Duplex UNS Research Articles

Environmentally Assisted Cracking of Duplex and Lean Duplex Stainless Steel Reinforcements in Alkaline Medium Contaminated with Chlorides

Herein, the corrosion performance of different stainless steel (SS) reinforcing bar grades in alkaline solution is presented, including UNS S32205 duplex stainless steel (DSS), UNS S32304 and UNS S32001 lean DDS (LDSS). The electrochemical dissolution kinetics were studied by potentiodynamic polarization and the Tafel slope method. The environmentally assisted cracking (EAC) mechanisms of the different SS grades in the presence of Cl− were revealed with the slow strain rate test (SSRT). The higher activation of the anodic branch and the loss of toughness were related to the austenite-to-ferrite phase ratio. UNS S32205 DSS presented the slowest anodic dissolution kinetics, mainly due to the higher austenite content compared to the other LDSS; however, it suffered a more severe EAC than the UNS S32304 LDSS. In the case of UNS S32001 LDSS, even while having the lowest Ni content (i.e., large ferrite α-phase ratio), it experienced the least decrease in elongation as well as low anodic dissolution kinetics for Cl− contents up to 8 wt.%, where the Cl− threshold was reached.

Integrity Assessment of Stress Corrosion Cracking Susceptibility of Duplex UNS S32205 and Austenitic UNS S31653 Stainless Steel Reinforcements

Herein, the stress corrosion cracking (SCC) susceptibility of duplex (UNS S32205) and austenitic (UNS S31653) stainless steel (SS) reinforcements was evaluated using integrity assessment criteria. Mechanical properties were analyzed and compared by different SCC susceptibility factors. The integrity assessment was conducted applying Cosenza, Creazza, and Ortega ductility criteria, following three different standards (ACI 318-19, ASTM A615, and FIB). A conventional carbon steel (UNS G10080) reinforcement was also evaluated for comparative purposes, whose high residual stress value (>280 MPa) promoted a high corrosion growth rate. Duplex UNS S32205 SS grade showed a significant decrease in elongation, leading to failure after ductility assessment at high chloride concentrations. Fractographic analysis of both SS grades, duplex and austenitic, revealed less than 40% brittle areas at 8 wt.% Cl−, while UNS G10080 had over 85% at 4 wt.% Cl−.

Influence of Pulsed Arc Parameters on the Tig Welding Process for the Stainless Steel Duplex UNS S31803.

The influence of parameters involved in the pulsed electric arc, used as an energy source in the tungsten inert gas (TIG)-mediated welding of Duplex UNS S31803 stainless steel, to attend the manufacture of flexible pipes for the extraction of oil and gas is presented. A fundamental part in the manufacturing process of flexible pipelines is the welding of these strips so that corresponding TIG welds will be subjected to the same process and work conditions. Therefore, it is necessary to maintain the same properties in the welded regions. Covering the effects of each parameter of the pulsed electric arc such as peak and base current as welds, cyclic ratio, and pulsation frequency is a desirable endeavor. The final objective is the mitigation of problems that have a great impact on production, such as weld breakage during the conformation of the strip in the process and test failures. With this, tensile, bending, and ferrite percentage tests were performed on 12 samples that qualified as satisfactory in the visual aspect. A minimum tensile strength of 734.57 MPa and a maximum of 775.77 MPa were obtained where all values found are above the tensile strength limit of the base material of 620 MPa. With the completion of the study, it is possible to understand not only the response of the process to each parameter but also the tendency when changing them. Moreover, it is possible to explore the possibility of guiding the changes to achieve results about the visual aspect and the mechanical properties of the welded material.

Microstructure, Corrosion Resistance, and Hardness of Simulated Heat-Affected Zone of Duplex UNS S32205 and Superduplex UNS S32750 Stainless Steels

Microstructure, Corrosion Resistance, and Hardness of Simulated Heat-Affected Zone of Duplex UNS S32205 and Superduplex UNS S32750 Stainless Steels

Strain Rate Sensitivity Analysis of Duplex and Superduplex Steels in Tensile Tests

