Superduplex Stainless Steel UNS S32750 Research Articles

A comparison of microstructure and mechanical characteristics correlation of the joint specimens for duplex stainless steel UNS S32304 and super-duplex stainless steel UNS S32750: The role of post-weld heat treatment

The present investigation studies the impact of post-weld heat treatment (PWHT) on the microstructure and mechanical characteristics of joints made by gas tungsten arc welding between duplex stainless steel and super-duplex stainless steel specimens. The PWHT process was considered at 1100 °C for 10 minutes followed by water quenching on UNS S32304 and UNS S32750 materials. Scanning electron microscopy, optical microscopy, X-ray diffractometry (XRD), and energy dispersive X-ray analyses have been applied in microstructure evolution, including recognizing local chemical contents and elemental distribution. To analyze the mechanical characteristics of samples, tensile and hardness tests were performed. It was shown that after applying PWHT, the morphology of the austenite varied becoming mostly intergranular/spheroidal in shape, accompanied by an improvement in the percentages of austenite. This investigation indicates the PWHT results in restoring an equal percentage of both phases. The phase percentages, based on both ASTM E1245 and ASTM E562, show a good agreement between the austenite and ferrite phases of weld materials. According to XRD, the phases are mainly ferrite and austenite with different lattice parameters with no evidence of unwanted intermetallic phases. It was concluded that the PWHT process plays a crucial role in the ductility of joint specimens due to the phase balance between austenite and ferrite.

Localized Corrosion of High-Grade Stainless Steels: Grade Selection in Chlorinated Seawater

Chlorination is widely used in seawater systems to avoid fouling and associated microbial-induced corrosion. Free chlorine is a strong oxidizing agent that prevents biofilm formation on immersed surfaces when used above a certain content. However, the presence of residual chlorine associated with the relatively high chloride content in seawater significantly increases the risk of localized corrosion for most stainless steels. In the present study, a module initially developed to quantify the formation of electroactive biofilms on stainless steels has been used to assess the corrosiveness of chlorinated seawater. Both the electrochemical potential and the cathodic current were measured on super-duplex stainless steel as a function of residual chlorine levels and seawater temperatures. In parallel, long-term localized corrosion tests have been performed in simulated environments to assess the environmental limits for the safe use of high-grade stainless steels in chlorinated seawater. It includes crevice corrosion exposure tests using adapted ISO 18070:2015 crevice formers and internal tube pitting corrosion exposure tests in model tube heat exchangers simulating heat flux from 35°C to 170°C. The synergetic effect of residual chlorine content and temperature on the risk of localized corrosion has been quantified. Corrosion resistance properties are correlated to the electrochemical monitoring data, and the environmental limits of selected base materials stainless steels have been established for duplex stainless steel UNS S32205, super-duplex stainless steel UNS S32750, hyper-duplex stainless steels UNS S32707 and UNS S33207, and the high-grade austenitic stainless steel UNS S31266.

Strength Enhancement of Superduplex Stainless Steel Using Thermomechanical Processing

The impact of microstructure evolution on mechanical properties in superduplex stainless steel UNS S32750 (EN 1.4410) was investigated. To this end, different thermomechanical treatments were carried out in order to obtain clearly distinct duplex microstructures. Optical microscopy and scanning electron microscopy, together with texture measurements, were used to characterize the morphology and the preferred orientations of ferrite and austenite in all microstructures. Additionally, the mechanical properties were assessed by tensile tests with digital image correlation. Phase morphology was not found to significantly affect the mechanical properties and neither were phase volume fractions within 13% of the 50/50 ratio. Austenite texture was the same combined Goss/Brass texture regardless of thermomechanical processing, while ferrite texture was mainly described by α-fiber orientations. Ferrite texture and average phase spacing were found to have a notable effect on mechanical properties. One of the original microstructures of superduplex stainless steel obtained here shows a strength improvement by the order of 120 MPa over the industrial material.

