Hydrogen embrittlement in metals

© Photo Fraunhofer IWM

Atomic hydrogen is able to significantly reduce the ductility of metallic raw materials. This can lead to the expected malfunction of components and is generally referred to as hydrogen brittleness. The diverse and distinct susceptibilities for hydrogen embrittlement in otherwise strong material properties limit the selection of raw materials.

Through various processes, hydrogen can penetrate into the raw materials; through loading with gaseous hydrogen and through electrochemical processes (e.g. corrosion or galvanic coating). The problem of hydrogen embrittlement in the traffic and energy sector is becoming increasingly important as hydrogen is viewed as an important energy carrier of the future. The associated development of new technologies to manufacture, distribute and use hydrogen as an energy supplier requires the qualification of available raw materials and those to be newly developed for these applications.

The Fraunhofer IWM supports its clients in selecting and qualifying raw materials and manufacturing procedures, assessing their lifetime expectancy and in cases of damage, identifying the causes of the component malfunction. Our employees have many years of experience in investigation and explaining the mechanisms of hydrogen brittleness and in the simulation of the underlying diffusion processes. Experimental equipment is available for the targeted loading of raw materials with hydrogen including with simultaneous mechanical and thermal loading to determine the hydrogen content, to characterize the microstructure and to determine the hardness-relevant properties.

Services

Damage investigations of components; developing corrective measures

Providing evidence of hydrogen brittleness

Selecting, qualifying and characterizing raw materials for hydrogen application

Residual stress optimization to avoid cold crack formation in welding joins

Avoiding hydrogen brittleness in additive manufacturing

Optimization of degassing heat treatments, e.g. after galvanic coating

Optimization of the welding and heat treatment processes

Developing new protective layers against hydrogen

Suitability of lubricants in rolling bearings, key term: “white etching cracks”

Developing hydrogen-resistant raw materials

Support of complex development projects in the area of hydrogen technology, e.g. gas and hydrogen as an energy carrier

Characterizing and simulating hydrogen brittleness

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Experimental investigations

Electrical mechanical test machines for:

Slow tensile tests (SSRT, CERT) with optional cells for electrolytical in situ hydrogen loading, e.g. according to ASTM G 129

Crack propagation and tiredness tests with optional cells for electrolytical in situ hydrogen loading

Notched tensile test, e.g. in consideration of ASTM F 519 or DIN EN 2832

As of mid-2017: Electrical mechanical test machines with integrated autoclave (1000 bar)

Carrier gas hot extraction

Measuring the total hydrogen content in metallic samples

Test apparatus: Eltra OH-900

Thermal desorption spectroscopy

Infrared oven up to 900°C

Wire-wound tube oven up to 1100°C

Measurement of the proportion of diffusible hydrogen

Measurement of trap binding energy

Test apparatus: Bruker Phoenix G4 with mass spectrometer from the company InProcess Instruments

Permeation tests

Measuring the diffusion coefficients

Trap density measurement

Testing to DIN EN ISO 17081

Microstructure and residual stress analysis, e.g. through

Light and electron microscopy

X-ray diffractometry

Metallography

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Simulation

Simulation of temporal and locational hydrogen distribution regarding the diffusion and trapping behavior in components and component areas (e.g. in welding joins and edge layers)

Models that predict the hydrogen-supported formation of cracks and calculate statistical stress limits for any component geometries

Lifetime assessment for hydrogen-supported crack expansion

Atomic simulation and binding energy of hydrogen in the crystal lattice and assessment of different trap areas

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© Photo Fraunhofer IWM

Calculation of cold crack probability via simulation and experimental cold crack criterion (AiF project with LFT of BTU Cottbus)

Calculation of hydrogen-influence cold crack risk

Using the numerical calculation of structures, residual stress and hydrogen diffusion when welding, it is possible to predict the cold crack resistance of complex components. Solutions allow users of thermal manufacturing processes, e.g. welding and heat treatment, the ability to address cold crack safety in the development phase when designing constructions. This takes place by coupling numerical simulation with experimentally determined risk criteria in consideration of process-typical loads.

Figure 1 shows a laser beam welding tube from high-strength steel, with which the cold crack probability was calculated from the numerical simulation of the temperature field, welding residual stress, and local hydrogen distribution. The calculation was based on a new methodology for determining quantitative cold crack criterion that was experimentally ascertained for the steel strength.

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Visualization of the diffusion of hydrogen atoms in a crystal lattice within a crack test

Detection of hydrogen in metal components using computer simulations

Since metal is “cooked”, rolled and later processed into sheet metal and components, the finest cracks and brittle areas can arise, leading to instability in the metal components. Atomic hydrogen is often at fault for this, as it moves through the metal and collects at imperfections or metal grain boundaries, thereby deteriorating the mechanical properties. The mechanism behind this is called hydrogen brittleness. Depending on the load and outside influence, such imperfections develop at fine cracks that can grow to a critical size - the component breaks.

Previously, scientists analyzed how cracks form and progress in order to understand hydrogen brittleness. However, these mechanisms strongly depend on how hydrogen moves in the metal and how high it is concentrated at critical areas of crack formation. The MultiHy project therefore focuses on local conditions in crack formation such as hydrogen concentration, material tension, temperature, and mechanical loading. It is important to understand the movements of hydrogen in the metal depending on external factors and the material structure in different size scales. “We now want to carry out computer simulations over many size scales overall, from the behavior of atoms across several interim steps to the behavior of the overall component”, explains the project coordinator, Dr. Nicholas Winzer, regarding the special features of the project. Atomic information such as diffusion barriers, activation energy and impurities in the crystal lattice can now directly flow into the prediction of the component lifetime under its use conditions. “Using the simulations, we can predict with greater accuracy how susceptible a material or component is to hydrogen brittleness under realistic conditions”, says Dr. Matuous Mrovec, coordinator of the atomic simulations. Experiments using material samples supplied by the project’s industry partners also occur. The Fraunhofer IWM, together with its project partners, optimizes simulations using these results.

Eleven partners from research and industry are working out application-oriented, industrially relevant problems in the “Multiscale Modeling of Hydrogen embrittlement MultiHy” project, which is promoted as part of the European Commission’s 7th framework program. The UK, The Netherlands, Norway, Austria and Spain are represented alongside Germany. The project will span four years in total.

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Literature

Casajus, P.; Winzer, N.; Intergranular stress corrosion crack propagation in rolled AZ31 Mg alloy sheet; Materials Science and Engineering A 602 (2014) 58–67 Link

Winzer, N.; Höpfel, H.; Casajus, P., Development of a generalised understanding of environmentally-assisted degradation of magnesium-aluminium alloys, in Proc. of Magnesium Technology 2014; Alderman, M.; Manuel, M.V.; Hort, N.; Neelameggham, N.R. (Eds.); John Wiley & Sons, Inc., Hoboken, NY, USA (2014) 365-372 Link

Winzer, N., Mrovec, M.; Di Stefano, D.; Katzarov, I.; Paxton, A., Industrially-relevant multiscale modelling of hydrogen assisted degradation, Hydrogen-Materials Interactions: Proceedings of of the 2012 International Hydrogen Conference ICH 2012; Somerday, B. ; Sofronis, P. (Eds.); ASME Press, New York, NY, USA (2014) 737-745 Link

Winzer, N.; Khader, I.; Hydrogen diffusion and trapping in bodies undergoing rolling contact; Wear 303/1-2 (2013) 451-458; 364/2013 Link