Abstract Duplex and super duplex stainless steels have high corrosion resistance, excellent mechanical properties, and high impact strength. Such characteristics make these steels able to operate in various segments of the industry, especially in aggressive environments. These alloys have higher strength than austenitic and ferritic stainless steels and their operation is generally restricted to temperatures lower than 300 °C. Most steels and alloys only exhibit elasto-viscoplastic behavior at temperatures higher than 1/3 of the absolute melting temperature. The objective of this work was to make an experimental study showing that the duplex and superduplex steels present a rate-dependent behavior even at room temperature (around 25 °C). Two different alloys: duplex stainless steel UNS 31803F51 and super duplex UNS S32760GRF55 were used in the study. Simple models were proposed to describe how the rate-dependent portion of the stress (called at this paper “the viscous term”) depends on the strain rate. A simple procedure to identify experimentally all material constants that appear in the theory is presented. The experimental results are in very good agreement with the model predictions.

Reproducibility of the Corrosion Resistance of UNS S32205 and UNS S32304 Stainless Steel Reinforcing Bars

One strategy to address the 75-year service life requirement of a number of highway bridge design codes is the use of more corrosion-resistant reinforcement. Over the last few decades, many stainless steel grades have become available for reinforcing concrete. Of these, two duplex alloys, UNS S32205 and UNS S32304, have become the preferred choices for major highway structures in North America. The purpose of the present study was to investigate the consistency, or variability, of the corrosion performance of bars of these grades from different steel manufacturers. Metallography, chemical analysis, corrosion resistance, and corrosion product evaluation were used to evaluate the bars. It was found that micropits exist to varying degree in the as-received conditions of these bars and that was the most dominant factor in determining the corrosion performance of the bars.

Effect of Grain Size on the Anodic Dissolution of Lean Duplex UNS S32202 Austenitic-Ferritic Stainless Steel

The effect of grain size on the anodic dissolution of lean duplex UNS S32202 dual-phase austenitic-ferritic stainless steel was evaluated. Grain coarsening was achieved by heat treatment, and grain size and grain boundary densities determined by automatic image analysis after etching. Potentiodynamic electrochemical testing in acidic chloride medium allowed isolating the anodic dissolution behavior of the crystallographic phases of the material. A relationship between grain boundary density (for grain sizes in the micrometer range) and dissolution rate has been found, showing that reducing grain size enhances active corrosion rates in environments that promote active behavior. This leads to new possibilities of industrial adjustment of the corrosion behavior of duplex stainless steels via grain size control.

Comparative study on the forming and reversion of strain-induced martensite in two duplex stainless steels: Developing a model for VSM analysis of powders or fine chips

Abstract An original feature of this work is the proposal of two equations to fit the volume fraction of ferromagnetic (α + α′) phases that can be applied in the measurement of magnetic saturation of comminuted duplex and superduplex stainless steels (for example, powders or filed chips). Duplex stainless steels contain similar volume fractions of austenite (γ) and ferrite (α) in their microstructure. Two steels exemplify this class, namely the most widely used duplex UNS S31803 and superduplex UNS S32520 stainless steels. The phenomena of work hardening, formation, and reversion of strain-induced martensite (α′) in austenite were compared in both stainless steels. Samples were work-hardened and annealed under identical conditions, and their behavior was evaluated mainly through X-ray diffraction and magnetic measurements. Notably, the volume fraction of strain-induced α′ in duplex stainless steel was as high as 32%, which indicated that this steel had a greater tendency to form α′ than superduplex stainless steel, for which the corresponding value equaled 15%. Annealing at 650 °C for 2 h promoted the reversion of strain-induced α′ into γ, decreasing the volume fraction of the former phase from 32 to 2% (duplex) and from 15 to 6% (superduplex).

Investigation of Stress Corrosion Cracking of Austenitic, Duplex and Super Duplex Stainless Steels under Drop Evaporation Test using Synthetic Seawater

Abstract Stress corrosion cracking (SCC) of UNS S31603 austenitic stainless steel (ASS), UNS S32205 duplex stainless steel (DSS) and UNS S32750 super duplex stainless steel (SDSS) was investigated. SCC tests were carried out at 110 oC for 500 h under drops of synthetic seawater (DET, drop evaporation test). Two loading conditions were investigated: 50 % and 100 % of the experimental yield strength of each steel. DSS and SDSS specimens showed no susceptibility to SCC under loading of 50 % of their yield strength, contrary to ASS, but all steels fractured at the highest load. SCC nucleated under the salt deposit formed on the surface of all specimens. SCC propagation was mainly transgranular, but SCC propagation of DSS also featured crack ramification in the austenite phase. In addition, SDSS also presented crack propagation along the ferrite/austenite interfaces. Transgranular cleavage fracture was also observed in all fractured specimens, but DSS also presented ferrite/austenite interfacial brittle fracture, while SDSS also featured intergranular brittle fracture.

