High Strength Steel Welds Research Articles

Metallurgical and mechanical properties of hydrogen charged carbide-free bainitic weld metals

Hydrogen embrittlement is a major concern during the welding of high-strength steels. The susceptibility of the welds to hydrogen embrittlement increases with increase in weld strength. The ever-increasing demand to increase the strength of steels necessitates the development of novel welding procedures and fillers to produce welds of high strength and with resistance to hydrogen embrittlement. In this current work, the susceptibility of carbide-free bainitic weld metals to hydrogen embrittlement is studied with varying volume fractions of constituent phases. Using three different weld metal compositions, six different weld metal microstructures of carbide-free bainite were generated. The hydrogen saturation behaviour of the various weld metals was studied by cathodic electrolytic charging and subsequent diffusible hydrogen measurements by the hot extraction method. Tensile tests were conducted on various weld metals with and without hydrogen charging to evaluate their susceptibility to hydrogen embrittlement. The results show that the carbide-free bainite weld metals are highly resistant to hydrogen embrittlement despite their very high strength.

Development of double-sided friction stir welding of advanced high strength steel sheets

Double-sided friction stir welding (FSW), performed with a pair of rotation tools, was studied in an attempt to increase the joining speed when applied to welding of automotive advanced high-strength steels (AHSS). The experiments were performed on butt joint of 1.6-mm-thick 1180 MPa grade steel sheets, and sound welds were obtained when the travel speed was no more than 4 m/min, fairly comparable with laser beam welding (LBW), one of the conventional automotive welding processes. It was clarified by experimental and numerical analyses that the enhancement in travel speed was due to the unique material flow interactively developed by a pair of rotation tools. Double-sided FSW was performed at 3 m/min of joining speed on butt joint of dissimilar steel sheets in a combination of 1.5 GPa and 590 MPa grades. The mechanical properties of the welded joints were evaluated by tensile, V-bending and Erichsen tests comparing with LBW, which verified that double-sided FSW exhibited sufficient mechanical properties for the press-forming to produce automotive parts. A prototype automotive part was successfully fabricated using the tailor-welded blank welded with double-sided FSW, which demonstrates its applicability to automotive production.

Process Characteristics of Laser Powder Filling Welding of B340LA High-Strength Steel

Process Characteristics of Laser Powder Filling Welding of B340LA High-Strength Steel

T-joint Fillet Laser Welding of High-Strength Stainless Steel for Curtain Wall Frames

The aim of this study is to develop a frame for exterior facades of architectural curtain walls using numerical and experimental analyses of stainless steel T-joint fillet laser welds. The main characteristics required for this frame are that it should have an aesthetic surface from the narrow bead width and exhibit improved tensile strength at the joints. In this study, a method for optimizing the welding conditions was developed to simulateneously improve two mutually exclusive properties. It is essential to achieve more than 80% penetration of the weld zone to guarantee tensile properties. Laser welding parameters such as power, welding speed and focus height are the main variables. In addition, when high-strength stainless steel is applied as a curtain wall frame, its structural performance improves compared to conventional stainless steel.

Welding Residual Stress Distributions in the Thickness Direction under Constraints Using Neutron Diffraction and Contour Methods

Using high-strength steel for offshore structures with a yield stress of 500 MPa, this study evaluated the distribution characteristics of welding residual stress in the thickness direction under the influence of constrained conditions during welding using cutting, neutron diffraction, and contour methods. Welding residual stress inevitably occurs during welding and impacts fracture stability in structures. As high-strength steel better reflects the effects of phase transformation, the behavior of welding residual stress is known to differ from that of general steel. This study fabricated fully constrained and unconstrained specimens and evaluated them under identical conditions to evaluate welding residual stress according to the influence of constraints on high-strength steel welds. The results indicated that the maximum tensile residual stress of the fully constrained specimen occurred in the first layer of the weld joint, while the maximum tensile residual stress of the fully unconstrained specimen occurred in the last layer of the weld joint. Additionally, the welding residual stress of the fully constrained specimen was larger. Although some errors occurred in the residual stress values in the thickness direction depending on the measurement method, both methods applied in this study exhibited nearly identical distributions. Meanwhile, a maximum angular deformation of about 6° occurred in the fully unconstrained specimen, and we considered that the residual stress decreased owing to the occurrence of angular deformation. The maximum welding residual stress is generally the degree of yield stress. When residual stress greater than the yield stress occurs, it changes to the plastic range and appears in the form of angular, longitudinal, and lateral deformation. Under the fully unconstrained condition, reduced residual stress is considered to appear in the form of angular deformation. The welding tensile residual stress decreased to around 42% in the unconstrained specimen compared to the constrained specimen, and at that time, angular deformation of approximately 6° occurred. Therefore, it is estimated that an angular distortion of about 2.4° occurs as the stress of 100 MPa decreases.

