High-strength Structural Steel Research Articles

An Investigation of the Effects of Flame Straightening Heating on the High-Strength Structural Steels Microstructure

AbstractIn this research, the effects of heating cycles that are performed correctly and excessively—well above the limitations given by the steel manufacturer—on flame straightening are assessed and analyzed. The paper focuses on the microstructural changes caused by the overheating during the flame straightening process. To create geometrical changes in a metal construction, a tiny section of the element or structure is heated to the straightening temperature during the flame straightening process. Strenx® 960 is typically produced through a combination of thermomechanical rolling and quenching and tempering, resulting in a fine-grained martensitic or bainitic microstructure. These processes are designed to achieve a balance of high strength and good toughness. The microstructural changes associated with different working conditions in the case of welding include HAZ softening and grain growth. The study aimed to highlight the influence of flame hardening on the microstructure of the material.

Thermal creep strain test and model of Q460GJ steel at elevated temperatures

Q460GJ steel is a typical high strength and high-performance structural steel. The thermal creep test on two thicknesses (8 mm and 12 mm) of Q460GJ steel plates at elevated temperatures (400–800 °C) was carried out. The test results showed that the thermal creep strain increases with the increases of temperature and stress. When the temperature exceeds about 500 °C and the stress ratio is greater than about 0.55, the Q460GJ steel plate specimens has obvious creep deformations, so it is necessary to consider the thermal creep deformation of steel at elevated temperatures for steel structural design. The difference in plate thickness does not affect the creep properties of the 8 mm and 12 mm Q460GJ steel plates at elevated temperatures. When the temperature is no more than 600 °C, the second stage creep strain rates of Q460 steel are obviously higher than that of Q460GJ steel while the stress levels are close. The Fields & Fields creep model is suitable for fitting the thermal creep strain-time curves for Q460GJ steel. The findings will contribute to providing theoretical support for design of high-performance steel engineering structures under fire.

Experimental study on shear mechanism of a new TU-type connector at HSS-UHPC interface

To maximize the benefits of high-strength steel (HSS) and ultra-high performance concrete (UHPC) composite structures, proper connections at HSS-UHPC interfaces are essential. Due to the significant variations in material strengths and component dimensions, traditional shear connectors could be unsuitable for HSS-UHPC interfaces. This study proposes a new TU-type connector (T-shaped rib with U-shaped holes), which has the advantages of high shear and tensile capacity, good slip capacity, low height limitation, and easy installation. Firstly, eighteen push-out tests of HSS TU-type connectors embedded in UHPC were performed to investigate the effect of connector heights and the contribution of connector flanges. The failure process, including the yielding of hole walls, the fracture of connector webs, and the development of fracture surfaces was discussed. Besides, the microstructures involving smooth areas, lamellar structures, and dimples on the fracture surfaces were presented by scanning electron microscope (SEM), and the effect of flange foam blocks on the out-of-plane displacements of UHPC slabs was investigated by digital image correlation (DIC). Secondly, the shear force–displacement curves and the load–strain relations of steel plates and rebars were discussed. In addition, the effects of considered parameters on shear capacity, peak slips, and shear stiffness were investigated. Finally, a strut-and-tie model was established to reveal the shear mechanism of TU-type connectors. Based on the principle of virtual work, the shear capacity equations of TU-type connectors were proposed.

Flexural behaviour of Q1100 ultra high strength steel welded I-sections

An experimental and numerical program investigating the in-plane flexural behaviour of Q1100 ultra high strength steel (UHSS) welded I-sections is presented in the current paper. The testing program comprised of material coupon tests, measurements of residual stresses, and in-plane 3-point bending tests. Subsequently, a numerical program was executed, involving the development of finite element (FE) models. The FE models validated against the experiments were subsequently employed for parametric studies, aiming at assessing the flexural behaviour of Q1100 welded I-sections across a broader spectrum of section geometries. The data obtained from the experiments and numerical simulations were used to assess the applicability of prevailing design provisions outlined in the European, Chinese and Australian codes for high-strength steel structures. These design provisions encompass classification limits for cross-sections and predicted methods for cross-sectional resistance in bending. Via reliability analyses, adjustments were made to the cross-section classification limits. Finally, a modified direct strength method was proposed to predict the in-plane bending resistance for Q1100 welded I-sections.

