Steel UNS S32750 Research Articles

Phase transformations of the duplex stainless steel UNS S31803 under non-isothermal conditions

Precipitation of the intermetallic phases embrittles duplex stainless steels. Currently, most studies in this subject simulated aging phenomena by first holding the material at different isothermal conditions and then, quenching down under high cooling rates to investigate the subsequent microstructures and mechanical properties. In this work, specimens of the duplex stainless steel UNS S31803 were submitted to cooling cycles at controlled rates under non-isothermal conditions. The volume fraction of sigma and chi formed was less than 1%. The value was lower than that assumed by the Sieurin and Sandstrom model. We relate this to the initial DSS morphology, with austenite islands in a ferritic matrix. Here, the number of nucleation sites for the deleterious phases is smaller than the lamellar microstructure of ferrite and austenite usually found in duplex stainless steels.

Effect of Ultrasonic Shot Peening on the Microstructure of Austenitic Stainless Steel 316 and Super Duplex Stainless Steel UNS S32750.

Shot peening for nanocrystallization of the surface region is a good way of improving corrosion-, fatigue-, and wear-resistance, etc. of metallic parts (Lu, K., 2014. Making strong nanomaterials ductile with gradients. Science, 345(6203), pp.1455-1456). The technique has been widely used for various materials as a method of surface modification. SUS316 has excellent corrosion and oxidation resistance with good formability However, its application is limited by the low mechanical strength and hardness. S32750 (duplex stainless steel) is one of most used tubing materials for oil/gas delivery systems in a corrosive environment under high pressure (Nilsson, J.O., 1992. Super duplex stainless steels. Materials Science and Technology, 8(8), pp.685-700). Thus, improving corrosion resistance is a key for the wider application and better maintenance of S32750. In our study, the alloy S32750 was heat-treated at 1070°C to obtain a precipitation-free microstructure (γ and δ dual-phase structure). It was then ultrasonic shot peened and microstructures were analyzed for: (1) surface nanocrystallization, (2) effect of the treatment processing parameters, and (3) the determination of microstructural evolution and the effect of the shot peening process.

Premature Failure of Grade-316Ti Stainless Steel Tubing in a Boiler Feed-Water Heat Exchanger in a Steel Complex

Premature failure of seamless tubes made of 316Ti stainless steel in a boiler feed-water (BFW) heat exchanger occurred at a local steel complex, which led to about 65–70% service-life reduction. The heat exchanger is a fixed-tube-sheet type with hydrogen gas flowing inside the tubes and water circulating through the shell side. The cooled hydrogen gas is purged to box annealing furnaces to create a reducing atmosphere. Failure analysis was conducted on the tubes to determine the contributing factors to premature failure of the heat exchanger. Four issues including corrosion fatigue, pitting corrosion attack, under-deposit corrosion, and fouling contributed to the failure of the tubes in the heat exchanger. The pitting corrosion attack was caused by chloride in circulating water. Chloride was also detected in accumulated deposits on the tubes causing the under-deposit corrosion issue. Contamination of boiler feed-water with Ca, Si, Mg, and Cl occurred during the water treatment process, a congruent phosphate–pH treatment in an in-house water treatment unit. The quality of the supplied water to the heat exchanger does not meet the recommended specifications. As a major recommendation, the tubes of the heat exchanger must be manufactured of super duplex stainless steel UNS S32750, based on specifications of the ASTM A789 standard.

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.

Influence of Electropulsing Treatments on Mechanical Properties of UNS S32750 Duplex Stainless Steel.

Prestrained at 5% and 15% duplex stainless steel UNS S32750 specimens have been subjected to electropulsing treatments with current density of 100 A/mm2 and 200 A/mm2 and 100 and 500 pulses for each current density value. Corrosion tests, X-ray diffraction, microhardness and residual stresses were collected before and after the electropulsing treatments. Tensile tests were performed after the electropulsing treatments in order to compare the mechanical response to reference tensile tests performed before pulsing treatments. Increase in fracture strain was observed after pulsing treatment in comparison to the reference tensile tests. A decrease in microhardness was also observed after electropulsing treatments for both degrees of prestrain. Electropulsing treatment almost eliminates the work-hardened state in the 5% prestrained specimens while partially recovered the 15% prestrained material increasing both uniform and fracture strain. Bulk temperature of the samples remained the same for all treatments duration. The effect are to be addressed to a combined effect of increase in atomic flux due to the electrical current and local joule heating in correspondence of crystal defects. Electropulsing treatment applied to metallic alloys is a promising technique to reduce the work hardening state without the need of annealing treatments in a dedicated furnace.

