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.

The effect of W and Cu on the microstructure and properties of Ni-saving 25Cr duplex stainless steels

Some new Ni-saving 25Cr duplex stainless steels(DSS)have been developed.The results indicate that the alloy has a balanced ferrite-austenite relation after hot forge and solid solution treatment at 1 000℃. The elements W and Cu have a marked effect on the microstructure of the alloy.The pitting potential of the steel adding W and Cu elements reaches the maximum value of 435 mV.The tensile strength and percentage of area reduction of all steels in this paper are 800 -900 MPa and 60%-70%,respectively.The tensile elongations of the alloys are all above 30%.The experimental steels have good corrosion and mechanical property.

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.

Comparative research on the mechanical property and precipitation of the 2101 and 2205 duplex stainless steels

The mechanical property and precipitation of the 2101 and 2205 duplex stainless steels were investigated. The results show that with nitrogen-content increasing from 0.12% to 0.26% to partly replace nickel ,the yield strength of the 2101 steel gains an increase of 80 MPa whilst its elongation proportion keeps unchanged. The impact energy at a low temperature is obviously reduced. The temperature at which the impact energy starts decreasing is lower than 20℃, -20℃ and - 40℃ for the 2101 steels containing 0.5 % Ni, 1.5 % Ni and 2.5 % Ni respectively, whereas it is - 70℃ for the 5 % Ni- containing 2205 steel. The nose temperature of precipitation is 700℃ for the 2101 steel and 850℃ for the 2205 steel. The scanning electronic microscope （SEM） ,the transmission electron microscope （TEM） and the X-ray diffraction （XRD） analyses show that the drop in the impact energy of the 2101 steel can be mainly attributed to the precipitation of Cr2N upon ageing while it is attributed to the sigma phase for the 2205 steel.

MICROSCOPIC CORROSION STUDIES OF DUPLEX STAINLESS STEELS

Electrochemical scanning tunneling microscopy and scanning electrochemical microscopy have been used for in situ monitoring of localized corrosion processes of different Duplex stainless steels (DSS) in acidic chloride solutions. The techniques allow imaging of local dissolution events with micrometer resolution, as opposed to conventional electrochemical techniques, which only give an overall view of the corrosion behavior. In addition, combined scanning Kelvin probe force microscopy and magnetic force microscopy were used for mapping the Volta potential variation over the surface of DSSs. A significant difference in Volta potential between the austenite and ferrite phases suggests galvanic interaction between the phases. A compositional gradient appears within 2 micrometers across the phase boundary, as seen with scanning Auger microscopy (SAM). In all, the studies suggest that higher alloyed DSS exhibit a more homogeneous dissolution behavior than lower alloyed DSS, due to higher and more similar corrosion resistance of the two phases, and enhanced resistance of the ferrite/austenite phase boundary regions.

INFLUENCE OF GRAIN SIZE ON STRENGTH AND HYDROGEN INDUCED CRACKING OF DUPLEX STAINLESS STEELS

The relation between grain size and strength of the duplex stainless steels and influence of grain size on properties of hydrogen induced cracking in these steels have been investigated. The Hall-Petch relation between grain size and strength of the steels is also followed.The susceptibility to hydrogen induced cracking of the steels increases with increasing grain size.

Progress in weldability research of duplex stainless steels

Duplex stainless steels(DSSs)show better corrosion resistance with higher strength than traditional austenite stainless steels in many aggressive environments,and can be welded properly with almost every welding processes,if proper heat input is provided.Progresses of research works on weldability of DSSs in recent years are reviewed in this paper.Balance control of ferrite/austenite phases is most important for DSSs welding.The phases balance can be controlled with filler materials,nitrogen addition in shielding gas,heat input,post weld heat treatment,and alternating magnetic field.Too high cooling rate results in not only extra ferrite,but also chromium nitride precipitation.While too low cooling rate or heating repeatedly results in precipitation of secondary austenite and intermetallic compounds.In both situations,mechanical properties and corrosion resistance of the DSS joints deteriorate.Recommended upper and lower limits of heat input and maximum interpass temperature should be observed.

