Microstructure Of Duplex Stainless Steel Research Articles

Microstructure, texture and mechanical properties of a nickel-free high nitrogen duplex stainless steel processed through friction stir spot welding

A nickel-free high nitrogen duplex stainless steel holding high Mn-content was friction stir spot welded under the various rotational speeds. The microstructure/texture evolutions of the joints and the correlated mechanical properties were investigated in detail. The ferrite and austenite constituent massive phases were refined down to the mean grain size of 0.9 μm and 1.1 μm, respectively. The microstructure evolutions revealed the activation of continuous dynamic recrystallization as the main restoration mechanism. This was further verified through the formation of Β- and Α-fiber shear texture in the specimens processed under the various processing condition. Besides the activation of different restoration micro-mechanisms in both ferrite and austenite, dynamic phase transformation of ferrite to austenite was also contributed in grain refinement. The transformation was facilitated by providing more diffusional paths through substructure development and grain refinement. The local and bulk mechanical properties of the joints were also assessed. The outstanding tensile-shear peak load of 12.8 KN obtained for the sample welded at 400 rpm was discussed relying on the developed ultrafine-grained microstructure and the maximum peak extension of 1.7 mm achieved at 600 rpm was attributed to the role of ferrite phase in strain accommodation.

Effect of laser beam welding on microstructure, tensile strength and fatigue behaviour of duplex stainless steel 2205

This study focuses on the microstructure, tensile and fatigue properties of laser beam welded butt joints in 4 mm thick sheets of duplex stainless steel 2205. The microstructural characteristics of the joints were investigated via optical microscopy and electron backscattered diffraction. A 300 μm wide heat affect zone with increased ferrite content and a nearly fully ferritic 800 μm wide fusion zone were found. No porosity could be found with X-ray radiography. Microhardness measurements revealed increased strength in the fusion and heat affect zones of the weldments. Uniaxial tensile and stress controlled fatigue tests were performed in order to characterize the mechanical properties of specimens containing laser beam welded joints and the base material. The specimens containing weldments were stronger, but less ductile than the base material, due to the weld metal restricting deformation. The as-welded joints exhibited worse fatigue performance than the base material due to the notch at the excess weld metal. The fatigue properties of the specimens containing joints could be elevated to the base material level by a laser surface remelting treatment.

Effect of post-weld heat treatment on microstructure and corrosion properties of multi-layer super duplex stainless steel welds

Abstract To investigate the influence of post-weld heat treatment on the microstructure and corrosion properties of super duplex stainless steel welded joints, multi-layer multi-pass welding of 2507 super duplex stainless steel by tungsten argon arc welding was performed using an ER2594 welding wire. The microstructures of the welded joints before and after post-weld heat treatment at 1150 °C, 1170 °C and 1190 °C were observed, and the mechanical and corrosion properties were tested. The post-weld heat treatment changed the austenite content and morphology of the welded joint and improved the corrosion resistance of different parts of the weld metal. The choice of various solution heat treatment temperatures affected the change in austenite content in the weld zone and the degree of diffusion and homogenization of the alloy elements. After post-weld heat treatment at 1170 °C, the two-phase ratios in each area of the weld were the most suitable and uniform, and the overall mechanical and corrosion properties of the joint were more uniform.

Effect of isothermal aging at 750 °C on microstructure and mechanical properties of UNS S32101 lean duplex stainless steel

UNS S32101 lean duplex stainless steel (DSS) has a two phase microstructure consisting of austenite and ferrite in near equal proportions. Aging at high temperature results in formation of σ phase reducing ductility and also corresponding depletion of Cr from the matrix. The effect of long term aging on the microstructure and mechanical properties is still an active area of research for lean DSSs. In the present work effect of aging at 750 °C (up to 480 h) on microstructure and mechanical properties has been systematically studied. Scanning electron microscopy (SEM) revealed that the precipitates were distributed along ferrite/austenite (δ/γ) interfaces and ferrite/ferrite (δ/δ) boundaries. Nitrides (mainly Cr2N) were observed in initial phases of aging and σ phase afterwards, mostly at longer aging times. The decrease of Ni and Mo and increase in N in UNS S32101 steel delayed the precipitation of σ, but could not avoid it completely. The room temperature absorbed impact energy of specimens decreased gradually as the aging time increased. The lowest value of absorbed impact energy was found to be 24 ± 2 J after 480 h of aging, which was only ~11% as that of solution annealed specimen. Fractography also showed that the fracture morphology changed from fibrous (ductile) to dominant brittle (which involved predominance of cleavage facets along with long, wide cracks i.e. delamination fracture) with increasing aging time. Tensile tests also showed a decrease in ductility and an increase in yield stress/ultimate tensile strength with aging time.

