Thermal aging effect on 2205 duplex stainless steel

The effect of isothermal aging treatment on the mechanical and corrosion properties of 2205 duplex stainless steel was investigated by means of impact toughness test and micro-hardness measurement in combination with the critical pitting temperature( CPT) technique. The corresponding fractography of the steel was then observed after the impact toughness test. The results demonstrated that,at the critical temperature for precipitation of the sigma( σ) phase,e. g.,850 ℃,the impact toughness decreased rapidly and the micro-hardness increased gradually with increasing aging time. The CPT decreased from 61 to 15 ℃ as the aging time increased from 4 min to 8 h. In addition,optical microscopy,transmission electron microscope( TEM) and X-ray diffraction studies showed that the ferrite in the steel transformed into secondary austenite and σ phase.     

A fractographic investigation of thermal embrittlement in cast duplex stainless steel

The fracture surface topography analysis (FRASTA) technique under development at SRI was applied to seek an explanation for severe thermal embrittlement observed in cast duplex stainless steel. By comparing topographic features of conjugate fracture surfaces, FRASTA showed that fracture in thermally embrittled cast duplex stainless steel occurs by microcrack initiation at delta-phase grain boundaries and at alpha-phase/gamma-phase interfaces, and by microcrack growth along these boundaries and interfaces. The critical crack tip opening displacement (CTOD) as measured from cross-sectional views generated by the FRASTA technique indicated a microcrack initiation toughness,J _Ic, of 287 KJ/m², in excellent agreement with measurements using conventional fracture mechanics procedures, and significantly less than the toughness of unaged material. Formerly with SRI International, Menlo Park, CA, is with the Department of Mechanical Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan 466.     

Influence of carbon content and ferrite morphology on the sensitization of duplex stainless steel

The influence of ferrite morphology and carbon content on the intergranular corrosion behavior of 308 stainless steel was investigated using four wrought alloys and six weld deposited alloys. The four wrought alloys were heat treated at four different annealing temperatures to introduce four different amounts of ferrite. The annealed samples along with the weld deposited alloys were aged at temperatures ranging from 480 to 700°C for times varying between 15 min and 1000 h and then tested for intergranular corrosion susceptibility in acidified copper-copper sulfate solution. For a given carbon content there exists a critical amount and distribution of α-γ boundary area above which the alloy is immune and below which it is susceptible to intergranular corrosion. For amounts and distributions of α-γ boundary area less than the critical value two types of sensitization behavior are possible. First, there may be a sufficient amount and distribution of α-γ boundary area to insure rapid healing of the sensitized microstructure. Second, there may be an inadequate amount or distribution of α-γ boundary area to produce either immunity or rapid healing and the alloy behaves as a fully austenitic alloy regardless of the amount of ferrite present. A model is presented which describes as a function of carbon content the critical amounts and distributions of α-γ boundary area required for rapid healing and immunity to sensitization.     

Influence of elastic and plastic anisotropy on the flow behavior in a duplex stainless steel

The load partitioning between two phases in a cold-rolled duplex stainless steel has been experimentally studied in situ by X-ray diffraction, for different loading directions. It was found that the load partitioning between the two phases is dependent on the loading direction. For loading in the rolling direction, both phases deform plastically to the same degree, while more plastic deformation occurs in the austenitic phase during loading in the transverse direction. For loading in the 45-deg direction, more plastic deformation occurs in the ferritic phase. The strong crystallographic texture in the ferritic phase makes the material anisotropic, with a higher stiffness and yield strength in the transverse direction compared to the rolling direction. The measured texture was used as input to theoretical predictions of both elastic and plastic anisotropy. The plastic anisotropy was predicted by assuming intragranular slip as the main deformation mechanism. The predicted anisotropic material properties were then used in finite-element simulations to study the flow behavior of the material in different directions. The predicted flow behavior was found to be in good agreement with the experimentally observed load partitioning between the phases for loading in the rolling and transverse directions. However, the yield strength of the ferritic phase during loading in the 45-deg direction was found to be lower than what was predicted. The reason for this is the difference in slip characteristics in different sample directions, because of the morphological texture.     

Influence of nitrogen addition on the microstructure and mechanical properties of duplex stainless steel weld metals

Duplex stainless steels find extensive applications in off‐shore, paper/pulp, refineries and petrochemical industries owing to their excellent combination of properties. Because of the thermal cycle experienced during the manufacturing/processing operations like welding, the weldment developed in this material is generally inferior in properties (mechanical and corrosion) when compared to the base material. The problem is more severe when the welding is done in the autogenous condition. The current work correlates the microstructure and property of UNS 31803 duplex stainless steel (material no. 1.4462) welded by autogenous GTA process with and without nitrogen in the shielding gas. It has been found that the nitrogen addition to the weld metal through the shielding gas mixture not only helps in achieving the proper phase balance but also improves the impact toughness of the resultant weld metal.     

