation of these phases for a particular Fe-Cr-Ni alloy has an adverse effect on its mechanical properties and corrosion resistance. In the present work, precipitation behavior of UNS S32205 duplex stainless steel (a Fe–Cr–Ni alloy) during controlled cooling and heating (isothermal aging) has been studied in the temperature range of 973–1073 K (700–800 °C). Scanning electron microscope (SEM), X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and energy-dispersive spectrometer (EDS) attached to SEM were used to characterize the microstructures. The effect of precipitation of σ and χ phases on the micro-hardness was also studied. The precipitation sequence for 1023 K (750 °C), when cooled from 12,000 to 5 °C/min, was δ → carbides → χ → σ, while for 1073 K (800 °C), it was found to be δ → χ → σ. The Mo-enriched metastable χ phase nucleates at the initial stage of aging which then transforms to stable σ precipitates. The amount of σ and χ phases increased with temperature and aging time, but temperature was found to have a dominant role than the cooling rate due to higher diffusion of solute atoms at high temperatures. EBSD studies did not show any orientation relationship between parent δ ferrite and σ phase.

Failure analysis on tee pipe of duplex stainless in an oilfield

During the hydraulic test in the station of an oil field, a tee-pipe of S32205 duplex stainless burst into several fragments without reaching the target pressure. In order to clarify the failure cause, visual inspection, chemical composition, mechanical test, hardness test, Charpy impact test, heat treatment, and finite element analysis were carried out in this paper. The results show the chemical composition of tee meets the requirements of the standard for S32205 duplex stainless steel, and the tensile property, hardness, Charpy impact and content of harmful phase do not meet the requirements of reference standard. The presence of sigma phase in the tee which induced by the improper heat treatment after cold forming makes the material hard and brittle. In the case of micro defects, the pressure bearing capacity of tee joint will be reduced, resulting in brittle cracking. Therefore, the quality control of tee should be strengthened, the heat treatment should be implemented strictly and nondestructive inspection of tee can be carried out such as field micro-hardness test and metallographic examination. Considering the burst failure case of tee pipe was rarely reported so far, the findings of this paper would provide useful quality control advice in the future.

Low-field magnetic analysis for sigma phase embrittlement monitoring in thermally aged 22Cr duplex stainless steel

The aim of this study is to evaluate a new nondestructive method designated as Low-Field Magnetic Analysis (LFMA) based on the measurement of magnetic flux density for monitoring sigma (σ) phase precipitation in duplex stainless steel (DSS). In addition, the effect of isothermal aging treatment at 800 °C on the metallurgical degradation of UNS S31803 DSS was investigated. It was found a change in the volume fraction of α-Fe/γ-Fe phases due to α-Fe (ferromagnetic phase) eutectoid decomposition to precipitate sigma and secondary austenite (paramagnetic phases). A significant and gradual reduction of saturation magnetization was observed with the increase of thermal treatment time, indicating a decrease in the quantity of α-Fe phase. Two magnetic methods were used to verify the evolution of the σ phase: M−H measurements at 300 K using a commercial magnetometer and LFMA testing. Both methods were sensitive to small percentages of σ phase in the microstructure. Furthermore, our developed technique presents accurate results for low magnetic fields with high sensitivity and reliability.

Application of electromechanical impedance technique in the monitoring of sigma phase embrittlement in duplex stainless steel

Duplex stainless steels (DSS) have a structure composed of austenite and ferrite in approximately equal volume fractions which provides an excellent integration between mechanical properties and corrosion resistance. However, the formation of intermetallic phases, such as sigma (σ) phase could become a major problem, once a small percentage of this phase originates a drastic deterioration of toughness and ductility, phenomenon known as σ phase embrittlement. In the present work, the effect of the isothermal aging treatment at 800 °C (temperature in which σ phase can be formed) on the degradation of mechanical properties of UNS S31803 DSS was investigated. The results showed that σ phase precipitation occurred regularly and by eutectoid decomposition. The effect of sigma phase on the mechanical properties was analyzed by the microhardness and impact tests at room temperature. The assessment of DSS also includes an σ phase embrittlement monitoring alternative method based on the electromechanical impedance (EMI), using piezoelectric sensors. The basic idea of the σ phase embrittlement monitoring method is that the increase in the hardness and the decrease in the impact toughness will leads to the variation in the EMI signatures by modifying structural stiffness. The proposed methodology is evaluated using the impedance module (|Z|), resistance (R), reactance (X) and by the statistical indexes: Relative Variation of Natural Frequencies (RVNF) and Root Mean Square Deviation (RMSD). This method is considered a nondestructive test, presents accurate results with high sensitivity and reliability providing capacity of determining sigma phase embrittlement and has promising application potential in this and other materials.