A short review on the role of alloying elements in duplex stainless steels

Duplex stainless steels consisting of ferrite and austenite are widely used due to their excellent mechanical properties and corrosion resistance.Compared with ferritic stainless steels,duplex stainless steels have better plasticity,toughness,and welding performance.They also possess higher strength and better resistance to stress,pitting,and crevice corrosion than austenitic stainless steels.In addition to the above-mentioned properties,there are cost-saving advantages in duplex stainless steels due to their lower nickel content.Today,the types of duplex stainless steel are mainly divided into four categories:lean duplex stainless steel,standard duplex stainless steel,super duplex stainless steel,and hyper duplex stainless steel.Alloying design of duplex stainless steel is an important strategy to achieve high performance.In the last two decades,significant progress has been made in both theoretical calculations and experiments.By adjusting alloying elements such as chromium,nickel,molybdenum,nitrogen,copper,tungsten and rare earth,etc.,the mechanical properties and/or corrosion resistance of the duplex stainless steels can be further improved.Summarizing the comprehensive progress of alloying design of duplex stainless steel is of great significance in providing a data basis for establishing the corresponding relationship between chemical compositions and properties.Therefore,this paper reveals the specific roles of alloying elements in the duplex stainless steels and provides a reference for alloying design with different performance requirements.

Non-isothermal Dissolution Modelling of Sigma Phase in Duplex Stainless Steels

In this work, the non-isothermal dissolution kinetics of the sigma phase in duplex stainless steels has been studied and modelled. A semi-empirical model is proposed to describe the kinetics of sigma phase precipitation/dissolution during continuous heating starting from the isothermal transformation kinetics. The proposed model, which presumes validity of the additivity rule, is validated by means of experimental investigations. A good agreement is found between experimental and analytical results.

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.

22Cr High-Mn-N Low-Ni Economical Duplex Stainless Steels

A new family of economical duplex stainless steels in which N or Mn was substituted for Ni with composi- tion of 22Cr-8.0Mn-xNi-1.0Mo-0.7Cu-0.7W-0.3N （x = 0.5 -- 2.0） have been developed by examining the micro- structure, mechanical and corrosion properties of these alloys. The results show that these alloys have a balanced ferrite-austenite relation. In addition, the alloys are free of precipitation of sigma phase and Cr-nitride when solution- treated at 750 to 1300℃ for 30 min. The yield strength, tensile strength and fracture elongation values of experi- mental alloys solution-treated at 1050 ℃ for 30 min are about 500, 750 MPa and 40.0%, respectively. Low-temper- ature impact properties can be improved distinctly with the increase of nickel content. Among the designed DSS al- loys, the alloy with Ni of 2.0% is found to be an optimum alloy with proper phase proportion, better low-tempera- ture impact properties and higher pitting corrosion resistance compared with those o~ other alloys. The mechanical and corrosion properties and lower production cost of the designed DSSs are better than those of AISI 304.

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.

Optimization of process parameters of friction welding of UNS S31803 duplex stainless steels joints

This article presents an approach based on Taguchi method and Grey relational analysis to optimize process parameters of friction welding of UNS31803 duplex stainless steel. The main objective is to maximize mechanical properties like tensile strength, hardness and impact toughness and to minimize corrosion rate. Heating pressure, heating time, upsetting pressure and upsetting time were the four process parameters taken each at three levels. According to Taguchi quality design concept, an L9 orthogonal array was selected for experiments. The best combination of process parameters was found by both Taguchi method and Grey relational analysis. The influ- ence of the process parameters on overall quality charac- teristics of the friction welding process was evaluated by the analysis of variance （ANOVA） method. The confir- mation test results with optimal parameters confirmed the effectiveness of the proposed method in this study. Later, comparison was done between Taguchi method and Grey relational analysis on the basis of improvement in multi- response signal to noise （S/N） ratio over initial process parameters. Grey relational analysis was proved to be a better technique than Taguchi method for optimization of multiple responses.

