Super duplex stainless steels (SDSSs) with lower nickel content are cost effective substitutes for higher alloyed austenitics and nickel alloys in demanding environments such as oil and gas production tubulars and pipelines due to their excellent corrosion resistance and high strength. The overall properties of SDSS are derived from its dual microstructure of equal ferrite and austenite, higher alloying additions of chromium, molybdenum, nitrogen and tungsten and its thermomechanical history. Higher alloying renders SDSS prone to secondary phase precipitation such as sigma phase during improper welding operations or fabrication, affecting the materials properties. Reports suggest that tungsten additions in SDSS delays sigma phase precipitation, hence the development of tungsten based SDSS such as UNS S39274. However, secondary phases cannot be entirely avoided in SDSS. Secondary phase evolution in DSS and the mechanical properties/corrosion behaviour of SDSS has been studied extensively yet their interaction is still not clear. In-service failures of SDSS components have identified gaps in the understanding of the link between secondary phase evolution and material properties, thus limiting the safe and efficient use of SDSS. The work presented in this thesis explored and quantified experimentally the role of aging on secondary phase evolution and ensuing effects on the mechanical properties, corrosion behaviour and toughness of UNS S39274 SDSS. The results revealed that chi phase precipitation occurred preferentially before the sigma phase, although chi phase was metastable in the studied alloy. Numerical modelling established that the measured concentration of the precipitated sigma phase follows the prediction by the Johnson-Mehl-Avrami kinetic model. The time-temperature -transformation was computed using experimental data, the results were compared with theoretical predictions. Results established that increase in both sigma and chi phase led to significant drop in the uniform strain and enhancement of the modulus, hardness and yield and tensile strengths. We note that the sigma phase was attacked by corrosion in comparison to other grades of 25Cr SDSS where the sigma phase remains inert to corrosion attack. Pitting corrosion resistance was influenced more by sigma phase than the chi phase composition. Chi phase was more damaging on the toughness than sigma phase. Another key finding is that the corrosion behaviour and fracture behaviour is more sensitive to lower secondary phase volume fraction than the tensile properties.

Cold-formed stainless steel sections have been increasingly used in architectural and structural applications. Yet the high price of stainless steel limits the application to construction projects. The lean duplex stainless steel (EN 1.4162) offers an opportunity for stainless steels to be used more widely due to its competitive in price, good mechanical properties and corrosion resistance. The lean duplex stainless steel is a relatively new material, and research on this material is limited. Currently, the lean duplex stainless steel is not covered in any design specification, and no design rules are available for such material. Therefore, the behaviour and design of cold-formed lean duplex stainless steel members are investigated in this study. The investigation focused on columns, beams and beam-columns of square and rectangular hollow sections. Both experimental and numerical investigations were performed and reported. Design rules for cold-form lean duplex stainless steel members are proposed. The experimental investigation included material tests, column tests, beam tests and beam-column tests at room and elevated temperatures ranged from 24 – 900 °C. The test specimens were cold-rolled from flat strips. The test program consists of two square hollow sections and four rectangular hollow sections. Coupon specimens were extracted from each hollow section, and their material properties were obtained from tensile coupon tests at room temperature and elevated temperatures. In this study, a modified design rule was proposed to predict the cold-formed lean duplex stainless steel material properties at elevated temperatures. The local and overall geometric imperfections were measured. A total of 38 column tests were conducted. The effective column length ranged from 75 to 1660 mm in order to obtain a column curve for each test series. The test program for beams included 10 pure bending tests, and the bending capacities of the specimens were determined. A total of 37 beam-column specimens were compressed between pinned ends at different eccentricities in order to obtain an interactive curve for each series of test. Numerical investigation on columns, beams and beam-columns at room temperature as well as elevated temperatures are also presented. Accurate finite element models were developed and verified against the experimental results for columns, beams and beam-columns at room temperature. The structural members at elevated temperatures were simulated by replacing the material properties with those obtained at elevated temperatures. Extensive parametric studies were carried out, including 150 columns, 126 beams and 150 beam-columns at room temperature, as well as 180 columns, 125 beams and 195 beam-columns at elevated temperatures. Column, beam and beam-column strengths obtained from the experimental and numerical investigations as well as available data were compared with the design strengths calculated using American, Australian/New Zealand, European specifications for stainless steel structures of duplex material, since lean duplex material is not covered by these specifications. In addition, direct strength method for carbon steel and stainless steel as well as continuous strength method for stainless steel were assessed for cold-formed lean duplex stainless steel. Modified design rules were proposed. The reliability of the current and modified design rules was evaluated using reliability analysis. