Open Access

Offshore wind turbines are an important goal in national energy strategies worldwide. Foundation structures are manufactured from submerged arc welded (SAW) plates with thicknesses up to 200 mm. In that connection, high-strength steels like the S420G2+M are more and more applied offering the possibility for increased stability and load-bearing capacity of the foundations. These offshore steel grades can show a susceptibility for delayed hydrogen assisted cold cracking of the weld joints. For that purpose, a minimum waiting time (MWT) of up to 48 h (dependent on applied standards) is recommended before non-destructive testing is allowed and conducted. But this concept is based on older steel grades that have been used for three or more decades. Nowadays, the metallurgical improvements (clean steels, proper rolling, and heat treatment) of base materials and well as welding consumables must be anticipated. Hence, the MWT concept should be critically discussed as it is assumed to be very conservative. For that reason, the focus of this study was to investigate the diffusion behavior in S420G2+M steel and its multi-layer SAW joint. Electrochemical permeation experiments were carried at room temperature. Boundary conditions were anticipated in terms of using different sample thicknesses. From the experimental data, hydrogen diffusion coefficients and absorbed diffusible hydrogen concentrations had been calculated. It was shown that hydrogen diffusion in the base material is increased compared to the weld metal. In addition, the sample thickness had a significant on the calculated diffusion coefficients. The minimum and maximum diffusion coefficients had been used for numerical modelling of the hydrogen diffusion in the welding joint. It became clear that a MWT must be always regarded together with a critical initial diffusible hydrogen concentration for the evaluation of a possible delayed cracking as diffusion times were mostly > 48 h due to the thick plates.

Hydrogen gas plays a key role in the European energy transition strategy. When transmitting and storing compressed hydrogen gas, safety is one of the most important conditions. With increasing hydrogen pressure and temperature, more hydrogen is absorbed by the steel components, such as pipelines or valves, and may lead to embrittlement. Although, a deep understanding of microstructure on the hydrogen solubility in steels is missing. Classical Sieverts’ law is only valid at high temperatures and low gas pressures. For that purpose, new theory is presented, which explains the role of microstructure on hydrogen solubility. Hydrogen trapping at microstructural defects is a thermally activated mechanism and causes an increase of the hydrogen solubility with decreasing temperatures. This mechanism has to be considered in cryogenic applications, such liquid or compressed hydrogen storage.

Die stetig steigende Nachfrage nach erneuerbaren Energieträgern führt dazu, dass große Mengen an Wasserstoff transportiert werden müssen. Da Pipelines eine kostengünstige Möglichkeit zur Verteilung von gasförmigem Wasserstoff bieten, muss die Wechselwirkung zwischen Wasserstoff und den Rohrleitungsmaterialien sorgfältig untersucht werden, da Wasserstoff unter bestimmten Bedingungen eine Verschlechterung der mechanischen Eigenschaften bewirken kann. Insbesondere Schweißnähte, von denen man annimmt, dass sie anfälliger für die Degradation durch Wasserstoff sind, sind von großem Interesse. Ziel dieser Studie ist es, die Auswirkungen von gasförmigem Wasserstoff auf die mechanischen Eigenschaften einer X65-Rohrleitung und der längs verlaufenden Unterpulverschweißnaht zu untersuchen. Die Prüfungen werden mit Hilfe der Hohlkörpertechnik an zwei Arten von Proben durchgeführt: eine aus dem Grundwerkstoff (GW) und die andere als Querzugprobe (QZ), bestehend aus Grundwerkstoff und Schweißnaht, entnommen. Die Proben werden in-situ mit einem Druck von 60 bar beaufschlagt und in Zugversuchen mit langsamer Dehnrate (SSR) und einer nominellen Dehnrate von 10-5 s-1 geprüft. Die durchgeführten Versuche zeigen eine Verringerung der Einschnürung (RA) von 72 % in inerter Atmosphäre auf 52 % in Wasserstoffatmosphäre für die QZ-Probe und einen Rückgang von 73 % in inerter Atmosphäre auf 51 % für die GW-Probe. Metallographische Analysen zeigten die Rissbildung zwischen der feinkörnigen Wärmeeinflusszone (FGHAZ) und des GW sowohl für die in Wasserstoffatmosphäre geprüften Proben als auch für die Vergleichsproben. Dies lässt den Schluss zu, dass sich der Ort der Rissentstehung durch die Anwesenheit von gasförmigem Wasserstoff nicht ändert.

Hydrogen has great potential into the decarbonization process of the energy and transport sectors, thus helping to mitigate the urgent issue of global warming. It can be sustainably produced through water electrolysis with potentially zero emissions, and efficiently used in fuel cell systems. Despite its environmental advantages, hydrogen is an extremely flammable substance and its interaction with most metallic materials could result in their mechanical properties degradation to an extent that could make them inherently unsafe. Extensive material testing under realistic operating conditions is required to determine the criteria under which hydrogen-induced damage is to be expected. In-situ slow strain rate tensile (SSRT) test is an option that allow the quantification of the behavior of metals in hydrogenated environments. The standardized procedure for testing in-situ the pressurized gaseous hydrogen effect on metals consists of the utilization of an autoclave as a containment volume. Testing inside an autoclave is difficult, expensive, and time-consuming, and requires specialized equipment and trained personnel. A relatively recent method to circumvent these issues and provide affordable and reliable test results consists in using hollow specimens as the gas containment volume, thus applying the hydrogen pressure inside rather than outside the specimen. This experimental setup allows us to minimize the volume of hydrogen and perform the tests safely and effectively. This study focuses on the evaluation of tensile properties of X65 pipeline steel, which was in vervice for natural gas transport, tested in a high-pressure hydrogen environment using hollow specimens. A constant nominal strain rate of 1ꞏ10-6 s-1 is applied. Tests are performed with different manufacturing techniques for the drilling process, which results in difeerent surface conditions. The effect of the roughness on the HE was investigated. For the evaluation the effect on the reduced area at fracture (RA) and the elongation loss were determined. Further fractographic analysis were performed. In this way, this study provides insights on the applicability of novel, reliable, and safer testing method which can be used to assess HE, particularly in relation with hydrogen-induced loss of ductility in metallic material.

The constantly increasing demand for renewable energy sources lead to the necessity of transporting large amounts of hydrogen. Since pipelines enable a cost-effective way for the distribution of gaseous hydrogen, the interaction of hydrogen and the pipeline materials must be carefully investigated as hydrogen can cause a degradation of the mechanical properties under certain conditions. Especially welds, which are assumed to be more susceptible to the degradation enhanced by hydrogen, are of great interest. The aim of this study is to investigate the effect of gaseous hydrogen on the mechanical properties of an X65 pipeline, and the longitudinal submerged arc welding (SAW) welded joint. The tests are conducted using the hollow specimen technique on two types of specimens: one extracted from the base material (BM) and the other extracted as a cross-weld (CW) specimen consisting of base material and weld seam. The specimens are charged in-situ under a pressure of 60 bar and tested using slow strain rate (SSR) tensile tests with a nominal strain rate of 10-5 s-1. The performed tests showed a decrease of the reduction of area (RA) from 72% in inert atmosphere to 52% in hydrogen atmosphere for the CW-specimen and a decrease from 73% in inert atmosphere to 51% for the BM. Metallographic analyses showed the crack initiation between fine grain heat affected zone (FGHAZ) and BM for the specimens tested in hydrogen atmosphere as well as for the reference specimens. This leads to the conclusion that the location of the crack initiation does not change due to the presence of gaseous hydrogen.