Behavior Of Duplex Stainless Steel Research Articles

Effect of Solution Annealing Temperature on Pitting Behavior of Duplex Stainless Steel 2204 in Chloride Solutions

The effect of solution annealing temperature ranging from 950 to 1200 °C on the microstructure and corrosion performance of duplex stainless steel (DSS) 2204 were investigated. The proportion of the ferrite phase increased while the austenite phase decreased and the ferrite stabilizing elements diluted in the ferrite phase with the increase of annealing temperature. The critical pitting temperature (CPT) of specimens annealed at 1000 °C was higher than those annealed at 950 °C, whereas further increasing the annealing temperature to 1200 °C decreased the CPT. The pitting initiation sites were observed in the austenite phase, at the boundary of ferrite/austenite phase and inside the ferrite phase for specimens annealed at 950, 1000 °C and exceeding 1100 °C, respectively. The evolution trend of the CPT and the pit initiation site were analyzed by the pitting resistance equivalent number.

Effect of strain rate and temperature on hot workability and flow behaviour of duplex stainless steel

The flow behaviour and processing map of a duplex stainless steel were studied via hot compressive tests in a temperature range of 1223–1473 K and a strain rate range of 0.01–30 s− 1. The effect of strain rate and temperature on the hot workability, strain partitioning and dominant flow behaviour of the alloy was systematically investigated. It is found that the softening mechanism of each constituent phase differs from each other. The ferrite is softened by dynamic recovery and continuous dynamic recrystallisation (CDRX), while the austenite is softened only by the limited discontinuous dynamic recrystallisation (DDRX). At lower strain rates (0.01 and 0.1 s− 1), the strain is mainly accommodated by ferrite due to its excellent softening capability, which causes the apparent activation energy Qp to decline continuously with the increase in true strain. In this case, plastic deformation of the austenite rarely occurs, and at this time, DDRX of austenite is not observed. When the strain rate increases, CDRX of ferrite is weakened at a relative low temperature, which prompts the strain transfer into austenite and induces the strain hardening due to its restricted softening. Accordingly, interactions between the strain hardening in austenite and weakened softening of ferrite leads to one or more platforms of Q formed at the medium stage of deformation (1–30 s− 1). The processing map shows that two flow instability regions appear at high strain rate due to the lack of sufficient response time for dynamic restoration at the early deformation stage. As the strain increases, dynamic softening mechanism is activated at a higher temperature, resulting in a gradually narrowed flow instability region. Differently, a decrease in temperature suppresses dynamic softening of the alloy with a high strain rate, which deteriorates the hot workability of the alloy and induces microcrack formation after straining of 0.8.

Super and hyper duplex stainless steels: structures, properties and applications

In oil-gas industry, the exploration and development are now targeted to the deep reservoirs with high pressures, high temperatures and extreme corrosive environments. This requires that the materials used should have a good combination of extra high strength and excellent corrosion resistance. In order to meet these challenges, hyper duplex stainless steels have recently been developed. These materials have nitrogen contents up to about 0.5% and PRE-values close to 50, and show both highest corrosion pitting resistance and highest strength among the existing duplex stainless steels.The purpose of this paper is to provide an overview on hyper duplex stainless steels. It will mainly focus on the material development, microstructures, corrosion properties such as critical pitting corrosion temperature and crevice corrosion resistance, heterogeneous deformation behaviour of duplex stainless steel, and mechanical properties such as tensile properties and fatigue properties. These properties and the ratios of strength/weight will then be compared with those of other type of duplex stainless steels. The potential applications for hyper duplex stainless steels are also discussed.

Mechanical, electrochemical and magnetic behaviour of duplex stainless steel for biomedical applications

Mechanical, electrochemical and magnetic properties of duplex stainless steel were analysed to evaluate its use as biomaterial, comparing the results with those obtained for austenitic stainless steel. Yield and ultimate tensile strengths are almost twice in duplex stainless steel, being the values 870 MPa and 564 MPa, respectively. The electrochemical test revealed that this material has lower susceptibility to localised corrosion because of its greater passive range, 1 V from the open circuit potential, while the austenitic stainless steel exhibited a passive region of 0.370 V. Both steels behave as soft magnetic materials, however, duplex stainless steel has higher magnetic saturation and remanence, while austenitic stainless steel is more prone to heating when exposed to a magnetic field.

