Steel Design Research Articles

The roles of Ce and Mn on solidification behaviors and mechanical properties of 7Mo super austenitic stainless steel

Effect of Ce and Mn on solidification behaviors and mechanical properties of 7Mo super austenitic stainless steel was investigated via in-situ observations, thermodynamic calculations and microstructural characterization. The results show that the network eutectic microstructure had higher hardness compare with ordinary cast structure. And increasing Ce and Mn can both improve the hardness which could be attributed to the effect of Ce and Mn on the solidification microstructure. Analyses indicate that Ce acted as inclusions and atoms to shorten the solidification interval and refine the microstructure to reduce the σ phase. This finer microstructure increased the average hardness. While increasing Mn content modified Ce–O inclusions into Ce–Mn–O compound inclusions which reduced nucleation sites. Mn also promoted the segregation of Cr and Mo in the liquid to decrease the solidification rate, leading to microstructure coarsening and high content of σ phase. This study may provide a theoretical basis for the composition design and applications of the 7Mo super austenitic stainless steel.

Prediction of compressive strength and elastic modulus for ultra-high-performance concrete

This study conducted compression tests to assess the behavior of ultra-high-performance concrete (UHPC) subjected to uniaxial compression. The primary test parameter is the fiber volume fraction in the 0 % to 2 % range. According to the test results, the presence of steel fibers in the matrix significantly affected the failure mode of UHPC. However, the compressive behavior of the test specimens appeared to be very similar, regardless of the fiber volume fraction. The compressive strength of UHPC was improved by approximately 10 % by the addition of the steel fibers. However, the fiber volume fraction and transverse strain had a strong association. The steel fibers significantly affected the volume change of the UHPC. Based on the test results and numerous test data sets, this study suggests that the compressive strength of the UHPC with steel fibers is a function of the UHPC compressive strength without steel fibers and design compressive strength. In addition, the elastic modulus for design is suggested. The proposed equations are applicable for the design and analysis of the UHPC with a fiber volume fraction of up to 6 % and a compressive strength between 120 and 255.9 MPa.

Research on the Fracture Behavior and Microstructure of T91 Steel at Ultrahigh Creep Temperatures

The creep behavior of T91 steel at ultrahigh creep temperatures (650–750 °C) at 100 MPa was investigated using small-size specimens, and the related microstructures were characterized. The results showed that the creep failure susceptibility of T91 steel increases significantly as the creep temperature increases from 650 °C to 675 °C. The creep life rapidly decreased from 7880 h to 224 h, and the fracture surface shrinkage increased from 61.9% to 86.5%. Thermodynamic calculations and microstructure observations showed that the increase in creep temperature has a significant effect on the morphology of M23C6-type carbides, which were distributed around grain boundaries, significantly pinning and hindering the movement of dislocations and the grain boundaries. As the creep temperature increases, the subcrystalline and M23C6-type carbides significantly coarsen, which is the main reason for the rapidly shortening creep life of T91 steel. This research provides guidance for the material design and microstructure regulation of T91 steel in service at ultrahigh environmental temperatures.

Whole life embodied emissions and net-zero emissions potential for a mid-rise office building constructed with mass timber

Increasing global interest in sustainable buildings has generated significant investment in the mass timber industry in recent years. Timber products including Cross-Laminated Timber (CLT) and glue-laminated timber beams (glulam) are a structurally sound, aesthetically pleasing, and potentially carbon reducing alternative to traditional steel or concrete framing. This study uses Life Cycle Analysis methodology to determine the total embodied carbon emissions for a four-story (10033 m2) office building using two design scenarios: a traditional design using structural steel framing and a mass timber alternative utilizing CLT and glulam. The work is founded on detailed material lists for the two building designs that were generated by an architectural contractor with both designs subject to strict evaluation regarding building and construction codes. In addition to a comparative embodied carbon emissions analysis, this study utilized building energy consumption models and existing utility data to estimate operational emissions throughout the lifetime of the building. Energy production with rooftop solar arrays was estimated to evaluate grid emissions avoidance and determine the potential to offset whole life embodied carbon emissions when evaluated over a 30-year time horizon with temporal grid emissions accounting. Three end-of-life scenarios were evaluated for the mass timber products to determine the fate of biogenic carbon stored in the wood biomass. For the three scenarios, different mass fractions were sent to the landfill, directly re-used, or processed into particleboard. Results for the cradle-to-grave embodied carbon emissions analysis show the steel design embodies 3999 t CO2-eq. (398.5 kg CO2-eq m−2) whereas the mass timber design offers an 80–99% reduction in embodied carbon totaling 50–795 t CO2-eq. (5–79 kg CO2-eq m−2) dependent on the end-of-life treatment of mass timber products. Of all the evaluated cradle-to-grave scenarios, the smallest required solar array to fully offset embodied carbon emissions in 30 years was 665 kW with the mass timber building and the end-of-life scenario where 75% of CLT and 95% of glulam were directly re-used with the remaining fractions manufactured into particleboard. Discussion focuses on the challenges and uncertainty in using solar arrays to offset emissions from building construction, operation, and end-of-life treatment, requirements for achieving true net-zero carbon buildings, and methodological limitations.

