9.4 Integrität von Schweißverbindungen
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This study examines the susceptibility of two filler materials (T46 6 M M21 1 H5 and G46 4 M21 4Si1, Ø 1.2 mm each) to solidification cracking, using an externally loaded hot cracking test. Although these filler materials are usually not critical in terms of hot cracking in engineering applications, cracking occurs when external loading is applied during welding. This was observed during welding under service conditions, e.g., repair welding of highway bridges under traffic loads. In order to characterize this behavior further, experiments were conducted on highly restrained fillet welds in an overlap configuration with 8 mm thick base material (S355J2 + N, 1.0570). The modified approach of the Transverse Motion Weldability (TMW) test is based on the transverse displacement of the lower plate relative to the welding direction at a controlled speed, corresponding to the induced deformation rate v in the mushy zone, initially set to 0.5 mm/s to initiate a crack and then linearly reduced to 0 mm/s. Gas metal arc welding was employed in this study, investigating energy per unit lengths of 0.8, 1.2, and 1.6 kJ/mm. The objective was to find and validate a critical deformation rate (vcr ), below which solidification cracking stops occurring. Therefore, a new force-based evaluation method is presented. While both filler materials exhibited low susceptibility to solidification cracking under standard welding conditions, an increase in cracking tendency was observed with higher energy per unit lengths. The externally loaded test provides a quantitative assessment of hot cracking susceptibility. The results enhance the understanding of solidification cracking mechanisms and provide valuable insights for welding applications under high restraint, including repair welding of loaded structures such as bridges.
In the near future, hydrogen will be transported from producers to consumers on the large scale exclusively by long-distance transmission pipelines, whereas the vast majority will be converted NG pipelines. Like in the NG grid, welding work on/onto pipelines as part of the “hot-tapping” is unavoidable e.g. for maintenance or in case of grid expansion. Meanwhile, the mere compatibility of low-alloyed pipeline steels under pressurized hydrogen was proven. But for repurposed NG pipelines (in use for decades), the transferability of in-service welding concepts is partially discussed. The reason is that NG does not penetrate the pipeline, whereas hydrogen can show the embrittling effects, which strongly interfere with possible defects in the legacy pipeline. The inner pipeline surface undergoes multiple short-term heating to high temperatures, which accelerates the hydrogen diffusion and increases the solubility. In particular for pipes with small wall thickness, the first welding passes can reach an austenitic transformation close temperature. Hence, the pipeline integrity vs. hydrogen embrittlement (HE) susceptibility is of vast interest, despite testing of the diverse material and thickness combinations is challenging. For the first time, welding experiments on pressurized DN300 pipeline-like demonstrators were conducted at approx. 85 bar hydrogen. For wall thicknesses between 5.6 and 6.3 mm this allowed realistic hoop stresses during welding of approx. 50 % of the individual SMYS. Using newly developed sample extraction routines, it was possible to quantify the hydrogen ingress in the material for both, the weld metal and the HAZ. It turned out that the existing surface oxides effectively limited the unavoidable hydrogen uptake during welding (compared to thermodynamic-based calculations). Hence, HE was unlikely to occur and confirmed by comprehensive NDT during and after welding. Accompanying numerical calculations suggest that the gas flow speed must be decreased during welding to limit the internal cooling effect of the gas on the pipeline weld.
Influence of Test Temperature and Test Frequency on Fatigue Life of Aluminum Alloy EN AW-2618A
(2026)
The aluminum alloy EN AW-2618A is a high-strength, precipitation-hardened material developed for applications at elevated temperatures. In service, components such as radial compressor wheels and pistons are exposed to thermal conditions that cause microstructural overaging, leading to mechanical softening. With future exhaust gas turbochargers expected to operate at temperatures up to 230°C and with increasing demands for efficiency and compression ratio, reliable data on fatigue behavior under these conditions is essential but currently lacking. This study aims to close this gap by conducting fully reversed (R = −1) high-cycle fatigue (HCF) tests at room temperature and at 230°C on specimens in an overaged condition (1000 h at 230°C), simulating the degradation that occurs in service due to the coarsening of the strengthening S-phase. Additionally, the influence of test frequency on fatigue performance is investigated.
LTT-Schweißzusätze (Low Transformation Temperature) mit niedriger Martensitstarttemperatur reduzieren Schweißeigenspannungen in der Schweißnaht sowie in der angrenzenden Wärmeeinflusszone (WEZ). Dies kann zu einer Erhöhung der Ermüdungsfestigkeit führen. Zur quantitativen Bewertung wurden Längssteifen aus hochfesten Baustahl als Versuchsproben verwendet. An den Stirnseiten wurden zusätzliche LTT-Schweißnähte aufgetragen. Zusätzlich wurden gehämmerte Proben (HFMI) als Vergleich herangezogen. Diese Proben wurden auf Eigenspannungen und Ermüdungsfestigkeit geprüft. In Abhängigkeit von der Nahtausführung und der XRD-bestimmten Eigenspannungen zeigte sich eine Steigerung der Ermüdungsfestigkeit von bis 140% im Vergleich zu Referenzproben mit herkömmlichen Schweißzusätzen. Außerdem stellte sich tendenziell eine lineare Abhängigkeit der Ermüdungsfestigkeit zu den Eigenspannungen heraus. LTT-Schweißzusätze sind somit grundsätzlich geeignet, um die Ermüdungsfestigkeit geschweißter Strukturen zu verbessern.
