Hydrogen interaction with structural materials, especially stainless steels, is of great importance due to the acute effect that it can have on them. Stainless steels have become very common in many ...applications, and in water and high pressure environments in particular, due to their high levels of corrosion resistance and broad range of strength. Steel's durability is very much dependent on its microstructure and interaction with hydrogen. The action of hydrogen can lead to changes in mechanical properties, phase transformation and eventually to environmentally-assisted failure, which is known as hydrogen embrittlement (fracture). The susceptibility of steels to this hydrogen fracture mechanism is directly related to the interaction between traps (defects) and hydrogen. In this research, we study hydrogen fracture mechanisms through hydrogen interaction with trapping sites by thermal desorption spectrometry (TDS), and the calculation of hydrogen trapping energies states. Microstructure effects on hydrogen were investigated by exploring different stainless steels, including: austenitic stainless steel (AUSS), ferritic-austenitic (duplex) stainless steel (DSS), and super martensitic stainless steel (SMSS). The objective of this study is to determine the influences of thermal desorption analysis on the crystal structure of different stainless steels in order to better understand the trapping mechanisms of hydrogen in a variety of structure materials. It was found that the AUSS has the greatest stability of austenitic (γ) phase– ∼22% higher than DSS and ∼45% higher than SMSS. Moreover, the AUSS presented the lowest hydrogen trapping values of ∼31% compared with DSS and ∼25% compared with SMSS.
Hydrogen fracture mechanism was found to be highly dependent on the hydrogen trapping states and even more on the γ-phase stability. The hydrogen trapping mechanisms are discussed in detail.
•Hydrogen trapping mechanisms in structure materials were investigated using TDS.•The relation between γ stability, trapping states and fracture mechanism is studied.•Low trapping levels were identified with severe surface damage by hydrogen.•High γ-phase stability was identified with severe surface damage by hydrogen.•The γ → α′ phase transition was related to irreversible trapping site for hydrogen.
The influence of varying the annealing temperature from 1000 °C to 1200 °C on the strain-hardening behavior of a lean duplex stainless steel with a metastable austenite phase was investigated by ...tensile deformation. The results indicated that the tensile properties of the test steel were sensitive to the annealing temperature. The test steel showed enhanced ultimate tensile strength (UTS) and elongation due to the transformation-induced plasticity (TRIP) or/and the twinning-induced plasticity (TWIP) effects. The optimum combination of UTS and ductility with approximately 60 GPa% was obtained at an annealing temperature of 1050 °C. The shape of the strain-hardening curve for the selected annealing temperature range could be divided into two cases: (i) a typical three-stage strain-hardening from 1000 to 1050 °C and (ii) a multiple-stage strain-hardening from 1100 to 1200 °C. Microstructural observations revealed that the typical three-stage hardening was mainly related to a strain-induced martensitic transformation with a sequence of γ→ε→α’, i.e., the TRIP effect. In addition to the strain-induced martensites, mechanical twins were observed in the deformed austenite of the specimens annealed at temperatures above 1100 °C. This result indicated that TRIP and TWIP occurred concurrently in the austenite at higher annealing temperatures. The synergy and mutual competition from the coexistence of TRIP and TWIP caused the multiple-stage strain-hardening.
•Cold-formed stainless steel RHS undergoing combined bending and web crippling was studied.•34 tests were conducted on specimens of lean-duplex and ferritic stainless steel grades.•Parametric studies ...of 312 specimens were performed.•Current combined bending and web crippling design provisions are shown to be conservative.•Improved design rules are suggested in this study.
The behaviour and resistances of cold-formed stainless steel rectangular hollow section (RHS) members undergoing combined bending and web crippling were studied based upon experimental and numerical investigations. A test program consists of 4 pure bending tests, 7 pure web crippling tests and 23 web crippling-bending interaction tests was conducted. The RHS specimens were cold-rolled from lean-duplex and ferritic stainless steel sheets. Numerical models were built and validated against the test results. Upon validation, parametric studies comprised of 312 finite element analyses were undertaken. The obtained test and numerical results were compared with nominal resistances predicted from the American, Australian/New Zealand and European design standards for stainless steel structures. Moreover, the provisions in the North American Specification for cold-formed steel members were also evaluated. The comparison results indicate that the codified design provisions are generally safe to use for design of cold-formed stainless steel RHS members undergoing combined bending and web crippling, among which the European provision yields overly-conservative predictions. The codified web crippling-bending interaction curves can be applied for designing the stainless steel RHS members undergoing combined bending and web crippling, whilst improved predictions could be achieved by employing recently proposed bending and web crippling design rules.
