High Strength Steel Joints Research Articles

Experimental and numerical study on seismic performance of Q690 high-strength steel plate reinforced joints

The steel structure joints in the seismic fortification areas should have good rotation capacity while having sufficient bearing capacity. The plate reinforced joint can not only improve the bending bearing capacity of the beam end,but also effectively ensure that the beam-end welds does not undergo brittle failure and improve the ductility of the joint. A low-cyclic loading test of 6 Q690 high-strength steel plate reinforced joints was carried out in this paper, revealing the failure mechanism and energy dissipation mechanism of the joints,the influence laws of different reinforcement forms, steel strength grades, joint region reinforcement measures and key control parameters on the joints seismic performances were discussed. The results indicated that Due to the direct welding between the cover plate and the beam flange and the cylinder, the connection stiffness of the “cover plate reinforced” joint is large, and the rotational restraint force on the joint is strong, so the deformation capacity of the joint is weaker than the “flange plate transition” joint,The use of ordinary steel for steel beams will reduce the bearing capacity of the joints, but it can achieve higher ductility and energy dissipation capacity. when the joint region welding steel plate is reinforced, the ultimate bearing capacity of the joints was elevated by 14.14% and 13.74% respectively, while the ductility coefficient was reduced by 14.09% and 31.42%, which limits the rotation capacity of the joints. For flange plate transition joints, it is suggested that the length of the flange plate should be (0.6–0.85) hb and the thickness should be (1.2–1.4) tb. For cover plate reinforced joints, it is suggested that the length of the cover plate should be (0.6–0.85) hb and the thickness should be (0.7–1.1) tb. The study provides a theoretical basis for the engineering application and promotion of high-strength steel joints in seismic fortification areas.

Welding stability and fatigue performance of laser welded low alloy high strength steel with 20 mm thickness

The butt joint of low alloy high strength steel with 20 mm thickness was joined by combining with high power laser welding and narrow gap hybrid laser-arc welding. For the high power laser welding, the laser power was a decisive factor of the weld formation: excessive laser power lead to unstable behavior of molten pool and key hole, while insufficient laser power resulted in the lack of penetration. For the narrow gap hybrid laser-arc welding, arc behavior was closely related to the dynamic behavior of the molten metal at tip of the wire. Unstable behavior of the molten metal at tip of the wire caused variable arcing positions, leading to the arc deflection and arc burning up. The narrow gap grooves were heated unevenly when the arc was whether deflection or burning up, leading to the formation of the sharp corner areas and lack of fusion defects. The fatigue fractures of both laser weld and hybrid weld were made up of the crack initiation areas, the crack propagation areas and the transient fracture areas. Fatigue property of the laser weld was better than that of the hybrid weld, which was thought to be strongly associated with the fine multidirectional grains within the laser weld and the precipitation of particle phases inside the hybrid weld.

Stress corrosion cracking behavior of welding joint of high strength steel

Nowadays, high-strength steel structures are increasingly being used in marine or soil environments, but low attention has been paid on the corrosion and stress corrosion cracking problem. In this paper, the susceptibility to stress corrosion cracking (SCC) of butt-welding joints of Strenx® 700 (S690QL high strength steel) both in marine and acid-polluted marine atmospheres was studied, by using slow strain rate tensile (SSRT) test. The seawater corrosion environment was considered and it was prepared following the ASTM D1141. To study the influence of the environment pH, NaOH 0,1M solutions and pure H2SO4 were used to adjust the pH of the simulated seawater solution to 8 and 4, respectively. The slow strain rate tensile tests (SSRT) with a crosshead speed of 0.000017 mm/s were carried out at room temperature (approximately 25 °C). After failure, the SCC susceptibility was evaluated acquiring the elongation loss rate and the reduction-in-area loss rate. The fracture surfaces were analyzed by SEM observation.