Corrosion Behavior of Austenitic and Duplex Stainless Steels in Thiosulfate- and Chloride-Containing Environments

The effect of thiosulfate concentration on the corrosion behavior of duplex stainless steels S32101 and S31803 was investigated in thiosulfate- and chloride-containing environments, of relevance to the paper machine environments and other chemical industrial processes. Electrochemical methods including cyclic potentiodynamic polarization and potentiostatic scratch test were used to study the localized corrosion behavior of these alloys. Corrosion behavior of tested duplex stainless steels was compared with austenitic stainless steels S30403 and S31603, which are typically used in relevant industrial equipment, like paper machines. Cyclic potentiodynamic polarization results indicated that the presence of thiosulfate ions considerably lowered the repassivation potential of all stainless steels tested. In presence of thiosulfate ions, different grades of duplex stainless steels behaved differently. S31803, with the highest molybdenum and chromium content, was the most resistant to pitting corrosion among th...

Effect of Secondary Phase Precipitation on the Corrosion Behavior of Duplex Stainless Steels.

Duplex stainless steels (DSSs) with austenitic and ferritic phases have been increasingly used for many industrial applications due to their good mechanical properties and corrosion resistance in acidic, caustic and marine environments. However, DSSs are susceptible to intergranular, pitting and stress corrosion in corrosive environments due to the formation of secondary phases. Such phases are induced in DSSs during the fabrication, improper heat treatment, welding process and prolonged exposure to high temperatures during their service lives. These include the precipitation of sigma and chi phases at 700–900 °C and spinodal decomposition of ferritic grains into Cr-rich and Cr-poor phases at 350–550 °C, respectively. This article gives the state-of the-art review on the microstructural evolution of secondary phase formation and their effects on the corrosion behavior of DSSs.

On the stress corrosion cracking of lean duplex steel in chloride environment

Duplex stainless steel having attractive combination of austenitic and ferritic properties is being used in industry such as petrochemical, pulp and paper mills. In this study, the corrosion and stress corrosion behavior of duplex stainless steel in 3.5% sodium chloride environment was investigated by weight loss measurements, electrochemical DC testing and slow strain rate test (SSRT). Weight loss data showed no significant corrosion after 1700 hours. Electrochemical polarization test in 3.5% NaCl solution exhibited a uniform corrosion rate of 0.008 mpy (calculated using Tafel analysis) showing passivity in the range of 735-950 mV. A comparison of the slow strain rate test in 3.5% NaCl solution with air shows almost a similar stress strain curve for duplex stainless steel. In comparison, the stress strain curves for 0.15% carbon steel show a loss of about 25% tensile elongation for the same comparison. The excellent corrosion and especially resistance to localized corrosion (pitting) is responsible for no loss of ductility in duplex stainless steel.

Corrosion behavior of electron beam welded duplex stainless steels

The corrosion behavior of duplex stainless steels is strongly affected by the austenite–ferrite ratio. With regard to corrosion resistance as well as mechanical properties a balanced ratio between these two microstructural components is desired. Weld seams manufactured by electron beam welding (EBW) generally exhibit very high amounts of ferrite due to a strong loss of nitrogen during the welding process and a very high cooling rate, which can affect the corrosion behavior adversely. Significant improvements can be reached by varying the electron beam guidance and applying multi‐beam technology. This paper considers the pitting corrosion as well as selective corrosion behavior of the welds as a function of the chosen process parameters during EBW. Comparisons with the corrosion performance of manual metal arc and tungsten inert gas welds are also made.

