High Strength Steel Welds Research Articles

Development of Welding Consumables for GMAW and FCAW

Welding consumables for arc welding are widely used in various industrial fields. Amongst all processes, the GMAW and FCAW can especially be seen as core processes as they cover approximately 80% in terms of usage.In regards to the FCAW, countless studies are done focusing on the diffusible hydrogen, resulting in developments such as seamless FCW in order to reduce the diffusible hydrogen and the development of welding torch devices that input hydrogen within the shielding gases. Furthermore, countless developments are being made for consumables in the field of construction related fabrications due to the plethora of steel types available and the trend of automation. Developments are also being made according to the automotive industry for the welding of high strength steels and zinc-primered steels. Key words: GMAW, Gas metal arc welding, FCAW, Flux cored arc welding, FCW

Hybrid laser/arc welding of thick high-strength steel in different configurations

In this investigation, hybrid laser/arc welding (HLAW) was employed to join 8-mm-thick high-strength quenched and tempered steel (HSQTS) plates in the butt- and T-joint configurations. The influences of welding parameters, such as laser power, welding speed, stand-off distance (SD) between the arc of gas metal arc welding, and the laser heat source on the weld quality and mechanical properties of joints, were studied to obtain non-porous and crack-free fully-penetrated welds. The weld microstructure, cross-section, and mechanical properties were evaluated by an optical microscope, and microhardness and tensile tests. In addition, a finite element model was developed to investigate the thermal history and molten pool geometry of the HLAW process to join the HSQTS. The numerical study demonstrated that the SD had a paramount role in good synergy between the heat sources and the stability of the keyhole. For the butt-joint configuration, the results showed that, at a higher welding speed (35 mm/s) and optimum SD between the arc and laser, a fully-penetrated sound weld could be achieved. A non-porous weld in the T-joint configuration was obtained at a lower welding speed (10 mm/s). Microstructural evaluations indicated that the formation of residual austenite and the continuous network of martensitic structure along the grain boundary through the heat affected zone were the primary reasons of the softening behavior of this area. This was confirmed by the sharp hardness reduction and failure behavior of the tensile coupons in this area.

Comparative evaluation of various experimental and numerical simulation methods for determination of t8/5 cooling times in HPAW process weldments

The aim of this article is to provide a quantitative comparison and efficiency verification of the methods of estimating t8/5 cooling time in the process of HPAW of S960QL steel.The measurements of t8/5 welding time were conducted at the face of weld with the use of thermoelectric, pyrometric and thermovision methods. A FEM model of the joint was made, and welding simulation was done. The results of the calculations were then confronted with experimental data, and measuring methods were evaluated.Differences in the results of t8/5 time measurements were determined for the analysed methods and arranged according to the precision of results presented; the applicability of FEM for predicting the value of t8/5 time was investigated.The usability of temperature measuring methods for determining cooling time was determined, the weaknesses of non-contact measurement in terms of diversification of cooling time in a section of a welded joint were shown, and the advantages of numerical method were demonstrated.It was established that joining experimental methods for measuring cooling time of a joint with FEM analysis allows to obtain a desired resolution of prediction. In this way, the technology for hybrid welding of advanced high-strength steels can be designed more efficiently.

Correlation Between Microstructure and Low-Temperature Impact Toughness of Simulated Reheated Zones in the Multi-pass Weld Metal of High-Strength Steel

A large fraction of reheated weld metal is formed during multi-pass welding, which significantly affects the mechanical properties (especially toughness) of welded structures. In this study, the low-temperature toughness of the simulated reheated zone in multi-pass weld metal was evaluated and compared to that of the as-deposited zone using microstructural analyses. Two kinds of high-strength steel welds with different hardenabilities were produced by single-pass, bead-in-groove welding, and both welds were thermally cycled to peak temperatures above Ac3 using a Gleeble simulator. When the weld metals were reheated, their toughness deteriorated in response to the increase in the fraction of detrimental microstructural components, i.e., grain boundary ferrite and coalesced bainite in the weld metals with low and high hardenabilities, respectively. In addition, toughness deterioration occurred in conjunction with an increase in the effective grain size, which was attributed to the decrease in nucleation probability of acicular ferrite; the main cause for this decrease changed depending on the hardenability of the weld metal.