Cyclic deformation behavior and failure mechanism of S32205 duplex stainless steel under torsional fatigue loadings

Torsional low cycle fatigue tests of ferrite-austenite duplex stainless steel S32205 at various shear strain amplitudes were conducted. The relationship between the evolution of dislocation structures in each constitutive phase and cyclic stress response and cyclic stress-strain curves was thoroughly studied. The surface damage evolution and short cracks nucleation behavior were also discussed. Results show that the cyclic softening occurred at all the strain amplitudes from 0.3% to 1.2% is due to the redistribution and localization of plastic strain and the development of strain transfer between phases, while the formation of regular dislocation cells contributes to achievement of cyclic saturation when the strain amplitude reaches 0.7%. The cyclic stress-strain curves show two-regimes with different cyclic strain hardening rates. As the plastic strain amplitude is smaller than 0.32%, the ferrite and austenite jointly accommodate the plastic strain within material matrix with the coexistence of representative planar slip and incipient wavy slip dislocation configurations in both phases, leading to a low cyclic strain hardening rate. By contrast, the ferrite dominates the plastic deformation with the formation of well-developed low-energy dislocation structures as the plastic strain amplitude is over than 0.32%, resulting in a high cyclic strain hardening rate. Moreover, microstructure observations reveal that the pronounced persistent slip markings in ferrite grains and phase boundaries are the preferential sites for microcrack nucleation and growth.

Critical assessment of the evaluation of thermal desorption spectroscopy data for duplex stainless steels: A combined experimental and numerical approach

The present study evaluates thermal desorption spectroscopy (TDS) data measured for UNS S32205 duplex stainless steel. Variations in the TDS spectra are obtained by electrochemical hydrogen charging for different times and by applying different heating rates for desorption to evaluate the desorption activation energy. Good agreement is found when comparing the experimental TDS curves with the desorption flux based on a numerical diffusion model using a homogeneous average hydrogen diffusion coefficient for the two-phase (ferrite-austenite) duplex microstructure. Trapping cannot be distinguished from the experimental TDS data since hydrogen diffusion in austenite is the rate-determining process during desorption. An average diffusion activation energy of 43.4 kJ/mol is determined from the experiments. Moreover, similar findings are obtained with a finite-element model that includes the heterogeneous hydrogen-related properties of the two phases of this duplex stainless steel.

Correlation between microstructural gradient and microindentation properties of dissimilar weld between INCONEL 625 and Duplex Stainless Steel

Microstructure evolution and microindentation properties of dissimilar weld between Inconel 625 nickel based alloy and UNS S32205 Duplex Stainless Steel have been investigated in the current study. Samples in cross-section areas were prepared to investigate microstructure and micromechanical properties in different regions of weld using optical microscopy, scanning electron microscopy and indentation measurements. Typical solidification microstructures were observed. Secondary phases were noticed in both the weld metal and heat affected zone of Inconel 625 alloy at cellular and interdendritic region in lamellar, rod shaped and cuboidal form. These secondary phases were identified as Laves phase and carbonitrides of Nb and Ti. Mechanical properties including elastic modulus and hardness were estimated across the weld joint. The results showed that the weld metal exhibit the lowest values of hardness and elastic modulus, however the Heat Affected Zone of the stainless steel exhibit the highest values owing to high δ-ferrite amount. In-situ nanoindentation coupled with Scanning Electron Microscope was carried out in Laves phase and the matrix weld in order to evaluate nanohardness. The result showed that the nanohardness of Laves phase is too great compared to the weld matrix.

Pitting Corrosion Behavior of UNS S31803 and UNS S32304 Duplex Stainless Steels in 3.5 wt% NaCl Solution

This work examined the pitting corrosion resistance of two types of duplex stainless steels: UNS S31803, with molybdenum, and UNS S32304, without molybdenum. Cyclic potentiodynamic polarization and critical pitting temperature (CPT) tests were performed in 3.5 wt% NaCl solution. Scanning electron microscopy (SEM) was used for analysis of the pits on the surface of samples. The two steels studied presented susceptible to pitting corrosion in 3.5 wt% NaCl solution. UNS S31803 steel showed a higher corrosion potential (Ecorr), pitting potential (Epit), and polarization resistance (Rp) compared to UNS S32304 steel. It was also observed that the UNS S31803 steel showed higher capacity to repassivation and CPT than the UNS S32304 steel. Keywords: Duplex stainless steels; Pitting potential; Electrochemical impedance spectroscopy; Cyclic potentiodynamic polarization; CPT.