Temperature and time effect of thermal aging treatment on microstructure and corrosion resistance of UNS S31803 duplex stainless steel

Duplex stainless steels correspond to a class of steel in which the microstructure is composed basically by the phases ferrite and austenite. Given the metastable character of ferrite, associated phase transformations can occur during thermal treatments, which can lead to a reduction in corrosion resistance. Therefore, the aim of this work was to evaluate the influence of the solution-treated at 1100°C for 30min and aging at 500 and 600°C for 1, 3 and 12h, on the microstructure and corrosion resistance of DSS UNS S31803. The steels were characterized by SEM, EDS, XRD, hardness, microhardness and thermal analysis. The steels aged at 500°C for 1h presented the phase alpha line dispersed in the ferrite, and the steels aged at 600°C for 12h presented the phases alpha line, sigma and chi. In the assays of double cycle potentiodynamic reactivation in solution of 0.5M H2SO4 and 0.01M KSNC, it was observed that no sensitization occurred. Furthermore, in the tests of cyclic potentiodynamic polarization, in 3.5% NaCl solution, it was verified that the precipitation of the alpha line phase did not alter corrosion resistance; nevertheless, the sigma phase reduced corrosion resistance.

VHCF damage in duplex stainless steel revealed by microbeam energy-dispersive X-ray Laue diffraction

Microstructure of austenitic-ferritic duplex stainless steel loaded in the Very High Cycle Fatigue regime was investigated using microbeam energy-dispersive X-ray Laue diffraction. Scanning electron microscopy analysis of the surface shows that damage in the form of fatigue cracks is initiated at grain boundaries assisted by slip bands observed in austenite grains. Energy-dispersive X-ray Laue diffraction was then used to scan a damaged area containing both a fatigue crack and slip bands, in order to measure the changes in microstructure. Results from the X-ray data from the austenite grain indicates slip activation of the most favored slip system tilted by 45° with respect to the external loading direction, dividing the grain into two regions on either side of the slip band. In the ferrite phase, in front of the crack, variations in the angle and energy spectra of the diffraction peaks indicate the presence of lattice curvature and a strain gradient. In regions around the crack, diffraction peaks spatially split into several sub-peaks indicating the presence of fine granular areas separated by polarized dislocation walls. Possible reasons for the observed structural evolution are discussed and the advantages of using energy-dispersive X-ray Laue diffraction in fatigue damage analysis are illustrated.

Effect of Friction Stir Welding on the Microstructure and Mechanical Properties of Super Duplex Stainless Steel

Effect of Friction Stir Welding on the Microstructure and Mechanical Properties of Super Duplex Stainless Steel

Duplex Stainless Steels—Alloys for the 21st Century

Duplex stainless steels were first manufactured early in the 20th century, but it was the introduction in the 1970s of the argon-oxygen decarburisation (AOD) steel making process and the addition of nitrogen to these steels, that made the alloys stronger, more weldable and more corrosion resistant. Today, duplex stainless steels can be categorised into four main groups, i.e., “lean”, “standard”, “super”, and “hyper” duplex types. These groups cover a range of compositions and properties, but they all have in common a microstructure consisting of roughly equal proportions of austenite and ferrite, high strength, good toughness and good corrosion resistance, especially to stress corrosion cracking (SCC) compared with similar austenitic stainless steels. Moreover, the development of a duplex stainless-steel microstructure requires lower levels of nickel in the composition than for a corresponding austenitic stainless steel with comparable pitting and crevice corrosion resistance, hence they cost less. This makes duplex stainless steels a very versatile and attractive group of alloys both commercially and technically. There are applications where duplex grades can be used as lower cost through-life options, in preference to coated carbon steels, a range of other stainless steels, and in some cases nickel alloys. This cost benefit is further emphasised if the design engineer can use the higher strength of duplex grades to construct vessels and pipework of lower wall thickness than would be the case if an austenitic grade or nickel alloy was being used. Hence, we find duplex stainless steels are widely used in many industries. In this paper their use in three industrial applications is reviewed, namely marine, heat exchangers, and the chemical and process industries. The corrosion resistance in the relevant fluids is discussed and some case histories highlight both successes and potential problems with duplex alloys in these industries. The paper shows how duplex stainless steels can provide cost-effective solutions in corrosive environments, and why they will be a standard corrosion resistant alloy (CRA) for many industries through the 21st century.

