Cold Cracking Research Articles

Diffusible Hydrogen Concentration in Drawn Arc Stud Weldments

AbstractThe mechanized drawn arc stud welding is a well established process in steel construction for joining technique for pin shaped elements, especially in the field of composite constructions. Therefore, the process is commonly applied on construction sites under different climatic conditions. The process is characterized by its relatively high heat input due to high current and a weld pool protection that is realized by a ceramic ferrule holding the metal vapor as a shielding gas. However, turbulences inside the ferrule during welding cause contaminations with atmospheric humidity. Furthermore, a high cooling rate results in a strongly hardened heat‐affected zone and a considerable residual stress state in the parent material. Thus, there is a risk of hydrogen assisted cold cracking, especially for stud welding on high strength steels or under adverse welding conditions. The knowledge about the expectable hydrogen concentration is fundamental for evaluating the cold cracking susceptibility. In this study, the effect storage conditions of the consumables and atmospheric conditions are investigated and evaluated regarding their effect on the resulting hydrogen concentration in weld metal. For determination of diffusible hydrogen concentration in stud weld metal a method based on the carrier gas hot extraction is used.

Numerical Alloy Development for Additive Manufacturing towards Reduced Cracking Susceptibility

In this work, we investigated the viability of established hot cracking models for numerically based development of crack-resistant nickel-base superalloys with a high γ′ volume fraction for additive manufacturing. Four cracking models were implemented, and one alloy designed for reduced cracking susceptibility was deduced based on each cracking criterion. The criteria were modeled using CALPHAD-based Scheil calculations. The alloys were designed using a previously developed multi-criteria optimization tool. The commercial superalloy Mar-M247 was chosen as the reference material. The alloys were fabricated by arc melting, then remelted with laser and electron beam, and the cracking was assessed. After electron beam melting, solidification cracks were more prevalent than cold cracks, and vice versa. The alloys exhibited vastly different crack densities ranging from 0 to nearly 12 mm−1. DSC measurements showed good qualitative agreement with the calculated transition temperatures. It was found that the cracking mechanisms differed strongly depending on the process temperature. A correlation analysis of the measured crack densities and the modeled cracking susceptibilities showed no clear positive correlation for any crack model, indicating that none of these models alone is sufficient to describe the cracking behavior of the alloys. One experimental alloy showed an improved cracking resistance during electron beam melting, suggesting that further development of the optimization-based alloy design approach could lead to the discovery of new crack-resistant superalloys.

Laser-based powder bed fusion of non-weldable low-alloy steels

ABSTRACT This study focuses on the processability of four low-alloy steels (AISI 4130, 4140, 4340 and 8620) via laser-based powder bed fusion (LB-PBF). In the as-built condition, the alloys consisted of tempered martensite that was the result of an intrinsic heat treatment (IHT) during LB-PBF. In terms of defects, a distinct transition in porosity was observed that correlated to the volumetric energy density (VED). At low VED, specimens contained a lack of fusion porosity, while at high VED, they contained keyhole porosity. Additionally, cold cracking was observed in 4140 and 4340 specimens produced at low/intermediate VEDs. This cracking could be mitigated by increasing the VED or laser power, as both enhance the IHT. This enhanced IHT lowered the material hardness below specific thresholds (<500HV 4340 and <460 4140), increasing ductility and allowing the specimens to avoid cracking. From these findings, crack-free, high-density (>99.8%) low-alloy steel specimens were produced without the requirement of build plate preheating.