In situ monitoring of high-temperature creep damage in CrMoV high-strength structural steel using acoustic emission

The accurate detection and evaluation of creep damage in structural steel is essential for ensuring the safety and reliability of high-temperature structures. In this work, the in situ monitoring and identification of high-temperature creep damage in CrMoV high-strength steel under different stress levels was conducted using the acoustic emission (AE) technique. A denoising procedure was proposed and applied to raw AE signals to reduce the noise unrelated to the growth of creep damage. To characterize the creep damage both qualitatively and quantitatively, multiple AE features were extracted from the creep-related signals, including peak amplitude, count, and information entropy. The results showed that a sudden increase in multiple cumulative parameters accompanied by high-entropy and high-count AE signals can accurately identify the transition from the secondary to the tertiary stage and can offer an early warning of the final rupture. In the log-log scale, the quantitative relationship between the AE rate and the minimum creep rate was found to be linear. Additionally, the substantial plastic deformation, and the nucleation, growth and coalescence of creep cavities were the dominating source mechanisms contributing to the generation of numerous high-count and high-entropy AE signals in the tertiary creep stage. Findings from this work will offer an approach for in situ monitoring and evaluation of the state of creep damage of high-temperature structures based on AE.

Influence of microalloying on precipitation behavior and notch impact toughness of welded high-strength structural steels

Microalloying elements such as Nb and Ti are essential to increase the strength of quenched and tempered high-strength low alloy (HSLA) structural steels with nominal yield strength ≥ 690 MPa and their welded joints. Standards such as EN 10025–6 only specify limits or ranges for chemical composition, which leads to variations in specific compositions between steel manufacturers. These standards do not address the mechanical properties of the material, and even small variations in alloy content can significantly affect these properties. This makes it difficult to predict the weldability and integrity of welded joints, with potential problems such as softening or excessive hardening of the heat-affected zone (HAZ). To understand these metallurgical effects, previous studies have investigated different microalloying routes with varying Ti and Nb contents using test alloys. The high-strength quenched and tempered fine-grained structural steel S690QL is the basic grade regarding chemical composition and heat treatment. To evaluate weldability, three-layer welds were made using high-performance MAG welding. HAZ formation was investigated, and critical microstructural areas were identified, focusing on phase transformations during cooling and metallurgical precipitation behavior. Isothermal thermodynamic calculations for different precipitations were also carried out. Mechanical properties, especially Charpy notch impact toughness, were evaluated to understand the influence of different microalloys on the microstructure of the HAZ and mechanical properties.

Investigation of the Fracture Behavior of High-Strength Structural Steel and Welds based on Micromechanical Models

Investigation of the Fracture Behavior of High-Strength Structural Steel and Welds based on Micromechanical Models

Fatigue lifetime predictions of notched specimens based on the critical distance and stress concentration factors

The theory of critical distances is used for evaluation of effects of notches on load-bearing capacity of notched components under static or cyclic fatigue loading. The theory can also be used to predict the fatigue life of notched components. One of the assumptions for a reliable prediction is that the critical distance is independent of the notch geometry and its stress concentration level. The study shows that the critical distance depends not only on the number of cycles to failure (under fatigue loading), but also on the notch radius. In this case, direct use of the critical distance leads to unreliable predictions. The study quantifies the relation between the model notch (from which the critical distance is determined) and the predicted notch by means of the ratio of their stress concentration factors. The predictions modified by the ratio of stress concentration factors provide results that are in satisfactory agreement with the experimental data. The predictions were made and verified on the basis of fatigue data measured on two stainless steels 1.4306 and 1.4307, and high strength structural steel S690QL. Smooth and notched specimens were tested on an ultrasonic fatigue machine in a symmetrical tension–compression mode. The results were evaluated in the regions of high cycle and very high cycle fatigue.