Investigation of Metallurgical and Mechanical Properties of Laser Beam Welded and Post-weld Heat Treated DP1400 Steel

In this paper, the effect of autogenous diode laser beam welding (LBW) and the influence of post weld heat treatment (PWHT) on microstructural changes and mechanical properties of dual phase DP1400 high strength steel (HSS) butt welded joint are studied and presented. LBW and PWHT were performed on 1 mm sheet thickness using 3 and 5 kW diode laser systems, respectively. The technology ensures high quality welded joints in HSS and facilitate the welding and PWHT by same process and equipment. Microstructure evaluation was performed using optical and scanning electron microscopy. Related to the mechanical properties, tensile tests, fractography of fractured tensile specimens and three-point bending tests were carried out. The microstructural examination presented the constituents of martensite and ferrite in the heat affected zone (HAZ) and fusion zone (FZ) consists of predominantly lath martensite with ferrite and some bainite. Tempered martensite was observed after PWHT in HAZ and FZ. The hardening peaks observed in coarse-grained and fine-grained subzones were significantly reduced by the novelty technology, i.e. PWHT and thereby cold cracking sensitivity. The fractography of the fractured tensile specimens showed characteristic features of ductile failure.

The Ultimate Load Estimation of Welded Joints of High-Strength Steels subject to Mechanical and Geometric Heterogeneity

In this paper we consider the problems arising in the numerical estimation of the ultimate load of welded joints of high-strength steels with slight hardening. The stress concentrator in the transition node from the deposited to the base metal is modeled based on the example of welding a roller wire on a plate made of high-strength steel. The use of welding wire with a yield point lower than that of the base metal allowed to simulate areas of the welded joint with heterogeneous mechanical properties. The geometry of three areas of the welded joint is studied, i.e. weld metal, heat-affected zone (HAZ) and the base metal. Mechanical properties of all three areas are determined by calculation and experimentally. For this purpose, it is proposed to consider the material in all sections as ideally elastic-plastic, and the yield strength is uniquely associated with the hardness in the indentation zone (a Rockwell diamond cone is used). Calculations of the inelastic indentation process by the finite element method (FEM) in axis-symmetric formulation allowed obtaining a linear relationship between the hardness and the yield strength with a coefficient of 0.418. Tests at a quasi-static three-point bend (with stretching in the surfacing area) were carried out on sample beams cut perpendicular to the direction of welding. The “force-deflection” diagrams are obtained and compared with the calculated curves (FEM in a three-dimensional formulation with an explicit consideration of the complex configuration of all sections and different yield stress in the areas determined by local hardness values). There is a good agreement between the calculated and experimental ultimate loads. The proposed method of the three-stage study (determination of local hardness, yield strength in the areas and the ultimate load) can be effectively used to assess the ultimate loads of the welded joints due to the low parametricity of the proposed models of materials inelastic deformation in areas for which it is impossible to manufacture standard samples for the study of mechanical properties. The experimental study of the strengthening effect of the seam with a stress concentrator in the form of an angle of 90 degrees on the value of the ultimate bending load showed that the removal of the deposited metal does not lead to an increase in the ultimate load of the welded joint when using the welding wire of low-carbon high-plastic steel.

Mechanical behaviour of longitudinal lap-welded joints of high strength steel: Experimental and numerical analysis

Mechanical behaviour of twenty-eight longitudinal lap-welded joints made of high strength steels (HSS) under tension load was investigated by experimental and numerical study. Weaknesses due to traditional deformation measurements for fillet welded joints can be perfectly solved by digital image correlation techniques (DIC). Distribution and development of strain field throughout the specimens along the whole loading process were recorded by DIC measurement. The effect of parameters (e.g. weld size, weld length and mismatch ratio) on mechanical properties (e.g. ultimate strength, failure modes, weld ductility and fracture angle) of longitudinal fillet welds and transverse fillet welds, which was introduced in detail in previous work by the authors, were compared. Generally, because of the difference on the combination of shear force and tension force, the fracture angle of longitudinal welded specimens (around 50∘) were much more divergent from transverse welded specimens (around 20∘) even though both of them failed at welded zone (welded zone only refers to weld metal in this paper), resulting that the mean strength of longitudinal welded specimens were only 0.58 time of transverse welded specimens. Conversely, the mean deformation capacity of longitudinal welded specimens was almost 4.0 times of transverse welded specimens. All longitudinal welded specimens have “platform” or “decline” feature while all transverse welded specimens have “rise” or “platform” feature load-deformation curves. Generally, the specimens with “rise” feature load-deformation curve are more likely to suffer sudden fracture, while specimens with “decline” or “platform” feature have satisfactory ductility performance. Based on the strain distribution and development features provided by DIC measurement, a new strength criterion, in which the ultimate strain obtained in relevant coupon tests was an indicator, was proposed and through the comparison between the numerical results and test results, its validation was verified by FE simulation. Moreover, it was confirmed that the predicted loads of EC3 and AISC Specification were close and slightly conservative for all specimens.