Stress intensity factor-based prediction of solidification crack growth during welding of high strength steel

A stress intensity factor (SIF)-based analysis was used to predict solidification crack growth patterns after welding. A coupled thermo-mechanical analysis was conducted to achieve the stresses induced by welding processes, which was used as the initial state in the SIF analysis. The SIF cracked model was established geometrically identical to an uncracked seam element in the thermo-mechanical analysis, based on the Lemaitre strain equivalence principle. Then the prediction of solidification crack growth employed the SIF solutions. The results show that propagation of longitudinal solidification cracks occurs in mode III (at high temperature), as validated by the non-equiaxial and elongated dimple-like crack profile of high-strength steel specimens. Simulations show that the longitudinal cracking switches to mode I as the temperature drops. Transverse orientations are naturally crack-resistant at high-temperature, however, they are prone to the open mode at low-temperature. The comparison of SIFs from different welding speeds shows that high-speed welding increases susceptibility to cracking in the open mode regardless of temperature.

Laser–arc hybrid welding of high-strength steel and aluminum alloy joints with brass filler

ABSTRACTThe laser–tungsten inert gas hybrid welding method was adopted to realize the welding process between Q460 high-strength steel and 6061 aluminum alloy. The influence of the dual heat source on the mechanical properties and microstructure of the welded joints are discussed. In addition, the effects of including a copper–zinc interlayer on the microstructure, elemental distribution, and mechanical properties of welded joints are also studied. The results show that the mechanical properties of the welded joints are influenced by the relative heat inputs of the two heat sources and the Cu-Zn interlayer. The braze welded joint fabricated without a Cu-Zn interlayer fractured at an Al-Fe intermetallic compound (IMC) layer formed at the interface, whereas the braze welded joint fabricated with a Cu-Zn interlayer fractured at an Al-Cu IMC layer formed at the interface. Comparisons show that the maximum tensile shear load of the brazed welded joint with the Cu-Zn interlayer was increased by about 20% relative to that formed without the interlayer. The formation of Al-Fe IMC layer in the deep penetration joint was inhibited by the combined effect of the dual heating sources and the Cu-Zn interlayer.

WELDOX960 Low Alloy High Strength Steel Welding Automation Manufacturability Research

This article combined with the practical production conditions,introduced the welding process parameter’s selection and operation technology of one-side welding with back formation for welding robot of WELDOX960 steel.The welding practices indicate that:when adopts the welding robot CO2+Ar to perform the one-side welding with back formation operation to WELDOX960 low alloy high strength steel,the setting of welding process parameters has the great impact to the high strength steel welding performance.

In situ neutron diffraction measurement of strain relaxation in welds during heat treatment

ABSTRACTNeutron diffraction (ND) is commonly used to investigate the stress redistribution before and after post-weld heat treatment (PWHT) in welded structures. However, there is a lack of information on the evaluations of strains during PWHT. The present work employed in situ ND to measure the relaxation of residual strains during conventional PWHT in multi-pass high-strength low-alloy steel welds. It was found that strain relaxation occurs principally during the heating stage of the heat treatment. The findings have important economic bearings and can be used to characterise comparable material combinations and optimise the PWHT process for high-strength low-alloy weld joints. This unique information also provides a valuable benchmark for the finite element modelling of this complex process.