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Abstract The evolutions of microstructure and texture in 2205 duplex stainless steel (DSS) welds produced by two high energy density (HED) processes, CO2 laser beam welding (LBW) and electron beam welding (EBW) were investigated. A variety of analytical techniques were applied for the study on microstructure and texture of the welds. In which, optical microscopy and electron microscopy were used to evaluate the detailed microstructure. X-ray diffraction (XRD) was put to investigate the crystallographic textures among the base metal, heat affected zone and fusion zone. Particular attention was focused on the determination of microtexture in HED welds by using electron backscatter diffraction (EBSD) technique. After that, an effort was put to compare the results by both of X-ray macro-texture and EBSD-microtexture. The recorded micrographs illustrates that the HED welds are mainly composed of d-ferrite grained structure, which is further decorated with allotriomorphic and Widmanst&#x00E4;tten austenite (g) at grain boundaries. With preheating treatment, the volume fraction of austenite in LB weld is gradually increased, and then leading to a completely different morphology. An apparent amount of transformation twins are found in g phase under TEM observations. No matter that they are Widmanst&#x00E4;tten austenite in nonpreheated welds or blocky austenite in preheated welds, all of the transformation twins have the same {111} twin boundary. Furthermore, modulated fringes composed of ferrite, secondary austenite and amorphous phase are also found in the nonpreheated LB weld. It is ascribed to the rapid cooling effect occurred in the nonpreheated LB weld. Two chromium nitrides (CrN and Cr2N) are also identified and attributed to their different driving forces. A remarkable texture gradient is found in the base metal along the thickness direction for both of austenite and ferrite phases in 2205 duplex stainless steel. The texture is governed separately by the {001}//ND-fibre, a-fibre, Goss and rotated cube components. Despite the analogous local texture evolutions revealing in both LB and EB welds, the global solidification textures in the two processes are considerably different. For which, the texture of LB weld is predominantly evolved with the Goss component. However, the texture of EB weld is mainly composed of the pronounced cube {001}<100>, while the Goss {011}<100> and rotated cube {001}<110> are weakened. The microtexture analysis shows that the centre region of the weld is dominated by oriented nucleation mechanism. Whereas, regions near the fusion boundaries are governed by oriented growth mechanism. The texture feature from EBSD does consist well with the XRD measured result. Moreover, the measurement of local texture from EB weld clearly indicates that a high percentage of high angle grain boundaries distributed in the crown. By contrary, a high percentage of low angle grain boundaries distributed in the root. Both of them again reflect the cooling effect of weld on the solidification mechanism. Throughout this study, the key factors to be responsible for the evolution of solidification texture of HED welded DSS are summarized. Those are thermal conductivity of the weld, turbulent flow in the molten pool, parent textures and the orientation relationship between ferrite and austenite.

Corrosion and stress corrosion cracking of structural components cost an estimated $300 billion annually in the United States alone and are a safety concern for a number of industries using hot alkaline environments. These process environments may contain different amounts of sulfide and chloride; however, the combined role of these ions on the stress corrosion cracking of duplex stainless steels, which are widely used because of their generally reliable performance, had never been studied. This study shows that chlorides in sulfide-containing caustic environments actually have a significant influence on the performance of these alloys. A mechanism for stress corrosion cracking of duplex stainless steels in hot alkaline environments in the presence of sulfide and/or chloride was proposed. Microstructural and environmental aspects were studied using mechanical, electrochemical, and film characterization techniques. The results showed that selective corrosion of the austenite phase depended on percent sulfidity, alkalinity, and chloride content. Chlorides enhanced crack initiation and coalescence along the austenite/ferrite phase boundaries. Unstable passivity of duplex stainless steels in hot alkaline-sulfide environments was due to anion adsorption on the surface leading to defective film formation. Chlorides and sulfide available at the electrolyte/film surface reduced the charge transfer resistance and shifted the response of the films to lower frequencies indicating the films became more defective. The surface films consisted of an outer, discontinuous layer, and an inner, barrier layer. Fe, Mo, and Mn were selectively dissolved in alkaline and alkaline-sulfide environments. The onset of stress corrosion cracking was related to the extent of selective dissolution and was consistent with a film breakdown and repair mechanism similar to slip-step dissolution. Recommendations for reducing the susceptibility of duplex stainless steels to stress corrosion cracking in sulfide-containing caustic environments include reducing the chloride to hydroxide ratio and alloying with less Mo and Mn. The results will impact the petrochemical, pulp and paper, and other process industries as new duplex grades can be developed with optimal compositions and environments can be controlled to extend equipment life.

