High throughput screening of localised and general corrosion in type 2205 duplex stainless steel at ambient temperature

Bipolar electrochemistry is used to produce a linear potential gradient across a bipolar electrode(BPE),providing direct access to the anodic and cathodic reactions under a wide range of applied potentials.The occurrence of pitting corrosion,crevice corrosion,and general corrosion on type 2205 duplex stainless steel(DSS 2205)BPE has been observed at room temperature.The critical pit depth of 10-20μm with a55%-75% probability of pits developing into stable pits at potential from+0.9 to+1.2 V vs.OCP(open circuit potential)are measured.All pit nucleation sites are either within ferritic grains or at the interface between austenite and ferrite.The critical conditions for pitting and crevice corrosion are discussed with Epit(critical pitting potential)and Ecre(critical crevice potential)decreasing from 0.87 and 0.80 V vs.OCP after150 s of exposure to 0.84 and 0.76 V vs.OCP after 900 s of exposure,respectively.Pit growth kinetics under different applied bipolar potentials and exposure times have been obtained.The ferrite is shown to be more susceptible to general dissolution.

Deformation Mechanism of Bimodal Structured 2205 Duplex Stainless Steel in Two Yield Stages

A kind of micro/nanostructured 2205 duplex stainless steel(DSS)with uniform distribution of nanocrystals was prepared via aluminothermic reaction method.The analysis of stress-strain curve showed that the fracture strength and elongation of the specimen were 946 MPa and 24.7%,respectively.At present,the research on microstructure of bimodal 2205 DSS at room temperature(RT)mainly depended on scanning electron microscope(SEM)observation after loading experiments.The test result indicates that there are two different yield stages in stress-strain curve of specimen during tensile process.The microstructure of duplex bimodal structured stainless steel consists of two pairs of soft hard regions and phases.By studying deformation mechanism of bimodal structured stainless steel,the interaction between soft phase and hard phase are discussed.The principle of composition design and microstructure control of typical duplex stainless steel is obtained,which provides an important research basis for designing of advanced duplex stainless steel.

Investigation on influence of alloying on phase transitions of duplex stainless steel based on thermochemical calculation

This paper investigated on influence of different alloying elements added into duplex stainless steel (DSS) on phase transitions using thermochemical methods in comparison with experiment.The results showed that the most possible species in the ferrite phase,austenite phase,σphase,Hcp phase,χphase,and carbide were Cr:Va-type,Fe:Va-type,Ni:Cr:Mo-type,Cr_(2)N-type,Fe_(24)Mo_(10)Cr_(24)-type,and Cr:Mo:C-type,respectively.Furthermore,the Ni,N,Cr,and Mo alloying had significant influences on the transition of each DSS phase.The Ni and N additions obviously raised the temperature at ferrite-1/austenite-1 balance while the Cr and Mo decreased the dual-phase balance temperature.In addition,the Ni addition can promote the precipitating ofσphase at relatively high temperature while the precipitating of Hcp phase at relatively low temperature.The Hcp phase andχphase can be obviously increased by the N addition.The introduction of Cr and Mo notably enhances the precipitation ofσphase.However,the promotion ofχphase precipitation is facilitated by the presence of Mo,while the Cr element acts as an inhibitor forχphase precipitation.Furthermore,the ferrite/austenite ratio tested by experiment was higher than that calculated by thermochemical methods,thus pre-designed solution temperature should be lower about 30-100℃than that calculated by thermochemical methods.

Literature Review of Phase Transformations and Cavitation Erosion of Duplex Stainless Steels

Phase transformation is one of the factors that would significantly influence the ability to resist cavitation erosion of stainless steels. Due to the specific properties of duplex stainless steel, the heat treatment would bring about significant phase transformations. In this paper, we have examined the previous studies on the phase transition of stainless steel, including the literature on the classification of stainless steel, spinodal decomposition, sigma phase transformation, and cavitation erosion of double stainless steel. Through these literature investigations, the destruction of cavitation erosion on duplex stainless steel can be clearly known, and the causes of failure of duplex stainless steel in seawater can be clarified, thus providing a theoretical basis for subsequent scientific research. And the review is about to help assess the possibility of using bulk heat treatment to improve the cavitation erosion (CE) behaviour of the duplex stainless steel 7MoPLUS.