Microstructure and mechanical properties of welded joints between nickel base alloy and 10Ni5CrMoV steel by MIG welding

• Welding of 10Ni5CrMoV steel using nickel-based alloy as filler metal. • The welding process adopted MIG welding and provided suitable welding parameters. • ERNiCrMo-4 had lower toughness, strength and higher hardness than ERNiCrMo-3. • The different precipitates will affect the mechanical properties of the weld. In order to solve the problem of cold cracks in welding of high strength steel with large thickness, austenitic stainless steel welding materials are usually selected in the industry, but there are few researches on the application of other austenitic welding materials such as nickel-based welding materials. In this paper, Metal Inertia Gas(MIG) welding was used to weld two typical nickel-based alloys (ERNiCrMo-3 and ERNiCrMo-4) with different contents of solid solution strengthening elements and 10Ni5CrMoV high-strength steel. The results showed that the welded joints of the two welding wires were well formed without obvious welding defects. In the impact toughness test, the average values of ERNiCrMo-3 and ERNiCrMo-4 welds at 73K were 134J and 96J respectively, while in the impact test of other parts, ERNiCrMo-3 was also higher than ERNiCrMo-4. In the tensile test, the tensile strength of both can reach 877MPa and 858MPa. In terms of precipitates, ERNiCrMo-4 containing more solid solution strengthening elements precipitated more TCP phase, which reduced the impact toughness of the weld to a certain extent. In the hardness test, it also proved that the hardness of ERNiCrMo-4 weld with more brittle and hard precipitates was higher. Overall, ERNiCrMo-3 welding wire has more advantages in welding large thickness 10Ni5CrMoV steel.

Quantifying influence of LME inner cracks on joint strength of resistance spot weld

It is well known that liquid metal embrittlement (LME) cracking can occur during the resistance spot welding (RSW) of zinc-coated high-strength steels. These cracks can be classified into two types, based on location, namely outer cracks inside or outside the electrode indentation region, and inner cracks around the weld area. Recent research has proposed that the latter cracks can be inhibited by controlling weld schedules, though there have been a few reports concerning especially the effects of these cracks on joint properties. In this study, an X-ray CT scan and a tensile-shear test were carried out consecutively with 10 mm-wide RSW joints in order to visualize the cracks in 3 dimensions, and to clarify their effects on tensile-shear strength (TSS). The cracks in the RSW joints of the zinc-coated ultra-high strength steel could be clearly observed by the scan. The results showed that the TSS decreased to 40% by the small interface cracks even with the maximum depth of 170 µm.

Role of the internal oxidation layer in the liquid metal embrittlement during the resistance spot welding of the Zn-coated advanced high strength steel

Advanced high strength steel (AHSS) with the Zn coating can produce high susceptibility of liquid metal embrittlement (LME) particularly when the resistance spot welding (RSW) is involved. The internal oxidation layer formed in the subsurface of AHSS is believed to be one of the most important factors to control the Zn-assisted LME susceptibility. The role of the internal oxidation layer in the formation of the LME for the resistance spot welded joint of the Zn-coated AHSS was comprehensively investigated in the present study. The results showed that increasing the dew point of the annealing atmosphere from −40 to −10 °C can successfully produce the internal oxidation layer in the subsurface of the steel. The Zn-coated AHSS without the internal oxidation layer revealed higher LME susceptibility during the RSW compared with the one with internal oxidation layer. In particular, the elements Si and O exhibit a synchronous segregation tendency, forming an internal oxidation layer in the subsurface of the steel. The internal oxidation layer together with the internal oxides distributed at the grain boundaries can decrease the penetration motivation of the liquid Zn, reducing the LME susceptibility.