Behaviour and design of high strength steel homogeneous and hybrid welded I-section beams

The behaviour and design of high strength steel (HSS) beams are addressed in the present study. Six in-plane three-point bending tests on three different welded I-sections − two homogeneous S690 steel welded I-sections and one hybrid welded I-section with S690 steel flanges and an S355 steel web, were first conducted. The beam tests were carried out in major axis bending and a bespoke restraint system was designed and employed in the test programme to prevent lateral-torsional buckling. Following the experimental investigation, a thorough finite element (FE) modelling programme was performed, which included a validation study confirming the accuracy of the developed FE models in replicating the flexural behaviour of HSS welded I-section beams, and a parametric study generating additional FE data on HSS welded I-section beams over a broader range of cross-sectional slendernesses, steel grades and loading configurations. The test results obtained in the present study and collected from the literature, together with the generated FE data from the parametric study, were used to evaluate the suitability of the current Eurocode 3 cross-section slenderness limits for HSS homogeneous and hybrid welded I-sections in bending. It is shown that the current Eurocode Class 2 and Class 3 slenderness limits are suitable for the classification of the outstand flange (in compression) and internal web (in bending) elements of both HSS homogeneous and hybrid welded I-sections subjected to major axis bending, while stricter Class 1 slenderness limits are considered necessary to achieve sufficient rotation capacity for plastic design. The findings from the present study indicate that plastic design can be used for HSS structures, provided the proposed stricter Class 1 slenderness limits are employed.

Effect of Intercritical Deformation on Microstructure and Mechanical Properties of Quenching and Partitioning Low Carbon Multiphase High-Strength Steel

Low carbon multiphase high strength steel is widely used in the automobile industry. In this work, the effect of intercritical deformation on the partitioning of alloying elements and the evolution of microstructure, as well as the effect of retained austenite stability on mechanical properties, were studied in a low carbon steel. The results demonstrate that the intercritical deformation enhances the C, Mn partition from ferrite to austenite during annealing at 770 ℃, and the volume fraction of the retained austenite increased from 8.8% to 12.3%. The DIQ&PB sample shows good balance between strength (1226.5 MPa) and ductility (24.4%), whose product of strength and elongation reached a larger value of 29926.6 MPa·% due to the intercritical deformation. This research provides theoretical guidance for the process design of automobile high-strength steels, considering the integration between rolling and heat cycles.

Computational Thermodynamics-Aided Design of (Cr-Mo-W-V) Steels with Enhanced Corrosion and Abrasion Resistance

Computational Thermodynamics-Aided Design of (Cr-Mo-W-V) Steels with Enhanced Corrosion and Abrasion Resistance

Design of sheathed built-up nested CFS channel studs in load-bearing LSF walls

Cold-formed steel built-up nested channel (NC) sections have been increasingly used to replace the commonly used lipped channel sections as the load-bearing studs of light gauge steel framed (LSF) walls with higher compression capacity. However, the related knowledge is limited and no research study has investigated such NC stud walls yet. In this study, an investigation was undertaken on the compression behaviour and capacity of LSF walls made of slender NC studs sheathed with gypsum plasterboards on both sides. Compression tests of sheathed NC studs were conducted first, which were then used in developing and validating suitable finite element models. The results obtained from tests and finite element models showed that plasterboards and stud-to-board screw connections had significant influence on the failure mode and compression capacity of sheathed NC studs, while the end fastener groups recommended in the current cold-formed steel design standards and the stud-to-stud screw connections had only marginal effects. Finally, a simple and conservative design method was proposed based on the Direct Strength Method equations in the current design standards. This paper presents the details of this research study, including the results, discussions and main findings.