Study of Formation and Reversion of the Martensitic Phase Induced by Deformation of Lean Duplex Stainless Steel

Lean duplex stainless steels consist of a low percentage of nickel and molybdenum, presenting twice as much resistance compared to austenitic stainless steel and their cost is about twice as low. However, this class of steels has microstructural instabilities, such as the formation of martensite induced by austenite deformation by cold rolling. This feature can significantly alter the properties of interest of this steel. The formation of the martensitic structure, as well as its reversion, is little studied in the steels of the austenitic–ferritic structure. The process of formation and reversal of the martensitic structure in cold rolled stainless steel duplex UNS S32304 was investigated through magnetic measurements, microhardness and X-ray diffraction analyzes. The deformation process allowed the formation of the -martensite phase from the austenite phase with an increase in the values of saturation magnetization, coercive field and micro-hardness values as well as a change in the intensity of the X-ray diffraction peaks. The heat treatment performed at $$650\,^\circ \hbox {C}$$ showed an increase in the peak intensity of the austenitic phase and a decrease in the saturation magnetization values, demonstrating a possible reversal of the martensitic structure. The SEM observations after annealing the Beraha’s etched samples revealed the possibility of a martensite transformation and reversion in a Lean duplex stainless steels.

A New Method to Recycle Stainless–Steel Duplex UNS S31803 Chips

Due to the increased consumption of raw materials, energy, and the waste it generates, recycling has become very important and fundamental for the environment and the industrial sector. The production of duplex stainless–steel powders with the addition of vanadium carbide in the high energy mechanical milling process is a new method for recycling materials for the manufacture of components in the industrial sector. This study aims to reuse the chips from the duplex stainless–steel UNS S31803 by powder metallurgy with the addition of Vanadium carbide (VC). The mechanical milling was performed using a planetary ball mill for 50 h at a milling speed of 350 rpm and a ball-to-powder weight ratio of 20:1, and the addition of 3 wt % of VC. The material submitted to milling with an addition of carbide has a particle size of less than 140 μm. After milling, the sample went through a stress relief treatment performed at 1050 °C for 1 h and the isostatic compaction process loaded with 300 MPa. The sintered powders and material was characterized by scanning electron microscopy, X-ray diffraction, and micro-hardness tests. The milling process with an addition of 3% VC produced a particle size smaller than the initial chip size. The measurement of micrometric sizes obtained was between 26 and 132 μm. The sintered material had a measurement of porosity evaluated at 15%. The obtained density of the material was 84% compared to the initial density of the material as stainless–steel duplex UNS S31803. The value of the microhardness measurement was 232 HV. The material submitted for grinding presented the formation of a martensitic structure and after the thermal treatment, the presence of ferrite and austenite phases was observed. Thus, in conclusion, this study demonstrates the efficacy in the production of a metal-ceramic composite using a new method to recycle stainless–steel duplex UNS S31803 chips.

Mechanistic study on micromilling of the super duplex stainless steel UNS S32750

Super duplex UNS S32750 is an important steel used in the deep oil industry that could be applied in the fabrication of microsensors and microactuators. However, there are no previous studies that could estimate cutting forces, no information in the literature about specific cutting force values for super duplex nor for similar stainless steels in micromilling. The present article analyzes three stainless steels and their behavior during micromilling: super duplex, which is a dual phase material, an austenitic and a ferritic stainless steel, chosen with similar mechanical properties to each phase of the first one. A set of experiments explored different feeds per tooth, as it is an important factor to identify mechanical behavior in micro scale, and cutting speed in order to develop a mechanistic force approach. The results showed that force coefficients for the dual phase material are located between the values of the austenitic and ferritic steels, indicating that heterogeneous super duplex steel microstructure can be modeled as a combination of ferritic and austenitic stainless steel coefficients. Lower feed per tooth experiments presented higher fluctuation of the maximum peak forces, maintaining average forces inside the predicted range.