Tensile Coupon Testing and Residual Stress Measurements of High-Strength Steel Built-Up I-Shaped Sections

High strength structural steels (with yield stresses greater than 65 ksi) may have notably different material characteristics when compared to structural steels conventionally used in building construction [i.e., ASTM A992/A992M (2022) or A572/A572M Gr. 50 (2021)]. This paper presents findings from an experimental program that investigated the material characterization of ASTM A656/A656M Gr. 80 (2024) plate steel. The results obtained were compared to conventional ASTM A572/A572M Gr. 50 steel. Two types of testing were performed for this work: tensile coupon testing and residual stress testing. The tensile coupon testing was carried out for both the A656/A656M Gr. 80 and A572/A572M Gr. 50 plate material. The A656/A656M Gr. 80 plate material showed more variation between the two different plate thicknesses in both mechanical behavior and microstructure due to differences in steel production. The 0.375 in. thick plate exhibited a clear yield plateau with an ultimate/yield stress ratio similar to the Gr. 50 material. In contrast, the 0.5 in. plate did not have a yield plateau and reached lower ultimate strain. The residual stress testing was performed using a sectioning technique for one A572/A572M Gr. 50 and five A656/A656M Gr. 80 built-up sections that were fabricated from 0.5 in. and 0.375 in. plate material. Residual stresses obtained from measurements were compared to previously published predictive models. The ECCS model and BSK99 models were found to be reasonable predictors of residual stresses for all specimens except the one section fabricated from 0.5 in. thick Gr. 80 plate. When comparing the Gr. 50 and Gr. 80 specimens of the same cross-sectional geometry, the residual stresses were similar, implying that cross-sectional geometry is more prevalent than the nominal yield stress in determining residual stresses in built-up I-sections.

Formation of Oxides and Sulfides during the Welding Process of S700MC Steel by Using New Electrodes Wires.

To receive a high-quality welding structure of high-strength S700MC steel for applications in the automotive industry, newly developed electrode wires with increased silicon and manganese content were used. The strength and structural tests of the obtained joints were performed. In the weld, we identified the beneficial oxides strengthening the joint structure and unfavorable MnS inclusions. The non-metallic inclusions were formed inside the weld. Their arrangement, morphology, and chemical composition is described. A view on the high-temperature mechanisms of the formations included during the welding process with new electrode wires is presented. It was found that the dominant mechanism of the inclusion formation and the temperature of the welding process impact the content and varied morphology of inclusions, thus determining the exploitation time of the welded joints obtained. The obtained MAG joints made S700MC steel, due to the formation mainly of oxide inclusions and a relatively small amount of MnS phase, were characterized by a high value of yield and tensile strength, which makes them a promising solution for the automotive industry, especially against the background of connections from the discussed steel grade presented in the literature.

Effects of Cr and Ni addition on critical crack tip opening displacement (CTOD) and supercritical CO2 corrosion resistance of high-strength low-alloy steel (HSLAs)

The effects of Cr and Ni content on the low-temperature toughness and supercritical CO2 corrosion resistance of high-strength low-alloy steel (HSLAs) were investigated by optical microscopy, scanning electron microscope, electron backscattered diffraction, transmission electron microscope, X-ray Photoelectron Spectroscopy, electron probe X-ray micro-analyzer, critical crack tip opening displacement, and supercritical CO2 corrosion experiments. In this study, the complex structure of high-strength low-alloy steel is identified and quantified, and related to the critical crack tip opening displacement (CTOD). The further addition of Ni and Cr promotes the formation of granular bainite (GB) and martensite-austenite (MA). The proportion of high-angle grain boundaries in the GB structure is small and does not significantly hinder crack expansion, while MA frequently leads to the initiation and expansion of cracks, deteriorating the low-temperature toughness of the material. Supercritical CO2 corrosion experiments displayed that further addition of Cr and Ni will improve the density and protection of corrosion products. But high-density dislocations will destroy the integrity of the lattice structure of the material and may serve as diffusion channels to affect the formation of the passivation layer and repair. Moreover, dislocation-dense areas are more likely to capture impurity elements, forming a micro-battery effect and accelerating local corrosion. This reduces the extent to which Cr and Ni alloy elements improve the corrosion resistance of HSLA steel.

