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Organisationseinheit der BAM
Für die Sicherheit von Spannbetonbauwerken ist eine Empfindlichkeitsprüfung von Spannstahlprodukten gegenüber der wasserstoffinduzierten Spannungsrisskorrosion zwingend erforderlich. Die derzeitig normativ festgelegten Prüfmethoden nach DIN EN ISO 15630-3 zeigen nachweislich Schwächen hinsichtlich der Reproduzierbarkeit der Ergebnisse und erschweren damit die eindeutige Bewertung der Empfindlichkeit. Ziel des Forschungsvorhabens der Förderinitiative WIPANO des BMWK war es, die Prüfmethode hinsichtlich der Reproduzierbarkeit und Aussagesicherheit zu verbessern. Im Projekt wurden durch materialseitige, elektrolytische und verfahrenstechnische Parameteränderungen konstante und reproduzierbare Prüfbedingungen geschaffen. Dies ermöglichte eine eindeutige Differenzierung zwischen Gut- und Schlechtprodukten. Innerhalb des Projekts wurde ein Ringversuch initiiert, welcher die Anwendbarkeit und Reproduzierbarkeit der neuen Prüfmethode bestätigte. Das neue Prüfverfahren wurde hinsichtlich der Anwendbarkeit für das Spannstahlerzeugnis Draht evaluiert. Ein entsprechender Normentwurf mit einem Vorschlag für ein Konformitätskriterium dieser Erzeugnisse wurde dem zuständigen ISO-Gremium präsentiert und zur Abstimmung überlassen. Nach aktueller Rückmeldung seitens des Gremiums wird der Normentwurf den Eingang in den informativen Anhang der Prüfnorm finden, dies könnte den Weg für eine Spiegelung auf europäischer und nationaler Normungsebene ebenen.
H2Sense - Cost-effective and reliable Hydrogen Sensors for Facilitating the Safe Use of Hydrogen
(2016)
The H2SENSE (Cost-effective and reliable hydrogen sensors for facilitating the safe use of hydrogen) project promoted hydrogen primarily, but not exclusively, for its use as an alternative fuel. It brought together different stakeholders including sensor manufacturers, end-users, certification bodies and independent evaluators to ensure the optimum use of low-cost and reliable hydrogen sensors.
Project partners analysed sensor performance in real-life applications in industrial environments and identified increased requirements for sensors and for regulations, codes and standards. H2SENSE also facilitated the safe use and implementation of hydrogen as an alternative fuel by ensuring the correct use of effective hydrogen detection devices.
European scientists worked together with colleagues from the National Renewable Energy Laboratory (NREL) in Colorado, USA. They pooled their knowledge of developments in hydrogen sensor technology as well as deployment and commercialisation strategies. These benefits will be continued through trans-Atlantic inter-laboratory sensor testing programmes in which EU and US laboratories perform complementary tests and exchange results.
Low-alloyed heat-resistant steels have a fundamental contribution to the currently applied steel grades in pressurized and temperature loaded components like membrane walls(water walls)or pressure vessels. Here, the main advantages of the low-alloy concept can be used in terms of superior high temperature mechanical properties, workability and decreased amounts of expensive alloy elements. The main challenge for the future is to further increase the power plant thermal efficiency independent of the type of power plant concept, i.e. fossil-fired or nuclear power plant, where the material selection can directly affect reduction of CO2 emissions.
In power plant design, welding is the most applied manufacturing technique in component construction. The necessary weld heat input causes metallurgical changes and phase transitions in the heat affected zone (HAZ) of the base materials and in the deposited weld metal. The weld joint can absorb hydrogen during welding or in later service - This absorption can cause degradation of mechanical properties of the materials, and in certain loading conditions, hydrogen-assisted cold cracks can occur. This cracking phenomenon can appear time delayed due to the temperature dependency of the hydrogen diffusion and
the presence of a “critical” hydrogen concentration. Additionally, each specific weld microstructure shows a certain hydrogen diffusion and solubility that contribute to susceptibility of the cracking phenomenon. Therefore hydrogen cannot be neglected as possible failure effect, which was identified recently in the case of T24 creep-resistant tubeto-tube weld joints. It is necessary to identify and assess the hydrogen effect in weld joints of low-alloyed steel grades for to improve further early detection of possible failures.
