Steel S960QL Research Articles

Low-cycle fatigue of welded joint from steel of X70 strength class

Steels of X70 strength class are particularly widely used in the field of heavy engineering. One of the most important issues when choosing steel for structures is its behavior under cyclic loads. It is difficult to find a description of the behavior of all zones of the welded joint under fatigue. The purpose of this study was to determine the fatigue characteristics of welded joints made of the Russian analogue of S690QL steel with fixation of acoustic and magnetic parameters for their use in the diagnosis. The objects of the study were the samples from domestic steel of X70 strength class. The chemical composition was determined using optical emission spectrometry. The grinds for microstructural analysis were prepared according to the standard technique with etching in the metal. The fatigue test was carried out on a specialized test bench. The authors used the acoustic system AIS NRK-3 for acoustic measurements and the acoustic parameter D – as an informative parameter. A MA-412MM coercitimeter was applied to evalua­te the magnetic characteristics. The following were evaluated: residual magnetization Br , coercive force Hc , Hc/Br ratio. The smallest number of cycles corresponds to the deposited metal zone. The decrease in amplitude showed a significant variation in the behavior of the material depending on the junction zone. However, the curves for heat affected zone (HAZ) and the deposited metal are practically the same. HAZ differs to a lesser extent from the base metal than the deposited metal zone. The graph of the acoustic parameter in its form is the reverse of the magnetic characteristics graph. Thus, there is a minimum for the acoustic parameter, depending on the operating time, and a maximum for the magnetic characteristics. For both graphs, the extremum is the point corresponding to the operating time of 0.6.

High strength low alloy steel joint fabricated by laser welding with real-time high-frequency resistance heating

The balance between softening and hardening is still a great challenge for the laser welding of the high strength low alloy steel. For example, welding with low heat input was commonly used to avoid softening, but it would inversely harden the joint and introduce the undesired welding defects such as hydrogen-induced cracks. This work proposed a simple and robust processing technique, denominated High Frequency electric cooperated Laser Welding. Laser heat source was combined with high-frequency electric heat source to balance heat input distribution and reconcile the contradiction between softening and hardening in the welding of high strength low alloy steel. As a structural material sensitive to welding softening and hardening, S690QL steel was adopted to show the fundamental advancement of the new process. Benefitted from the skin effect and proximity effect of high-frequency current that improve the heat input distribution and the regulation of the high-frequency heat input proportion, the martensite content in the heat affected zone was greatly reduced, whilst the grain size was limited to a low value. Under the high-frequency heat input proportion of 47.1 %, lower bainite distributed throughout the heat affected zone. The softening and hardening issues of the joint therefore have been improved simultaneously: the tensile strength of the joint was almost equal to that of the base material, the impact energy of weld seam and the heat affected zone reached 77.6 J and 66.2 J respectively.

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.

Research on the Influence of HMFI and PWHT Treatments on the Properties and Stress States of MAG-Welded S690QL Steel Joints.

The aim of this study was to analyze the effect of the HFMI (high-frequency mechanical impact) treatment of each weld bead on the properties of a butt joint with a ceramic backing welded by robotic method 135 (MAG-metal active gas welding method) and to determine the effect of HMFI on the stress level. This analysis was based on a comparison of three butt joints made of a S690QL plate, in the as-welded condition, with the HFMI of each bead and with the heat treatment carried out with PWHT stress relief annealing. The high-frequency (90 Hz) peening of each weld bead was linked with a stress reduction in the weld via the implementation of compressive stresses into the joint. The HFMI pneumatic hammer was used for this. The correctness of treatment was achieved when 100% of the surface of each bead including the face was treated. As part of the post-welding tests, basic tests were carried out based on the standards for the qualification of welding technology, and as a supplementary test, a stress state analysis using the Barkhausen effect was carried out. The tests carried out showed that the use of high-frequency peening after each pass did not affect the negative results of all the required tests when qualifying the welding technology of S690QL sheet metal compared to the test plates in the as-welded condition and after heat treatment-stress relief annealing. Inter-pass peening of the welded face and HAZ (heat-affected zone) resulted in a reduction in post-weld residual stresses at a distance of 12 mm from the joint axis compared to the stress measurement result for the sample in the as-welded condition. This allowed for a positive assessment of peening in the context of reducing the notch, which is the concentration of tensile stresses in the area of the fusion line and HAZ. The tests carried out showed that the peening process does not reduce the strength properties of welded joints, and the results obtained allow the technology to be qualified based on applicable standards.