Weldment Nondestructive Testing Using Magneto-optical Imaging Induced by Alternating Magnetic Field

This paper introduces an innovative Nondestructive testing (NDT) approach by using dynamic magneto-optical imaging (MOI) system to diagnose weld defects. MOI mechanism was explained by Faraday magneto-optical effect and magnetic domain theory. Two Q235 specimen MOI experiments based on excitation of permanent magnet and alternating electromagnet (alternating current driven electromagnet) were performed, thus the feasibility of MOI system for weld defects detection was verified and the relation between alternating magnetic field (AMF) and dynamic MO images was discussed as well. In this research, AMF of welded high-strength steel (HSS) weldment was excited by an alternating electromagnet, and dynamic MO images of HSS seam were acquired for weldment NDT. Finally, a pattern recognition method including three steps was proposed. Dynamic MO images were fused periodically and the features of fused images were extracted by principal component analysis. A classifier based on error back propagation (BP) neural network was established to identify these weld features. It proved that typical weld features such as incomplete penetration, sag, crack and no defect can be classified by the proposed method with an accuracy of 93.5%.

Microstructure, Composition, and Impact Toughness Across the Fusion Line of High-Strength Bainitic Steel Weldments

This paper analyzed the evolution of microstructure, composition, and impact toughness across the fusion line of high-strength bainitic steel weldments with different heat inputs. The main purpose was to develop a convenient method to evaluate the HAZ toughness quickly. The compositions of HAZ were insensitive to higher contents of alloy elements (e.g., Ni, Mo) in the weld metal because their diffusion distance is very short into the HAZ. The weld metal contained predominantly acicular ferrite at any a heat input, whereas the main microstructures in the HAZ changed from lath martensite/bainite to upper bainite with the increasing heat input. The evolution of HAZ toughness in relation to microstructural changes can be revealed clearly combined with the impact load curve and fracture morphology, although the results of impact tests do not show an obvious change with heat input because the position of Charpy V notch contains the weld metal, HAZ as well as a part of base metal. As a result, based on the bead-on-plate welding tests, the welding parameter affecting the HAZ toughness can be evaluated rapidly.

Investigating the effects of welding process on residual stresses, microstructure and mechanical properties in HSLA steel welds

One of the important steps in the design and fabrication of welded structures is the selection of the welding process and the filler consumables. This is because these two factors control the mechanics of thermal distribution and the chemistry of the welded join, which in turn affect weld integrity through the resulting microstructure and residual stresses. The present study employed neutron diffraction to investigate the effects of welding process on the residual stresses in high-strength low-alloy steel weld joints made by SMAW (shielded metal arc welding) and combined MSAW (modified short arc welding) and FCAW (flux cored arc welding) processes. A significantly higher level of residual stress was found in the MASW+FCAW combination which was shown to be in line with the microstructural and mechanical properties. Higher levels of residual stresses may be related to the formation of bainite and Widmanstätten ferrite in the weld metal and HAZ of the combined MSAW and FCAW processes.

Mixed mode fatigue crack growth behavior of Ni-Cr-Mo-V high strength steel weldments

The extensive use of Ni-Cr-Mo-V high strength steels in modern marine environment poses increasing requirements for accurate characterization of the fatigue crack growth (FCG) behavior of their welded joints. The present work reported a study on a new generation Ni-Cr-Mo-V high strength steel concerning the mixed mode FCG behavior of its welded joint. Effects of microstructure and welding residual stresses (WRSs) were emphasized. It was found that the 4% compliance offset criterion in the conventional ASTM procedure and the 10% offset criterion in the normalized ASTM procedure were suitable for determination of the crack opening force of high strength steel weldments. A new effective stress intensity factor range parameter as the crack growth driving force was proposed, which yielded fairly good correlation for the mixed mode FCG data. Acicular ferrite with basket-weave microstructure in the weld metal exhibited favorable crack growth resistance relative to the base material and was preferred to the welded joints. The attained master or intrinsic FCG curves for the base material and the weld metal on the basis of the proposed parameter provided a basic database for the design of high strength steels employed in modern marine structures.