Effect of nitrogen in shielding gas of keyhole GTAW on properties of duplex and superduplex welds

Keyhole gas tungsten arc welding (GTAW) is a relatively new variant used for fabrication of pressure equipment where up to 10-mm-thick material can be autogenously welded in one pass. As the shielding gas used for keyhole mode is typically Ar + 5% H2, the suitability of this method for welding of duplex stainless steels has been investigated. Duplex UNS S31803 and superduplex UNS S32750 schedule 40 seamless pipe was welded using a closed square butt joint configuration. Shielding gas compositions were applied with increasing nitrogen content in argon or argon-hydrogen mixtures. An investigation of the completed welds included examinations of the microstructure, corrosion and mechanical properties of the weld metal. All shielding gases resulted in high-quality welds free from defects, showed high strength and excellent corrosion resistance. Hydrogen additions increased the weld metal nitrogen loss and resulted in lower austenite formation. Ar + 5% H2 showed up to 20 vol.% less austenite than the gas types containing nitrogen and also showed the lowest impact toughness. At least 2% nitrogen would be recommended for optimizing the phase balance and impact toughness. Taking all aspects into account, Ar + 2% N2 is the preferred shielding gas for welding duplex and superduplex with the keyhole GTAW process.

Crevice Corrosion of Duplex Stainless Steels by Cyclic Potentiodynamic Polarization and Potentiostatic Techniques

Crevice corrosions of duplex stainless steels were investigated under the aggressive condition of sea water pumps in offshore petroleum platforms. The study aims to compare crevice corrosion behavior of three grades of duplex stainless steels, UNS S32101 (2101), UNS S31803 (2205) and UNS S32750, in seawater with 200 ppm of hypochlorite. Duplex steels were compared with a widely used austenitic steel UNS S31603 (316). Specimens were tested under synthetic sea water prepared at pH4, pH6 and pH8 by the cyclic potentiodynamics polarization technique and the potentiostatic technique. The results show that the duplex stainless steels had higher corrosion resistance compared with the 316. The released charges and the corrosion rates of the 2101 and the 2205 were similar, but much lower than those of the 2507especially after 3 months period.

Brazing of UNS S32101 and UNS S32304 lean duplex stainless steels and UNS S32750 super duplex with AWS A5.3 BNi-7 (Ni–Cr–P) filler metal

Due to their corrosion resistance, duplex stainless steels are widely applied in various industries in which corrosion resistance is a required property. Unlike ferritic and austenitic stainless steels, duplex stainless steels have superior mechanical strength and can be used to produce lighter equipment. When thermally cycled, the properties of these steels can change, especially when using joining processes such as brazing, and when the austenite/ferrite volume fraction becomes imbalanced. This study aims to describe the brazing of lean duplex and superduplex stainless steels, verifying the effect of the joint gap and brazing time on the formation of continuous intermetallics inside the brazed joint. In this study, samples of duplex stainless steel UNS S32101, UNS S32304 and UNS S32750 were brazed in a continuous hydrogen atmosphere furnace, using BNi-7 (Ni–Cr–P) filler metal for brazing at a temperature of 1100°C, for 12 and 32 min, using joint gaps of 0.0 and 0.3 mm, followed by in-furnace cooling. The samples were characterized using optical microscopy, scanning electron microscopy with energy dispersion and X-ray diffraction. The results showed that the best condition for the in-furnace brazing used was a joint gap of 0.0 mm. Both of the joint gaps had different microstructures, yet the joint gap of 0.0 mm showed a continuous centred face cubic phase rich in nickel, which is frequently called γ-Ni. This region is composed of intermetallic phases rich in phosphorus. Sigma phase precipitation in the base metal on the duplex stainless steel UNS S35750 (32750?) was observed in the brazing conditions studied.

Martensitic transformation induced by cold deformation of lean duplex stainless steel UNS S32304

Lean duplex UNS S32304 is an austenitic ferritic stainless steel with pitting resistance equivalent number around 24-25. Its corrosion resistance is similar to AISI 316L steel, but with the advantage of a yield limit two times superior to conventional austenitic grades. Deformation induced martensitic transformations (γ→e and γ→ α') have been extensively studied in austenitic steels, but few works were published about these reactions in austenitic ferritic grades. In the present work, deformation induced martensitic transformation in lean duplex UNS S32304 was investigated by means of magnetic measurements and X-ray diffraction. The curves of α' martensite volume fraction against true deformation by cold rolling were constructed and compared to duplex and austenitic grades. It was found that lean duplex UNS S32304 is much more susceptible to martensitic transformation than duplex UNS S31803, which can be explained by its lower Mo and Ni contents.