Corrosion resistance of welded joints made by underwater wet welding

One of the reasons of coming out of order of metal structures, operating in water environment, including sea conditions, is the corrosion. Thus, for example, the corrosion wear of metal of the ship hull underwater part can reach from 0.3 up to 0.5 mm/year. The corrosion of welds is the ever more critical situation. The rate of their fracture exceeds the rate of the base metal corrosion and in some cases it may reach 1-3 mm/year [1, 2]. The great selective corrosion in the form of fissures along the welds (on both sides) was observed in heat-affected zone (HAZ) metal of slot, butt welds and welds for welding-on to the basic workpiece, i.e. up to 1 mm/year, in some cases a through corrosion fracture of the fusion line (formation of blowholes) was observed [1]. In the opinion of the work authors [2] the welds are subjected to fracture due to the occurrence of thermal electromotive force between the parts welded under conditions of high electric conductivity of the sea water (Seebeck effect).&#x0D; To repair the corroded welded joints of metal structures, operated under the water, a wet underwater welding is used. At the E.O.Paton Electric Welding Institute the specialized flux-cored wires have been developed, which are designed for welding low-carbon and low-alloyed steels, including those of a higher strength. In the latter case the electrode materials of an austenite type are used to provide resistance against cold crack formation in the HAZ [3]. The work was aimed at study of corrosion resistance of welded joints with ferrite and austenite deposited metal, made by the underwater wet welding, under the conditions, which simulate the service conditions in sea water.

Study of cracking susceptibility in similar and dissimilar welds between carbon steel and austenitic stainless steel through finger test and FE numerical model

Hot cracking susceptibility and the formation of brittle martensite phase are the main factors that limit the weldability of a dissimilar joint between carbon steel (CS) and austenitic stainless steel (SS). In this study, the self-constraint finger test was used to correlate the welding thermo-mechanical field with the crack susceptibility of a dissimilar weld between the CS ASTM A36 and SS AISI 304L. The finger test allowed to intercalate fingers (portions) of tested materials in the weld samples to produce dissimilar welds. The heat dissipation and the distortion behavior were related to the crack susceptibility, critical weld regions extension, and chemical species diffusion. Four samples were welded (two similar welds and two dissimilar welds) using the filler metals ER70S-6 and EC410NiMo. Welds were analyzed through light optical microscopy (LOM) and scanning electron microscopy (SEM) to characterize phases, detect cracks, microstructural changes, and element diffusion. A finite element (FE) numerical model was applied to simulate the welding thermo-mechanical field. FE estimations of distortion and residual stress helped to predict induced crack propagation (the initial gap between fingers) towards the fusion zone. Additionally, electrochemical tests were carried out to assess the corrosion susceptibility of the dissimilar welds. The observed cracks were produced due to different factors such as residual stress distribution, the formation of brittle and untempered martensitic phase in the fusion zone (FZ), and hot cracking associated with the weld sample distortion behavior. According to the FE estimations, the high thermal expansion of the SS was responsible for the bending curvature change in welds 2 and 4, which produced a gap between fingers and increased the crack extension in the FZ of weld 4. The dilution contributed to the formation of δ-ferrite in the FZ, which limited the growth of cold and hot cracks. The decarburization and sensitization were not observed in dissimilar welds due to the low element diffusion.

Effect of the Root Gap on the Structure and Properties of High Strength Steel S700MC Welds

Nowadays, the use of high strength (HS) and ultra-high strength steels (UHS) increases, notably in welded constructions. These steels are mainly exploited in heavy loaded welded constructions such as bridges, cranes and excavators, in pressure vessels, vehicles, ships, drilling rigs etc. working at room or lower temperatures. As the welded constructions have specific requirements, the development of high strength and ultra-high strength steels imposes the need for research on the factors influencing their weldability. Among the possible negative implications are: cold cracks formation, softening of the heat affected zone, brittleness in the coarse grained zone. When complying with the generally accepted rules for welding, HS and UHS are readily welded by all conventional welding methods. Recommendations for welding of steels after normalization, thermo-mechanical treatment and quenching and tempering are given in the standard EN 1011 -1, 2. The use of thermo-mechanically treated (hot-rolled) steels with low carbon equivalent, such as S700MC, allows reduction in time required for welding as the preheating temperature is lowered or even preheating is not necessary. A more pronounced negative effect on the weld quality has the presence of different defects. S700MC can be welded by all conventional methods, and a reduction in the softened zone can be achieved by using appropriate welding parameters. Joint preparation for welding of HS and UHS steels is described in the standards EN ISO 9692-1:2013 and EN ISO 9692-2:2001. Nevertheless, the root gap is often the closing part in constructions and does not comply with the standard recommendations. That is why the effect of the root gap on welds has to be researched. The present work introduces results of a research studying the effect of the root gap on the structure and some mechanical and technological properties of S700MS welds, welded by submerged arc welding.