One approach to increase the service life of welded structures is the use of so-called Low Transformation Temperature (LTT) welding consumables. By reducing the martensite start temperature, the volume expansion from the austenite-martensite transformation is efficiently utilized to reduce residual stresses in weld and heat affected zone (HAZ). This article focuses on the use of LTT in transverse stiffeners. Hardness mapping results showed increased hardness in the LTT weld metal. The residual stress analysis in the HAZ using X-ray diffraction indicated a tendency that the LTT filler could reduce the residual stresses in fatigue test direction. Fatigue tests showed a 50% improvement at 2 million cycles when using LTT welding consumables.
Mikrolegierungselemente wie Niob (Nb) und Titan (Ti) spielen eine entscheidende Rolle bei der Einstellung der gewünschten mechanischen Eigenschaften vergüteter hochfester Feinkornbaustähle mit einer Nennstreckgrenze von ≥ 690 MPa. Aktuelle Spezifikationen der chemischen Zusammensetzung definieren für diese Elemente lediglich Obergrenzen und gewähren den Herstellern damit einen gewissen Spielraum. Bereits geringfügige Abweichungen in den Legierungskonzepten können jedoch die resultierenden mechanischen Eigenschaften erheblich beeinflussen. Infolgedessen wird die zuverlässige Vorhersage der Schweißeignung sowie der Integrität geschweißter Verbindungen aufgrund von Zusammensetzungsvariationen und den damit verbundenen mikrostrukturellen Veränderungen erschwert oder sogar unmöglich. Mögliche nachteilige Effekte umfassen eine Aufweichung der Wärmeeinflusszone (WEZ) oder umgekehrt lokale Aufhärtungsphänomene. Zur Bewältigung dieser Herausforderungen werden erstmals verschiedene Mikrolegierungsstrategien mit unterschiedlichen Ti- und Nb-Gehalten systematisch anhand speziell hergestellter, im Labor gegossener Legierungen untersucht. Jeder Legierungsansatz basiert auf dem häufig verwendeten Stahl S690QL, wobei eine konsistente chemische Zusammensetzung sowie identische Wärmebehandlungsparameter beibehalten werden.
Zur Bewertung der Schweißeignung wurden dreilagige Schweißverbindungen mittels Metall-Schutzgasschweißen durchgeführt und kritische mikrostrukturelle Bereiche – insbesondere solche innerhalb der Wärmeeinflusszone (WEZ) mit ausgeprägter Aufweichung oder Aufhärtung – identifiziert. Der Einfluss der aufgeweichten WEZ-Bereiche auf das Versagensverhalten wurde durch Querzugversuche untersucht. Zur In-situ-Analyse lokaler Dehnungsverteilungen in verschiedenen WEZ-Regionen wurde die digitale Bildkorrelation (DIC) eingesetzt. Darüber hinaus wurden Kerbschlagbiegeversuche (Charpy) an Grundwerkstoff, Schweißgut und WEZ durchgeführt, um die Kerbschlagzähigkeit zu bestimmen.
S690 steel is increasingly employed in modern steel construction, including building, plant, and mobile crane applications, due to their high strength and weldability. Submerged arc welding (SAW) is commonly used for these thick-walled structures but poses a risk of delayed hydrogen-assisted cold cracking (HACC). The influence of microstructure-dependent diffusion coefficients (DH) on hydrogen accumulation and distribution during welding and cooling remains poorly understood. In this study, the HACC susceptibility of SAW joints of thermomechanically rolled (MC) and quenched and tempered (Q) variants of S690 steel were compared, followed by electrochemical hydrogen permeation tests to determine the microstructure-specific DH. A numerical model was developed to analyze hydrogen diffusion as a function of temperature, time, and microstructure. Results showed that the MC-grade exhibited slightly faster hydrogen diffusion than the Q-grade. However, simulations indicated that higher welding heat input and increased plate thickness had a significantly greater impact on hydrogen retention than microstructure-dependent diffusion effects. These findings suggest that while differences in hydrogen diffusivity exist between S690MC and S690Q, microstructure-specific diffusion plays a minor role in HACC risk. Instead, proper control of welding parameters is crucial for mitigating HACC, particularly in thick-plate, multi-layer SAW joints.