Microstructure evolution and mechanical behavior of a lean duplex stainless steel aged at 475oC

In this research, the effects of isothermal treatments at 475 °C on the microstructure and mechanical properties of a UNS S32304 lean duplex stainless steel were investigated. Samples of the alloy under as received and aged conditions were analyzed by atomic force microscopy and magnetic force microscopy in order to evaluate the ability of these techniques to identify important aspects that characterize microstructural changes caused by aging. The mechanical behavior was evaluated by tensile tests, hardness tests, Charpy impact tests, crack extension resistance tests, force controlled axial fatigue tests and fatigue crack growth rate tests. The fracture surfaces of all tested specimens were analyzed by scanning electron microscopy. The results indicate that the α-phase spinodal decomposition occurred, with a tendency to saturation for a not too long aging time. The analysis by atomic/magnetic force microscopy allowed to identify characteristics regarding the preferential phase dissolution of the α phase that distinguish the alloy in the as received condition from the aged alloy. As a consequence of the formation of the α’ phase, the absorbed impact energy and the fracture toughness of the alloy decreased substantially. On the other hand, the tensile strength and the fatigue resistance increased significantly. • Possibility to effectively follow spinodal decomposition in a LDSS using the Scanning Probe Microscopy technique. • Short aging time after which the spinodal decomposition and the mechanical behavior of the steel tend to saturate. • Higher values for LDSS impact strength and fracture toughness compared to other duplex stainless steel grades. • An adequate heat treatment can provide interesting values of tensile, toughness and fatigue resistance for the LDSS.

Preservation of Natural Phase Balance in Multi-pass and Wire Arc Additive Manufacturing-Made Duplex Stainless Steel Structures

This work investigates the effects of Ar-based shielding gases on the microstructure and phase balance of duplex stainless steel (DSS) depositions. The results are then used for fabrication of DSS walls using wire arc additive manufacturing (WAAM), and the microstructural characteristics and mechanical performance of the products are assessed. WAAM currently poses as one of the most interesting methods of manufacturing by enabling joining of materials to fabricate structures layer by layer. However, the extreme conditions imposed by the process, such as high heat inputs and re-melt and solidification of the deposited layers, could compromise the performance of the products. A detailed microstructural analysis is conducted on single layer and double layer depositions of DSS to assess the effects of different shielding gases on the ferrite/austenite phase balance in the specimens. The results show that the depositions shielded with a small fraction of carbon dioxide (CO2) present better microstructural features in comparison to depositions shielded with the other selected gases. A mixture of 98%Ar + 2%CO2 was employed for fabrication of a 60-layer DSS WAAM-made wall and some mechanical characteristics were studied. The mechanical examinations demonstrate that the properties of the parts fabricated using WAAM are comparable with their wrought counterparts.

Effect of heat treatment on the microstructure and corrosion properties of cast duplex stainless steels

The effect of the quenching temperature on the pitting corrosion resistance of lean duplex stainless steel (DSS) were examined. Using thermodynamic modeling of phase formation processes in steel during solidification and subsequent cooling was shown that the equal amount of austenite and ferrite is achieved at 1210°С for examined composition. Experimental steel samples were quenched from 1100 and 1200°С. It was found that as the temperature rises, the pitting potential increases significantly due to the achievement of a favorable phase ratio. The results of this experimental study made it possible to develop a heat treatment that provides high corrosion properties of lean DSS.