Corrosion Behavior of Duplex Stainless Steels (2205) in Seawater in the Presence of Deep Sea Bacteria

The corrosion behavior of duplex stainless steel immersed in nutrient-rich simulated seawater without deep sea bacteria and with deep sea bacteria (Erythrobacter pelagi sp.nov) was studied. The effect of chloride ions and presence of deep sea bacteria on corrosion resistance was investigated. The occurrence of localized corrosion (Viz. pitting and crevice corrosion) was examined using visual inspection and SEM-EDX. Electrochemical impedance spectra were used to study the effects of deep sea bacteria on duplex stainless steel and inferences were made. Most significantly was the reduction of pitting resistance potential with increase in exposure time.

Influence of surface preparation and microstructure on the passivity and corrosion behaviour of duplex stainless steels

The influence of various surface preparation methods (mechanical polishing, electropolishing and etching) on the passivity and the corrosion behaviour of a duplex stainless steel (UNS S32202) was studied using Auger spectroscopy and the electrochemical microcell technique. From surface analyses, the thickness and chemical compositions (ratios Cr/Fe and O2−/OH−, distribution of chloride and nitrogen) were determined. The corrosion behaviour of samples was investigated from local polarization curves. The presence of oxidation peaks and stable pitting was discussed considering the specimen microstructure and the properties of the passive film.

The Influence of Chromium Nitrides Precipitation on the Fatigue Behavior of Duplex Stainless Steels

This paper studies the fatigue behavior at room temperature of UNS S32205 and UNS S32750 duplex stainless steels (DSSs) under two thermal treatments. In both types of DSSs, thermal treatments at high temperature followed by water quenching (TTW) produces the precipitation of Cr2N within the ferrite phase. The amount of Cr2N increases mainly with the ferritic grain size independently of the nitrogen content. This nitride precipitation hardens the ferritic phase and produces a detrimental effect on the fatigue life of both steels.

Study of corrosion behavior of a 22% Cr duplex stainless steel: Influence of nano-sized chromium nitrides and exposure temperature

• Nano-sized quenched-in Cr 2 N particles were formed in a duplex stainless steel by heat treatment. • Finely dispersed quenched-in Cr 2 N particles have higher relative nobility than ferrite matrix. • Nano-sized quenched-in Cr 2 N particles do not cause localized corrosion of the steel in 1 M NaCl. • Exposure temperature has a strong influence on the corrosion behavior of duplex stainless steel. Chromium nitrides may precipitate in duplex stainless steels during processing and their influence on the corrosion behavior is of great importance for the steel performance. In this study, the influence of nano-sized quenched-in chromium nitrides on the corrosion behavior of a heat treated 2205 duplex stainless steel was investigated at room temperature and 50 °C (just above critical pitting temperature). The microstructure was characterized by SEM/EDS and AFM analyses, and quenched-in nitrides precipitated in the ferrite phase were identified by TEM analysis. Volta potential mapping at room temperature suggests lower relative nobility of the ferrite matrix. Electrochemical polarization and in-situ AFM measurements in 1 M NaCl solution at room temperature show a passive behavior of the steel despite the presence of the quenched-in nitrides in the ferrite phase, and preferential dissolution of ferrite phase occurred only at transpassive conditions. At 50 °C, selective dissolution of the austenite phase was observed, while the ferrite phase with the quenched-in nitrides remained to be stable. It can be concluded that the finely dispersed quenched-in nitrides do not cause localized corrosion, whereas the exposure temperature has a strong influence on the corrosion behavior of the duplex stainless steel.