Microalloying elements such as niobium (Nb) and titanium (Ti) play a crucial role in tailoring the mechanical properties of high-strength, quenched and tempered fine-grained structural steels with yield strengths ≥ 690 MPa. While current standards typically specify only upper limits for these elements, different microalloying strategies provide significant flexibility in alloy design. However, even minor variations in Nb and/or Ti content can substantially alter transformation kinetics, precipitation behavior, and tempering response, thereby significantly affecting weldability.
Against this background, the present study investigates the influence of various microalloying strategies (Nb-microalloyed, Ti-microalloyed, and combined Nb+Ti microalloyed concepts) on the development and interpretative value of welding Continuous Cooling Transformation (weld CCT) diagrams for high-strength structural steels, using an S690QL-type alloy concept as a model system. The objective is to assess the extent to which microalloying-induced differences in phase transformation behavior and precipitation dynamics necessitate alloy-specific adaptation of weld CCT diagrams.
Conventional weld CCT diagrams are typically established for single-pass weld simulations and represent the phase transformation behavior for a defined heat input or cooling time (e.g., Δt8/5-time). In practical applications, however, welded joints are predominantly produced using multi-pass welding procedures, resulting in repeated thermal cycling of the heat-affected zone (HAZ). These complex thermal histories significantly modify phase transformation sequences, tempering reactions, and precipitation phenomena compared to single-pass conditions. Consequently, the direct transferability of classical weld CCT diagrams to real multi-pass welding scenarios is limited and must be critically evaluated.
Pipeline steels are widely applicated for long-distance transmission pipelines. However, the welded joints of these steels can be susceptible to hydrogen-assisted cold cracking (HACC) during welding and after cooling especially in the heat-affected zone (HAZ). HACC in welds basically involves a critical combination of local, mutually dependent parameters consisting of: (1) a crack-critical microstructure; (2) sufficiently high mechanical stress; and (3) a diffusible hydrogen concentration (HD). In this context, thick-walled steel weld joints typically involve multi-layer welding with several passes. This leads to an effective, empirically known reduction in the global HD in the weld seam. This reduction is due to the repeated reheating of subsequently welded beads or layers, which reduces the local HD in the individual weld beads or layers. However, this has not yet been adequately quantified or described. Bead-on-plate tests, such as ISO 3690, cannot correctly reproduce the local HD distribution in individual welding passes (and thus, the global HD in the entire seam). Therefore, these tests lead to an extremely conservative evaluation of hydrogen ingress and the potential for "self-reduction" of HD due to increased interpass temperature during multi-layer welding, which has not yet been adequately addressed in the literature. Ideally, the local HD in each pass and the global (average) HD of the multi-layer weld would be known immediately after welding. For this reason, the study proposes an approach to address the local welding pass and layer-dependent, as well as global HD of multi-layer welds via a modified ISO 3690 test. To this end, representative welding parameter combinations for selected practical welding processes (e.g., SAW, GMAW, or SMAW) will be carried out as conventional bead-on-plate samples. These samples will be systematically extended by an increasing number of welding passes. Several sample series with an increasing number of beads or layers will be examined. Additionally, the initial HD is varied by methods such as targeted shielding gas variation (e.g., addition of hydrogen in GMAW) or moistened flux/coating (e.g., SAW/SMAW). Based on experimental data (temperature field measurements during welding and determination of temperature-dependent HD coefficients), a numerical hydrogen diffusion model is created. This model calculates the local HD (in the beads) and the HD across the weld cross-section. Additional variation calculations represent heat transfer conditions that were not recorded experimentally and their influence on HD distribution. Finally, we investigate the potential use of the modified ISO 3690 multi-layer geometry for application cases such as predicting the effectiveness and necessity of hydrogen removal heat treatment procedures for given welding parameter sets. Finally, an international round robin should be initiated once the concept has been successfully confirmed and verified.
Influence of Residual Stresses and Heat Control on Stress Relief Cracking in a CrMoV Steel Mockup
(2026)
Preventing failures in the manufacture or operation of petrochemical reactors made from creep-resistant, low-alloy steels such as 13CrMoV9-10 still requires research, despite over 60 years of investigation into the field of stress relief cracking. Welding this steel grade requires care. This is due to its low toughness and high strength in the as-welded condition if not heat treated after welding, combined with an increased susceptibility to cracking during stress relaxation.
Previous research on cracking has focused on metallurgical factors; little knowledge has been gained on cracking under real-life restraint conditions. In this work,the influence of welding heat control on cracking was investigated by simulating the manufacturing conditions prevailing in the construction of petrochemical reactors using a weld mockup. Cracks formed during post-weld heat treatment in a temperature range between 300 °C and 500 °C. Compared to small scale samples, the toughness of the restrained welds was significantly lower. SEM and TEM analyses of the samples revealed early ageing due to precipitation of carbides during post-weld heat treatment under restraint. An overview of how a comprehensive assessment of the integrity of submerged arc welded CrMoV steels can be achieved is provided.