Microstructural changes during heating of super duplex stainless steel and its thermal deformation behavior

Microstructural changes during heating of highly alloyed Cr26Ni7 type super duplex stainless steel( SDSS2607) and its thermal deformation behavior were investigated. At different heating rates,the mechanism of phase transition from γ phase to δ phase and grow th modes of δ phase differed. Variations in microstructures for ascast SDSS2607 during heat preservation at 1 220 ℃ indicated two kinds of transformations from γ phase to δ phase.In-situ observations of microstructural changes during the tensile process at 1 050 ℃ showed a mutual coordination betw een γ and δ phases. When the true strain increased,the mutual coordination between γ and δ phases was damaged. Subsequently,cracks nucleated at the γ/δ interface. With the increase in temperature,the strength of ascast SDSS2607 decreased while its plasticity increased. Its thermoplasticity was poor,and the reduction in area of tensile specimens was less than 80%. When the deformation strain of hot compression increased,the stable deformation zone in the heat processing maps enlarged gradually. Moreover,the unstable deformation zones were extended.     

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.     

Room-temperature aging of manganese-alloyed high nitrogen duplex stainless steels

The room-temperature aging behavior of two duplex stainless alloys with different austenite stability was investigated. Both alloys readily aged at room temperature. Even for aging times as short as 30 seconds, the originally continuous yielding behavior becomes discontinuous upon reloading after prestraining. The magnitude of the stress increase due to aging was higher in the presence of strain-induced martensite, even though it was shown that aging also occurred in the austenite phase. The aging response was shown to be thermally activated, with increasing age hardening associated with increasing aging times. The results could be explained by the combination of aging phenomena in the bcc phases by interstitials and the aging by interstitial-vacancy complexes in the fcc phase, where the interstitials are thought to be immobile during the short aging times used and aging would occur due to short-range migration of vacancies instead.     

Influences of superheat and cooling intensity on macrostructureand macrosegregation of duplex stainless steel studied by thermal simulation

The influences of superheat and cooling intensity on macrostructure and macrosegregation of one new kind duplex stainless steel (DSS) were studied. Thermal simu...     

Numerical method applied to duplex stainless steel welding

Abstract

Microstructure plays an essential role in the attractive properties of the duplex stainless steels (DSSs) such as toughness and corrosion resistance. These properties are obtained by an adequate balance between the fractions of the ferrite and austenite phases, which can be modified when DSSs are welded. Besides the unbalanced fractions of ferrite and austenite, the precipitation of deleterious compounds at high temperatures such as sigma phase can also occur during DSS welding. In this work, a model based on transport equations was numerically implemented by the finite volume method in a computational code in order to simulate the 2205 DSS welding. It was able to evaluate qualitatively the sigma phase precipitation and the formation of the ferrite and austenite phases by calculating the cooling rates reached during 2205 DSS welding. The results are discussed in light of the previous work, and good agreement between numerical and experimental results was obtained.     

2205双相不锈钢表面起皮缺陷分析

对2205双相不锈钢冷轧NO.1板表面起皮缺陷进行扫描电镜和能谱分析,并结合其冶炼-热轧-退火酸洗整个生产工艺,得出:缺陷的实质是热轧过程中咬入氧化铁皮经过退火酸洗后的表现形式。通过对2205双相不锈钢板坯加热后金相组织、热塑性、热轧板金相组织以及热轧板氧化铁皮结构的分析,得出:板坯边部柱状晶的存在以及热轧加热炉加热温度过高会影响材料的热塑性,导致轧制过程边部出现微裂纹,热轧完成后演变为咬入式氧化铁皮缺陷。提高2205双相不锈钢等轴晶率、控制加热温度在1260℃以下可以有效避免起皮缺陷的发生。     

Fracture toughness of the lean duplex stainless steel LDX 2101

Fracture toughness testing was performed on the recently developed lean duplex stainless steel LDX 2101 (EN 1.4162, UNS S32101). The results were evaluated by master curve analysis, including deriving a reference temperature. The master curve approach, originally developed for ferritic steels, has been used successfully. The reference temperature corresponds to a fracture toughness of 100 MPa√m, which characterizes the onset of cleavage cracking at elastic or elastic-plastic instabilities. The reference temperature, T ₀, was −112 °C and −92 °C for the base and weld materials, respectively. In addition, the fracture toughness is compared with impact toughness results. Complementary crack tip opening displacements (CTODs) have also been calculated. The toughness properties found in traditional duplex stainless steels (DSS) are generally good. The current study verifies a high fracture toughness for both base and weld materials and for the low alloyed grade LDX 2101. Even though the fracture toughness was somewhat lower than for duplex stainless steel 2205, it is still sufficiently high for most low-temperature applications.     