Evaluation of Hot Deformation Behaviour of UNS S32750 Super Duplex Stainless Steel (SDSS) Alloy

The super-duplex stainless steel UNS S32750 consists of two main phases, austenite and ferrite, which differ not only by their morphology, physical, and mechanical properties, but also by their deformation behaviour. A heterogenous deformation can be obtained during thermomechanical processing, generating internal stresses and sometimes fissures or cracks on sample lateral surfaces, due to ferrite’s phase lower potential of plastic deformation accommodation in comparison with austenite phase. The research objective is to determine the optimum range of the applied deformation degree, during hot deformation processing by upsetting of the super-duplex steel (SDSS) UNS S32750. In the experimental program several samples were hot deformed by upsetting, by applying a deformation degree between 5–50%, at 1050 °C and 1300 °C. The most representative hot-deformed samples were selected and analysed by scanning electron microscope-Electron Backscatter Diffraction (SEM-EBSD), to determine the main microstructural characteristics obtained during thermomechanical processing. When considering the experimental results, the influence of the applied deformation degree on the microstructure has been evaluated. Microstructural features, such as nature, distribution, morphology and relative proportion of constituent phases, Grain Reference Orientation Deviation (GROD), and recrystallization (RX), were analysed, in correlation with the applied deformation degree. Finally, it was concluded that the UNS S32750 alloy can be safely hot deformed, by upsetting, at 1050 °C and 1300 °C, with a maximum applied deformation degree of 20% at 1050 °C and, respectively, by 50% at 1300 °C.

Sigma and Chi Phases Analysis in UNS S32750 Superduplex Stainless Steel by Optimized Linear Sweep Voltammetry in Alkaline Medium

Linear sweep voltammetry (LSV) test parameters in alkaline medium were optimized by Doehlert matrix design in order to quantify the deleterious phases in a superduplex stainless steel UNS S32750. The microstructural analysis was performed, in several heat treated specimens, by Light Optical (LOM) and Scanning Electron (SEM) Microscopies and correlated with the electrochemical tests. In these tests, optimized parameters were obtained for tests in aqueous solutions of KOH. The concentration of 3.55 mol l−1, scan rate of 3.42 mV s−1 and initial potential of −0.818 V, showed a good correlation between the deleterious phases precipitated and charge density values. Differently from LOM characterization, chi and sigma deleterious phases can be distinguished by LSV optimized test. Finally, this test can be a non-destructive powerful tool of quality control to detect embrittlement and corrosion resistance decay that commonly affected this stainless steel as consequence of inadequate fabrication processes.

Effect of heat input on microstructure and mechanical properties of Nd: YAG laser welded super duplex stainless steel-numerical and experimental approach

This study reports the effect of different heat input (126 J/mm to 330 J/mm) on the microstructure and mechanical properties of pulsed Nd: YAG laser welded super duplex stainless steel UNS S32750 (SDSS) butt weld joints. Macrostructure exhibit incomplete, complete and complete penetration with a crater at the top of the weld bead, for low, middle and higher heat input respectively. Experimental macrostructure is in agreement with the numerical simulation performed by SYSWELD - a 3D software package. Maximum temperature is obtained at the mid section of the weld for all attempted heat inputs. Micro-analysis display an enhancement in the ferrite concentration in the weld zone for an increase in the heat input. Sigma phase, a reaction compound, is observed for the highest heat input. Higher micro-hardness is witnessed at the interface for the mid range heat input. The results of tensile test show that the tensile strength tends to reach a peak and then decrease with the increasing heat input.

Precipitation condition and effect of volume fraction on corrosion properties of secondary phase on casted super-duplex stainless steel UNS S32750