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.

Fatigue tests of S690 as-welded and HFMI-treated details with longitudinal and transverse attachments

The complex nature of the fatigue strength of welded steel details, which is influenced by a multitude of factors, can be improved by extending the crack initiation phase of the total fatigue life. This enhancement can be achieved through the use of the High-Frequency Mechanical Impact (HFMI) method. This study discusses the fabrication and laboratory cyclic testing of as-welded and HFMI-treated transversal and longitudinal attachments made from S690QL steel. Assessment of a two-stage model (TSM) for the evaluation of their fatigue life, previously developed by the authors, is also performed. The TSM's estimated fatigue lives of the details under consideration align well with the fatigue test results. Moreover, both the TSM model results and the corresponding fatigue test results demonstrate that the HFMI method significantly prolongs their fatigue life. The fatigue resistance of HFMI-treated details compared to as-welded details increased by 1.67 and 1.91 times for longitudinal and transversal attachments, respectively.

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.

Stresses in repair welding of high-strength steels—part 2: heat control and stress optimization

In welding of high-strength steels, e.g. for foundations and erection structures of wind energy plants, unacceptable defects can occasionally be found in the weld area, which should be removed by thermal gouging and subsequent re-welding. High shrinkage restraint of repair welds may lead to crack formation and component failure, predominantly in interaction with degraded microstructures and mechanical properties due to repair cycles. This study aims for elaboration of recommendations for repair concepts appropriate to the stresses and materials involved to avoid cold cracking, damage and expensive reworking. In part 1 [1] of this study, systematic investigations of influences of shrinkage restraint on residual stresses and cold cracking risk during repair welding of two high-strength steels S500MLO for offshore application and S960QL for mobile crane structures were focussed. In this part 2, the microstructure, particularly hardness, and residual stresses due to gouging and influences of heat control parameters in repair welding are analysed. A clear reduction in residual stress after gouging can be observed, especially for the specimens with restrained transverse shrinkage. Gouging to a depth of approx. 2/3 of the seam height does not lead to a complete relaxation of the observed reaction forces. Particularly for the higher strength steel S960QL, there are pronounced areas influenced by the gouging process in which a degradation of the microstructure and properties should be assumed. Overall, the repair welds show a significant increase in the width of the weld and HAZ compared to the original weld, especially in the case of S960QL/G89. The repair welds show higher welding-induced stresses than the original welds, especially in the areas of the HAZ and the base metal close to the weld seam. This behaviour can be attributed overall to increased restraint conditions due to the remaining root weld or shorter gouge grooves. In good agreement with earlier investigations, the residual stresses transverse to the weld can be significantly reduced by upwardly limited working or interpass temperatures, and the reaction stresses resulting from high restraint conditions can be effectively counteracted. The influence of the heat input on the stress formation is low compared to the interpass temperature for both test materials.

High-cycle rotating-bending fatigue performance of S690QL welded joints

In recent decades, high-strength steels have become increasingly popular in the construction industry. Materials like S690QL steel are mainly characterised by their enhanced mechanical properties, including high yield and tensile strength. As a result, this leads to improved performance, durability, and safety of steel components, making such materials commonly used in several offshore applications due to their exceptional strength and cost-efficiency. Within the offshore industry, the most critical connections are usually the welded joints, thus rising significant concerns regarding the characterisation of the material in the area close and affected by the welding process. Furthermore, this material is highly prone to the appearance of cold cracks. To address these issues, rotating-bending fatigue tests were successfully conducted on S690QL steel to evaluate and compare the fatigue life within different regions of the investigated welded joints, specifically the weld material, heat-affected zone and base material. Increasing thickness of the weld plates was found to have a negative impact on the fatigue resistance, which aligns with the lower static strength for the thicker case. Also, variations in fatigue strength across the thickness were observed. The heat-affected zone exhibited a heterogeneous microstructure, leading to increased fatigue variability during testing, but the fatigue resistance observed aligns with the findings in the literature for comparable steel grades. Detailed experimental results are presented, particularly regarding this heat-affected zone, where the majority of fatigue failures occur, which provide bases for the development of new normative material-scale SN curves.