Duplex stainless steel (DSS) is a dual-phase material with approximately equal volume amount of austenite and ferrite. It has both great mechanical properties (good ductility and high tensile/fatigue strength) and excellent corrosion resistance due to the mixture of the two phases. Cyclic loadings with high stress level and low frequency are experienced by many structures. However, the existing study on corrosion fatigue (CF) study of various metallic materials has mainly concentrated on relatively high frequency range. No systematic study has been done to understand the ultra-low frequency (10-5 Hz) cyclic loading effect on stress corrosion cracking (SCC) of DSSs. In this study, the ultra-low frequency cyclic loading effect on SCC of DSS 2205 was studied in acidified sodium chloride and caustic white liquor (WL) solutions. The research work focused on the environmental effect on SCC of DSS 2205, the cyclic stress effect on strain accumulation behavior of DSS 2205, and the combined environmental and cyclic stress effect on the stress corrosion crack initiation of DSS 2205 in the above environments. Potentiodynamic polarization tests were performed to investigate the electrochemical behavior of DSS 2205 in acidic NaCl solution. Series of slow strain rate tests (SSRTs) at different applied potential values were conducted to reveal the optimum applied potential value for SCC to happen. Room temperature static and cyclic creep tests were performed in air to illustrate the strain accumulation effect of cyclic stresses. Test results showed that cyclic loading could enhance strain accumulation in DSS 2205 compared to static loading. Moreover, the strain accumulation behavior of DSS 2205 was found to be controlled by the two phases of DSS 2205 with different crystal structures. The B.C.C. ferrite phase enhanced strain accumulation due to extensive cross-slips of the dislocations, whereas the F.C.C. austenite phase resisted strain accumulation due to cyclic strain hardening. Cyclic SSRTs were performed under the conditions that SCC occurs in sodium chloride and WL solutions. Test results show that cyclic stress facilitated crack initiations in DSS 2205. Stress corrosion cracks initiated from the intermetallic precipitates in acidic chloride environment, and the cracks initiated from austenite phase in WL environment. Cold-working has been found to retard the crack initiations induced by cyclic stresses.

Os aços inoxidáveis duplex (AID) caracterizam-se pela sua microestrutura composta por austenita numa matriz ferrítica, com fração volumétrica média de 50% para cada fase. A combinação destas características confere-lhes excelente resistência mecânica e à corrosão. A soldagem/junção destes aços é frequentemente uma operação crítica, já que, ao sofrer ciclos térmicos, estes aços têm suas propriedades alteradas. Portanto, processos de junção com ausência de gradientes de temperatura, como a brasagem, mostram-se uma solução prática para a junção destes aços. No entanto, o adequado desenvolvimento do processo de brasagem em AID envolve considerações importantes a respeito da escolha dos parâmetros de processamento e metal de adição em conjunto com os ciclos térmicos de aquecimento e resfriamento. O presente estudo pretende avaliar a brasabilidade dos AID UNS S32101(baixa liga), UNS S32304 (baixa liga), UNS S32507 (superduplex) e UNS S32707(hiperduplex), mediante a caracterização da junta brasada. Estes aços foram brasados em forno continuo com metal de adição BNi-7 (Ni-Cr-P), na temperatura de 1100oC, por tempos de 32 min e 12 min, seguidos de resfriamento em forno, utilizando-se folgas de 0,5, 0,3 e 0,0 mm. A junta brasada foi caracterizada utilizando-se microscopia ótica e microscopia eletrônica de varredura (MEV). A identificação microestrutural foi realizada via Microanálise Química de Energia Dispersiva de Raios-X (EDS) e difração de Raios-X. Foi identificada na junta brasada dos diferentes AID a fase rica em níquel, assim como fases prejudiciais compostas por fosfetos de níquel e cromo. A fase rica em níquel, que usualmente é denominada de -Ni, apresenta-se preferencialmente em toda a região da junta, sem a presença de fase intermetálica contínua no centro da junta. No caso dos aços inoxidáveis super e hiperduplex esses apresentaram a formação de fase sigma na região da interface da junta, do lado do metal de base, devido aos ciclos térmicos de brasagem / The duplex stainless steels (DSS) are characterized by its microstructure consisting of austenite in a ferritic matrix with mean volumetric fraction of 50% for each phase. The combination of these features gives them excellent mechanical strength and corrosion resistance. The welding / joining of these steels are often considered as a critical operation, since, subjected to thermal cycles, they have their microstructures changed and, consequently, their properties. Therefore, joining processes without temperature gradients, like brazing, are shown as a practical solution for joining these steels. However, the proper development of brazing process of DSS involves important issues concerning the choice of processing parameters and the brazing filler metal together with heating and cooling brazing thermal cycles. This study aims at evaluating the brazeability of DSS UNS S32101 (lean duplex), UNS S32304 (lean duplex), UNS S32507 (superduplex) and UNS S32707 (hyperduplex) by characterizing the brazed joint. These steels were brazed in a furnace with filler metal BNi-7 (Ni-Cr-P) at 1100°C for times of 32 min and 12 min, followed by cooling in a continuous brazing furnace, with joint gaps of 0.5 , 0.3 and 0.0 mm. The brazed joint was characterized using optical microscopy and scanning electron microscopy (SEM). The phase identifications were performed by microanalysis using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction. Results showed, for non-ideal joint gaps, in all duplex used in this work, the Ni rich solid solution, as well as deleterious phases composed of nickel and chromium phosphides. The ideal gap presented Ni rich solid solution, usually called -Ni, was found continuously without a continuous intermetallic region in the joint center. In the case of super duplex and hyperduplex brazing, the brazing thermal cycles produced the formation of sigma phase in the region of the joint interface, in the base metal.