Aspects of Changing the Structure and Hardness of Metal of Heat-Affected Zone During Welding of Low-Carbon High-Strength Steels for Pipes with Increased Deformability

Aspects of Changing the Structure and Hardness of Metal of Heat-Affected Zone During Welding of Low-Carbon High-Strength Steels for Pipes with Increased Deformability

Dissimilar welding of high nitrogen stainless steel and low alloy high strength steel under different shielding gas composition: Process, microstructure and mechanical properties

Ar–N2-O2 ternary shielding gas is employed in dissimilar welding between high nitrogen steel and low alloy steel. The effect of O2 and N2 is investigated based on the systematical analysis of the metal transfer, nitrogen escape phenomenon, weld appearance, nondestructive detection, nitrogen content distribution, microstructure and mechanical properties. There are two nitrogen sources of the nitrogen in the weld: high nitrogen base material and shielding gas. The effect of shielding gas is mainly reflected in these two aspects. The change of the droplet transfer mode affects the fusion ratio, N2 in the shielding gas can increase nitrogen content and promote the nitrogen uniform distribution. The addition of 2% O2 to Ar matrix can change the metal transfer from globular transfer to spray transfer, high nitrogen base material is thereby dissolved more to the molten pool, making nitrogen content increase, ferrite decrease and the mechanical properties improve. When applying N2-containing shielding gas, arc stability becomes poor and short-circuiting transfer frequency increases due to the nitrogen escape from droplets and the molten pool. Performance of the joints is improved with N2 increasing, but internal gas pores are easier to appear because of the poor capacity of low alloy steel to dissolve nitrogen, The generation of pores will greatly reduce the impact resistance. 4–8% N2 content in shielding gas is recommended in this study considering the integrated properties of the dissimilar welded joint.

Study on microstructure and impact toughness of laser hybrid welding joints

In the study, laser-MIG hybrid welding of high-strength shipbuilding steel was conducted. The microstructure and impact toughness were studied. The microstructure of the Filling and capping weld metal (FCWM) is widmanstatten and acicular ferrite. The microstructure of the backing WM (BWM) and the heat-affected zone (HAZ) are bainite. The impact energy of the joints is greater than 110 J at different charpy impact test positions. The reason for the different toughness is related to the proportion and distribution of the microstructure at the V-notch of the impact specimen. Through the observation of the section, it is also found that the section corresponding to the microstructure of the different HAZ will show significant differences in fracture morphology.

Hardox 450 Weld in Microstructural and Mechanical Approaches after Welding at Micro-Jet Cooling.

The demand for high-strength steel welds, as observed in civil and transport engineering, is related to a mass reduction in vehicles. Container-type trucks are examples of this kind of transport means because their boxes are able to be produced using Hardox grade steels. Therefore, this study reflects on the properties of welds in the MAG welding of Hardox 450, obtained through an innovative micro-jet cooling process with helium. This joining technology aims to reduce the formation of defects and to obtain a joint with very good assumed mechanical properties. Structural components of grade steel require welds with acceptable mechanical parameters with respect to operational loading conditions. That is, this study focuses on selecting welding parameters for the Hardox 450 steel and determining the weld quality with respect to microstructural observations and mechanical tests, such as the Charpy, tensile and fatigue tests. Weld fracturing under increasing monotonic force was examined and was strongly related to both stress components, i.e., axial and shear. The joint response under fatigue was expressed through differences in the fracture zones, i.e., at a stress value lower than the proportional limit, and weld degradation occurred in the shear and axial stress components. The data indicate that the hourglass specimen, with the weld in the centre zone of the measurement section, can be directly used to determine a weld response under cyclic loading. The impact test results showed attractive behaviour in the tested joint, as represented by 47 J at −20 °C. The recommended MAG welding parameters for Hardox 450 steel are low-oxygen when using an Ar + 18% CO2 shielding mixture. The collected results can be directly used as a guide to weld thin-walled structures (6 mm) made of Hardox grade steel, while the data from mechanical tests can support the modelling, designing and manufacturing of components made from this kind of steel grade.