Hot deformation behavior of microstructural constituents in a duplex stainless steel during high-temperature straining

The hot deformation characteristics of 1.4462 duplex stainless steel （DSS） were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800-1100~C and strain rates of 0.001-1 s-1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-HoUomon parameter （Z） increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6. x 1015 （lnZ---35.5）, a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction （EBSD） confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.

Effect of Temperature and Cl^- Concentration on Pitting of 2205 Duplex Stainless Steel

The electrochemical behaviors of 2205 duplex stainless steel in NaCl solution with different temperatures and concentrations were studied by gravimetric tests, potentiodynamic polarization, electrochemical impedance spectroscopy and scanning electron microscopy. The experinental results show that temperature and chloride concentration have a great influence on the pitting resistance of 2205 duplex stainless steels. They not only effect the corrosion rate of pitting, but also change the shape of the pits. When NaCl solution was in low concentration and temperature below the critical pitting temperature, pits were very small and scattered with hemisphere-like shape. On the contrary, the pits of 2205 duplex stainless steel were large and sometimes had a lacy cover when the NaCl concentration was higher and the temperature was 70℃.

Optimization of process parameters of the activated tungsten inert gas welding for aspect ratio of UNS S32205 duplex stainless steel welds

The activated TIG(ATIG) welding process mainly focuses on increasing the depth of penetration and the reduction in the width of weld bead has not been paid much attention.The shape of a weld in terms of its width-to-depth ratio known as aspect ratio has a marked influence on its solidification cracking tendency.The major influencing ATIG welding parameters,such as electrode gap,travel speed,current and voltage,that aid in controlling the aspect ratio of DSS joints,must be optimized to obtain desirable aspect ratio for DSS joints.Hence in this study,the above parameters of ATIG welding for aspect ratio of ASTM/UNS S32205 DSS welds are optimized by using Taguchi orthogonal array(OA)experimental design and other statistical tools such as Analysis of Variance(ANOVA) and Pooled ANOVA techniques.The optimum process parameters are found to be 1 mm electrode gap,130 mm/min travel speed,140 A current and 12 V voltage.The aspect ratio and the ferrite content for the DSS joints fabricated using the optimized ATIG parameters are found to be well within the acceptable range and there is no macroscopically evident solidification cracking.

Effect of Heat Sink and Cooling Mediums on Ferrite Austenite Ratio and Distortion in Laser Welding of Duplex Stainless Steel 2205

In order to control the ferrite and austenite percentage in duplex stainless steel welding, many researchers try to change the laser welding parameters and cooling medium, but ignore to study the influence of heat sink effect on weld strength. In this work, the effect of aluminium heat sink and varying cooling medium on the laser welding of duplex stainless steel (DSS) 2205 is studied. The 2 mm thick DSS sheets welded with pulsed Nd: YAG laser welding machine by varying the cooling medium (air and oil) and an aluminium plate used as a heat sink. The welded specimens tested for tensile strength, micro-hardness, distortion, microstructure and radiography analysis. The faster cooling rate in the oil quenching process enhances the ferrite percentage compared with air-cooled samples. But the faster cooling rate in oil quenching leads to more distortion and using aluminium as a heat sink influenced positively the distortion to a small extent. The lower cooling rate in air quenching leads to a higher tensile strength of the welded specimen. The objective of this work is to analyse experimentally the effect of cooling medium and heat sink in the mechanical and metallurgical properties of laser welded duplex stainless steel.