Effect of the Welding Thermal Cycles Based on Simulated Heat Affected Zone of S1300 Ultrahigh Strength Steel

In the automotive industry there is an increasing demand for the wider application of high strength steels due to their favourable mechanical properties. The steel producers continuously developing new generations of high strength steels to insure higher strength and toughness properties. Since in most cases these steels are joined in welded structures, great attention must be taken to their weldability. The weldability of high strength steels has still challenges which are as follows: cold cracking sensitivity; reduction of strength and toughness of heat affected zone (HAZ); filler metal selection. Because the mechanical properties of ultrahigh strength steels are provided by using various alloying elements, micro alloys, and by different metallurgical methods, the steels may lose their outstanding properties during welding. In real welded joints the critical parts of the HAZ have small extent so their properties can be limitedly analysed by conventional material testing methods. With the help of physical simulators, the different parts of the heat affected zone can be produced in an adequate size for subsequent tests. In our research work the weldability, especially the HAZ properties of an ultrahigh strength structural steel (Rp0.2 = 1300 MPa) were investigated on thermal simulated samples with the help of Gleeble 3500 physical simulator. Three relevant technological variants for gas metal arc welding (GMAW), t8/5 = 5 s, 15 s and 30 s were applied during the HAZ simulations in the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones. Both the microstructure was studied by optical microscope and the mechanical properties were analysed by Vickers hardness tests and Charpy V-notch impact tests at -40 °C. According to the results the investigated ultrahigh strength steel was softened on account of the welding heat cycles, besides that the strength of the investigated ultrahigh strength steel can be better with the application of shorter t8/5 cooling time.

Influence of Different Alloying Strategies on the Mechanical Behavior of Tool Steel Produced by Laser-Powder Bed Fusion.

Additive manufacturing is a high-potential technique that allows the production of components with almost no limitation in complexity. However, one of the main factors that still limits the laser-based additive manufacturing is a lack of processable alloys such as carbon martensitic hardenable tool steels, which are rarely investigated due to their susceptibility to cold cracking. Therefore, this study aimed to expand the variety of steels for laser powder bed fusion (L-PBF) by investigating an alternative alloying strategy for hot work tool steel powder. In this study, a comprehensive investigation was performed on the powder and L-PBF processed specimen properties and their correlation with the existing defects. Cubical specimens were created using the following two alloying strategies by means of L-PBF: conventional pre-alloyed gas-atomized powder and a mixture of gas-atomized powder with mechanically crushed pure elements and ferroalloys. The influence of the particle parameters such as morphology were correlated to the defect density and resulting quasi-static mechanical properties. Micromechanical behavior and damage evolution of the processed specimens were investigated using in situ computed tomography. It was shown that the properties of the L-PBF processed specimens obtained from the powder mixture performs equal or better compared to the specimens produced from conventional powder.

Semi-automatic welding in the environment of protective gases of welded structures from hardening steels with regulation of thermal cycles

In order to increase the technological and operational strength of welded joints of type X12CrMo5 steels in the process of welding in a protective gas environment, it is possible to use vibration deformation treatment of a welded joint with a pearlite joint to form a metal structure of a welded joint with minimal sensitivity to the formation of cold cracks due to an increase in the rate of heating and cooling, reduction of residence duration and provision of conditions of high-temperature decay of austenitic metal structures in seam and near-seam zones above critical temperatures. The work carried out research on the process of semi-automatic welding in the medium of protective gases in the mixture of carbon dioxide and argon with pearlite and austenitic welding materials of welded structures from hardening steels of grades 12Kh2M1 and X12CrMo5 with regulation of thermal cycles (RTC) due to the use of vibration deformation treatment and accompanying forced cooling.