Effects of low temperature aging on the mechanical properties and corrosion resistance of duplex and lean duplex stainless steels UNS S32205 and UNS S32304

Duplex stainless steels (DSS) with austenite-ferrite microstructure are high strength and corrosion resistant steels frequently used as pipes and accessories in chemical/petrochemical on/and off-shore industries. Low temperature heat treatments (400-475ºC) may increase the hardness and wear resistance of duplex steels, due to a spinodal decomposition reaction of the ferrite, whose small particles α' precipitate in the matrix. In this work, several heat treatments at 400ºC and 475ºC with short duration (<24h) were performed on UNS S32304 and UNS S32205 grades. The kinetics of a' precipitation was studied by hardness measurements. Tensile tests were also carried out for some heat treatment conditions to evaluate the effect of the aging on the ductility of both grades. The hardening of duplex UNS S32205 was more intense than that of UNS S32304, although both steels have shown an interesting increase in tensile strength. Specimens of duplex steel heat treated at 475ºC for 4 h and 8 did not show any detectable decrease of corrosion resistance in anodic polarization and critical pitting temperature tests were carried out in NaCl media.

Erosion–corrosion behaviour of lean duplex stainless steels in 3.5% NaCl solution

The interaction between corrosion and erosion has often been a subject of debate among scientists and engineers. The total material degradation when corrosion and erosion interact is often more than the individual components acting separately. Much effort has been directed towards the study of erosion–corrosion behaviour of carbon steels, standard austenitic and duplex stainless steels, highly alloyed austenitic and super duplex stainless steels. However, the subject of erosion–corrosion of lean duplex stainless steels is still rarely reported. This present work studies the erosion–corrosion behaviour of lean duplex stainless steels UNS S32304 and UNS S32101 in an aerated 3.5% NaCl environment. The erosion–corrosion behaviour of these alloys is compared with an austenitic stainless steel, UNS S30403 and a duplex stainless steel, UNS S32205. Sub-surface microstructure modifications induced by work-hardening as a result of the sand impingement were analysed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Results show that lean duplex stainless steels, UNS S32101 and UNS S32304, have higher resistance to pure erosion than UNS S30403 and UNS S32205; better erosion–corrosion resistance than UNS S30403 austenitic stainless steel; and equivalent erosion–corrosion resistance to UNS S32205 duplex stainless steel. Also, erosion with 500mg/l sand at 15m/s in de-aerated tap water was able to cause subsurface deformation, phase transformation and an increase in hardness on the stainless steels.

Tensile properties of duplex UNS S32205 and lean duplex UNS S32304 steels and the influence of short duration 475 ºC aging

Duplex stainless steels are high strength and corrosion resistant steels extensively used in the petrochemical and chemical industries. The aging at 475 °C for long periods of time provokes embrittlement and deterioration of corrosion resistance. However, short duration aging at 475 °C may be used as heat treatment to improve mechanical resistance with small decrease in the other properties. In this work the flow stress curves of lean duplex UNS S32304 and duplex UNS S32205 steels were modeled with Hollomon’s equation and work hardening exponents (n) were determined. The analyses were conducted in specimens annealed and heat treated at 475 °C for short periods of time. The aging at 475 °C for 4 hours, 8 hours and 12 hours promoted significant hardening with small decrease of ductility. The work hardening exponents of both steels were compared, being higher in the duplex steel than in the lean duplex grade.

Spinodal decomposition mechanism study on the duplex stainless steel UNS S31803 using ultrasonic speed measurements

This work, focuses on the spinodal decomposition mechanism study on the duplex stainless steel duplex UNS S31803, composed by austenite (γ) and ferrite (α) phases, at 425 °C and 475 °C temperatures by ultrasonic speed measurements. This temperature range is responsible for the transformation mechanism of α initial phase to α phases (poor in chromium) and α′ (rich in chromium) by spinodal decomposition. The techniques to accomplish this analysis are based mainly on X-ray diffraction measures and ultrasonic speed. The obtained results show that it is possible to conclude that the use of ultrasonic speed measurements indicates a promising technique for following-up the phase transformation and spinodal decomposition on the steel studied.