Characterizations of dissimilar S32205/316L welds using austenitic, super-austenitic and super-duplex filler metals

UNS S32205 duplex stainless steel plates were welded to AISI 316L stainless steel using the pulsed gas tungsten arc welding process with three different filler metals: ER2594, ER312, and ER385. The microstructures of the welds were characterized using optical and scanning electron microscopy, and all of the specimens were evaluated by ferrite measurements. The mechanical properties were studied through hardness, tensile, and impact tests. In addition, the pitting resistance equivalent number was calculated and cyclic polarization tests were performed to evaluate the corrosion resistance of the weld metal. The results showed that chromium nitride was formed in the heat-affected zone of the duplex side, whereas no sigma phase was detected in any of the specimens. The ferrite number increased from the root pass to the final pass. The absorbed energies of the impact test decreased with increasing ferrite number, whereas the tensile strength was enhanced. The fully austenitic microstructure of the specimen welded with ER385 exhibited the highest resistance to pitting corrosion at 25°C, and the super-duplex weld metal presented superior corrosion resistance at 50°C.

Evaluation of microstructural and mechanical properties of dissimilar Inconel 625 nickel alloy–UNS S32205 duplex stainless steel weldment using MIG welding

Inconel 625 nickel alloy and UNS S32205 duplex stainless steel (DSS) were welded with ER2209 filler metal using MIG (Metal Inert Gas) welding method. The weld metal obtained by DSS filler metal was subjected to mechanical and microstructural evaluation. Toughness and hardness properties had been examined by mechanical and microstructural characterization of weld metal, fusion line, and HAZ (Heat Affected Zone) region, and precipitations were investigated by light microscopy (LM), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). Corrosion behavior of base metals and face and root of weld metal were examined by using potentiodynamic polarization test. Additionally, detailed elemental analysis and mapping of weld metal with both optical emission spectrometer (OES) and X-Ray spectrometer were attained. Results demonstrate a significant decrease in toughness of the welding due to the presence of Nb and Mo rich intermetallic precipitations in the Inconel HAZ and root region, although there is no significant increase in hardness. Potentiodynamic polarization test shows that the dilution-induced microstructural transformation in the root of weld has the worst corrosion resistance in the weld metal. Therefore, this dissimilar welding does not have optimum properties for neither toughness nor corrosion.

Effects of carbon substitution for nitrogen on the pitting corrosion resistance of type UNS S32205 duplex stainless steel

The effects of C-substitution for N on the pitting corrosion resistance of type UNS S32205 duplex stainless steel (DSS) were investigated. It was found that the C-substituted DSS possessed higher resistance to pitting corrosion than the normal DSS. X-ray photoelectron spectroscopy and transmission electron microscopy analyses on the passive films of the DSSs revealed that the passive films formed on the both α and γ phases in the C-substituted DSS contained higher Cr content than those in the normal DSS, which was considered to be the reason for the enhanced resistance to pitting corrosion.

Investigation of burr formation and tool wear in micromilling operation of duplex stainless steel

Despite progress in the study of micro-machining processes, there is still no consensus on the effect of cutting conditions or standardization of procedures for comparison of the results, due to the typical difficulties of micro-scale processes. In this context, this work has as main objective the investigation of tool wear and burr formation in micro milling operation of UNS S32205 duplex stainless steel. Experimental tests were carried out micro machining slots using a CNC micro mill machine tool, with a maximum spindle speed of 60,000 rpm. The performance of cemented carbide microtools of two different diameters (200 μm and 400 μm) was analyzed. The results showed that the tools with a diameter of 200 μm presented an excellent performance concerning the machined length. However, increased cutting speed led to excessive wear and high burrs. The use of cutting fluid attenuated the wear and the height of the burrs. Finally, the best machinability was obtained using tools of bigger diameter (400 μm). Lower wear and smaller burrs were obtained compared to tools of smaller diameter (200 μm).

Microstructure and Impact Toughness of Local-Dry Keyhole Tungsten Inert Gas Welded Joints

In this paper, the microstructure and impact toughness of a S32101 duplex stainless steel underwater local-dry keyhole tungsten inert gas welded joint were studied. The impact toughness value of the underwater weld metal reached 78% of the onshore weld metal, which is in accordance with the underwater welding standards. The proportion of austenite in the underwater weld metal was 0.9% lower than that of the onshore weld metal. The proportion of the Σ3 coincidence site lattice boundaries and random phase boundaries in the underwater weld metal, which significantly influence the impact toughness of the weld metal, were smaller than that of the onshore weld metal.

The influence of microstructure and chromium nitride precipitations on the mechanical and intergranular corrosion properties of K-TIG weld metals

Abstract S32101 duplex stainless steel (DSS) without filling metal was welded by applying a keyhole tungsten inert gas welding (K-TIG) system at a heat input range of 1.99–2.46 kJ/mm. The coincidence site lattice (CSL) grain boundaries and the chromium nitride precipitations in the weld metal (WM) were analyzed and calibrated on the basis of the electron backscatter diffraction (EBSD) technique. The mechanical and intergranular corrosion (IC) properties of the WM were evaluated. Experiments were carried out to investigate and establish the relationship of the microstructure, chromium nitride precipitation and mechanical and intergranular corrosion properties of the WM. The EBSD analysis and experimental results indicated that the chromium nitride precipitations and microstructure have an influence on the mechanical properties. The frequency of the Σ3 CSL grain boundary and the chromium nitride precipitations can be applied to evaluate the IC property of the WM.