Metastable precursor structures in hydrogen-infused super duplex stainless steel microstructure – An operando diffraction experiment

We report the evolution of metastable precursor structures during hydrogen infusion in the near-surface region of a super duplex stainless steel. Grazing-incidence x-ray diffraction was employed to monitor, operando, the lattice degradation of the austenite and ferrite phases. Electrochemical hydrogen charging resulted in the splitting of the diffraction peaks of the austenite phase, suggesting the evolution of a metastable precursor structure. This may be explained by the formation of quasi-hydrides, which convert back into the austenite parent structure during hydrogen effusion. The ferrite showed less lattice deformation than the austenite and no phase transformation.

Effects of Ce on Microstructure and Mechanical Properties of LDX2101 Duplex Stainless Steel

In the present study, the effects of rare earth metal (REM) Ce on the number, size distribution and type of inclusions, as well as the grain size, tensile and impact properties of LDX2101 duplex stainless steel, were investigated using thermodynamic analysis, optical microscope, image software, scanning electron microscope, tensile and impact testing machines. The results indicate that when the Ce content in steel is 0%, the main inclusions are irregular and large size Al2O3 and Al2O3-MnS. When the Ce content in steel is 0.005% or 0.02%, the inclusions are mainly spherical and small size inclusions CeAlO3, Ce2O2S and Ce2O3. With the increase in Ce content, the proportion of small size inclusions gradually increases; the number of inclusions per unit area decreases from 122 to 58 and the average grain size reduces from 16 ± 0.7 μm to 12 ± 0.3 μm. Moreover, the mechanical properties of steels are improved, and the tensile strength, yield strength and elongation are elevated by 4.69%, 2.83% and 4.9%, respectively. The impact fracture mode of steel without Ce is cleavage fracture, however, the fracture mode is transformed into a mixed fracture of cleavage and dimple after adding 0.005% and 0.02% Ce.

Metallographic screening of duplex stainless steel weld microstructure with a bipolar electrochemistry technique

A novel bipolar electrochemistry screening method to reveal corrosion susceptibility of weld microstructures is introduced. This metallographic method is applied to characterise the weld microstructure of a lean duplex stainless steel. The corrosion behaviour of the Fusion Zone (FZ), Heat Affected Zone (HAZ) and Base Metal (BM) was revealed, with the most severe etch response found at the interface between Fusion Zone (FZ) – Heat Affected Zone (HAZ). Transpassive and crevice corrosion were more significant in the FZ, with the highest corrosion resistance observed in the BM. The observed behaviour was linked to differences in local element distributions and the ferrite-to-austenite phase fraction. Corrosion pits formed via preferential dissolution of the ferrite, with the austenite preferentially dissolving in the transpassive region.

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.

Operando time- and space-resolved high-energy X-ray diffraction measurement to understand hydrogen-microstructure interactions in duplex stainless steel

Local strain development in the microstructure of a commercial 25Cr-7Ni super duplex stainless steel was mapped using high-energy x-ray diffraction during cathodic hydrogen charging under constant uniaxial load. The infusion of hydrogen resulted in tensile strains in austenite grains, one order of magnitude larger than those in the ferrite. Most strain evolution occurred at the near-surface, with compensating compressive forces developed in underlying regions, with up to two-times more compression occurring in the ferrite than the austenite. The strains along the loading axis were more pronounced than in the transverse direction, in which mostly compressive strains developed in the ferrite.

Effect of heat treatment on the microstructure of duplex stainless steel 2205

The Duplex stainless steel (DSS) is required to maintain its exceptional mechanical and corrosion resistance properties during its utilization. It is a dual micro-structured material consisting of δ-ferrite and γ-austenite in almost equal proportion as established by the international quality standards. It owes its exceptional properties to the optimum heat treatment and the partitioning of additional elements such as Cr, Mn, Mo, Ni, and N during its manufacturing. Unfortunately, due to drastic working conditions requiring elevated temperatures and sudden cooling, the DSS is subjected to noticeable phase equilibrium shifting and the formation of intermetallic phases. These changes are responsible for eventual weakness observed in most structures made with the DSS. To alleviate the inconvenience, the present paper focuses on the investigation of the effect of cooling method on the microstructure of the DSS exposed to solution annealing at a temperature of 1100 ℃. The DSS was cooled in the furnace and air and quenched in oil and water. The microstructure generated by the different cooling methods was assessed using the light optical microscopy (OM), scanning electron microscopy (SEM-EDS) and microhardness measurement. The initial lamellar structure transformed to clustered or blocky structure and intermetallic precipitates are revealed.