Cyclic Void Growth Model Parameter Calibration of Q460D Steel and ER55-G Welds after Exposure to High Temperatures

When high-strength steel is heated to high temperatures and then cooled naturally, its ductility decreases. In earthquake-prone areas, it is necessary to evaluate the ultra-low cycle fatigue fracture (ULCF) behavior of high-strength steel structures after a fire if these structures are used continuously. However, the ULCF fracture model of high-strength steel subjected to high temperatures followed by natural cooling has not been deeply studied. In view of this, twelve notched, round bar specimens fabricated from Q460D steel and ER55-G welds were heated to 900 °C followed by natural cooling and then cyclic loading experiments and finite element analyses (FEA) were performed on these specimens. The fracture deformation obtained from the experiments was used in the FEA to calibrate the damage degradation parameter of a Cyclic Void Growth Model (CVGM) of Q460D steel and ER55-G welds under this condition. The calibrated values were 0.30 and 0.20, respectively. The calibrated CVGM was employed to predict the number of cycles and the force and displacement at the fracture moment of the notched round bar specimens. The predicted results aligned closely with the experimental results, indicating that CVGM is effective in predicting the fracture of Q460D steel and ER55-G welds following exposure to 900 °C and subsequent natural cooling.

Comparison of fatigue crack growth design curves on GMAW and EBW joints of high strength steels

There is a growing demand in the industrial sector for the use of high-strength structural steels (HSSSs), which can achieve a significant weight reduction in structures. These structural steels are usually produced by quenching and tempering (Q + T) or thermomechanical treatment (TM), and their applications in welded structures pose several challenges for the users. In industrial practice, gas metal arc welding (GMAW) is basically the most commonly used fusion welding process, which has a relatively high heat input. However, at HSSSs, there is a need for low heat input but, at the same time, productive welding processes. High-energy density welding processes, e.g., electron beam welding (EBW), offer a unique opportunity to weld these steels. The widespread use of HSSSs is also hampered by the fact that the benefits of high strength can be exploited primarily under static loading. At the same time, different welded structures made of HSSSs are often subjected to cyclic loading, and possible weld defects and material discontinuities are major risks in this case. During our experiments, GMAW and autogenous EBW processes were applied to make welded joints from S960 Q + T and TM structural steels. The fatigue resistance of the welded joints was characterized by fatigue crack growth (FCG) tests, considering the increased crack sensitivity of HSSSs. A statistical approach was followed both in the design of the experiments and in the evaluation of their results. Based on the test results fatigue crack propagation design curves were determined for the investigated GMAW and EBW welded joints. The design curves were compared with each other, with design curves of lower strength material (S690QL) and with the recommended fatigue crack growth laws of BS 7910.

A numerical model simulating cyclic behavior of high-strength steel

High-strength steel (HSS) can effectively and economically design structural members that withstand large forces induced by extreme events such as earthquakes. To evaluate the seismic performance of HSS members and structures using nonlinear finite element (FE) analyses, using an accurate material model is important, which can simulate the inelastic cyclic behavior of the HSS. The peculiar material behavior of HSS needs to be considered in the model. This study used a combined hardening model to construct the material model. The configuration of the model and the constituent model parameter values are determined to precisely simulate the low-cycle fatigue (LCF) behavior of HSS. An efficient particle swarm optimization (PSO) algorithm is used to determine parameter values with LCF test data of 54 individual HSS coupons. In additions, to conveniently determine the model parameter values with only monotonic tensile test data instead of conducting expensive LCT tests, empirical equations are proposed. It is shown that the LCF curves of HSS can be accurately simulated using the constructed material model with the proposed equations.