Influence of hot rolling on the microstructure of lean duplex stainless steel 2101

In this work, the microstructure and the strain partitioning of lean duplex stainless steel 2101 (LDX 2101) during different hot-rolling processes are investigated by optical microscopy and electron-backscattered diffraction (EBSD). The results show that the LDX 2101 exhibits poor thermoplasticity at high temperature. The four-pass hot-rolled plates show fewer edge-cracking defects and superior thermoplasticity compared with the two-pass hot-rolled plates prepared at different temperature. The phase boundary is the weakest site in the LDX 2101. The cracks are initiated and propagated along the phase boundaries during the hot-rolling process. According to the EBSD analysis, the increase of the hot-rolling pass can dramatically improve the strain distribution in ferrite and austenite phases and promote the strain transmission in the constituent phases, thereby improving the coordinated deformation ability of the two phases. This effect further in- creases the thermoplasticity and reduces the formation of edge cracks in LDX 2101.

Welding of 2205 duplex stainless steel natural gas pipeline

There are abundant natural gas resources in western China, but many oil and gas fields are rich in chloridion, sulfureted hydrogen, carbon dioxide and other corrosive medium, which have strong corrosivity to pipeline. One gas field possesses abundant natural gas with great pressure, and the chloridion concentration in the water separated from gas is about 10% , so the medium has great corrosivity. In order to ensure the safety of the pipeline, about 13 km length pipeline and the internal pipes of a gas treatment plant that purifies gas about 12 billion cubic meter a year are made of 2205 duplex stainless steel （2205 DSS ） . 2205 DSS has many characteristics in welding with complex welding process; and because of high quality requirements for the construction of natural gas pipeline and restriction of on-site conditions, the site welding is very difficult. Around the engineering application, a large number of experimental researches have been carried out on the material microstructure, properties and weldability. Finally welded joints which conform to the requirements of standard are obtained, contributing to the first large-scale application of this material in the field of oil and gas pipelines. Considering the engineering application and the latest research development, the welding and key factors affecting the joint properties of 2205 DSS pipes are summarized and analyzed.

Effect of sigma phase precipitation on microstructure and properties of cast ZG0Cr26Ni5Mo3Cu3 duplex stainless steel under different heat treatments

ZG0Cr26Ni5Mo3Cu3 Duplex Stainless Steel(DSS) was solution treated at 1,060 ℃ for 3 h, followed by water cooling. Tempering treatments were conducted at 720, 750 and 780 ℃, respectively, for 16 h, followed by air cooling. The microstructures of ZG0Cr26Ni5Mo3Cu3 duplex stainless steel samples treated at different tempering temperatures were observed by scanning electron microscope(SEM) and energy dispersal spectroscopy(EDS), and the phase consitutions were analyzed using X-ray diffraction(XRD). The effects of the precipitation of sigma(σ) phase on the duplex phase percentage, hardness, impact toughness and corrosion resistance of the DSS were studied. Results showed that microstructures of ZG0Cr26Ni5Mo3Cu3 after solution treatment consists of ferrite(α) phase and austenite(γ) phase; after being tempered at different temperatures, σ phase appeared due to a eutectoid-type reaction of α→σ+γ2 during tempering treatment. It was observed that σ phase distributed along the grain boundary. The volume fraction of σ and γ phases increased with increasing tempering temperature in the range of 720 to 780 ℃, whereas the volume fraction of α phase showed the opposite trend. When the percentage of σ phase increased, the hardness of steel also increased. In the solution treated steel, hardness was measured to be only 244.0 HB, because σ phase did not appear. However, it increased to 391.8 HB when the DSS was tempered at 780℃because a great of deal of σ phase appeared. The impact toughness and corrosion resistance of DSS decreased when the percentage of σ phase increased.