Microstructural and Corrosion Resistance of Lean Duplex Stainless Steel UNS S32304 Welded by SAW with Cold Wire Addition

The results of the corrosion tests, in chloride environment, of lean duplex stainless steel UNS S32304 welded by submerged arc welding (SAW) process variant with cold wire, indicated that the sample with higher heat input heat input (HI) (2.7 kJ/mm) had higher corrosion resistance, which can be explained by higher amount of austenite and lower amount of precipitates. Therefore, the results indicate that the use of HI above the limit recommended by literature (2.5 kJ/mm) may produce welds with adequate corrosion resistance with SAW with cold wire addition, because this technique has the potential to deliver higher productivity still controlling the process HI.

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.

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.

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.

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.

Effect of ageing time on the toughness and the corrosion properties of duplex stainless steel UNS S31803

Duplex stainless steels are materials with high mechanical strength, toughness, and corrosion resistance, properties that make them rather appealing for the application on chemical and petrochemical industries. However, the exposure of such materials to high temperatures promotes the precipitation of deleterious intermetallic phases that cause significant damage to the mechanical and corrosion properties of these materials. In this study, the UNS S31803 duplex stainless steel received a thermal treatment at a temperature of 830 °C for 30, 90, and 180 minutes. After the treatment, the precipitates were characterized by backscattered electrons (BSE) and chemical composition mapping, and the properties of impact toughness were assessed by Charpy V-notch test for the different treatment conditions. The corrosion resistance properties were assessed by cyclic potentiodynamic polarization study. The thermal treatments at 830 °C resulted in a considerable reduction of impact toughness, and the corrosion resistance was also reduced in longer treatment times along with the loss of the passivation ability of the materials treated.

A New methodology of Nondestructive Testing for Quantitative Evaluation of Sigma Phase in Duplex Stainless Steels

Duplex Stainless Steels (DSSs) are widely used in the oil and gas industry. When the steel is subjected to a thermal gradient, as in welding may occur precipitation of intermetallic phases. Among these phases, there is the sigma phase, which even in small quantities reduces drastically the mechanical and corrosion resistance properties. According to some studies in the literature, the limit amount of sigma phase present in steel in welded joints must be in a maximum volume fraction of 2,5%. In search to improve the detection sensitivity of the sigma phase, this paper presents results obtained by Linear Sweep Voltammetry (LSV) in Duplex Stainless Steel UNS S31803 as a Nondestructive Testing (NDT) model. The innovation in the application of this test is to use a microcell-based system that allows a reduced area of ​​analysis obtaining density of currents in the order of microamperes and in the use of selective electrolytes to the elements of the oxides present in the intermetallic phases. With the use of these systems and through the optimization of control parameters it was possible to detect very low amounts of deleterious phases such as the sigma and chi phases.

Produção e caracterização de compósito metálico laminado produzido por laminação à frio

O crescente interesse pelas indústrias por compósitos metálicos laminados resultou da necessidade de se desenvolver tecnologias de fabricação cada vez mais eficientes. Visto que é um assunto de pouco debate no Brasil e resultou no atraso do conhecimento científico do assunto, o presente trabalho objetivou a produção de um composto metálico laminado por meio do processo de laminação a frio na tentativa de melhorar as condições de competitividade do material nacional com o mercado exterior. O primeiro composto proposto era constituído por alumínio comercial AA1050 e aço inoxidável duplex UNS S32304. Uma vez as tentativas de sua produção foram ineficazes, o segundo foi substituído pelo aço inoxidável ferrítico AISI 430 com o objetivo de entender os mecanismos de ligação. O uso desse metal permitiu a produção de compósitos metálicos laminados e posterior estudo com ensaios de aderência (peeling) e análises microestruturais do mecanismo de aderência dos materiais.

Microstructural and Corrosion Properties of Cold Rolled Laser Welded UNS S32750 Duplex Stainless Steel