Performance optimization and investigation of metal-cored filler wires for high-strength steel during gas metal arc welding

Abstract This study examines the utilization of metal-cored filler wire in conjunction with the gas metal arc welding (GMAW) technique for welding high-strength S690QL steel. Since welding parameters significantly impact the bead quality and weld joint integrity, the main objective was to identify the optimal welding parameters. To achieve this, the input variables including the current (A), voltage (V), and gas flow rate (GFR), and their effects were evaluated for reinforcement (R), width (W), depth of penetration (DOP), and the width of the heat-affected zone (HAZ). For a more efficient and cost-effective investigation, a Box–Behnken design, which is based on response surface methodology, was used for bead-on-plate trials. Mathematical regression models, derived from experimental data, were rigorously validated using the analysis of variance, main effects plots, residual analysis, and the R 2 and Adj. R 2 values. Additionally, the heat transfer search (HTS) algorithm was employed for process optimization. While single-objective optimization provided optimal settings for individual responses, simultaneous optimization aimed to strike a balance between multiple, sometimes conflicting, objectives. This comprehensive approach resulted in specific values, including a reinforcement (R) of 4.285 mm, a width (W) of 9.906 mm, a DOP of 2.039 mm, and an HAZ width of 2.020 mm. These values were achieved with specific input parameters: current (221 A), voltage (24 V), and GFR (21 L·min−1). The Pareto solutions offered a nuanced selection of the most suitable configuration, taking into account the desired values for R, W, DOP, and HAZ. The close alignment between predicted and experimentally measured values for the responses highlights the precision and suitability of the HTS algorithm in estimating critical bead geometries during GMAW of S690QL plates.

In situ localised post-weld heat treatment with electron beam welding of S690QL steel

The objective of this research is to assess the influence of in-situ localised electron beam post-weld heat treatment (LEB-PWHT) on the microstructure and mechanical properties of high-strength S690QL steel welded joints utilising highly sophisticated electron beam welding (EBW) technology. Although EBW is well-known for creating high-quality welds, the addition of the novel LEB-PWHT technique may significantly improve the characteristics of the welded joints. To systematically analyse the impacts of PWHT on the welded joints, the study employs microstructural analysis like optical microscopy and mechanical properties like microhardness testing, bending, tensile, and impact tests. Furthermore, tensile fractography analysed by scanning electron microscopy (SEM) to provide detailed insights into the fracture behaviour of the welded joints. The results show that the in-situ PWHT approach improves the microstructure and mechanical characteristics of the welded joints, such as a lower hardness peak in the heat affected zone (HAZ), increased tensile strength (5%), and improved toughness by 7 times. LEB-PWHT fractured surface revealed smaller, more sharply defined dimples and micro-voids. These results suggest that in-situ PWHT is an attractive option for improving the quality and performance of welds in a wide range of applications incorporating S690QL steel.

Influence of shielding gas on filler wire mixing at laser hybrid welding of thick high strength steels

The laser hybrid welding process offers many advantages during welding of thick-walled steels, such as the increased penetration depth and, thus, reduced number of layers, reduced heat input and decreased distortion compared to arc-based welding processes. Especially, when welding high-strength steels (HSS), the reduced heat input plays an essential role. However, a major challenge when laser hybrid welding of thick-walled steels is the limited filler wire mixing over the entire seam thickness, which can lead to changed mechanical properties over the depth. To overcome this issue, the add of oxygen into the shielding gas and its influence on the filler wire mixing and finally to the mechanical properties were investigated within this work. Therefore, 20 mm thick S690QL steels were laser hybrid welded in a single-pass. A contactless electromagnetic backing was used to avoid sagging. The admixture of oxygen was performed by a gas mixer, where the oxygen content was varied between 0 % and 7.2 %. The experiments were also accompanied by laser beam welding tests in steel/glass configuration, where the melt pool geometry as well as the melt flow characteristics were captured by a high-speed camera. It can be concluded, that adding of approx. 4 % oxygen into the shielding gas had a positive effect on the filler wire mixing, were up to a depth of 18 mm elements of the filler wire could be observed.