•Flat and corner material properties of the stainless steel tubes are presented.•Using ABAQUS, 3D finite element models CFFSSTs under axial compression are developed.•The fundamental behaviour of the ...CFFSST columns is discussed in detail..•The feasibility of current design codes to the design of the CFFSST columns is evaluated.•The proposed design model is shown to predict well the strengths of CFFSST columns.
Concrete-filled stainless steel tubes (CFSSTs) are increasingly applied in current composite structures owing to their excellent performance characterized by aesthetic appearance, high corrosion resistance and durability, and ease of maintenance. Compared with austenitic and duplex stainless steels, ferritic stainless steels have not or contain very low nickel content, achieving lower and more stable material costs. This provides a more viable alternative for structural applications. Additionally, it has been used successfully in corrosive marine environment in a few countries, such as South Africa. However, structural applications of concrete-filled ferritic stainless steel tubular (CFFSST) columns are still limited owing to the dearth of performance data and design guidance. To understand their structural behaviour and to include relevant design recommendations in current design specifications, this paper aims at investigating the compressive behaviour of axially loaded rectangular CFFSST short columns using the nonlinear finite element (FE) modelling. The FE models are developed using ABAQUS software, where two different material constitutive models for the flat and corner portions of the rectangular ferritic stainless steel tubes are incorporated. Close agreement is found between the outputs of FE analysis and existing test results with respect to the failure modes, cross-sectional strengths and axial load-shortening curves. Parametric analysis is then conducted to ascertain the impacts of key variables on the compressive behaviour of the studied CFFSST columns. The obtained numerical results as well as available test results are employed to evaluate the feasibility of current design specifications to design the examined CFFSST columns. Finally, a novel strength design model is proposed and shown to achieve a higher degree of accuracy and consistency of the design predictions.
This work aims at studying the electrochemical behavior, by Electrochemical Impedance Spectroscopy (EIS), of austenitic (AISI 304 and AISI 316) and duplex (SAF 2205 and SAF 2304) stainless steels, ...when embedded in concrete specimens. Concrete specimens were exposed to chloride containing solutions simulating the aggressive conditions found in sweater environments. Samples were fully immersed and submitted to periodic immersion/emersion cycles. EIS and open circuit potential (OCP) were monitored in a monthly basis. Samples containing carbon steel (C-steel) rebars were also prepared for comparison of the corrosion rates between the different materials.
The results showed that the austenitic and duplex SAF 2205 stainless steels were passive for all the testing period, due to the formation of protective oxide layer, with the AISI 316 presenting the higher corrosion resistance value. The EIS results suggest an increase of more than one order of magnitude in the corrosion resistance of the duplex steels and AISI 316 comparatively to C-steel rebars.
The tensile flow properties of austenitic (S316-L) and martensitic (S410-L) stainless steel wall structures deposited by powder-fed laser additive manufacturing (LAM) process are evaluated. The ...properties obtained by the LAM process are compared to commercial rolled sheets of these metals. Strain-rate sensitivity, work hardening, and fracture behavior are assessed by conducting uniaxial tensile testing at different strain rates (0.001, 0.01, 0.1, and 1.0 sec−1). Moreover, a correlation between the final microstructure and mechanical properties is established for the LAM products through detailed analyses of grain structures and hardness indentation measurements. The results indicate a strong dependency for the strain rate in martensitic alloys compared to austenitic alloys produced by the LAM process. Interestingly, the tensile strength of commercial rolled martensitic stainless steel sheet doubles (∼100% increase) by increasing the strain rate, while preserving its elongation to failure. Comparing the two manufacturing methods, a lower strain-rate sensitivity factor is recorded for the additive manufacturing material (m of ∼0.0336) compared to the commercial sheet (m of ∼0.0775). This lower sensitivity is attributed to coarser grain structure and greater microstructural heterogeneity of the LAM product, which stems from directional solidification and cooling phenomenon during the layer-upon-layer deposition process. In contrast, the work hardening exponent (n value) varies little (0.1834–0.2854) for the different materials and manufacturing methods. Fractographic studies reveal that the fracture mode varies from ductile rupture towards ductile-brittle with the formation of greater martensitic phases, which is in combination with the failure component changing from shear to tensile at high strain rates.