Study on welding thermal cycle and residual stress of UNS S32304 duplex stainless steel selected as external shield for a transport packaging of Mo-99

Thin plates of duplex stainless steel UNS S32304 were welded using the pulsed gas tungsten arc GTAW process (butt joint) without filler addition. The used shielding gas was pure argon and 98% argon plus 2% of nitrogen. The thermal cycles were acquired during welding, in regions near the melting pool. This alloy is candidate for the external clad of a cask for the transport of high activity radiopharmaceuticals substances. For the residual stress measurements in austenite phase an X-ray diffractometer was used in a Bragg-Brentano geometry with CuKα radiation (λ= 0.154 nm) and for ferrite phase was used a pseudo-parallel geometry with CrKα radiation (λ= 0.2291nm). The results of residual stress using sin2y methodology shown that the influence of the high welding temperature leads to compressive stresses for both phase of the duplex steels mainly in heat-affected zone. It was observed a high temperature peak and an increase of the mean residual stress after addition of nitrogen to the argon shielding gas.

Impact Toughness, Fatigue Crack Growth and Corrosion Behavior of Thermally Aged UNS S32205 Duplex Stainless Steel

The aim of the present work was to study the impact, fatigue crack growth and corrosion behavior of 2205 duplex stainless steel (DSS) when subjected to one of the most influential thermal aging condition that promotes deleterious secondary precipitation. Formation of Cr- and Mo-rich σ-phase was confirmed by EDX analysis which deteriorated impact toughness causing the material to fail following quasi-cleavage mode of fracture. Susceptibility of DSS 2205 to pitting corrosion assessed though Potentiodynamic polarization technique was enhanced by thermal aging, but sensitization performance evaluated via double-loop electrochemical potentiokinetic reactivation (DLEPR) technique did not show much degradation. However, fatigue crack growth behavior testing divulged drastic enhancement in impediment to crack propagation by the matrix after thermal aging.

Understanding the relation between pitting corrosion resistance and phase fraction of S32101 duplex stainless steel

The relation between phase fraction and pitting corrosion resistance of S32101 duplex stainless steel was interpreted by considering the stable and metastable pitting potential and galvanic corrosion rate of the duplex-phased matrix. As the ferrite fraction increased, the potential for the stable pitting corrosion was linearly decreased which was found to be closely related to the increase in the galvanic corrosion rate between the two constituent phases. It was also revealed that the pit propagation rate was more important than the pit initiation probability in determining the overall pitting corrosion resistance of the S32101 duplex stainless steel.

The temperature-dependent pitting and repassivation behaviors of UNS S31803 duplex stainless steel in chloride solutions

An improved electrochemical method named thermal pulse technique was designed to measure the current flowing from a potentiostatically maintained UNS S31803 electrode in 1 M NaCl solution, while various thermal pulses were applied on the solutions. Both the critical pitting temperature (CPT) and the repassivation temperature (Tr) were measured in one thermal pulse. The Tr data presented an inverted V-shape trend as the cooling rate decreased. Both the CPT and Tr values declined after multiple thermal pulses. The cyclic potentiodynamic polarization curves present a sharp transition of pitting and repassivation potential at the temperature ranged between the CPT and Tr.

Effect of Warm Rolling on the Corrosion and Mechanical Properties of UNS S32205 Duplex Stainless Steel

Duplex stainless steel (DSS) has been considered as an excellent alternative for applications where high corrosion resistance and high mechanical strength are required. Therefore, microstructure and properties of DSS remain topics of much interest. For this purpose, the effect of warm-rolling was studied in a duplex stainless steel. The focus of this work was 2205 which is one of the most useful type of DSS. Although evolution of microstructure, texture and tensile properties during warm rolling of DDS have been reported, there is no study on the effect of warm rolling on corrosion resistance of 2205 DSS. In this context, the objective of this work is to evaluate microstructure, texture and mechanical and corrosion properties of UNS S32205 DSS after warm rolling. The duplex stainless steel was warm-rolled at 600°C up to 60 and 80% thickness reduction and submitted to electrochemical tests. The electrochemical behaviour of warm-rolled 2205 in the chlorine ion environment was evaluated using cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS). The samples exhibited an excellent corrosion resistance in 3.5% NaCl solution. The study showed that the thermomechanical treatment used favoured the formation of the passive film and led to a greater polarization resistance. This behaviour is consistent with the pits density observed by scanning electron microscopy (SEM) and the crystallographic microtexture of the steel.