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 simultaneous mechanical and thermal loading to determine the hydrogen content, to characterize the microstructure and to determine the hardness-relevant properties.
Hollow samples – an alternative to autoclave technology
Tensile tests with hollow samples can deliver significant results for the evaluation of a material’s sensitivity to hydrogen. The samples have a through bore which is pressurized with a hydrogen containing gas mixture during testing. The significance of hollow sample technology is illustrated by current results of tensile tests on a pipeline steel. There is a considerable reduction of elongation at break/fracture strain through hydrogen embrittlement compared to reference tests in air. Tests with hollow samples are quicker, simpler and cheaper than tests in pressurized hydrogen autoclaves with full samples. Furthermore, they reflect the real/actual hydrogen gradient within a pipeline.
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.
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 load and outside influence, such imperfections may develop along fine cracks and can grow to a critical size - at which point 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.