Overlap welded joint strength of 2.0 GPa-strength steel sheets using single-mode laser wobbling

During laser overlap welding of high strength steels, a wide interface-bead width is a prerequisite for ensuring joint strength. However, a wide weld bead is accompanied by thermal effects such as thermal deformation and softening of the heat-affected zone owing to the high heat input during welding. Hot-press-forming steel with a strength of 2.0 GPa is the highest strength steel sheet in the automotive industry. When laser-welded, the minimum hardness in the heat-affected zone is less than 2/3 of the base metal hardness. In this study, single-mode laser and beam wobbling was employed to obtain a proper bead width while minimizing the heat input in the lap welding of steel sheets with a strength of 2.0 GPa. Two strategies—high frequency wobbling/high travel speed and low frequency wobbling/low travel speed—were evaluated with a laser power fixed at 1 kW. In the high frequency wobbling/high travel speed condition, the load-carrying at the overlap joint increased as the travel speed and wobbling frequency decreased. However, even in the case with the maximum fracture load, the fracture location in the tensile–shear test was the weld metal. The low frequency wobbling/low travel speed strategy was more effective in ensuring joint strength, and the fracture location in the tensile–shear test moved to the heat-affected zone. An equivalent tensile strength of 1 GPa or more was achieved by selecting appropriate parameters. Under optimal conditions, multiple weld penetrations and sufficient interface beads were confirmed on the cross section.

Optimization Process of Double Spots Welding of High Strength Steel Using in the Automotive Industry

Resistance Spot Welding (RSW) is one of the most important welding techniques used in the automotive industry because it is an economic process and is suitable for many materials. Many parameters affect the mechanical and microstructural properties of nugget formation and its strength, like welding current, electrode force, and welding time. Therefore, optimizing the RSW process to get the optimum welding parameters is necessary for automobile manufacturing companies. High-strength steel is widely used in the automotive industry because of its superior characteristics such as high strength-weight ratio, ductility, fatigue, and corrosion resistance. This paper presents an optimization process for RSW using the Taguchi method for high strength low alloy steel (HSLA) DOCOL 500 LA, considered a new steel grade. Two spots were used in this work.The mechanical and microstructural tests are achieved to get the maximum nugget strength, nugget diameter, different welding zones microstructures, microhardness values, and failure modes. The results showed that optimum welding parameters were welding current of 8800 Amp, welding time of 20 cycles, and electrode force of 1900 N. The failure mode for optimum conditions was a full pullout with tearing of the welded sheets because of high plastic deformation and absorbed energy. The maximum microhardness value is in the fusion zone, the heat-affected region, and finally, in the base material due to the nugget zone's rapid melting and solidification process.

VANADIUM ALLOYING IN S355 STRUCTURAL STEEL: EFFECT ON RESIDUAL AUSTENITE FORMATION IN WELDED JOINTS HEAT AFFECTED ZONE

The inter-critical heat affected zone (ICHAZ) appears to be one of the most brittle sections in the welding of high-strength micro-alloy steels (HSLA). Following multiple heating cycles in the temperature range between Ac1 and Ac3, the ICHAZ undergoes a strong loss of toughness and fatigue resistance, mainly caused by the formation of martensite-austenite constituent (MA). The presence of micro-alloying elements in HSLA steels induces variations in the formation of some microstructural constituents, more or less beneficial, which allow to improve the mechanical performance of a welded joint. The behavior in the inter-critical region of a S355 grade steel with 0.1wt% V addition is reported in this paper. Five double-pass welding thermal cycles were simulated using a dilatometer, with the maximum temperature of the secondary peak in the inter-critical area, in the range between 720 ° C and 790 ° C. The residual austenite dependence on inter-critical temperature is analyzed and related to the hardness behavior.

Study on microstructure evolution and mechanical properties of high-strength low-alloy steel welds realized by flash butt welding thermomechanical simulation

Defects would occur in the weld joint of the wheel rims during the post-flash butt welding (FBW) process suffering from poor plasticity, which will deteriorate the quality and lifecycle of finish products. Therefore, the FBW process of the 440CL high-strength-low-alloy (HSLA) steel was physically simulated and the influence of flash parameters on FBW joints was systematically evaluated in this study. The results showed that the width of heat affected zone increased with accumulated flash allowance (δf) while declined with accelerated flash speed (vf). The recrystallization level would be intensified with increased δf. Meanwhile, the acceleration in vf populated the WZ with a more homogeneous microstructure, higher recrystallization degree and lower dislocation density. The hardness in WZ slightly reduced (202 → 195 HV) as increased δf but obviously dropped (192 → 177 HV) as increased vf. All tensile samples were fractured at the BM location and the tensile properties of FBW joints exhibit a good match with those of BM, with a slight increase in strength (UTS: 468 ~ 493 MPa; YS: 370 ~ 403 MPa) but a mild decrease in plasticity (EL: 39 ~ 44%; RA: 74 ~ 79%). Furthermore, both the joint strength and ductility showed a downward tendency with the increment of δf. However, the strength slightly decreased while the ductility increased with the advancement of vf. These findings would be valuably referential to the real FBW of HSLA steels with optimized microstructure and mechanical performance.