Revealing the microstructure and mechanical attributes of pre-heated conditions for gas tungsten arc welded AISI 1045 steel joints

Gas tungsten arc welding (GTAW) is considered a well-established process in the manufacturing industry. Despite, certain challenges associated with high hardness of heat affected zone and cold cracking susceptibility of joints, are the main barriers for this process to be implemented successfully within high integrity structure. By using a combined procedure of experiments and modelling (response surface methodology (RSM) and multi-objective optimization: multi-objective genetic algorithm (MOGA)) allows obtaining good enhancement over uniform heating, cooling and the heat-affected zone which enable major progress in obtaining high quality welded parts. Therefore, this research study combines the experiments and modelling in a systematic manner considering for the first type the pre-heated treatment and without- pre-heating conditions of GTAW manufacturing. It leads to optimizing the process parameters of GTAW when manufacturing AISI 1045 medium carbon steel. The effects of critical parameters i.e. welding current: WC, welding speed: WS, and gas flow rate: GFR on the mechanical properties (ultimate tensile strength (UTS) and hardness) were investigated and evaluated against the microstructure of weld fracture. The multi-objective genetic algorithm corroborated with experimental observation enables to obtain a maximum UTS of approx. 625 MPa and hardness of 80.19 HRB for preheat condition. The results highlight an improvement in UTS of 0.2%–6.7% and a decrease in hardness of 0.1%–21.5% by implementing the preheating condition.

Effect of Mg Addition on the Microstructure and Properties of a Heat-Affected Zone in Submerged Arc Welding of an Al-Killed Low Carbon Steel.

To reveal the effect of Mg treatment on the microstructure evolution behavior in the actual steel welding process, the microstructure and properties of Al-deoxidized high-strength ship plate steel with Mg addition were analyzed after double-side submerged arc welding. It was found that the Al–Mg–O + MnS inclusion formed under 26 ppm Mg treatment could promote acicular ferrite (AF) nucleation in the coarse-grained heat-affected zone (CGHAZ) and inhibit the formation of widmanstätten ferrite and coarse grain boundary ferrite. In the fine-grained heat-affected zone (FGHAZ) and intercritical heat-affected zone (ICHAZ), polygonal ferrite and pearlite were dominant. Al–Mg–O+MnS cannot play a role in inducing AF, but the grain size of ferrite was refined by Mg addition. The impact toughness in HAZ of the Mg-added steel was higher than that of Mg-free steel. With the heat-input rising from 29.55 to 44.11 kJ/cm, it remained relatively stable in Mg-treated steel. From the fusion line to the base metal, the micro-hardness of the fusion zone, CGHAZ, ICHAZ and FGHAZ decreased to some extent after Mg addition, which means the cold cracking tendency in the welding weak zone could be reduced. Finally, the mechanisms of Mg-containing inclusion-induced AF were also systematically discussed.

Process-related influences and correlations in wire arc additive manufacturing of high-strength steels

High-strength fine-grained structural steels have great potential for weight-optimized, efficient structures in many modern steel applications. Further advances in efficiency can be achieved through additive manufacturing and bionic design. Commercial high-strength filler materials for wire arc additive manufacturing (WAAM) are already provided by the consumable producers. Today, application would be strictly limited due to absence of quantitative findings or any guidelines for the industry regarding welding-related stresses and component safety during manufacturing and service. Hence, process- and material-related influences and design-related restraint conditions associated with formation of residual stresses and cold cracking risk are investigated. The aim is the accessibility of special WAAM self-restraining cold cracking tests and easy applicable processing recommendations, enabling an economical, fit-for-purpose and crack-safe WAAM of high-strength steels. This first study focuses on determination of interactions between WAAM process parameters, resulting layer geometry, microstructure and residual stresses, analyzed via X-ray diffraction. Defined reference specimens are automated welded using a special WAAM solid wire (yield strength >820 MPa). Geometric properties can be specifically adjusted by wire feed and welding speed, but cannot be varied arbitrarily, since a high heat input causes local overheating, inadmissible changes of microstructure and mechanical properties, defects and comparable high tensile residual stresses.

