Open Access

Der Einsatz hochfester Stähle wie S690 erlaubt durch geringeren Materialeinsatz nicht nur eine immer wichtiger werdende Verringerung von CO₂-Emissionen, sondern auch eine effektive Kosten- und Gewichtsreduktion dickwandiger Bauteile. Insbesondere bei Wandstärken von bis zu 200 mm ist das Unterpulver- (UP-)Mehrdrahtschweißen aufgrund seiner hohen Effizienz eine gängige Praxis. Allerdings steigt bei hochfesten Stählen, hier vorliegend S690, die Gefahr wasserstoffunterstützter Kaltrisse (HAC), aufgrund ihrer Mikrolegierungskonzepte im Zusammenspiel mit hohen Eigenspannungen aus dem Schweißprozess und resultierend aus hohen Bauteilsteifigkeiten. Zusätzlich kann die erhebliche Aufmischung von Grund- und Zusatzwerkstoff beim UP-Schweißen zu risskritischen Gefügen führen, in denen der diffusible Wasserstoff besonders schädlich wirkt. Für Gefüge UP-geschweißter Bauteile liegen keine gesicherten Daten bezüglich Wasserstoffdiffusionskoeffizienten bzw. HAC-Rissanfälligkeit vor. Insbesondere die mikrostrukturabhängige Diffusion von durch den UP-Schweißprozess eingebrachtem Wasserstoff war nicht hinreichend gesichert. Ziel des Forschungsvorhabens war es daher, einen Beitrag zur kaltrisssicheren UP-Schweißverarbeitung hochfester Dickbleche zu leisten. Hierzu wurden systematisch unterschiedliche GW (S690 TM/QL) untersucht, die sich insbesondere in ihren Mikrostrukturen unterscheiden. Diese zeigten in Voruntersuchungen stark divergente Härteverteilungen im Besonderen in der letzten Lage der Schweißung, sodass ein ebenfalls stark divergentes Diffusionsverhalten postuliert wurde. Zunächst wurde der Wasserstoffeintrag über die Draht-Pulver-Kombination gemäß ISO 3690 ermittelt. Anschließend erfolgten mehrlagige Schweißungen sowohl unter freiem Schrumpfen als auch unter äußerer Zwängung. Eine detaillierte Gefügecharakterisierung und mechanisch-technologische Prüfungen, sowie Eigenspannungsmessungen ermöglichten die Bewertung der Rissanfälligkeit bei variierter Wärmeführung (schweißgeschwindigkeitsgesteuert). Zur quantitativen Beschreibung der Wasserstoffdiffusion wurden das Schweißgut (SG), die Wärmeeinflusszone (WEZ) und die Grundwerkstoffe (GW) mittels elektrochemischer Beladung und Trägergasheißextraktion (TGHE), sowie Permeationsversuchen untersucht. Basierend auf den ermittelten Diffusionskoeffizienten wurden numerische Modelle erstellt, um den Einfluss verschiedener Diffusionskoeffizienten auf die Wasserstoffverteilung in der Schweißnaht zu evaluieren. Entgegen dem Postulat wurden keine signifikanten Unterschiede in der Wasserstoff-Diffusionsgeschwindigkeit gemessen. Beide GW-Klassen (QL vs. TM) als auch das SG und die WEZ wiesen für diesen Werkstofftyp charakteristische Diffusionskoeffizienten mit nur geringen Unterschieden auf. Dies zusammen mit den nur sehr geringen Unterschieden in der Ausprägung der Eigenspannungen und mechanisch-technologischen Eigenschaften der Nähte, weisen auf eine hohe Kaltrisssicherheit hin. Die in allen Untersuchungen geringen Unterschiede zwischen QL und TM sprechen, hinsichtlich des HAC-Risikos aufgrund einer differenten Wasserstoffdiffusion, für die Austauschbarkeit der beiden Werkstoffe in der Produktion.