Electrochemical Corrosion Behavior of the Laser Continuous Heat Treatment Welded Joints of 2205 Duplex Stainless Steel

The electrochemical corrosion behaviors of the welded joints of 2205 duplex stainless steel with the laser continuous heat treatment were investigated. The secondary austenite formation is the outcome of thermodynamic equilibrium breach of the alloy during heat treatment and the result of the continuous heat treatment which has the most important effect on the weld material. The partitioning behaviors of chromium and molybdenum as well as the volume fraction of ferrite and austenite have a remarkable influence on the composition of the individual phase. Mechanical examination of the laser trated weld demonstrates that the tensile strength and yield strength increase with increasing the amount of the secondary austenite. It is shown that the ultimate tensile strength of the 6 kW laser-treated weld is higher about 20 MPa than no heat treatment weld and the ductility can be further improved without compromising strength. The results indicate that the welding alters the corrosion behavior because of different post heat treatment power and the broad active peak is not identified which is attributed to the dissolution of the secondary austenitic in the ferrite phase. It is indicated that pitting resistance equivalent （PRE） values of base metal and 6 kW weld are higher than that of other welds; base metal is 33.7, 6 kW weld 33.3, no treatment 32.4, 4 kW weld 32.8, 8 kW weld 32.5. The extent of corrosion resistance improvement after reheating treatment is mainly caused by the removal of nitrogen from ferritic regions, which occurred as a consequence of secondary austenite growth.

Hot Deformation Behavior of an As-cast Duplex Stainless Steel

The hot deformation behavior of an as-cast OCrl7Mnl4Mo2N duplex stainless steel has been studied by hot compression test at the temperature range from 1000℃ to 1200℃, and the strain rates are 0.1 s-1, 1 s-1 and 5 s-1, respectively. It was found that during hot deformation there is only dynamic recovery taking place within the δ-ferrite phase, but the γ-austenite phase undergoes dynamic recrystallization. The activation energy of the steel for hot compression is estimated to be 480 kJ/mol.

Mechanism of hot-rolling crack formation in lean duplex stainless steel 2101

The thermoplasticity of duplex stainless steel 2205（DSS2205） is better than that of lean duplex steel 2101（LDX2101）, which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy（OM）, electron backscatter diffraction（EBSD）, Thermo-Calc software, and transmission electron microscopy（TEM）. The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.

Influence of isothermal aging onσprecipitation in super duplex stainless steel

The time-temperature-transformation （TTT） curve of the 00Cr25Ni7Mo4N duplex stainless steel was obtained with a Formastor-digital thermal dilatometer, and the influence of isothermal aging on o precipitation was studied by metallographic observation, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. The results show that the decomposition of ferrite phase is accompanied by the formation of σ phase at 750-1000℃, especially in the range of 800-900℃. The longer the aging time, the higher the amount of o precipi- tation. The area fxaction of various phases remains at a certain value upon the completion of ferrite deformation. The temperature of 850℃ is the most sensitive transaction temperature, the incubation time for the formation of o precipitation is less than 1 min, and aging for 20 min leads to the complete transformation of ferrite. The o phase is formed preferentially at the α/α/γjunction, and then grows along the α/α boundary in the matrix.

Quantitative deformation measurements and analysis of the ferrite-austenite banded structure in a 2205 duplex stainless steel at 250 ℃

The deformation process of the microstructure in 2205 duplex stainless steel （DSS） under thermo-mechanical coupling at 250℃ was investigated using digital image correlation （DIC）. A thermal tension test of duplex stainless steel （2205DSS） with a banded structure was carded out to observe the initial deformation of the microstructure. It was found that inhomogeneous strain fields occurred primarily in austenite. The maximum normal strain in austenite was almost pos- itive, while that in ferrite was almost negative. In addition, a thermal cyclic-loading test was conducted, and the strain field was characterized by e11. Strain heterogeneities were induced after 400 cycles, which spread within the austenite and at the phase boundaries with the load increasing. The high tensile-strain regions were always located adjacent to regions of intense compressive strain. Based on the strain matrix sum vs. cycle number, we found that hardening occurred in the early cycles followed by softening.