A Study of the Effects of Amorphous/Crystalline Oxide Films on the Cold Spray Behavior of 6061 Al Alloy Powders

This paper presents the effects of nanoscale surface oxide layers on the cold spray behavior of 6061 Al alloy powders. The surface oxide films are revealed via Transmission Electron Microscopy (TEM), and Energy Dispersive X-ray Spectroscopy (EDS). Phenomena associated with surface contacts, contact-induced elastic-plastic deformation, heating, and cracking are then simulated using a combination of analytical models, finite element analysis and Molecular Dynamics (MD) simulations. MD simulations are used to provide insights into the effects of powder impact on deformation and fracture at powder impact velocities that are consistent with previously reported critical velocities and predictions. MD simulations are also used to obtain estimates of oxide film moduli, toughness, strains to failure, and the fracture energies of aluminum oxide films with crystalline and amorphous structures. These are incorporated into finite element simulations of cold spray contact-induced deformation and cracking. The impact between the powder particles with nanoscale oxide layer and the substrate is modeled using a bi-linear Johnson-Cook model. The powder impacts are shown to result in localized splat deformation and heating, and the cracking of the oxide layers in ways that can expose fresh metallic surfaces to high temperature contacts (above the recrystallization temperature) that can give rise to bonding and mechanical interlocking. The implications of the results are discussed for the design of cold spray processes for the fabrication and repair of 6061 Al structures.

Weldability of Underwater Wet-Welded HSLA Steel: Effects of Electrode Hydrophobic Coatings.

The paper presents the effects of waterproof coatings use to cover electrodes on the weldability of high-strength, low-alloy (HSLA) steel in water. With the aim of improving the weldability of S460N HSLA steel in water, modifications of welding filler material were chosen. The surfaces of electrodes were covered by different hydrophobic substances. The aim of the controlled thermal severity (CTS) test was to check the influence of these substances on the HSLA steel weldability in the wet welding conditions. The visual test, metallographic tests, and hardness Vickers HV10 measurements were performed during investigations. The results proved that hydrophobic coatings can reduce the hardness of welded joints in the heat-affected zone by 40–50 HV10. Additionally, the number of cold cracks can be significantly reduced by application of waterproof coatings on the filler material. The obtained results showed that electrode hydrophobic coatings can be used to improve the weldability of HSLA steel in underwater conditions.

Analysis and treatment of cracks in LPG spherical tank

During the first periodic inspection of the LPG spherical tank, a crack was inspected when the inner shell was detected by fluorescent magnetic powder test. The cause of the crack was analyzed by macromorphology examination, chemical composition analysis, hardness test and metallographic examination. Analysis results show that unreasonable welding procedure, no preheating process, improper storage of weld rods result in martensite structure of the welded joint, welding residual stress and diffusible hydrogen, which is the typical characteristic of the welding cold crack. The repair measures are taken to solve the problem and reduce the hidden dangers.

Development and evaluation of welding repair for threaded hole of Reactor Pressure Vessel flange

Temper bead welding repair method was proposed to repair the damage of stud holes with threads in the flanges of the Reactor Pressure Vessel. The particular welding procedure was developed to avoid cold crack. And the filler material matched with the base metal was developed to achieve the basic engineering mechanical characteristics. Furthermore, fatigue properties and welding residual stresses were evaluated. Based on the design curve defined in the design code, a kind of evaluation method was established, and used for assessing the fatigue strength of the weld metal. The welding residual stresses induced by temper bead welding were estimated from engineering view, based on analyses of multiple perspectives. In addition of the mechanical performance, a series of tests were designed and conducted to evaluate the application performance.