Strain rate-temperature effects and constitutive models for Q690D QT steel

This paper investigates the mechanical properties of the high-strength structural steel Q690D QT (Quenched and Tempered) over a wide temperature range and a wide strain rate range. Firstly, a series of quasi-static and dynamic tests adopting the universal testing machine and Split Hopkinson Pressure Bar were carried out to investigate the mechanical properties at strain rates from 0.00025 s−1 to 5000 s−1 and temperatures from room temperature (20 °C) to 700 °C. Stress-strain curves and dynamic yield strength increase factors were extracted and investigated. Test results show that Q690D QT exhibits different strain rate effects at different temperatures, more significant at the temperatures of 600 and 700 °C, as well as the high-temperature softening effect. Dynamic strain aging effect is identified at temperatures between 300 − 600 °C for plastic strain rates of 800 s−1 and 2300 s−1, nevertheless, this effect is observed at a higher temperature range of 500–700 °C for higher plastic strain rates of 4000 s−1 and 6400 s−1. From the analysis, the adiabatic thermosoftening effect cannot be ignored when the plastic strain rates reach 4000 s−1. Finally, an improved form of the Johnson-Cook model is proposed for capturing the adiabatic temperature effects at different temperatures and validated by the test data. A parameter dynamic temperature factor is proposed to describe the comprehensive effect of strain rate, test temperature, and adiabatic thermosoftening.

Revealing the mechanism of crack initiation and propagation in CGHAZ of S690QL welded joints through low-temperature impact tests with gradient setting of pendulum angle

As the most brittle areas of welded S690QL high-strength steel structures, coarse-grained heat-affected zone (CGHAZ) was crucial for the structural integrity of components. This paper designed an innovative low-temperature impact testing method with a gradient setting of pendulum angle to investigate the micro-mechanism of crack initiation and propagation in the CGHAZ of S690QL welded joints. The results reveal that continuous blocky M−A constituents were distributed along prior austenite grain boundaries (PAGBs), leading to stress concentration and crack initiation. During low-temperature impact processes, cracks propagate in a transgranular mode along {100} or {110} crystal planes. Due to the interaction between compressive stress and the crack tip, regions with high kernel average misorientation (KAM) such as PAGBs can divert the crack. The coarsening of prior austenite grains in the CGHAZ leads to a diminished hindering effect of PAGBs on crack propagation. The findings elucidate the complex relationship between microstructure and impact toughness of CGHAZ, providing insights into crack behavior and mechanisms in CGHAZ.

Assessment of fatigue crack growth resistance of newly developed LTT alloy composition for the repair of high strength steel structures

Tensile residual stress (TRS) is a well-known factor that deteriorate the integrity of welded joints. Fatigue failure is accelerated by the existence of TRS introduced during the welding process. There have been efforts in the last two decades to develop filler alloys that can reduce TRS by introducing compressive residual stress (CRS) to oppose the TRS in high strength steel welded joints. These works are based on the theory of austenite (γ) to martensite (α’) transformation and the filler is often called a low transformation-temperature (LTT) alloy. Many studies have reported that the fatigue strength (FS) of weld joint made with LTT alloy is many times better than that of the conventional fillers. It is reported to be particularly useful in the repair of high strength steel structures. However, studies on the fatigue crack growth (FCG) behaviour of these LTT alloys is scarce. In this work, we developed Fe-CrNiMo based LTT weld metal composition, assessed its FCG behaviour and compared the results with that of a conventional welding wire (ER70S-6). It is found that ER70S-6 weld metal obtained under relatively fast cooling is extremely tough, but the associated heat affected zone (HAZ) has poor resistance to FCG which obscured the benefit of the tough weld metal. High heat input or condition that results to slow cooling of the ER70S-6 weldment deteriorates its resistance to FCG. Unfortunately, despite its low martensite start temperature of 231±7 and the anticipated beneficial effect of induced CRS, the LTT alloy studied had the lowest FCG resistance. The LTT alloy appears to have an intrinsic microstructural feature or a ‘fault line’ that reduced its resistance to FCG. While the LTT alloy weld metal has poor resistance to FCG, the associated HAZ resisted FCG more than the HAZ associated with ER70S-6 weld metal. It is observed that aligning the ER70S-6 weld metal perpendicular to the crack front produced the highest resistance to fatigue crack initiation and propagation. In the case of ER70S-6, it is believed that the weld metal induced a CRS at the notch tip which resulted to the high fatigue resistance. In the case of the LTT alloy, perpendicular alignment of the weld metal produced slight improvement.