PurposeThe secondary phase decreased the corrosion resistance because of the segregation of Cr and Mo. Therefore, this paper aims to study the precipitation condition and the effect of secondary phase with volume fraction on corrosion behavior.Design/methodology/approachSecondary phase precipitated approximately from 375°C to 975°C because of saturated Cr and Mo at grain boundary by growth of austenite. Therefore, heat treatment from 800°C to 1,300°C was applied to start the precipitation of the secondary phase.FindingsThe secondary phase is precipitated at 1,020°C because of segregation by heterogeneous austenite. The growth of austenite at 1,000°C needs the time to saturate the Cr and Mo at grain boundary. When the volume fraction of austenite is 56 per cent (14 min at 1,000°C), the secondary phase is precipitated with grain boundary of austenite. The secondary phase increased the current density (corrosion rate) and decreased the passivation. That is checked to the critical pitting temperature (CPT) curves. The 1 per cent volume fraction of secondary phase decreased CPT to 60°C from 71°C.Research limitations/implicationsThe precipitation of secondary phase not wants anyone. Casted super-duplex stainless steel (SDSS) of big size precipitates the secondary phase. This study worked the precipitation condition and the suppression conditions of secondary phase.Social implicationsManufacturers need precipitation condition to make high-performance SDSS.Originality/valueThe corrosion resistance of SDSS is hard the optimization because SDSS is dual-phase stainless steel. The precipitation of the secondary phase must be controlled to optimize of the corrosion resistance of SDSS. Anyone not studied the precipitation condition of secondary phase and the effect of secondary phase with volume fraction on corrosion behavior of SDSS.

Pulsed Nd: YAG laser welding and subsequent post-weld heat treatment on super duplex stainless steel

This study attempts to explore the weldability of super duplex stainless steel UNS S32750 (SDSS), using a pulsed Nd: YAG laser welding machine with an average power of 600W. The process was performed by varying welding speeds (200mm/min–400mm/min in steps of 50mm/min) and keeping the other parameters unchanged, so as to alter the heat input (75J/mm–150J/mm). Microstructural analysis on the joints reveal higher volume percentages of ferrite (>50%) affecting the phase equilibrium of base alloy. The as-weld joints were subjected to post weld heat treatment (PWHT) at 1050°C for two hours to enhance the levels of the austenite phase. This alleviates the phase imbalance and improves the corrosion properties of SDSS laser welds. Vickers microhardness and tensile strength of as-weld joints were found to be higher than PWHT joints due to the higher ferritic phase in the weld zone.

Characterization of weld strength and toughness in the multi-pass welding of Inconel 625 and Super-duplex stainless steel UNS S32750

The present study investigated the weldability of dissimilar metals involving Inconel 625 and Super-duplex stainless steel obtained from continuous current (CC) and pulsed current (PC) gas tungsten arc welding (GTAW) processes employing ER2553 and ERNiCrMo-4 fillers. A comparative analysis on these dissimilar weldments was carried out to establish the structure-property relationships. Microstructure examination was carried out using optical microscopy (OM) and scanning electron microscopy (SEM) techniques. Grain coarsening was observed at the HAZ of UNS S32750 for all the cases. Mechanical tests ascertained that the PCGTA weldments employing ERNiCrMo-4 offered better weld strength and impact toughness. Elaborative studies on the structure – property relationships of these dissimilar weldments were discussed.

Effect of carbon dioxide and temperature on passive film parametersof superduplex stainless steel

Superduplex stainless steel has been frequently employed in new sites of Brazilian Pre-Salt. In these environments, chloride concentration, temperature and carbon dioxide are normally present in higher levels than those at sea water at room temperature. In these conditions, it is expected that the passive films of stainless steel also show modifications. To better understand such modifications, samples of superduplex stainless steel UNS S32750 were submitted to electrochemical impedance measurements in brine media, at two temperatures and under presence/absence of carbon dioxide. The electrochemical impedance results were initially tested using the Kramers-Kronig transform and subsequently fitted by equivalent circuit employing constant phase elements - CPE. Moreover, to quantify the effect of each factor (temperature, chloride, carbon dioxide and microstructure) on the equivalent circuit, their parameters were tested applying statistical analysis. Significant effect of carbon dioxide and temperature was found on related parameters of passive film for heat-treated samples.