Duplex Steels. Part II: Carbides and Nitrides

As mentioned in Part I, duplex stainless steels have a very complex precipitation behavior due to the high amount of alloying elements they contain. Thermodynamically, this leads to the formation of many phases (Fig. 1). Diffusion rates for interstitial elements in ferrite are approximately 100 times higher and have solubilities that are 100 times lower than those in austenite. This leads to the result that formations of carbides, nitrides, and intermetallic phases occur in the ferritic phase only. The precipitated phases can be distinguished regarding their kinetic formation, controlled due to element solubility or diffusion. The growth of phases, which are dependent on element solubilities, cannot be avoided due to technically practicable cooling rates, in general (austenite, carbides, nitrides, and e-phase). In contrast, diffusion-dependent phase formation is associated with prolonged treatment time at critical temperatures (v-, r-phase, and 475 C embrittlement) and can be suppressed by rapidly quenching. The precipitation behavior of duplex stainless steels shown in Fig. 1 can be divided into two temperature ranges. In the upper temperature range between 600 and 1,000 C, the nitrides of type CrN, Cr2N, carbides of type M23C6, and vand r-phases can develop. In the lower temperature range of 300–550 C, the formations of p-, Zand a0-phases take place (475 C embrittlement). In the same temperature range, e-phase can occur in coppercontaining duplex steels. The solubilities of carbon and nitrogen in duplex steels are limited and become lower with decreasing temperature. Both elements prefer to concentrate in the austenite, due to their higher solubility for interstitial elements and to a greater number of tetrahedral sites. The ferrite can dissolve very small amounts of carbon and nitrogen in octahedral sites and, for this reason, the formations of carbides and nitrides occur exclusively in the ferritic phase and at the ferrite–austenite phase boundaries. An overview of carbides and nitrides, which have been identified in duplex steel by various studies, is shown in Table 1. Precipitation of carbides and nitrides greatly reduces the corrosion resistance and toughness of the duplex steels and thus should be avoided. Figures 2, 3, and 4 show examples of the negative effects of carbides and nitrides on corrosion behavior.

Effects of prior solution treatment on thermal aging behavior of duplex stainless steels

The influence of solution temperature on thermal aging behavior was studied in duplex stainless steels. With increasing solution temperature, the ferrite contents remarkably increase, Cr and Ni elements redistribute. During thermal aging, the impact properties of higher solution temperature treated materials suffer a serious degradation, which is not only related with ferrite content but also the alloy compositions in ferrite. Enrichment of Ni in ferrite can accelerate the spinodal decomposition kinetics. Thermal aging-inducing strain fields in ferrite cause the embrittlement of DSS.

Effect of stress and strain on corrosion resistance of duplex stainless steel

The interplay of the mechanical and electrochemical phenomena has been a subject of active research. In this paper, corrosion resistance studies about SAF2205 and SAF2507 duplex stainless steel were carried out under elastic stress applied (100 MPa, 300 MPa, 500 MPa) and pre-strain (5%, 10%, 15%) in 3.5% NaCl and 2 mol/L HCl solution. Potentiodynamic anodic polarization study revealed that corrosion resistance of SAF2205 duplex stainless steel decreases slightly with increasing of elastic stress level and noticeably with increasing of pre-strained level. Scanning electron microscopy investigation on surface of the electrochemical tested SAF2205 duplex stainless steel samples indicated that pitting is always located in austenite grains when pre-strain level is below 5% (including different elastic stress level) and located on intersection of ferrite and austenite grain when pre-strain level is above 5%. For SAF2507 duplex stainless steel, elastic stress and pre-strain have no effect on general corrosion and pitting corrosion. Based on deformation mechanism of duplex structure and the relationship of mechanical load and corrosion potential, Pitting corrosion behavior of duplex stainless steel is explained and discussed.

Study of hot deformation behaviour of 2205 duplex stainless steel through hot tension tests

The hot working behaviour of duplex stainless steels has been studied in the literature mainly through hot torsion or hot compression tests. The aim of this paper is to investigate the hot deformation behaviour of a duplex stainless steel type 2205 (F51) through hot tension tests, which are easier to carry out and can offer some additional information about the maximum sustainable strain in tension before starting of irreversible damage. In fact, even under the uniaxial compressive action of a forging press, tensile stresses may develop in some zones of the forged product. These zones become most critical and the knowledge of the limiting conditions are important. Three different test temperatures were selected, namely 900, 1000, and 1100 °C. The hot working behaviour was characterised by fitting the mean flow stress, strain rate, and temperature relationship with the hyperbolic sine function defined by Sellars and Tegart. An activation energy Q equal to 430 kJ/mol was obtained for plastic straining at high temperature. It corresponds to values, obtained with hot torsion or hot compression tests, reported in the literature where a large variation in activation energy is also found for duplex stainless steels. It was explained by considering the intrinsic two-phase nature of the investigated steel. The ductility in the different hot working conditions was characterised by the true strain at the onset of damage formation under prevailing tensile stress conditions. The role of the microstructure as well as of damage formation during deformation on the shape of the flow stress curves was analyzed.