For each specific weld joint microstructure, it is necessary to separate the interdependencies between mechanical load and the hydrogen concentration. The
diffusivity and solubility must be considered to identify hydrogen quantities in the material at any given time. In this case, the effects of mechanical loading were dealt with independently. For the characterization of the mechanical properties, hydrogen charged tensile specimens were investigated for the base materials and thermally simulated HAZ
microstructures. The hydrogen diffusion was characterized with the permeation technique at room temperature and at elevated temperature ranges up to 400°C - It was investigated by interpreting the hydrogen effusion behavior with carrier gas hot extraction technique (CGHE). For realistic determination of the hydrogen diffusion coefficients, an improved
method was developed encompassing accelerated specimen heating and hydrogen determination via mass spectrometer (MS). Simultaneously, the corresponding temperature
dependent trapped and total hydrogen concentrations were determined.
The determined experimental results showed increased susceptibility to the hydrogen affected
degradation of the HAZ compared to the base material, which is independent of the investigated alloy composition. In particular, the martensitic coarse grain HAZ is the most susceptible microstructure to hydrogen-affected degradation. The results of the tensile
tests allowed the definition of consistent microstructure specific failure criteria (envelope curves) versus quantified hydrogen concentrations for the reactor pressure vessel 16MND5 steel (20MnMoNi-5-5) and the creep-resistant T24 steel (7CrMoVTiB10-10). The procedure of quantifying hydrogen concentrations in HAZ microstructures is novel and supports a new method of analysis for hydrogen degradation effects. Further investigations with the T22
steel (10CrMo9-10), as compared to the creep-resistant T24 steel (7CrMoVTiB10-10),
confirmed the beneficial effect of Vanadium as an alloying element to improve the resistance to degradation. In general, Mn-Mo-Ni base material grades show a higher resistance compared to Cr-Mo steels that do not include Vanadium alloying.
The investigations showed the decreased diffusion coefficient of the HAZ microstructure compared to the base material microstructure. This is caused by the stronger trapping effects that are present which simultaneously increase the hydrogen solubility as well. In
general, trapping effects above 100°C are negligible. It is noted that after testing the T24 grade, these trapping effects were observed above 100°C and must be considered. At elevated temperatures, the calculated hydrogen diffusion coefficients are sometimes greater than those in literature. This is primarily due to the unique applied specimen heating procedure resulting in a varied hydrogen effusion from the specimen.
The significance of the obtained results can be characterized in three perspectives. First, the direct comparison of the degradation was possible in terms of microstructure-specific hydrogen effects on the mechanical properties. Second, consistent failure criteria were established to quantify degradation vs. the hydrogen concentration. Third, the determination of more accurate hydrogen diffusion coefficients is now available.
From a scientific point of view, important contributions were made to further interpret the hydrogen effects on the macroscopic mechanical properties, with respect to the alloy composition and the microstructure. From a procedural standpoint, the mentioned deviation in the elevated temperature diffusion coefficients can be caused by the calculation method. This can be an explanation for the reported data scatter in the references.
In terms of an economic view, the presented experimental results contribute to a safe and reliable weld workability of the steel grades. Thus, the identified temperature levels of hydrogen trapping can be applied in the definition of minimum preheat, interpass or postheat temperatures. In addition, recommendations for suitable dehydrogenation heat treatment (DHT) procedures, with accurate temperature values and holding times, can be derived from these results. In the future, the application of the mechanical and diffusion data is intended to support numerical analysis methods to provide an improved prediction of hydrogen effects on material degradation in weld microstructures.