Inhomogeneous microstructure and fatigue crack propagation of thick-section high strength steel joint welded using double-sided hybrid fiber laser-arc welding

The inhomogeneous microstructure and fatigue crack propagation of 30 mm thick-section high strength steel welded joint by double-sided hybrid fiber laser-arc welding were investigated in detail. The results indicated that the average effective grain size of the laser zone was only 1/2 of that of the arc zone, due to the faster cooling rate of the laser resource. The base metal consisted of massive polygonal ferrites and small granular carbides, while fine grained region, the coarse grained region and weld metal were all composed of martensite with a high dislocation density. Compared with the arc zone, the percentage of grain boundaries with high misorientation angle increased 24% for the laser zone, as the average grain size of the laser zone was smaller than that of the arc zone. The results also revealed that the fatigue crack propagation resistance of the welded joint was higher than that of the base metal. Meanwhile, a significant increase in the fatigue crack propagation resistance of the laser zone occurred, as compared with the arc zone, due to the refined grains and the high proportion of the grain boundaries with high misorientation angle (>15°) in the laser zone.

Capability of martensitic low transformation temperature welding consumables for increasing the fatigue strength of high strength steel joints

Abstract The use of low transformation temperature (LTT) filler materials represents a smart approach for increasing the fatigue strength of welded high strength steel structures apart from the usual procedures of post weld treatment. The main mechanism is based on the effect of the low start temperature of martensite formation on the stress already present during welding. Thus, compressive residual stress formed due to constrained volume expansion in connection with phase transformation become highly effective. Furthermore, the weld metal has a high hardness that can delay the formation of fatigue cracks but also leads to low toughness. Fundamental investigations on the weldability of an LTT filler material are presented in this work, including the characterization of the weld microstructure, its hardness, phase transformation temperature and mechanical properties. Special attention was applied to avoid imperfections in order to ensure a high weld quality for subsequent fatigue testing. Fatigue tests were conducted on the welded joints of the base materials S355J2 and S960QL using conventional filler materials as a comparison to the LTT filler. Butt joints were used with a variation in the weld type (DY-weld and V-weld). In addition, a component-like specimen (longitudinal stiffener) was investigated where the LTT filler material was applied as an additional layer. The joints were characterized with respect to residual stress, its stability during cyclic loading and microstructure. The results show that the application of LTT consumables leads to a significant increase in fatigue strength when basic design guidelines are followed. This enables a benefit from the lightweight design potential of high-strength steel grades.

Strength Assessment of Welded Joints of High-Strength Alloy Steels by Indentation Method

The paper presents the indentation method for strength assessment of the welded joints in high-strength alloy steels, including base metal, heat-affected zone, and welded joint, in production after application of selected flaw detection methods as a final stage of non-destructive testing methods. Non-destructive testing methods are mainly used to detect defects, their location, size, and nature. The indentation method allows one to determine both the joint strength and the strength and dimensions of the heat-affected zone during the production process. The authors performed the comparative analysis on the hardness and strength limit of welded joints of 30Kh2CN2MFA and 28GRA alloy steels as delivered of sheet metal of 6 mm in thickness (for the production of special structures) to corroborate the method efficiency. It is shown that the Brinell hardness values obtained by the TSH-2 stationary hardness tester, ERNST Computest SC portable hardness tester, and instrumented indentation imply the same nature of the hardness distribution along the length of the specimen cut across the axis of the welded joint. The distribution nature of the strength limit values determined by the indentation method is similar to that of the Brinell HBW hardness distribution. It is established that the deviation of strength limit values determined by the indentation method did not exceed 3.7%.