The main goal of this work was to study the effect of plastic deformation on weldability of duplex stainless steel (DSS). It is well known that plastic deformation prior to thermal cycles can enhance secondary phase precipitation in DSS which can lead to significant change of the ferrite-austenite phase ratio. From this point of view one of the most important phase transformation in DSS is the eutectoid decomposition of ferrite. Duplex stainless steels (DSSs) are a category of stainless steels which are employed in all kinds of applications where high strength and excellent corrosion resistance are both required. This favorable combination of properties is provided by their biphasic microstructure, consisting of ferrite and austenite in approximately equal volume fractions. Nevertheless, these materials may suffer from several microstructural transformations if they undergo heat treatments, welding processes or thermal cycles. These transformations modify the balanced phase ratio, compromising the corrosion and mechanical properties of the material. In this paper, the microstructural stability as a consequence of heat history due to welding processes has been investigated for a super duplex stainless steel (SDSS) UNS S32750. During this work, the effects of laser beam welding on cold rolled UNS S32750 SDSS have been investigated. Samples have been cold rolled at different thickness reduction (ε = 9.6%, 21.1%, 29.6%, 39.4%, 49.5%, and 60.3%) and then welded using Nd:YAG laser. Optical and electronical microscopy, eddy’s current tests, microhardness tests, and critical pitting temperature tests have been performed on the welded samples to analyze the microstructure, ferrite content, hardness, and corrosion resistance. Results show that laser welded joints had a strongly unbalanced microstructure, mostly consisting of ferritic phase (~60%). Ferrite content decreases with increasing distance from the middle of the joint. The heat-affected zone (HAZ) was almost undetectable and no defects or secondary phases have been observed. Both hardness and corrosion susceptibility of the joints increase. Plastic deformation had no effects on microstructure, hardness or corrosion resistance of the joints, but resulted in higher hardness of the base material. Cold rolling process instead, influences the corrosion resistance of the base material.

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.

Effect of heat input on the corrosion resistance of the cold worked 2304 lean duplex stainless steel GMAW welded joint

The effect of the welding GMAW, with heat input of 1.4 kJ/mm, on the microstructure and corrosion resistance of the steel UNS S32304 prior cold worked (0, 10, 30, and 30% thickness reduction) was evaluated. The welding process generated, in the weld joint, the regions: fusion zone (FZ), heat‐affected zone (HAZ), an annealed region around the HAZ, and the metal base zone (BM) deformed by cold working. Excessive ferritization and high precipitation of chromium nitrides in the HAZ were identified. The corrosion resistance of the welded joints was not significantly changed for low deformation levels by cold work (up to 30% thickness reduction). However, for 50% thickness reduction a decrease in the corrosion resistance of the welded joint was observed. The annealing of the BM around the HAZ increased its corrosion resistance, making this region immune to localized corrosion. The heat input used in this study promoted higher ferritization and precipitation of chromium nitrides, lower annealing of the base metal near the HAZ and lower corrosion resistance when compared with the higher heat input used in our previous work (about 2.5 kJ/mm).

Formation of Secondary Phases and Their Effect on the Mechanical Properties of Joints Formed by TLP Bonding Using Fe–B–Si Insert Metal in Duplex Stainless Steel

We studied the effects of secondary phases on the mechanical properties of joints formed by transient liquid-phase bonding in nitrogen-containing duplex stainless steel UNS S32750. Transient liquid-phase bonding was carried out using Fe–B–Si amorphous insert metal over the temperature range of 1423–1473 K for 0–1000 s under a vacuum of 5 × 10−5 Torr. Area fraction of austenite (γ) in the joint region decreased with increasing bonding temperature and holding time. In particular, when the holding time was sufficiently prolonged, γ phase-depleted region were observed in the base metal adjacent to the joint. Secondary phases formed at the joint after short holding times were predominantly chromium borides. For specimens bonded with longer holding times (up to 1000 s), boron nitrides were formed at the interface with the joint. Formation of secondary phases in a joint area during the bonding process was expressed by diffusion equation. Tensile strength was improved in accordance with increase of bonding temperature and holding time. Especially, the bonding efficiency was over 98% for the specimen held for 1000 s at 1473 K. Tensile strength of the joint after short holding times was affected by brittle residual eutectic constituents and borides, whereas tensile strength after longer holding time was influenced by boron nitride formed at the joint interface. The results showed that the mechanical properties were strongly related to the secondary phases (CrB, BN) formed during the bonding process.

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.

Comparison of the effect of carbide addition on particle size reduction on UNS S31803 steel chip millings

This study aims to analyze the efficiency of additional carbides in the high-energy milling of duplex stainless steel chips. At the end of the life of a mechanical component, it is discarded in deposits or recast. Chips originating from machining processes are raw materials for products obtained through powder metallurgy. To obtain the duplex structure for a stainless steel, rolled and annealing are necessary subsequent steps, making this cycle costly for the industry, and its disposal leads to environmental problems. Components produced by powder metallurgy from corrosion-resistant alloys are considered advantageous. The addition of carbides aims to increase the efficiency of the milling process and improve the mechanical properties of the steels produced by powder metallurgy. The granulometric analysis and the scanning electron microscopy were used in the characterization and identification of the particle size. Results indicated that the milling with the addition of carbides showed a higher efficiency in the milling process. The particle size obtained with the addition of TiC and NbC in the milling of UNS S 31803 steel chips was 98 and 85 times smaller than the original chip.

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.