•26 ferritic stainless steel beam-column tests undertaken.•Over 2000 numerical results generated for austenitic, duplex and ferritic stainless steel CHS beam-columns over a range of local and global ...slenderness values.•Existing Eurocode 3 design provisions are evaluated and new design proposals are determined and verified.•Reliability analyses are undertaken for existing and proposed stainless steel CHS beam-column design methods using the new experimental and numerical dataset.
The present work was prompted by shortcomings identified in existing design provisions for stainless steel circular hollow section (CHS) beam-columns. First, addressing a lack of existing experimental data, a series of ferritic stainless steel CHS beam-column tests was undertaken at the cross-section and member levels. In total, 26 beam-column tests, including two section sizes (a non-slender class 3 and slender class 4 cross-section), two member slenderness values for each cross-section type and a wide range of loading eccentricities were carried out to investigate the interaction between local and global buckling. Following validation of finite element (FE) models, a numerical study was then undertaken to explore the buckling response of stainless steel CHS beam-columns, covering austenitic, duplex and ferritic grades with a wide range of local and global slendernesses and applied loading eccentricities. Over 2000 numerical results were generated and used to assess new design proposals for stainless steel beam-columns, featuring improved compression and bending end points and new interaction factors. The new proposals are more consistent and more accurate in their resistance predictions than the current EN 1993-1-4 (2015) design approach. The reliability of the new proposals has been verified by means of statistical analyses according to EN 1990 (2005).
Passive films were grown at constant potential in acidic (pH∼2) and alkaline (pH∼13) solutions on chromium, AISI 304L, AISI 316L and Duplex stainless steels. Passive films on chromium grow following ...a high field mechanism considering the presence of dissolution phenomena. According to the photoelectrochemical characterization, passive films on Cr have a bandgap of 3.4 eV when formed in acidic solution, and of 2.4 eV when formed in alkaline solution due to the formation of Cr(OH)3. These films result to be poorly stable against anodic dissolution due to a very anodic flat band potential. Conversely, impedance and photoelectrochemical measurements proved that passive films on stainless steels are chromium rich oxide n-type semiconductors with a very high polarization resistance. Their band gap depends on the pH of the passivation solution and on the SS composition.
In the present study, a 304 stainless steel was deformed at different temperatures of room temperature (cold rolling) and 200 °C (warm rolling) for elucidating the distinct deformation mechanisms, ...and then, the influence of the deformed microstructures on the subsequent microstructural evolution and texture development during annealing treatments was studied. The results indicated that deformation-induced martensite transformation was the dominant deformation mechanism during cold rolling, and a mixed microstructure consisting of ~60% volume fraction of α'-martensite and deformed austenite was obtained. Deformation twinning of austenite dominated the deformation during warm rolling. The underlying reason was the increasing SFE with deformation temperature, which inhibited the martensitic transformation but promoted the formation of deformation twins. On annealing, the recrystallization of retained austenite in cold-rolled samples was retarded because of martensitic reversion compared to the warm-rolled samples. Completely recrystallized structure with average grain size of ~0.7 µm was obtained in both cold and warm-rolled samples after annealing at 800 °C. Texture evolution indicated that martensitic reversion enhanced the intensity of {110}< 112 > texture in cold-rolled and annealed samples. However, the final recrystallization texture components were consisted of {110}< 112 > and {113}< 211 > regardless of rolling process. Furthermore, structure-property relationship was established, and combination of high strength and excellent plasticity can be obtained in both cold and warm-rolled sheets by tuning the subsequent annealing parameters.
Stainless steels represent a quite interesting material family, both from a scientific and commercial point of view, following to their excellent combination in terms of strength and ductility ...together with corrosion resistance. Thanks to such properties, stainless steels have been indispensable for the technological progress during the last century and their annual consumption increased faster than other materials. They find application in all these fields requiring good corrosion resistance together with ability to be worked into complex geometries. Despite to their diffusion as a consolidated materials, many research fields are active regarding the possibility to increase stainless steels mechanical properties and corrosion resistance by grain refinement or by alloying by interstitial elements. At the same time innovations are coming from the manufacturing process of such a family of materials, also including the possibility to manufacture them starting from metals powder for 3D printing. The Special Issue scope embraces interdisciplinary work covering physical metallurgy and processes, reporting about experimental and theoretical progress concerning microstructural evolution during processing, microstructure-properties relations, applications including automotive, energy and structural.