Traditional fatigue design is based on S-N curves, providing a robust and well-established tool for structural design. However, the applicability of this approach to remaining life assessment is limited, as the presence and growth of fatigue cracks cannot be explicitly considered. In inspection-based assessments, cracks below the detection limit must often be assumed to exist in the structure, requiring methods that allow the prediction of crack propagation and thus residual fatigue life. In contrast, approaches based on linear elastic fracture mechanics provide a suitable framework for such assessments, although the practical implementation depends strongly on the chosen modeling strategy and underlying assumptions. In this contribution, three fracture-mechanics-based fatigue assessment approaches of increasing modeling complexity are compared using 10 mm thick butt-welded dogbone specimens manufactured from structural steel S355NL. High-resolution laser scanning is used to capture the as-built weld geometry, providing the geometric input for all approaches considered. The methods differ primarily in how this geometric information is incorporated into the fatigue assessment. The first approach is based on a fracture mechanics-based assessment following BS 7910, in which the weld geometry is represented by single or averaged geometric parameters derived from the scan data. The second approach employs the IBESS methodology, which evaluates discrete local weld geometry measurements along the weld seam, typically at millimeter-scale spacing, and describes fatigue damage using a phenomenological crack initiation and growth concept. The third approach introduces a newly developed three-dimensional numerical framework in which the complete, continuous weld geometry obtained from reverse engineering is directly integrated into linear elastic crack propagation simulations. All approaches aim at specimen-specific fatigue life prediction but differ in their treatment of weld geometry, crack modeling concepts, and computational effort. Experimental fatigue tests with crack monitoring based on strain measurements and beach marks are used to validate the obtained theoretical results and support the comparison. The results illustrate the influence of modeling assumptions and geometric representation on fatigue life predictions and highlight the advantages and limitations of the different fracture-mechanics-based assessment approaches.

The growing application of high-strength steels in lightweight structures requires a detailed understanding of weldability and microstructural evolution in the heat-affected zone (HAZ). This study investigates the influence of niobium (Nb) and titanium (Ti) microalloying strategies on phase transformations, HAZ softening, and mechanical performance of S690QL steels. Welded joints were produced using a three-layer GMAW process and analysed by hardness mapping, metallography, dilatometry, Weld-CCT diagram development, EPMA, and mechanical testing. The results revealed significant differences in HAZ behaviour depending on the microalloying concept. Nb-microalloyed steel showed superior tempering resistance and reduced softening, whereas Ti-microalloyed steel exhibited more pronounced hardness loss in the intercritical and subcritical HAZ. However, thermally stable Ti precipitates provided an effective pinning effect, limiting austenite grain growth and reducing the width of the coarse-grained HAZ. Physical simulations identified maximum softening at peak temperatures of approximately 750 °C, particularly for the Ti-containing grade. EPMA analyses demonstrated a local enrichment of Mo in the HAZ of Nb-microalloyed steel. The interaction between Nb and Mo promotes secondary carbide formation and delays tempering reactions, contributing to improved hardness retention. Weld-CCT diagrams further revealed distinct transformation kinetics for the investigated steels. While both Nb and Ti increased the martensite start temperature compared with the reference grade, the Ti-microalloyed steel showed particularly stable transformation behaviour over a wide range of cooling conditions. Overall, the results indicate that HAZ softening is governed primarily by precipitation behaviour, tempering resistance, and interactions between microalloying and alloying elements rather than by heat input or grain coarsening alone. The findings provide valuable insight for the optimisation of welding procedures and the development of advanced Weld-CCT diagrams for modern high-strength steels.

Pipelines require maintenance and expansion of the grid, which involves the well-known "hot tapping" and "plugging" processes of welding onto pipelines while they are in service. Compared to natural gas, the challenges can be broadly divided into the possible austenitization of inner pipe materials exposed to hydrogen and the welding process itself. Both result in a significant increase in hydrogen solubility and could pose challenges in terms of HE. Emphasis is placed on the word "could" because knowledge of "hot tapping" on hydrogen pipelines is scarce due to a lack of service experience. This article therefore provides an overview of research conducted over the past three years as part of the DVGW/BAM 'H2SuD' project. As part of this project, a representative portfolio of existing and new pipeline materials and their welded joints was investigated. Circumferential pipe girth welds, for example, are of central importance for “hot tapping”. To this end, extensive welding tests were conducted, with the respective temperature fields being measured during welding. These measurements were then used to evaluate the inner pipe wall temperature. Tests on hydrogen-pressurized demonstrators demonstrated the safe feasibility of in-service welding on pressurized hydrogen pipelines. These results are being incorporated into applicable regulations. Numerical modelling was carried out to simulate hydrogen diffusion and distribution during welding and after cooling. It was shown that the inner surface pipeline temperature can be effectively reduced by a certain gas flow during in-service welding, which limits the ingress of hydrogen into the welded joint.

Pipeline steels are widely applicated for long-distance transmission pipelines. However, 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. Several sample series with an increasing number of beads or layers will be examined. Based on experimental data (temperature field measurements during welding and determination of temperature-dependent HD coefficients), a numerical hydrogen diffusion model is created.