Research on influence of new structural design regulations on general design of steel towers

This work describes the main revision content of the power industry standard (DL/T 5486-2020) (Technical specification for the design of steel supporting structures of overhead transmission line). And the effects of the new revision standard (DL/T 5154-2012) Technical code for the design of tower and pole structures of overhead transmission line (DL/T 5154-2012), on the general design of 35kV~750kV steel towers are analyzed in depth. Subsequently, the optimization suggestions of general design of the steel tower are proposed.

Inelastic Behaviour and Strength of Steel Beam-Columns with Applied Torsion

Abstract: This dissertation presents the outcome of an experimental and theoretical study o f the inelastic behavior and strength of steel beam-columns with applied torsion. Although the international steel design specifications contain interaction relations for biaxially loaded beam-columns, the influence of applied torsion on such members has been completely ignored in the past. A series of hollow square section steel members are tested using an apparatus specially designed to apply torsion in the presence of an axial load and biaxial bending. The theoretical analysis involves formulation of a system of materially nonlinear differential equations and their solution based on a finite integral formulation. The predicted member response agreed quite well with the experiments. A set of new yield limit and strength interaction expressions are then developed which include the influence of applied torsion. In addition, beam-column interaction relations used in Australia, Canada, China, Great Britain, Japan, Russia, the U.S. and Eurocode are modified to account for applied torsion. Finally, this dissertation presents new load-moment-torsion interaction relations for possible adoption in the international steel design specifications.

Effects of state‐of‐the‐art residual stress models on the member and local stability behaviour

AbstractResidual stresses have considerable effect on both the local and the member stability behaviour of steel structures. Previous studies have shown that the structural stability behaviour depends on both the distribution and the amplitude of the residual stresses. The residual stress distribution is affected by the cross‐section geometry, the steel grade and the manufacturing process, e.g. flame cutting and the welding procedure. A realistic consideration of residual stresses is necessary for an efficient and safe design of steel structures.This article presents an experimental and numerical study on the influence of residual stresses on the stability behaviour of structural members, i.e. steel columns and beams considering a variety of cross‐sectional slenderness, i.e. plastic, compact and slender cross‐sections are taken into account. A novel residual stress model for welded I‐shaped sections was developed and evaluated using test data. This model takes into account the main influencing parameters, i.e. the cross‐sections geometry, the steel grade as well as manufacturing process with thermal cuts or non‐thermal cuts of the steel plates. The novel residual stress model is then used to investigate the stability behaviour in terms of a numerical simulation study. The result of the study allows to propose an improvement of the buckling curve selection for welded high strength steel columns and beams.

Novel Apex Connection for Light Wood Frame Panelized Roof.

Panelized fabrication of light-frame wood buildings has higher productivity than the traditional stick-built method. However, the roof production process is not very efficient due to the structural system and construction method. This study proposes a novel apex connection that allows for a folding mechanism in a panelized light wood frame roof system. Proof of concept of the proposed connection assembly is presented by a 3D printout of the developed connection. Following the steel design code and timber code, the initial estimation of different parameters, such as the pinhole diameter and number screws, were established. A detailed finite element analysis (FEA) was performed to determine the connection strength requirement for different load case scenarios. The results of the FEA and 3D printout of the assembly show that the proposed connection can provide the required folding mechanism before roof installation and can withstand the load in the unfolding state at service.