Beneficial use of hyperbaric process conditions on the welding of high-strength low alloy steels

Hyperbaric welding is used for different steels in many underwater applications as a repair welding process. A difference between wet and dry welding processes can be made. Due to the increased ambient pressure, these processes have special features inherent in the process that influence the cooling and penetration behaviour. The positive use of these effects outside underwater applications is currently rarely addressed in science and application. The presented work establishes these advantages on the basis of a higher strength structural steel and characterizes the effects on the microstructure of a joined S700MC steel and on the mechanical properties of the joint. It will be shown that a hyperbaric environment can be used to orient the weld more towards the depth of the sheet. Furthermore, it will be shown that this change leads to modified cooling, which in itself influences the mechanical properties of the weld seam.

The Influence of Electrode Indentation Rate on LME Formation during RSW

During resistance spot welding of zinc-coated advanced high-strength steels (AHSSs) for automotive production, liquid metal embrittlement (LME) cracking may occur in the event of a combination of various unfavorable influences. In this study, the interactions of different welding current levels and weld times on the tendency for LME cracking in third-generation AHSSs were investigated. LME manifested itself as high-penetration cracks around the circumference of the spot welds for welding currents closely below the expulsion limit. At the same time, the observed tendency for LME cracking showed no direct correlation with the overall heat input of the investigated welding processes. To identify a reliable indicator of the tendency for LME cracking, the local strain rate at the origin of the observed cracks was analyzed over the course of the welding process via finite element simulation. While the local strain rate showed a good correlation with the process-specific LME cracking tendency, it was difficult to interpret due to its discontinuous course. Therefore, based on the experimental measurement of electrode displacement during welding, electrode indentation velocity was proposed as a descriptive indicator for quantifying cracking tendency.

Non-destructive testing of physical and mechanical properties of local zones in welded joints

The influence of the “saw-tooth” type electron beam sweep on the penetration shape during electron-beam welding of 30KhGSA high-strength steel with different welding modes was investigated. Using the instrumented indentation method, the distribution of Young’s modulus, as well as the characteristics of strength and plasticity in the welded joints cross-sections, was obtained. It has been found that in the weld metal there is a sharp increase in strength characteristics, while the plasticity ones are significantly reduced. The values of the Young’s modulus also varied over the cross-sections. The considerable decrease in this characteristic (up to 20%) was registered in the weld metal in comparison with the parent metal results.

Influence of Heat Control on Properties and Residual Stresses of Additive-Welded High-Strength Steel Components

Advanced high-performance filler metals for wire arc additive manufacturing (WAAM) exist on the market already. Nevertheless, these high-strength steels are not yet widely used in industrial applications due to limited knowledge of cold-cracking susceptibility, welding residual stresses, and therefore sufficient safety in terms of manufacturing and operation. High residual stresses promote cold-cracking risk, especially in the welding of high-strength steels, as the result of a complex interaction between the applied material, process conditions, and component design. The focus of the present investigation was the determination of the influence of the process parameters on the ∆t8/5 cooling time, mechanical properties, and residual stresses to correlate, for the first time, heat control, cooling conditions, and residual stress for WAAM of high-strength filler materials. This contributed to the knowledge regarding the safe avoidance of cold cracking. In addition to a thermophysical simulation using a dilatometer of different high-strength steels with subsequent tensile testing, reference WAAM specimens (open hollow cuboids) were welded while utilizing a high-strength filler metal (ultimate tensile strength > 790 MPa). The heat control was varied by means of the heat input and interlayer temperature such that the ∆t8/5 cooling times corresponded to the recommended processing range (approx. 5 s to 20 s). For the heat input, significant effects were exhibited, in particular on the local residual stresses in the component. Welding with an excessive heat input or deposition rate may lead to low cooling rates, and hence to unfavorable microstructure and component properties, but at the same time, is intended to result in lower tensile residual stress levels. Such complex interactions must ultimately be clarified to provide users with easily applicable processing recommendations and standard specifications for an economical WAAM of high-strength steels. These investigations demonstrated a major influence of the heat input on both the cooling conditions and the residual stresses of components manufactured with WAAM using high-strength filler materials. A higher heat input led to longer cooling times (∆t8/5) and approx. 200 MPa lower residual stresses in the surface of the top layer.