Open Access

A large explosion occurred at an oil refinery after a desulfurization reactor cracked, releasing hot, pressurized gasoline. The resulting explosions and fire caused extensive damage and injured some employees. A 1.4 m crack adjacent to a weld seam at a reactor support bracket caused the release. The reactor shell exhibited distinctive circumferential bulging. The reactor was erected using mild steel. All material properties of the reactor shell complied with the regulations in effect at that time. Fractographic analysis of the main crack and smaller ones at the other support brackets revealed stepwise ductile fracture resulting from static loading. All well-known failure mechanisms for pressure vessel burst had proven wrong: overpressure, pressure cycles, overtemperature, creep, corrosion, external (cyclic) mechanical loads… Due to small spherical indentations on the inside of the reactor shell, a completely new failure mechanism was established and investigated: During service, the reactor was partially filled with ceramic ball grading and catalyst. Gasoline was processed at 150–250 °C and ∼ 24 bar. Each of the reactor’s ∼ 20 operating cycles ran for several months until the catalyst was spent, after which the reactor was cooled, depressurized, emptied, and refilled. Upon heating, the thermal expansion of the steel shell (∼3 times greater than ceramic) created gaps that were filled as ceramic balls settled. During operation, the catalyst degraded and agglomerated with the ceramic balls, forming a rigid mass. Upon cooling, the rigid mass resisted the vessel’s thermal contraction, inducing circumferential tensile stresses and plastic deformation, resulting in permanent bulging of the vessel. Charpy impact energy near the welds was significantly reduced, attributed to thermal and strain aging in the bulged region. Progressive embrittlement and increasing plastic strain led to crack initiation and incremental ductile crack propagation at the support bracket welds over successive cycles. The fracture features were reproduced in laboratory tests at 200 °C. FEA analysis confirmed the proposed mechanism. Two identical vessels showed similar damage but had not yet failed. To prevent future damage in pressure vessels, this new failure mechanism needs to be incorporated into design, operating and inspection codes for pressure vessels possibly/partially filled with solids.

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.

In the SimuCrown project, ageing mechanisms within the crown-cement-tooth complex (CCTC) are investigated, with a focus on high-resolution structural characterisation. Although dental crowns are widely used indirect restorations worldwide, links between clinical complications such as debonding, fractures and time-dependent cement degradation remain insufficiently understood. To address this, we employ high-resolution X-ray µCT and synchrotron X-ray refraction radiography (SXRR) to identify microstructural ageing processes which are inaccessible with conventional imaging. CCTCs subjected to controlled chewing simulation are examined using SXRR to detect structural alterations, interfacial defects, and the progression of damage within the cement layer and neighbouring materials. Our research explores crack initiation, pore evolution, and interfacial integrity using SXRR, capturing degradation phenomena across multiple time scales. By integrating synchrotron imaging with functional ageing simulation and computational modelling, the SimuCrown project offers a robust framework for evaluating the long-term performance of dental restorations. The outcomes contribute to the optimisation of materials and cementation approaches aimed at improving the clinical durability of dental restorations.

Unter Passivität versteht man den Korrosionsschutz durch dünne, festhaftende Schichten aus Korrosionsprodukten. Für den schadensfreien Einsatz nichtrostender Stähle ergeben sich daraus zwei grundsätzliche Anforderungen. Erstens muss sich die Passivschicht vollständig ausbilden können. Dabei sind die Oberflächenausführung, die mechanische Bearbeitung, die Elementverteilung, Ausscheidungen, die Wärmeeinbringung z.B. bei der schweißtechnischen Verarbeitung, Anlauffarben, sowie die chemische Nachbehandlung bzw. Konditionierung der Oberflächen von großer Bedeutung. Zweitens muss die Repassivierung von Defektstellen unter den Einsatzbedingungen möglich sein. Für das Repassivierungsvermögen sind die Legierungszusammensetzung, mögliche kritische Betriebszustände, prozessbedingte Besonderheiten, wie etwa Stagnationsbedingungen und Reinigungsprozesse von Bedeutung.