Characterisation of super duplex stainless steel by optimisation of EBSD parameters

Electron backscatter diffraction (EBSD) was used to characterise the microstructure of a super duplex stainless steel (SDSS) alloy Zeron 100 and the intermetallic phases sigma and chi in this material. The material was analysed in three conditions; as-received, heat-treated at 900 °C for 8 min and heat-treated at 750 °C for 4 h. Heat treatments were performed to introduce the intermetallic phases in the steel. It is difficult to get reliable indexing of the sigma and chi phases due to noisy and overlapping patterns. To get the best results, the Hough transformation parameters were optimised and an averaging technique was used. The optimisation was done by removing some of the reflectors in the sigma- and chi-phases in the original TSL material files used for pattern indexing. By changing Hough parameters, considerable improvements in the indexing of the patterns were observed along with the increase of average confidence index (CI) value for both intermetallic phases. The applied SEM, Zeiss Ultra 55 FESEM was equipped with a NORDIF UF-1100 EBSD detector to acquire and stream patterns to HD. Indexing and data processing were performed by TSL/OIM 7.3. In the present study, at the most four different phases are present in the SDSS.

Influence of hot isostatic pressing post-treatment on the microstructure and mechanical behavior of standard and super duplex stainless steel produced by laser powder bed fusion

This study investigates the properties of standard duplex steel 2205 and super duplex steel 2507 processed by laser powder bed fusion. The focus of the analysis is on the changes in microstructures and the mechanical properties caused by hot isostatic pressing (HIP) as post treatment. For that purpose, samples produced by laser powder bed fusion (LPBF) were exposed to hot isostatic pressing post-treatment followed by solution annealing. The properties of this condition were compared to properties of the duplex steels without post-heat treatment and of those of samples with only solution annealing as post-treatment.Both duplex steels had a predominantly ferritic structure after the LPBF process. Subsequent heat treatments were able to establish a ferritic-austenitic duplex structure. HIP applied before solution annealing leads to a coarsening of austenite grains and a reduction in porosity.All post-heat treated samples meet the minimum requirements of the standard DIN EN 10088-3 regarding tensile properties. An effect of hot isostatic pressing was seen in the fatigue strength. An increase in the fatigue strength was measured in both steels, which was mainly attributed to the decrease in inner defects size.

The effect of chemical composition and annealing condition on the microstructure and tensile properties of a resource-saving duplex stainless steel

Edge cracking phenomenon occurs in ‘Mn + N′ duplex stainless steels because of inferior thermoplasticity that is mainly induced by the non-uniform deformation between ferrite and austenite phase with different strength during hot rolling, limiting its application. To address this problem, a Fe–19Cr-0.6Al–1Ni–12Mn duplex stainless steel with an optimized alloy composition was trial-prepared, where Ni is substituted by Mn and Cr is partially substituted by Al. This enabled a high content ferrite to be present during hot rolling to reduce the adverse effect of austenite on thermoplasticity. Thus, the edge cracking of hot rolled plates was solved. Furthermore, the formation of austenite was governed by annealing after cold rolling. In view of this, it is significant to study phase transformation and precipitation in duplex stainless steel during annealing. A large number of experimental results indicated that there were two types of transformation during the annealing process: the transformation of ferrite to austenite and recovery and recrystallization of ferrite. After annealing at lower temperature, the austenite was characterized by the banded structure at the grain boundaries of ferrite. Besides, the formation of austenite restricted recrystallization of ferrite. With the increase of annealing temperature, the growth of austenite grain would be enhanced, and the banded austenite evolved into bamboo morphology or island morphology. When the annealing temperature was greater than 900 °C, recrystallization of ferrite was promoted, and transformation of ferrite to austenite was inhibited. However, the precipitation of σ phase increased with the decrease of annealing temperature below 800 °C, reducing plasticity and pitting resistance for the investigated steel.