Welding metallurgy of martensitic advanced high strength steels during resistance spot welding

ABSTRACTThe paper addresses the process–microstructure–performance relationships in resistance spot welding of a martensitic advanced high strength steel. Significant softening was observed in the heat affected zone (HAZ) due to allotriomorphic ferrite formation in the inter-critical HAZ and tempering of martensite in sub-critical HAZ (SCHAZ), with the latter plays more important role in mechanical properties of the spot welds. The strain concentration associated with the HAZ softening promotes initiation of pullout failure from the soft SCHAZ. While, the peak load in the interfacial failure mode is governed by the fusion zone size, that of the pullout mode is significantly affected by the HAZ softening. To improve weldability of martensitic steels, the HAZ softening should be minimised via modifications in welding process or steel chemistry.

Comparative study of phase transformation temperatures in high strength steel weld metals

For the determination of phase transformation temperatures in metallic materials, dilatometric analysis (DA) is one of the most commonly applied thermoanalytical methods. Since it depends on elaborate sample preparation, in-situ methods are likely to support the application of welding process investigations. In this study, several gas metal arc (GMA) welded all-weld metal samples were examined regarding their solid state phase transformation temperature during cooling by means of dilatometry and single-sensor differential thermal analysis (SS DTA). The results show the applicability of the SS DTA method during GMA welding and good agreement with dilatometric values.

Microstructure, microhardness and tensile properties of FSWed DP 800 steel

Purpose: Dual phase (DP) steels are widely used in the automotive industry due to their properties of a high balance of strength and formability. However, it is known that conventional welding of high strength steel leads to some undesirable results such as hardness decrease in the heat affected zone. Friction stir welding (FSW) is a new solid state joining method, which is used to join these steels due to its advantage of low heat input. The aim of this study is to evaluate the microstructural change and mechanical properties of friction stir welded DP800 steel. Design/methodology/approach: DP 800 steels with 1.5 mm thickness were subjected to friction stir welding, by using a tungsten carbide (WC) tool. The tool was tilted 2°, and downforce of the tool was kept constant at 6 kN. During processing, the tool rotation and traverse speed were fixed at 1600 rpm and 170 mm∙min-1, respectively. Findings: The friction stir welded region comprises martensite, bainite, refined ferrite. The average microhardness of stir zone has increased from 260 HV0.2 to about 450 HV0.2. The tensile sample shows a decrease in the ultimate tensile strength (σUTS) about 3%, from 827 MPa to 806 MPa for the joint. The yield strength (YS) of the joint is about 566 MPa and the value is near that of DP800. Research limitations/implications: The tungsten carbide tool used for the friction stir welding has suffered deterioration in the pin profile after 1 meter welding operation. It may be advisable to drill a pre-hole in the specimens for a longer tool life. Practical implications: Tool wear for industrial applications will be a major problem. Therefore, the use of tools with high wear resistance such as polycrystalline cubic boron nitride may be recommended. Originality/value: Works on friction stir welding of dual phase steels are limited and they mostly focus on spot welding. Also, this study systematically investigates the microstructure and mechanical properties of dual-phase 800 steels after the friction stir welding.

Effects of evolution of nanoscale copper precipitation and copper content on mechanical properties of high-strength steel weld metal

Nanoscale copper precipitation in high-strength weld metal was characterised by using atom probe tomography (APT) and transmission electron microscopy. In as-deposited zone, there was no copper precipitation. During welding thermal cycling, nanoscale copper of 2–3 nm in radius precipitated with face-centred cubic (FCC) and body-centred cubic (BCC) structure. During post-weld heat treatment (PWHT), FCC copper precipitate became coarsen and BCC transformed into FCC. The highest hardness was in the reheated zone which subjected to the peak temperature below 500°C; while the lowest hardness was both in deposited zone and the reheated zone which subjected to the peak temperature greater than 810°C. After PWHT, the hardness became homogenous as well as decreased. With increasing copper content, the hardness was increased and the toughness was slightly decreased. After PWHT, the toughness decreased due to the change in the structure of copper precipitates.