Gleeble Simulation of HAZ in S690QL Steel: Microstructural and Mechanical Properties

When investigating the heterogeneous properties of welded joints, mechanical testing of certain Heat Affected Zone (HAZ) regions, using standard approach test specimens, is very difficult or sometimes even impossible. Inability to precisely position and extract mechanical test specimens, even ones of the subsize dimensions, from the narrow HAZ regions is a limiting factor in the mechanical testing implementation. Detailed investigation of the HAZ is made possible by the use of thermo-mechanical simulations on the Gleeble welding simulator. In scope of this paper several characteristic HAZ microstructures of S690QL grade High Strength Steel (HSS) are being simulated. Multi-pass welding simulations are done on special 10x10 mm square section bar specimens in order to reproduce thermal gradients and characteristic microstructures at any location in a weld. Such simulated HAZ microstructures are of a sufficiently large volume, with homogeneous and repeatable properties, that standard specimen methods for mechanical testing can be readily implemented. Metallographic optical examinations, as well as hardness measurements were done initially. Mechanical properties are focused on determining stress-strain curves for each characteristic weld region. The paper investigates whether the mechanical properties of Gleeble simulated hard-soft combined HAZ regions are better in comparison to exclusively hard or soft HAZ regions. The obtained results can subsequently be used for the material model development.

Investigation of Hardfacing on Ultra-High Strength Steel Base Material

The application of high-strength steels is increasing rapidly nowadays, and steels with more than 1000 MPa yield strength are usually used in welded structures. The welding of these materials has many difficulties, so very important the precise technology planning, and disciplined work during welding. The weldability of these materials is commonly investigated field in case of joining. The application of ultra-high strength steels expands rapidly, and in the last years, it started to use them as a base material for hardfacing. Besides the wearing, there is a claim about higher strength of base materials in case of relatively extremely loaded machines. Because this ultra-high strength steel appears as a base material for hardfacing and it brings new challenges for welding technologists. In case of joining, the welding technology is complicated, usually need preheating before welding, is important to calculate and to use the right t8/5 cooling time, and basically necessary to decrease the heat input as much as possible. The bad effect of welding heat input can be compensated by the filler material too in some cases. In contrast in case of hardfacing the base material itself usually has a big thickness, and no joint preparation, additionally important to reach deep fusion on the surface. It basically determines the heat input which has a different heat cycle as in case of joining. Therefore, the heat affected zone (HAZ) differs from the HAZ in case of joining application. In this investigation, four different hardfacing were made with four different technological parameters by robotic gas metal arc welding on S1100QL steel. During the welding parameter determination, we try to find a series of heat inputs from the lowest to the practically usable highest heat input. For the experiments, two filler materials used, one for the buffer zone, and for the hardfacing itself. Microstructural evaluation and hardness tests were made on the specimens which can show the differences between the heat affected zones.

Applied research of high-strength steel utilization for a track of demining machine in terms of mechanical properties

The aim of this paper is to investigate welded joints of high-strength steel S960 QL manufactured by using three different welding technologies, namely the electron beam, the laser beam, and the metal active gas (MAG) technologies. The experimental part included tensile strength evaluation, microstructural analysis of welded joints, and hardness measurement. Welded joints (WJ) have consisted of the identical steels and the identical thickness (10 mm). Destructive tests confirmed that welded joints are characterized by the tensile strength similar to the base material. Upon further observation, we can conclude that microhardness was characterized by the lowest value in the softening zone (SZ) and the highest value in the hardening zone (HZ). The degree of softening was 11% for the electron welding, 13% for the laser welding, and up to 27% for the conventional MAG welding. This also corresponds with the size of the SZ, which was the widest in welds made with the MAG technology. The laser beam weld produced up to 50% lower heat-affected zone (HAZ) compared to the conventional MAG technology. In case of the electron beam, this number is even higher. On the contrary, highest hardness was observed for the electron beam technology, where the hardness in the hardening zone increased by up to 40% when compared with the base material. Tests show the possibility of production of reliable welded joints, which meet the complex requirements for lifetime and quality (according to the standard EN 6520–1 focusing on defects categorization and EN 5817 dealing with defects tolerance).