Optimal design of steel–concrete composite bridge based on a transfer function discrete swarm intelligence algorithm

Bridge optimization can be complex because of the large number of variables involved in the problem. In this paper, two box-girder steel–concrete composite bridge single objective optimizations have been carried out considering cost and CO_{2} emissions as objective functions. Taking CO_{2} emissions as an objective function allows to add sustainable criteria to compare the results with cost. SAMO2, SCA, and Jaya metaheuristics have been applied to reach this goal. Transfer functions have been implemented to fit SCA and Jaya to the discontinuous nature of the bridge optimization problem. Furthermore, a Design of Experiments has been carried out to tune the algorithm to set its parameters. Consequently, it has been observed that SCA shows similar values for objective cost function as SAMO2 but improves computational time by 18% while also getting lower values for the objective function result deviation. From a cost and CO_{2} optimization analysis, it has been observed that a reduction of 2.51 kg CO_{2} is obtained by each euro reduced using metaheuristic techniques. Moreover, for both optimization objectives, it is observed that adding cells to bridge cross-sections improves not only the section behavior but also the optimization results. Finally, it is observed that the proposed design of double composite action in the supports allows to remove continuous longitudinal stiffeners in the bottom flange in this study.

The hydrogen-resistant surface of steels designed by alloy elements doping: First-principles calculations

The energetics of hydrogen atoms (H) adsorption and absorption on the bcc-Fe(100) surface are investigated based on DFT. We conclude that H permeate from the surface goes into bulk via hopping between nearest-neighbor T-sites is an endothermic process determined by the surface permeation barrier (Ep). Combined with thermodynamic analysis, we predict that Mn, Cr, and V doped surfaces have smaller H coverage (θ) and higher Ep, while W, Nb, Mo, Ti, Co, and C doped surfaces have higher θ and Ep compared with the pure-Fe surface. Doping these alloy elements inhibits H diffusivity, guiding the design of steel’s hydrogen-resistant surfaces.

Cleavage fracture micromechanisms in thick-section quenched and tempered S690 high-strength steels

For structural assessment and optimal design of thick-section high-strength steels in applications under harsh service conditions, it is essential to understand the cleavage fracture micromechanisms. In this study, we assess the effects of through-thickness microstructure of an 80-mm-thick quenched and tempered S690 high-strength steel, notch orientation, and crack tip constraint in cleavage nucleation and propagation via sub-sized crack tip opening displacement (CTOD) testing at −100 °C. The notch was placed parallel and perpendicular to the rolling direction, and the crack tip constraint was analysed by varying the a/W ratio: 0.5, 0.25, and 0.1. The notch orientation does not play a role, and the material is considered isotropic in-plane. Nb-rich inclusions were observed to act as the weak microstructural link in the steel, triggering fracture in specimens with the lowest CTOD values. While shallow-cracked specimens from the top section present larger critical CTOD values than deep-cracked ones due to stress relief ahead of the crack tip, the constraint does not have a significant influence in the middle due to the very detrimental microstructure in the presence of Nb-rich inclusions. Some specimens show areas of intergranular fracture due to the combined effect of C, Cr, Mn, Ni, and P segregation along with precipitation of Nb-rich inclusions clusters on the grain boundaries. Several crack deflections at high-angle grain boundaries were observed where the neighbouring sub-structure has different Bain axes.

Sublattice alloy design of high-strength steels: Application of clustering and nanoscale precipitation of interstitial and substitutional solutes

Recent advances in high-strength steels for automobile applications or surface-hardened steels have attracted interest on controlling the nanoscale dispersion of alloy carbides/nitrides in ferrite. Particularly, B1-type carbide/nitride precipitation and metastable alloy clustering can be understood as the coupled ordering and compositional phase separation of interstitial alloying elements i and substitutional alloying elements s, i.e., "spinodal ordering". In this paper, the behavior of i–s clustering and alloy carbide/nitride precipitation is revealed by nanoscale characterization, theoretical calculations, and evaluations of mechanical properties. Physical measurements and density function theory-based analyses revealed that strong i–s attractive interaction is responsible for metastable i–s clustering, which clearly indicated "hidden" nanoscale inhomogeneity in solid solutions. Further, we discuss a concept termed interstitial sublattice engineering in bulk and at interphase boundary for strengthening of alloyed steels.