A Study on Fatigue Crack Growth Parameters for Fatigue Life Assessment based on Fracture Mechanics

Recent studies on the fatigue assessment of high strength steel weld based on the fracture mechanics have frequently raised the problems related to the conservatism in the fatigue crack growth rate specified in the relevant design code. The purpose of this study is to evaluate the effect of the fatigue crack growth parameter on the fatigue life for the low carbon steel weld. In order to do it, the fatigue tests with the constant stress ratio were performed to evaluate the fatigue crack growth rate in the butt weld of SM490. And the fatigue crack growth parameters of the weld were evaluated in accordance with ASTM E647. From the comparative fatigue assessment results, it was found that the fatigue crack growth rate specified in the relevant design code was too conservative to estimate the residual fatigue life of welded structure. So, in order to get the more reliable results, it was recommended that the fatigue life estimation based on the fracture mechanics be performed with the fatigue crack growth parameter specified by test. Key words: Fatigue crack growth parameters, Fatigue crack growth analysis, BS7910

Low cycle fatigue life assessment of welded high-strength structural steels based on nominal and local design concepts

Fatigue life curves of butt joints, transverse stiffeners and sample components with tube-clevis connections, manufactured by active gas metal arc welding (GMAW) of S960QL, S960M and S1100QL high-strength steels, show a linear extension into the low cycle fatigue (LCF) regime with a constant slope up to the yield strength of the material. An extended S-N design curve based on fatigue class FAT 160 (k=5) gives conservative results for high quality butt welds below the ultimate tensile strength. In comparison to high frequency hammer peening and grinding, TIG dressing of S1100QL butt joints is most favorable for the increase of fatigue life.The effective notch stress approach with rref=1.00mm and measured radii of post treated welds leads to a reliable fatigue assessment up to at least N=5·103cycles to failure, which confirms FAT 225. The stress averaging approach using the microstructural support increases the assessment quality. A similar assessment reliability for damage parameter Wöhler lines, derived from elastic-plastic FE-analysis, is achieved, if support effects are considered.

Effects of weaving laser on scanning laser-MAG hybrid welding characteristics of high-strength steel

Scanning laser-MAG hybrid welding with weaving laser was developed for the bead-on-plate welding of high-strength steel. Weaving laser exerted little effect on the droplet transfer behaviour during the scanning laser-MAG hybrid welding process. In order to achieve the keyhole mode of laser welding, the weaving amplitude should be no more than 2 mm. The weld penetration decreased with the increasing weaving frequency. Compared to laser-MAG hybrid weld, the ratio of arc zone width to laser zone width decreased significantly. Scanning laser-MAG hybrid welding could suppress the porosity defects of weld efficiently. The optimal weaving frequency for the porosity defects suppression was 20 Hz. However, the porosity defects increased sharply with the weaving frequency of laser over 40 Hz. The impact-absorbed energy value of specimen fabricated by scanning laser-MAG hybrid welding was about 46 J, increased by about 31.4% compared to about 35 J of laser-MAG hybrid welding.

Twin-spot laser welding of advanced high-strength multiphase microstructure steel

The study addresses the results concerning the laser welding of TRIP (TRansformation Induced Plasticity) steel using a beam focused at two spots (also referred to as twin-spot laser welding). The analysis involved the effect of variable welding thermal cycles on the properties and microstructure of welded joints. The tests were performed using a linear energy of 0.048 and 0.060kJ/mm and the laser beam power distribution of 50%:50%, 60%:40% and 70%:30%. The tests also involved welding performed using a linear energy of 0.150kJ/mm and the laser beam power distribution of 70%:30%. In addition, the research included observations of the microstructure of the fusion zone, heat affected zone and the transition zone using light microscopy and scanning electron microscopy. The fusion zone was composed of blocky-lath martensite whereas the HAZ (heat-affected zone) was characterised by the lath microstructure containing martensite, bainite and retained austenite. The distribution of twin-spot laser beam power significantly affected the microstructure and hardness profiles of welded joints. The highest hardness (480–505HV), regardless of welding variants used, was observed in the HAZ.