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Os aços inoxidáveis duplex (AID) caracterizam-se pela sua microestrutura composta por austenita numa matriz ferrítica, com fração volumétrica média de 50% para cada fase. A combinação destas características confere-lhes excelente resistência mecânica e à corrosão. A soldagem/junção destes aços é frequentemente uma operação crítica, já que, ao sofrer ciclos térmicos, estes aços têm suas propriedades alteradas. Portanto, processos de junção com ausência de gradientes de temperatura, como a brasagem, mostram-se uma solução prática para a junção destes aços. No entanto, o adequado desenvolvimento do processo de brasagem em AID envolve considerações importantes a respeito da escolha dos parâmetros de processamento e metal de adição em conjunto com os ciclos térmicos de aquecimento e resfriamento. O presente estudo pretende avaliar a brasabilidade dos AID UNS S32101(baixa liga), UNS S32304 (baixa liga), UNS S32507 (superduplex) e UNS S32707(hiperduplex), mediante a caracterização da junta brasada. Estes aços foram brasados em forno continuo com metal de adição BNi-7 (Ni-Cr-P), na temperatura de 1100oC, por tempos de 32 min e 12 min, seguidos de resfriamento em forno, utilizando-se folgas de 0,5, 0,3 e 0,0 mm. A junta brasada foi caracterizada utilizando-se microscopia ótica e microscopia eletrônica de varredura (MEV). A identificação microestrutural foi realizada via Microanálise Química de Energia Dispersiva de Raios-X (EDS) e difração de Raios-X. Foi identificada na junta brasada dos diferentes AID a fase rica em níquel, assim como fases prejudiciais compostas por fosfetos de níquel e cromo. A fase rica em níquel, que usualmente é denominada de -Ni, apresenta-se preferencialmente em toda a região da junta, sem a presença de fase intermetálica contínua no centro da junta. No caso dos aços inoxidáveis super e hiperduplex esses apresentaram a formação de fase sigma na região da interface da junta, do lado do metal de base, devido aos ciclos térmicos de brasagem / The duplex stainless steels (DSS) are characterized by its microstructure consisting of austenite in a ferritic matrix with mean volumetric fraction of 50% for each phase. The combination of these features gives them excellent mechanical strength and corrosion resistance. The welding / joining of these steels are often considered as a critical operation, since, subjected to thermal cycles, they have their microstructures changed and, consequently, their properties. Therefore, joining processes without temperature gradients, like brazing, are shown as a practical solution for joining these steels. However, the proper development of brazing process of DSS involves important issues concerning the choice of processing parameters and the brazing filler metal together with heating and cooling brazing thermal cycles. This study aims at evaluating the brazeability of DSS UNS S32101 (lean duplex), UNS S32304 (lean duplex), UNS S32507 (superduplex) and UNS S32707 (hyperduplex) by characterizing the brazed joint. These steels were brazed in a furnace with filler metal BNi-7 (Ni-Cr-P) at 1100°C for times of 32 min and 12 min, followed by cooling in a continuous brazing furnace, with joint gaps of 0.5 , 0.3 and 0.0 mm. The brazed joint was characterized using optical microscopy and scanning electron microscopy (SEM). The phase identifications were performed by microanalysis using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction. Results showed, for non-ideal joint gaps, in all duplex used in this work, the Ni rich solid solution, as well as deleterious phases composed of nickel and chromium phosphides. The ideal gap presented Ni rich solid solution, usually called -Ni, was found continuously without a continuous intermetallic region in the joint center. In the case of super duplex and hyperduplex brazing, the brazing thermal cycles produced the formation of sigma phase in the region of the joint interface, in the base metal.