Study of deleterious phase precipitation under continuous cooling of superduplex stainless steel UNS S32750

Duplex and superduplex stainless steels have high strength and high corrosion resistance with a crescent demand in chemical and petrochemical industries. The best mechanical properties of this family of steels are obtained with approximately equal parts of ferrite and austenite. Grain refinement increases the toughness and resistance to environmentally assisted cracking. The precipitation of tertiary phases, such as σ, χ and secondary austenite, decreases the mechanical and corrosion resistance properties. The σ phase is the most dangerous of these deleterious phases and the one that precipitates in higher amounts. The majority of research works on deleterious phases in duplex and superduplex steels evaluate the kinetics of precipitation in specimens isothermally treated. In the present work, the precipitation of deleterious phases during continuous cooling from 1100°C was evaluated by optical and scanning electron microcopies, hardness measurements and electrochemical corrosion tests. Two superduplex stainless steel steels UNS S32750 with similar composition but quite different grain sizes were analysed in order to evaluate the influence of this microstructural feature.

Influence of heat treatments on the micro-abrasion wear resistance of a superduplex stainless steel

Abstract Duplex and superduplex stainless steels are corrosion resistant materials with increasing applications on chemical, petrochemical, power generation and storage and transport industrial sectors. This work investigates abrasive wear resistance of a superduplex stainless steel UNS S32750 carried in a micro-abrasion test. The microstructure of the steel was changed by heat treatments at 475 °C, 850 °C and 950 °C. These treatments induced a hardening effect on steel due to the precipitation of tertiary phases (α′, χ and σ). Micro-abrasion tests were conducted in a fixed-ball apparatus. Topographical analysis showed that the wear scar reproduced the counter-body shape. The cap diameter was measured using an image acquisition system attached to the equipment, which captures an amplified image of the wear scar. The abrasion was evaluated after 14 min of testing, regarding a well described steady state criterion. Results showed that the abrasion wear resistance increased 50% in comparison to the non-treated material, due to the presence of phase precipitation. The heat treatments leaded to an interesting increase of mechanical strength and wear resistance. Results showed a clear tendency of the decrease of abrasive wear coefficient with the increase of hardness. The effects on toughness and corrosion resistance were also analyzed and discussed. The sample heat treated at 475 °C and 8 h (presence of α′ precipitate) resulted in the same toughness and corrosion resistance of the non-treated sample. Analysis carried out using scanning electron microscopy (SEM), showed different wear mechanisms when comparing precipitation phases and the austenite phase.

Sigma phase precipitation and properties of super-duplex stainless steel UNS S32750 aged at the nose temperature

The nose temperature for σ-phase precipitation in super-duplex stainless steel (SDSS) UNS S32750 was evaluated by hardness method. Color-optical microscopy, scanning electron microscopy, energy spectrum analysis, impact and corrosion testing were carried out to investigate characteristics of microstructure and properties of the SDSS aged at the nose temperature. The experimental results indicate that the nose temperature of precipitation is 920 °C and aging at this temperature tiny σ phases can precipitate at phase interfaces or ferrite grain boundaries within 2 min. Prolonging aging duration the amount of σ-phase increases and a dual structure with σ and γ is obtained when aging for 120 min. The precipitation of σ-phase leads to severe deterioration in impact toughness (longitudinal/transverse direction) and corrosion resistance of SDSS.

Deleterious phases precipitation on superduplex stainless steel UNS S32750: characterization by light optical and scanning electron microscopy

Deleterious phases precipitation in superduplex stainless steels is the main concern in fabrication by welding and hot forming of this class of material. Sigma, chi and secondary austenite phases are considered deleterious phases because they produce negative effects on corrosion resistance. Besides, sigma and chi phases also promote strong decrease of toughness. In the present work, the precipitations of sigma, chi and secondary austenite under aging in the 800-950 °C interval were studied in two UNS S32750 steels with different grain sizes. The deleterious phases could be quantified by light optical microscopy, with no distinction between them. Scanning electron microscopy was used to distinguish the individual phases in various aging conditions. The results elucidate the influence of the aging temperature and grain size on the kinetics precipitation and morphology of deleterious phases. The kinetics of deleterious phases is higher in the fine grained material in the initial stage of aging, but the maximum amount of deleterious phases is higher in the coarse grained steel.