Capability of martensitic low transformation temperature welding consumables for increasing the fatigue strength of high strength steel joints

Abstract The use of low transformation temperature (LTT) filler materials represents a smart approach for increasing the fatigue strength of welded high strength steel structures apart from the usual procedures of post weld treatment. The main mechanism is based on the effect of the low start temperature of martensite formation on the stress already present during welding. Thus, compressive residual stress formed due to constrained volume expansion in connection with phase transformation become highly effective. Furthermore, the weld metal has a high hardness that can delay the formation of fatigue cracks but also leads to low toughness. Fundamental investigations on the weldability of an LTT filler material are presented in this work, including the characterization of the weld microstructure, its hardness, phase transformation temperature and mechanical properties. Special attention was applied to avoid imperfections in order to ensure a high weld quality for subsequent fatigue testing. Fatigue tests were conducted on the welded joints of the base materials S355J2 and S960QL using conventional filler materials as a comparison to the LTT filler. Butt joints were used with a variation in the weld type (DY-weld and V-weld). In addition, a component-like specimen (longitudinal stiffener) was investigated where the LTT filler material was applied as an additional layer. The joints were characterized with respect to residual stress, its stability during cyclic loading and microstructure. The results show that the application of LTT consumables leads to a significant increase in fatigue strength when basic design guidelines are followed. This enables a benefit from the lightweight design potential of high-strength steel grades.

Improvement of a rivet geometry for the self-piercing riveting of high-strength steel and multi-material joints

As a result of lightweight design, increased use is being made of high-strength steel and aluminium in car bodies. Self-piercing riveting is an established technique for joining these materials. The dissimilar properties of the two materials have led to a number of different rivet geometries in the past. Each rivet geometry fulfils the requirements of the materials within a limited range. In the present investigation, an improved rivet geometry is developed, which permits the reliable joining of two material combinations that could only be joined by two different rivet geometries up until now. Material combination 1 consists of high-strength steel on both sides, while material combination 2 comprises aluminium on the punch side and high-strength steel on the die side. The material flow and the stress and strain conditions prevailing during the joining process are analysed by means of numerical simulation. The rivet geometry is then improved step-by-step on the basis of this analysis. Finally, the improved rivet geometry is manufactured and the findings of the investigation are verified in experimental joining tests.

The Assessment of Selected Properties of Welded Joints in High-Strength Steels

The use of technologically advanced structural materials entails the necessity of adjusting typical welding processes to special requirements resulting from the limited weldability of certain material groups. Difficulties obtaining high-quality joints may be the consequence of deteriorated mechanical properties and structural changes in materials (beyond requirements of related standards). One of the aforementioned materials is steel characterised by a guaranteed yield point of 1300 MPa, where high strength is obtained through the addition of slight amounts of carbide-forming elements and the application of complex heat treatment processes. A heat input during welding may worsen the aforesaid properties not only in the weld but also in the adjacent material. The tests discussed in the article revealed that the crucial area was that heated below a temperature of 600°C, where the hardness of the material decreased from approximately 520 HV to 330 HV.

Effect of adhesive on laser-arc hybrid welding of aluminum alloy to high-strength steel joint

Aluminum alloy 6061 and high-strength steel Q460 were joined by laser-arc-adhesive hybrid welding technology successfully. A Cu alloy interlayer was added between Al alloy and high-strength steel. The effect of the adhesive on laser-arc-adhesive hybrid welding of Al alloy to high-strength steel was discussed. The optical microscope, scanning electron microscope and electron probe micro-analysis were applied to observe the microstructural evolution and phase transition at Al–Fe interface of laser-arc-adhesive hybrid welded joints. The results showed the maximum tensile shear strength of the joint without adhesive was 256 MPa. After adding adhesive, the maximum tensile shear strength reached 282 MPa. The combination of the mechanical bonding and the metallurgical effect could improve the mechanical performances. The shape of the joint in Al alloy changed into a canine-like morphology. There was no porosity in welds because the molten pool of Al alloy and the special keyhole phenomenon of laser welding provided a channel for the decomposed gas to escape from fusion zone.