High-strength and high-toughness weld of S690QL HSLA steel via a high-speed high frequency electric cooperated arc welding without additional heating measures

Problems in balancing joint strength and toughness, caused by the strict requirements of welding heat, have been a long-troubling problem that attracts close attention in the High Strength Low alloy (HSLA) steel welding field. Additional heating measures have been considered as an only and necessary solution with limited effect to deal with the above issue, which not only complicate the welding process but also have some incidental impacts. In this paper, a novel method named High Frequency Electric Cooperated Arc Welding (HFAW) was employed to join typical HSLA steel S690QL without additional pre- or post-heating measures. Relationships between the welding thermal characteristics, microstructure and mechanical properties of different parts in the joint were revealed and compared. Results showed that sound HFAW joint was achieved with a welding heat input and speed of 3630.6 J/cm and 2.3 cm/s, while that in conventional GMAW was 7077.1 J/cm and 0.5 cm/s, respectively. The width of the softening zone in the optimal HFAW joint was narrow to 0.95 mm. Lath Martensite and Bainite were generated in the coarse grain zone with an average grain size of 14.9 μm, rather than coarse feathery Bainite in the GMAW joint (with doubled grain size). Benefitted from the fine grain size, better phase composition, narrow and slight softening, the HFAW joints exhibited excellent tensile strength, moreover, the impact toughness at hardened zone and softening zone reached 0.87 J/mm2 and 1.15 J/mm2, which were at least 60% higher than that in the GMAW joint (0.58 J/mm2 and 0.61 J/cm2).

Low-cycle fatigue behavior of differently matched welded joints of quenched and tempered steel

The appropriate matching of base and filler material is a complex task, where yield strength matching is the most general aspect. As the strength properties of structural steels have significantly improved in the past decades, the matching problem has become more relevant today. The mismatch phenomenon significantly affects the behavior of welded joints under dynamic and cyclic loading. Among cyclic loading, low-cycle fatigue (LCF) often occurs in welded steel constructions; furthermore, the LCF resistance of these advanced steels and their welded joints is limitedly known. In this paper, welding experiments are presented for the analysis of the LCF behavior of differently matched butt-welded joints made from two grades of quenched and tempered (Q + T) high-strength steels. For S690QL steel, matched and overmatched consumables were applied, while for S960QL steel, matched and undermatched filler materials were used. Material tests were performed to determine the mechanical properties of the different welded joints. In both examined steel grades, the welded joints tolerated a smaller number of cycles until failure than the base materials. In the case of S690QL, the LCF resistance of the matched welded joints was higher than the overmatched filler material. At S960QL in the higher strain amplitude range, the number of cycles to failure was higher at the same total and plastic strain amplitudes when undermatched filler material was used; however, an opposite ratio can be observed at lower strain values, compared to the matching filler material.

Wear behavior of innovative niobium carbide cutting tools in ultrasonic-assisted finishing milling