Synchrotron diffraction residual stresses studies of electron beam welded high strength structural steels

In recent years, there's been a strong focus on enhancing high-strength structural steels (HSSSs), making them tougher, stronger, lighter, and more weldable. However, these steels face challenges like cold cracking sensitivity (CCS) and reduced toughness with higher heat input. Therefore, exploring HSSSs with advanced welding techniques like electron beam welding (EBW) is crucial. EBW known for its innovation and precision, is gaining popularity in HSSSs applications for vehicles, construction industries etc. offering advantages like narrow welds, reduced heat-affected zone (HAZ) and low distortion. It is essential to understand the influence of residual stresses (RS) in HSSSs which ultimately affect structural integrity and fatigue life. Using EBW on HSSSs, coupled with synchrotron XRD for residual stress studies, has great potential to advance this research significantly. In this work, a 15-mm-thick EB-welded joint of S960QL and S960 M steels was used to study the RS. At an energy level of 20 keV, RS data from the synchrotron XRD method and its result on the top of welded joints for both types of steel were compared. S960 M steel showed a longitudinal tensile stress of 250 MPa at the weld toe, approximately 1.8 times higher than the 138 MPa observed in S960QL steel under similar conditions. In transverse stress, S960QL exhibits a higher stress of 279 MPa compared to S960 M 46 MPa at the weld toe, approximately 83% lower. The synchrotron method measured 76% lower LRS for S960QL compared to conventional XRD, while S960 M exhibited 55% lower LRS using the same approach.

Machine-learning-assisted design of high strength steel I-section columns

High strength steel has been attracting attention in the building industry due to its superior mechanical properties. The accurate design of high strength steel structures is crucial to boost its wide application. In this paper, an accurate and unified design approach for high strength steel I-section columns with different material grades, boundary conditions, geometric dimensions (including cross-section sizes and member lengths) and failure modes is proposed based on machine learning. Firstly, 871 experimental and numerical data were collected from the literature to establish a database. Then, seven machine learning algorithms, including Decision Tree, Random Forest, Support Vector Machine, K-Nearest Neighbour, Adaptive Boosting, Extreme Gradient Boosting and Categorical Boosting, were applied to establish machine learning regression models to predict buckling resistances of high strength steel I-section columns. The model performance was then evaluated through statistic indices, with the evaluation results indicating that the Categorical Boosting trained model yields the highest level of accuracy. Based on the data in the collected database, the regression model trained by Categorical Boosting and existing codified design provisions, as given in the European code and American specification, were assessed and compared. The European code and American specification were found to yield scattered and inaccurate failure load predictions, while the Categorical Boosting trained model led to substantially more accurate and consistent failure load predictions for high strength steel I-section columns with different material grades, boundary conditions, geometric dimensions and failure modes.

Unified machine-learning-based design method for normal and high strength steel I-section beam–columns

High strength steel is regarded as a promising construction material due to its superior mechanical properties. However, the codified failure load predictions for high strength steel welded I-section beam–columns are not accurate in some cases owing to the lack of relevant design codes for high strength steel structures. In addition, current design codes ignore the interaction effect of the cross-section plate elements, leading to the inaccuracy of codified predictive ultimate resistances for both normal and high strength steel beam–columns. In this paper, a unified and accurate design method was proposed based on machine learning for both normal and high strength steel welded I-section beam–columns failing by different failure modes. Firstly, a total of 812 experimental and numerical data on welded I-section beam–columns with various steel grades, geometric dimensions, including cross-section dimensions and member lengths, and failure modes, including global buckling, local buckling and local–global interactive buckling were collected to establish a database. Seven machine learning algorithms, including Decision Tree, Random Forest, Support Vector Machine, K-Nearest Neighbour, Adaptive Boosting, Extreme Gradient Boosting and Categorical Boosting were applied to develop regression models to predict failure loads. Based on the established database, each machine learning model was then trained and key hyperparameters were optimised. The model performance was evaluated through a series of statistical indicators and the feature importance analysis, and the evaluated results indicated that the XG-trained regression model had the highest level of accuracy. Finally, the predictions given by the XG-trained regression model were compared with those of the existing codified design provisions, as given in Eurocode and American codes. The evaluation results indicated that the codified predictive bearing capacities of I-section beam–columns were scattered and inaccurate, while the XG-trained regression model provided substantially improved failure load predictions.