Nanoscale Structures of Laser–Arc Welded Joints of High-Strength Low-Alloy Steels

Nanoscale Structures of Laser–Arc Welded Joints of High-Strength Low-Alloy Steels

Fatigue properties of as-welded and post-weld-treated high-strength steel joints: The influence of constant and variable amplitude loads

In HSS welded structures, HFMI allows fatigue strength improvement factors by inducing compressive residual stresses, cold-worked surface region and modifying the weld toe shape. According to the literature, the factors are proposed based on failures at CAL and also checked with a limited dataset at VAL, the latter being more realistic of service loadings. Therefore, this paper investigates the behaviour of HFMI welds at CAL and VAL for R = −0.43. Studies are also performed at the microstructural level for locating the crack initiation and hardness. Fatigue damage sums are evaluated and discussed with the recommended values in the literature.

Structural Transformations of the Metal of Heat-Affected Zone of Welded Joints of High-Strength Armor Steels

We construct the thermokinetic diagrams of austenite decomposition and analyze the influence of the rate of cooling of specimens simulating the metal of the heat-affected zone (HAZ) on the structure and durability of Quardian-500 and Armstal-500 high-strength steels. The regularities of the influence of carbon on the structure formation in the HAZ metal are determined for these steels and the susceptibility to brittle fracture is estimated. It is shown that, in Quardian-500 steel with a carbon content of 0.26%, a predominantly martensitic structure with a hardness of 4850–4890 MPa is formed in the zone of overheating for cooling rates of 20–30°C/sec, whereas in Armstal-500 steel with a carbon content of 0.29%, the transformation of overcooled austenite in the HAZ of simulating specimens occurs at cooling rates of 7–30°C/sec and is accompanied by the formation of martensitic structure with a hardness of 5160–5390 MPa, which improves the static strength of the products.

초고장력 강판의 CMT 용접 조건과 용접 이음부 분석

초고장력 강판의 CMT 용접 조건과 용접 이음부 분석

Subminiature eddy-current transducer for studying defects in weld joints of high-strength steel

The article describes the design of a hardware-software system intended for studying the properties of high-strength steels. Within the scope of the study, defects in the welds of the transformer tanks made of high strength 08G2B steel were being located. Such materials belong to ultra-low-carbon steels, and thus, it is possible to study their properties applying the eddy-current method. In the course of the work, a subminiature eddy-current transducer with three coils wound on a core made of 80NM3 ferrite was designed and optimized. In the work, samples with model continuity defects (cracks and holes) were examined, and a study was conducted in order to simulate weld corrosion defects.

Analysis of Influence of the Welding Procedure on Impact Toughness of Welded Joints of the High-Strength Low-Alloyed Steels

This paper presents results of comparison of two welding procedures’ influence on selected properties of the welded joints of high-strength low-alloyed steel (HSLA), specifically the impact toughness and the hardness distribution in the specific zones of “single V” butt multiple-pass welded joints. Based on results obtained from experiments, the two applied welding technologies were evaluated. They differ by the welding grove geometry and by the applied root pass welding procedure. Both procedures use MAG (Metal Active Gas) welding for execution of the filling and covering passes, while the root passes are executed by the MMA (Manual Metal Arc) procedure in the first case and by the MIG (Metal Inert Gas) procedure in the second. Experimentally obtained values of the fracture energy of the welded samples for both procedures were smaller than the values for the parent metal, which confirms the fact that welding causes degradation of the mechanical properties of HSLA steel; thus, any welding technology parameters must be so selected to mitigate this deficiency.

Evaluation of the effect using possibility of contact strengthening in welded joints of high-strength low-alloy steels

ABSTRACT The use of the contact strengthening effect at narrow bevel welding of high-strength steels with less durable welding materials is a promising solution to many technological problems. However, the method of quantitatively accounting for this effect at the analysis of the static equal strength of the welded joint remains not developed. In this regard, tests of samples with variable bevel parameters and strength combinations of the base metal and weld metal, as well as numerical studies of the FEM processes of their deformation in a 3D setting are performed. On the basis of the obtained results, interpolation relations, which make it possible to predict the strength of a mechanically inhomogeneous welded joint, are proposed.