The resources of niobium exceed the ones of tungsten by an order of magnitude. With 92%, Brazil is today the main global producer of niobium. Hence, niobium carbides (NbC) are a sustainable and economic alternative to conventionally used cutting materials, especially tungsten carbides (WC). Moreover, NbC can be used in Ni alloy matrix and thus offer significant advantages by substituting WC in Co matrix as cutting materials in terms of health risks and raw material price and supply risk. Based on recent studies which found an increased performance of NbC compared to WC cutting tools in machining higher strength steels, the composition NbC12Ni4Mo4VC was chosen for finish machining of a high-strength steel S960QL in this study. The experiments were carried out on an ultrasonic-assisted 5-axis milling machine using NbC tools specially made to benchmark them with commercially available coated WC cutting inserts. In addition, the influence of a coating system for the NbC inserts is tested and evaluated for its performance in the cutting process. Tool wear and cutting force analyses are implied to identify optimal parameter combinations as well as tool properties for the novel NbC tool. Together with the oscillation of ultrasonic-assisted milling, the loads on the component surface and the tool can be reduced and the wear behavior of the novel NbC tool can be refined. These milling tests are accompanied by standardized wear tests, i.e., pin-on-disc, between the aforementioned material combinations, and the results are correlated with each other. Finally, the behavior when using hard-to-cut materials such as Ni alloys, or innovative materials such as iron aluminide is also being tested, as these are constantly in the focus of machining optimization. With this strategy, comprehensive knowledge is achievable for future efficient application of NbC for milling tools, which have already been researched for decades using WC.

End-One-Flange Web Crippling Behavior of Cold-Formed High-Strength Steel Channels with Web Holes at Elevated Temperatures

This paper investigates the web crippling strength of cold-formed high-strength steel (CHS) channels with centered web holes subjected to end-one-flange (EOF) loading at elevated temperatures, considering both flanges fastened and unfastened to load plates conditions. The stress-strain curve and material properties for CHS (S690QL steel grade) channels were adopted from the literature, where the temperatures ranged from 20 to 800 °C. The material characteristics were incorporated into finite element (FE) models using ABAQUS. The developed FE model was then validated against the published test results to evaluate the effects of various parameters including web hole diameter, bearing length, cross-section sizes, and flange fastening conditions of such channels at elevated temperatures, and a comprehensive parametric investigation including a total of 1710 validated finite element models was performed. From the parametric study results, it was found that the web crippling strength reduction factor is sensitive to the changes of the hole size and the bearing length, with the parameters of hole size having the largest effect on the web crippling reduction factor; however, the web crippling strength reduction factor remains stable when the temperature is changed from 20 to 800 °C. According to the FEA results, new reliable web crippling strength reduction factor equations for such CHS channels were proposed. In the comparison of proposed design strengths to the numerical failure load, the proposed design equations are suitable to predict the web crippling strength for CHS channels subject to EOF loading at ambient and elevated temperatures.

Effect of welding processes on the fatigue behaviour of ultra-high strength steel butt-welded joints

In the last decades, advances in steel manufacturing made possible the use of high-strength steel (HSS) and ultra-high strength steel (UHSS) for several applications, such as bridges, cranes, offshore structures, oil pipelines and automotive parts. Capacity of withstanding loads with reduced cross-section and minimum weight could be efficiently increased. Since most structures need to be joined, welding procedures are a major issue in mechanical design of HSS elements. Particularly in construction codes and design documents, it is normally assumed that fatigue resistance of as-welded joints is independent of strength level. Nevertheless, fatigue loaded as-welded components with high quality welds or post-weld treated joints could experience benefits from the use of HSS as the base material (BM).The purpose of the present work is to analyse fatigue behaviour of ultra-high strength steel butt-welded joints, by means of experimental testing and a fracture mechanics approach. Sheets of steel S960MC and S960QL were joined with different welding techniques: Gas Metal Arc Welding (GMAW), Laser Hybrid Welding (LHW) and Electron Beam Welding (EBW). Fatigue tests were performed with stress ratio R=0.1, under four points bending loading.All specimens exhibited fatigue crack initiation and subsequent propagation from the weld toe area, near the heat affected zone (HAZ). Different SN curves were obtained for the different welding processes. The Resistance Curve methodology was employed to assess the effect of microstructure, defect size, hardness, and joint geometry resulting from each technique. This fracture mechanics approach allowed to estimate the relative influence of the different geometrical and mechanical parameters of the weld and showed that joint geometry could not explain by itself the differences in fatigue strength. It was observed that microstructure and the size of defects played an important role in early crack growth, and they can reduce the benign effect of a high-strength base material.