SAF 2205 듀플렉스 스테인레스강의 미세조직 및 기계적 특성에 미치는 냉각속도의 영향

Duplex stainless steel, which is a kind of stainless steel with a mixed microstructure of about equal proportions of austenite and ferrite, is generally known as a unique material with excellent corrosion resistance and high strength. However, toughness, strength, and corrosion resistance of the steel could be reduced due to precipitation of topologically closed packed phases such as sigma phase during cooling. In case of large forged products, they have strong possibility that <TEX>${\sigma}$</TEX>-phase precipitates due to difference of cooling rate between surface and inner of the products. Investigation on sigma phase precipitation behavior of duplex stainless steel with change of cooling rate was carried out in this study. Forged SAF 2205 duplex stainless steel was used as specimens to examine the cooling rate effect. Dissolution behavior of sigma phase was also discussed through resolution test of duplex stainless steel containing lots of sigma phase. Experimental results revealed that impact energy was very sensitive to precipitation of small amount sigma phase. However, sigma phase could be removed by short term resolution treatment and impact resistance of the duplex stainless steel was restored.

Improving surface properties and wear behaviors of duplex stainless steel via pressure carburizing

In this research, the surface properties and wear behavior of duplex stainless steel (DSS) carburized using pressure carburizing (PC) method are studied. Carburizing on DSS having different microstructures was carried out using conventional carburizing and pressure carburizing methods. The surface properties namely, the microstructure, hardness and wear of DSS prior to and following the carburizing process were investigated. The results showed that the initial microstructure condition affects the carburizing process wherein the DSS with a finer microstructure exhibits better surface properties than the coarser one. It was found that the surface hardness and wear properties of DSS are enhanced significantly for the sample carburized via PC method. The enhancement was attributed to the formation of a thicker and harder carburized layer. Elemental analyses revealed that the concentration of carbon atoms in the PC sample was highly packed and that the atoms diffused further into the substrate compared with the conventional carburizing sample.

Ageing effects on the lattice strain behaviour of duplex stainless steel

The mechanical properties of duplex stainless steel (DSS) are known to display embrittlement due to ageing effects. Knowledge of the micro behaviour of aged DSS is important for understanding the mechanisms of deformation and failure, but it has proved difficult to obtain due to the complex effects created by the duplex structure. Lattice strain measurements of DSS aged under various conditions of temperature and time have been performed in order to determine the strain behaviour of individual ( hkl) planes in the α- and γ-phases using white synchrotron radiation X-rays at SPring-8. It was found that the lattice strain of α-Fe(200) depends strongly on the ageing conditions and that the lattice strain behaviour in the γ-phase is more stable than that in the α-phase. These results suggest that the changes in macro mechanical properties, including yield strength and ultimate strength, which are revealed by standard tensile tests, are solely due to ageing effects in the α-phase. The results further suggest that the lattice strain of α-Fe(200) can be used as an index to evaluate 475 °C ageing effects in DSS.

Effect of annealing temperature on the corrosion behavior of duplex stainless steel studied by in situ techniques

The effects of the annealing temperature on the microstructure and the corrosion behavior of duplex stainless steel 2507 were investigated by means of magnetic force microscopy (MFM), scanning Kelvin probe force microscopy (SKPFM), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The SEM results indicated that the volume fraction of the austenite phase decreased with the increased annealing temperature. SKPFM/MFM measurements conducted in air at the room temperature and an ambient relative humidity of about 25% showed a higher Volta potential of the paramagnetic austenite than that of the ferromagnetic ferrite. The in situ AFM observation in a hydrochloric acid solution provided solid evidence that corrosion preferentially occurred in the ferrite phase. The sample annealed at 1100 °C exhibited a greater Volta potential difference between the ferrite and austenite and a higher corrosion rate in the ferrite, while that annealed at 1150 °C had a smaller Volta potential difference and a lower corrosion rate. The relative nobility and microstructure change of two phases, as well as their corrosion behavior, can be explained by the effect of the composition of alloying elements.