The development and design of the repair welding procedure of the thick wall duplex stainless steel piping

In this paper,the performance characteristics of ASTM A790 S31803 and ASTM A182 F51 duplex stainless steel were introduced,and the weldabilities were analyzed. The welding repair procedures of thick wall duplex stainless steel were developed by welding procedure qualification. Research shows that the repair welding has less effect on mechanical properties of welded joint, and great influence on phase proportions and pitting corrosion resistance. The test results meet the requirements of project specifications. The repair welding procedure can be used in project,and only one time repair shall be used.

Effect of cooling rate and forced convection on as-cast structure of 2205 duplex stainless steel

To forecast the as-cast structure and ferrite-austenite phase ratio of 2205 duplex stainless steel(DSS), the effects of cooling rate and forced convection were observed in a high-vacuum resistance furnace in which the forced convection was created by the rotation of the crucible. The as-cast structure of all 2205 DSS samples is full equiaxed grains, and the microstructure consists of a great amount of desirable intra-granular austenite inside the continuous ferrite grain matrix, besides Widmanstatten austenite and grain boundary austenite. The ferrite grain size decreases gradually with the increase in the cooling rates(20 to 60 oC·min-1) or the forced convection, while the ferrite grains of the samples solidified with a strong convection are barely changed when the cooling rate is below 50 oC·min-1. Moreover, a small grain size is beneficial for the austenite formation but the influence is not very obvious under the cooling rates in the range of 5 to 50 oC·min-1. Compared with grain size, the cooling rate has a greater influence on the final ferrite content. A model based on the experimental results is established to predict the ferrite content, which could be approximated by δ(%) = 20.5·exp(c/80.0) + 0.34 d +34.1, where cis the cooling rate in oC·min-1 and d is the grain size in mm. By using this model, the dependence of the final ferrite content on cooling rate and grain size is well described.

Effect of cooling rate on solidification structure and linear contraction of a duplex stainless steel

Cooling rate is a key factor that can drastically affect the phase transformation and thermal stress of duplex stainless steels. Therefore, in this research, different sand moulds were used to explore the influence of cooling rate on the solidification of the 2304 duplex stainless steel (DSS). The macro and micro structures of the 2304 DSS were investigated. Small equiaxed grains are obtained in chromite sand mould sample with a lower pouring temperature and a higher cooling rate, whereas coarse columnar and equiaxed grains are found in silica sand and refractory powder mould samples. The size of austenite phase is significantly increased with decreasing cooling rate, while the ferrite phase content ranging from 51.6% to 53.9% does not change obviously. In addition, the linear contraction of the 2304 DSS decreases from 2.34% to 1.09% when the mean cooling rate above 1,173 K increases from 0.99 K·s-1 to 3.66 K·s-1.

Electrochemical extraction and the analysis of inter-metallic phases and nitrides in tempered duplex stainless steels

During aging at a temperature ranging from 650 -950 ℃,the ferric matrix in duplex stainless steels undergoes various decomposition processes which could form the precipitates of the Sigma （σ） and Chi （X） phases, as well as nitrides. It is well known that these precipitates lead to a reduction in creep ductility and adversely affect toughness and corrosion properties of steel. This experiment carded out qualitative and quantitative analyses of intermetallic phases and nitrides and established an analytical procedure, including specimen preparation, the choosing of the electrolyte and electrolytic systems,electrolytic isolation,wet chemical separation, and physical and chemical analysis, etc. The residues were collected by ultrasonic cleaning and filtration after galvanostatic electrolysis. Dynamic laser scattering sizer （DLS- sizer） ,scanning electron microscope （SEM） and transmission electron microscope （TEM） were used to examine their structure,modality and size. Qualitative and quantitative analyses were performed by using X-ray diffraction （XRD）, oxygen-nitrogen analyzer and wet chemical analysis. Furthermore, there is a discussion on the effect of isothermal treatment on precipitation that occurs at different temperatures for different periods of time.