Effect of vibration treatment during welding on technological strength of chromium martensitic steel P91

The paper shows the possibility of using the technology of vibration treatment in the process of welding of high-temperature chromium steel of martensitic class P91 applied to the product to increase the technological strength of welded joints and the effect of this technology on mechanical properties before and after thermal treatment. Application of vibration action on welding bath and weld seam in process of manufacturing of welded structure may allow to reduce inhomogeneity of weld metal structure in liquid phase, as well as increase its microchemical inhomogeneity within crystallites during crystallization, reduce grain size due to more efficient heat removal and increase of number of crystallization centers. On the example of manufacturing the turbine support housing of the gas turbine drive, it is shown that it is necessary to replace part of the heat treatment operations with other more energy - efficient and less labor-intensive operations. Results of studies of influence of vibration treatment of welded joint on tendency of welded joint metal to form cold cracks, strength, hardness and impact toughness are given. Based on the obtained results, diagrams of average values of parameters are constructed and conclusions are drawn on the possibility of using the technology of vibration treatment of the turbine casing in the process of welding in order to reduce the number of heat treatment operations during its manufacture.

Waiting time before NDT of welded offshore steel grades under consideration of delayed hydrogen-assisted cracking

Offshore wind turbines (OWT) are a major goal of the energy strategy of Germany encompassing the increase of the installed wind power. OWT components are manufactured from welded steel plates with thicknesses up to 200 mm. The underlying standards and technical recommendations for construction of OWTs encompass specifications of so-called minimum waiting time (MWT) before non-destructive testing of the weld joints is allowed. Reason is the increased risk of time-delayed hydrogen-assisted cold cracking as hydrogen diffusion is very slow due to the very thick plates. The strict consideration of those long MWT up to 48 h during the construction of OWTs leads to significant financial burden (like disproportionately high costs for installer ships as well as storage problems (onshore)). In this study, weld joints made of S355 ML were examined in comparison with the offshore steel grade S460 G2+M. The aim was to optimize, i.e., reduce, the MWT before NDT considering varied heat input, hydrogen concentration and using self-restraint weld tests. This would significantly reduce the manufacturing time and costs of OWT construction. To quantify the necessary delay time until hydrogen-assisted cold cracks appear, acoustic emission analysis was applied directly after welding for at least 48 h.

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.

Cold cracking, microstructure, notch toughness, transverse and longitudinal hardness to the weld joint of P460NL1 steel [Fisuracion en frio, microestructura, tenacidad a la entalla, dureza transversal y longitudinal a la union de soldadura del acero P460NL1

Microstructural development, longitudinal HV10 hardness as transversal to the fusion zone and notch toughness in root and filling pass were evaluated at preheating temperatures of 100 °C, 140 °C, 180 °C, 220 °C and 260 °C in the weld joint of P460NL1 steel, by FCAW in root pass and for filling-finishing by means of the SAW process, the control of the start and interpass preheating temperatures was carried out by thermal blankets and digital pyrometer. The NDT of magnetic particles justify the acceptance or rejection of the specimens for the study. In selected points of the weld joint they showed the dependence of the hardness as a function of the microstructure. At room temperature, immediate cold cracking was observed. In the HAZ, the microconstituents generated areas of high hardness under the cord and areas of low hardness due to overturning, with the presence of Windmastatten ferrite, the increase in preheating determined the decrease and sometimes the absence of hardened structures. The profiles determined a decrease in hardness as the preheating temperature increased. Columnar grains with a high presence of WF ferrite in their different morphologies and FP (C) and refined areas with typical microconstituents were observed in the Fusion Zone. Homogeneity was evidenced with respect to notch toughness at all temperatures and the effect of passes on the longitudinal microstructure, which shows the benefit of the preheating temperature by reducing the hardness in critical areas and homogenizing the welded joint.