Cracking of duplex stainless steel due to dissolved hydrogen

Ferallium 255 duplex stainless steel was cathodically precharged with hydrogen at 265 °C in a molten salt electrolyte. Sustained load tests were carried out in air at 0 °C, 25 °C and 50 °C with average hydrogen contents from 3 to 15 wt ppm. The DC potential drop method was calibrated with optical measurements to continuously monitor the crack position and allow calculation of crack velocity and stress intensity. The crack velocityvs stress intensity (K) curves generally rose gradually over a large range inK and had definite thresholds for subcritical crack growth. Second and third stages were not always clearly delineated. Threshold stress intensities decreased as hydrogen content increased. An identifiable stage II occurred most often for alloys containing about 10 wt ppm dissolved hydrogen. The crack growth velocities generally increased with increasing temperature or hydrogen content. As the dissolved hydrogen increased, the fracture mode changed from microvoid coalescence (MVC) to microcrack coalescence (MCC) with some tearing ridges. At high hydrogen content, both ferrite and austenite phases showed brittle morphology, which was identical to the fracture surface of the uncharged specimens tested in hydrogen gas at 108 kPa pressure. Comparing the embrittling effect of internal hydrogen with that of external hydrogen it is found that the threshold stress intensity in hydrogen gas at 1 atm is lower than that at the highest internal hydrogen concentration (15 wt ppm). In the case of external hydrogen, the hydrogen source is at the crack tip, whereas for the internal hydrogen case, outgassing reduces the hydrogen content in this region, even when the bulk hydrogen content is fairly high.     

Development of Chinese duplex stainless steel in recent years

The development in research, production, applications, and national standards of Chinese duplex stainless steel (DSS) in recent years was introduced in light of the worldwide development in the field of DSS.The results showed that the output of Chinese DSS increased greatly, and at the same time its grade gradually evolved into a collaborative developing series including the main grade type 2205 and other DSS types in recent years.Economical DSS and super DSS underwent rapid development, especially after 2010.In re-cent years, the application of Chinese DSS has been expanded further not only in traditional application ar-eas such as the petrochemical industry, but also in diverse new fields such as oil and gas transportation, chemical tanker manufacturing, nuclear power plants, and construction.Moreover, due to the increase in output and improvement in quality, as well as applications in Chinese projects, Chinese DSS has also been exported to the Middle East, Eastern Europe, and other regions.     

Precipitation in CF-8M duplex stainless steel welds

Welds of CF-8M, a cast 316-type stainless steel which normally solidifies as primary delta-ferrite, were induced to solidify as primary austenite by the addition of nitrogen to the shielding gas used during gas tungsten arc welding. Those welds which experienced a shift in solidification mode formed eutectic ferrite during the terminal transient stage of solidification. Primary delta-ferrite and eutectic ferrite are differentiated by their location in the dendritic microstructure. The shape of the ferrite/austenite interface tends to be rounded for primary delta-ferrite and more angular for eutectic ferrite. Elemental profiles were plotted from STEM/EDS measurements across the two types of ferrite, and showed differences between the composition of the austenite immediately adjacent to the primary delta-ferrite, as opposed to the eutectic ferrite. In addition, while the primary delta-ferrite/austenite interfaces are largely devoid of precipitation, the eutectic ferrite/austenite interfaces are densely covered with small precipitates ofx-phase. The mean stoichiometry of this phase has been calculated from STEM/EDS data on extraction replicas, and approximates Fe₅₀Cr₃₂Mo₁₃Ni₅. Intragranular inclusions were also examined and found to be complex, with most of them containing varying quantities of Mn, Si, and S.     

双相不锈钢无缝钢管生产裂纹浅析

双相不锈钢无缝钢管生产所面临的最大问题就是产生裂纹 ,裂纹的产生与钢的塑性、韧性有密切关系 ;而生产加工中的双相钢的塑性、韧性直接受钢的脆性相以及低温脆性影响     

2205双相不锈钢边裂缺陷形成原因分析

采用金相显微镜及扫描电镜观察2205双相不锈钢热轧板边裂缺陷,通过夹杂物和微观组织分析,研究了边裂缺陷的形成原因.结果表明,2205双相不锈钢热轧边裂缺陷是由于边部奥氏体晶粒粗大、铁素体和奥氏体两相比例不协调引起的.根据上述原因提出了相应的改进建议.     

Deformation behaviour of duplex stainless steel during industrial hot rolling

Industrially processed duplex stainless steel sheet was investigated after the reheating, the roughing mill and the finishing rolling. Light optical microscopy revealed that the cast structure that existed before rolling was removed and changed into a banded microstructure. During the process, the ferrite‐austenite volume fraction ratio changed from 72 %a + 28 %γ to 48 %α + 52 %γ. The microhardness was measured for both phases after each process step. Texture measurements were executed by means of the electron backscattering diffraction technique (EBSD). These measurements revealed that extensive static recrystallisation of the austenite occurred during the roughing but that recrystallisation was inhibited during the finishing. Transmission electron microscopy (TEM) on the finish rolled material indicated that the ferrite softened efficiently by extended dynamic recovery. The dislocations in ferrite grains with rotated cube orientation were often found to be straight screw dislocations which were inclined 55° to the surface of the sheet. The importance of the partitioning of the alloying elements and the strain partitioning on the deformation behaviour is highlighted.     

Gas metal arc welding of duplex stainless clad steel plates

The 2205 duplex stainless + DH36 clad steel plate was welded by gas metal arc welding (GMAW),and the welding performance of the clad steel plate was investigated.The results show that the adaptability of the welding procedure for the base metal of carbon steel,the transition layer,and the cladding material is excellent.The test results indicate that the phase proportion and component dilution of the GMAW-welded joints of clad steel plate can be effectively controlled to yield joints with good mechanical properties and corrosion resistance.