Annealing temperature effects on superduplex stainless steel UNS S32750 welded joints. II: pitting corrosion resistance evaluation

This study deals with the effect of the annealing temperature on the pitting corrosion resistance of UNS S32750 submerged-arc welded joints. In a companion article (Part I), the influence of post-weld annealing temperature on microstructure evolution and chemical composition of austenite and ferrite was analyzed; this study can thus be considered directly connected with the previous one. The pitting corrosion resistance of the heat-treated welded joints was evaluated by using both electrochemical measurements and ASTM G48 standard gravimetric tests; examinations of initiation sites of pitting attack were carried out in order to correlate the experimental data obtained in this study with the predicted pitting corrosion behavior obtained by using the results described in Part I. Generally, the post-weld annealing treatment enhances the pitting corrosion resistance of UNS S32750 welded joints. By using PREN analysis of single phases, a correlation between the chemical composition evolution of ferrite and austenite and the experimental pitting behavior of the welded joints was found, in relation to welding and post-welding heat treatment temperature. In particular, an exponential relationship between PREN of weaker phase and pitting potential in 3.5% NaCl solution at 80 °C for the weld metal was obtained. The most favorable annealing temperature for the analyzed welded joints was found to be 1100 °C.

Influence of the grain size on deleterious phase precipitation in superduplex stainless steel UNS S32750

In the present work, the effect of grain size on deleterious phase precipitation in a superduplex stainless steel was investigated. The materials studied were heat treated isothermally at 800 °C, 850 °C and 900 °C for times up to 120 min. Hardness tests, light optical microscopy, scanning electron microscopy and X-ray diffraction were carried out to detect sigma and other harmful precipitate phases. The ferritic and austenitic grain sizes in the solution treated condition of the two steels analyzed were measured by electron backscattered diffraction (EBSD). Cyclic polarization corrosion tests were performed to evaluate the effect of grain size on the corrosion resistance. The results presented show that the precipitation of deleterious phases such as χ, σ and γ 2, which can occur during welding and forming operations, is retarded by grain growth.

Electrochemical noise characterization of heat-treated superduplex stainless steel

Three microstructures of superduplex stainless steel UNS S32750 were evaluated in which concerns their corrosion resistance in a 70,000 ppm chloride medium at 25 and 60 °C. These environmental conditions are found in ultra-deep offshore oil exploitation. The microstructures are related to as-received, as-welded and hot-worked steel processing. The corrosion behavior was characterized for 30 min and 24 h by means of polarization curves and mainly electrochemical noise, EN, measurements performed with a zero resistance ammeter setup. The results show that, in this highly aggressive environment, time and temperature affect the corrosion resistance of each microstructure differently. The presence of sigma precipitates at grain boundaries trigger the breakdown of passive films at lower potential as well as in the case of coupling different microstructures. The as-received homogeneous one is nonetheless also susceptible to undergoing corrosive attack at long-lasting high temperature exposition.

Characterization of microstructure, chemical composition, corrosion resistance and toughness of a multipass weld joint of superduplex stainless steel UNS S32750

The superduplex stainless steels have an austeno-ferritic microstructure with an average fraction of each phase of approximately 50%. This duplex microstructure improves simultaneously the mechanical properties and corrosion resistance. Welding of these steels is often a critical operation. In this paper we focus on characterization and analysis of a multipass weld joint of UNS S32750 steel prepared using welding conditions equal to industrial standards. The toughness and corrosion resistance properties of the base metal, root pass welded with gas tungsten arc welding, as well as the filler passes, welded with shielded metal arc welding, were evaluated. The microstructure and chemical composition of the selected areas were also determined and correlated to the corrosion and mechanical properties. The root pass was welded with low nickel filler metal and, as a consequence, presented low austenite content and significant precipitation. This precipitation is reflected in the corrosion and mechanical properties. The filler passes presented an adequate ferrite:austenite proportion but, due to their high oxygen content, the toughness was lower than that of the root pass. Corrosion properties were evaluated by cyclic polarization tests in 3.5% NaCl and H 2SO 4 media.