Effect of heat treatment process on impact toughness of a Ni-containing stainless steel 3Cr13 Precision Die

Nowadays 3Cr13 martensitic stainless steel is increasingly used for precision molds. However, its low-temperature impact toughness below room temperature requires further improvement. This study investigates the effect of heat treatment process of a Ni-containing at low temperature impact toughness of 3Cr13 stainless steel used for precision die. Various characterization techniques are combined in order to unveil different heat treatment processes on impact toughness. The test results show that after three different heat treatment processes, the low-temperature impact toughness of the investigated steel at 0 ℃ has been greatly improved. After the investigated steel is heated at 1040 ℃ for 15 minutes, then water-cooled quenched, and then heated to 540 ℃ for 30 minutes, the impact fracture is a dimple shaped plastic one, and its low-temperature impact absorbed energy is higher than that of other heat treatment processes. It shows that the holding time under quenching heating temperature and tempering temperature also have important effects on the improvement of low-temperature impact toughness, which provides technical support for heat treatment design for the Ni-alloyed 3Cr13martensitic stainless steel.

Effect of heat treatment process on impact toughness of a Ni-containing stainless steel 3Cr13 Precision Die

Nowadays 3Cr13 martensitic stainless steel is increasingly used for precision molds. However, its low-temperature impact toughness below room temperature requires further improvement. This study investigates the effect of heat treatment process of a Ni-containing at low temperature impact toughness of 3Cr13 stainless steel used for precision die. Various characterization techniques are combined in order to unveil different heat treatment processes on impact toughness. The test results show that after three different heat treatment processes, the low-temperature impact toughness of the investigated steel at 0 ℃ has been greatly improved. After the investigated steel is heated at 1040 ℃ for 15 minutes, then water-cooled quenched, and then heated to 540 ℃ for 30 minutes, the impact fracture is a dimple shaped plastic one, and its low-temperature impact absorbed energy is higher than that of other heat treatment processes. It shows that the holding time under quenching heating temperature and tempering temperature also have important effects on the improvement of low-temperature impact toughness, which provides technical support for heat treatment design for the Ni-alloyed 3Cr13martensitic stainless steel.

Design and characterization of a cobalt-free stainless maraging steel for laser-based powder bed fusion

This study presents a new Co-free stainless maraging variant for laser-based powder bed fusion developed using a computational alloy design approach. The goal was to develop an easily printable material with similar performance to 18Ni-300. After screening numerous compositions, Fe-13.2Cr-9.1Ni-1.1Al-0.6Mo-0.5Nb-0.23Ti-0.5Mn-0.5Si (wt.%) was selected. This composition showed excellent printability with low porosity levels. The precipitation strengthening response was evaluated by aging at 500 °C for 15 min, 3 h and 18 h, measuring hardness, tensile strength, and by characterization using atom probe tomography. After 15 min of aging, 90% of the maximum hardness was reached, thanks to formation of (Ni, Al, Nb, Ti, Mn, Si) clusters with a density of 1.5 × 1024 m-3. Between 15 min and 3 h, distinct precipitates formed with a radius of ∼1.4 nm. The precipitates underwent a splitting phenomenon after 18 h, forming several unique Ni-rich precipitates including Ni16Si7(Ti, Nb)6 and Ni3(Al, Ti, Nb, Si). The splitting can be a reason for the slow coarsening rate, as the average precipitate radius after 18 h was only 2 nm. Simulations of the precipitation sequence using PRISMA indicated very rapid and dense precipitation of L12-Ni3X precipitates with a slow coarsening rate, in agreement with experimental observations.

Numerical simulation and design of ferritic stainless steel bolted T-stubs in tension

Following the experimental study on EN 1.4003 ferritic stainless steel T-stubs in tension discussed in the companion paper, this study reports the development and validation of an advanced FE model that can predict the overall behaviour and failure modes of ferritic stainless steel bolted T-stubs subjected to tension. Key simulation strategies regarding the modelling of bolt geometry and overcoming numerical instabilities are discussed. Following the determination of material properties in the longitudinal, transverse and diagonal direction reported in the companion paper, the effect of allowing for anisotropy in the FE simulations is investigated and modelling recommendations for its inclusion in FE models are made. Moreover, the effect of bolt end and edge spacing on the joint plastic resistance, ultimate capacity, ductility as well as overall response is comprehensively discussed by inspecting the stress distribution through the plate thickness at various locations along the T-stub, thus revealing both the flexural and the membrane component of the load transfer mechanism. The numerical results were validated against the experimental results reported in the companion paper in terms of predicted plastic and ultimate resistance, ductility and obtained failure modes. On the basis of the obtained results and the discussion, modelling recommendations for the simulation of stainless steel T-stubs are made.