A New Series of Mo-free 21.5Cr-3.5Ni-xW-0.2N Economical Duplex Stainless Steels

A new series of economical Mo-free duplex stainless steels 21.5Cr-3.5Ni-xW-0. 2N （x = 1.8 -- 3. 0, mass%） have been developed. The effects of W on mechanical properties and corrosion resistance were investigated, and the microstructures were analyzed by optical microscopy, X-ray diffraction, transmission electron microscopy and electron backscatter diffraction. The designed steels have a balanced ferrite-austenite relation and are free of sigma phase after solution treatment at 750--1 300 ~C for 30 min followed by water-quenching, whereas a small number of Cr23 C~ precipitates were found after solution treatment at 750 ~C. After solution treatment at 1050 ℃, the steel with 1.8~ （mass percent） W exhibits the highest room temperature tensile strength due to the strongest work hardening effect, while the steel with 3.0% （mass percent） W exhibits the highest fracture elongation owing to the transformation-in- duced plasticity （TRIP） effect. The ductile-brittle transition （DBT） and martensite transformation are respectively found in the ferrite and austenite, which deteriorates the impact properties of the steels with the increase of W con- tent. The corrosion resistance of the designed steels is improved with the increase of W content. The pitting resistance of austenite is obviously better than that of ferrite for the designed alloys. Among the designed steels, the steel with 1.8% （mass percent） W is found to be an optimum steel with excellent comprehensive properties and lowest production cost.

A New Resource-Saving,High Chromium and Manganese Super Duplex Stainless Steels 29Cr-12Mn-2Ni-1Mo-xN

A new family of resource saving, high chromium and manganese super duplex stainless steels （DSSs）, with a composition in mass percent, % of Cr 0.29, Mn 0.12, Ni 2.0, Mo i. 0, and N 0. 51-0. 68, has been developed by examining the effect of N on the microstructure, mechanical properties and corrosion properties. The results show that these alloys have a balanced ferrite-austenite relation. The austenite volume fraction decreases with the solution treatment temperature, but it increases with an increase in N content. The increases in nitrogen enhance the ultimate tensile strength （UTS） and reduce the ductility of the material slightly. The pitting corrosion potential increases first and then decreases with an increase in nitrogen content when the amount of N arrives to 0.68%. The yield stress and ultimate tensile strength of solution treated samples were more than 680 and 900 MPa, the elongation of experimental alloys are higher than 30%, respectively, what is more, the pitting potentials were beyond 1 100 mV.

The 80-year history of duplex stainless steel

80 years has passed since duplex stainless steels were first produced and now they have developed into an integral series with the efforts on R & D and development of technology.In the recent decade, duplex stainless steels have been accepted by more and more customers and increasingly used. The first duplex grade produced in Sweden was 453E(26Cr-5Ni) in the 1930s,and then developed into 329.These two grades were characterized by high carbon content and called the first generation of duplex stainless steels.At that time,it was very difficult to add nitrogen into the steels and maintain the phase equilibrium,thus influencing the application properties,for example,intergranular corrosion post welding. One method to solve this problem is to alter the chemical composition,like adding nitrogen,etc.And that came to reality with the development of AOD and metallurgical theory of stainless steels.New series of duplex grades,called the second and third generations duplex,have successively emerged since the 1980s. These grades are characterized by high amounts of alloying elements,like chromium,molybdenum and nitrogen.Furthermore,super duplex stainless grades,like S32750,S32760 and S32707,were developed for various harsh service environments with their outstanding corrosion resistance and workability.These grades possess corrosion resistance corresponding to super austenitic grades,or close to nickel-base alloys, and are used in ocean-engineering,sea water desalination and oil industries,etc. And the application of duplex stainless steel is expending into other industries.For instance,453E is used in the pulp & paper industry.2205(S32205),a medium-alloyed grade,has become the most typical one in the duplex stainless steel family and widely used in many industries like pulp & paper,chemical and oil.New applications are emerging with better understanding of the duplex grades. Modern duplex stainless steels features most the corrosion resistance and strength,making them most cost-efficient in more and more projects. In this paper,the history of duplex stainless steels is recalled and reviewed from R&D,production to application,and latest grades like S82441 are also introduced.

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℃.

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.