Cloud III Hydrogen Management

Dr.-Ing. W. Song (Institut für Eisenhüttenkunde, RWTH Aachen University), Prof. Dr. rer.nat. R. Spatschek (Thermochemie von Energiewerkstoffen am Forschungszentrum Jülich, RWTH Aachen University)

 

Despite intensive investigations, the phenomenon of hydrogen embrittlement has been a significant technological problem for more than a hundred years, which is discussed scientifically controversially and is considered unresolved. In this phenomenon, embrittlement takes place in the material, which frequently leads to spontaneous failure without warning. This phenomenon occurs in a wide range of different metals and steels of different crystal- and microstructures, with a particular susceptibility to high-strength steels. As necessary boundary conditions for the occurrence of hydrogen embrittlement a high strength, a high internal stress as well as the presence of dissolved hydrogen in the material are considered.

It is commonly assumed that different effects are combined in a complex manner, which makes the understanding and thus the further development of hydrogen-resistant high-strength steels more difficult. A particular challenge of this topic is given by the fact that hydrogen in metals is not directly detectable quantitatively, and that its diffusion can take place so quickly that under all known experimental conditions, a static state can not be reliably established within the analysis.

Current explanations for hydrogen-induced embrittlement are based on the theories of hydride formation with brittle fracture, hydrogen-induced decohesion (HEDE), and hydrogen-induced local plasticity (HELP) (Fig.). In addition, hydrogen supports the excessive formation of vacancies (superabundant vacancies) and pores, which can also initiate crack formation. In hydride-forming metals, such as vanadium, niobium or titanium, the embrittlement is generally caused by the stress-induced formation of hydrides close to crack tips. These inclusions grow and bond so that the metal becomes susceptible to fracture in the brittle hydrides. Crack formation is presumed either at the phase boundary between the low-hydrogen metal and the hydride inclusions, or alternatively in the brittle hydride itself. TEM and SEM of the fracture surfaces could confirm these processes.

 

Zentrale Mechanismen der Wasserstoffversprödung
Schematische Darstellung der Wasserstoff-Wirkung

 

Spannungsinduzierte Hydridbildung und Sprödbruch: Relevant in hydridbildenden Systemen und solchen, in denen Hydride beispielsweise durch Spannungen stabilisiert werden

HELP (Hydrogen Enhanced Localised Plasticity): Wasserstoff erhöht die Versetzungsbeweglichkeit und dadurch das plastische Verhalten

HEDE (Hydrogen induced Decohesion) –

Schwächung der atomaren Bindung an Rissspitzen durch Wasserstoff in der Kohäsivzone (CZ)

Schematische Darstellung der Wasserstoff-Wirkung Theoretische Ansätze

In non-hydride-forming metals such as the iron-manganese system, the decohesion model (HEDE) is the most widely used theory, but the situation here is much more unclear. It is assumed that the hydrogen accumulates both in volume and at grain boundaries and thereby reduces the binding strength of the metal atoms among one another. The extent of this reduction, however, is rather unexplained and depends on the enrichment of hydrogen in the dilated areas at the crack tips. As a result of this process, brittle fracture formation becomes more favorable compared to ductile processes. This image is supported experimentally by the correlation between the hydrogen partial pressure and the beginning of the crack formation  and on the other hand theoretically by the segregation of hydrogen at grain boundaries.

Recent simulations show complementarily that the formation of dislocations as carriers of plastic deformation is suppressed by hydrogen enrichment. Contrariwise, signatures of plastic deformation in fracture surfaces are often found in hydrogen-embrittled samples. To explain this, the model of hydrogen-induced local plasticity (HELP) was developed. The model is based on a reduction of the dislocation-dislocation interaction by the hydrogen in the distortion field of step dislocations, whereby the strain caused by the lattice defect is partially compensated by the compressive behavior of the hydrogen in the interstitial lattice sites. The hydrogen is stored in the form of nanohydrides. Together with a reduction of the shear modulus, this results in shielding effects reducing the dislocation distance. In situ TEM studies confirm this effect, with an increased mobility of the dislocations going along. The reduced dislocation distances in pile-ups are then responsible for crack initiation. It should be noted here that the area of plastic deformation remains very small so that the fracture surfaces appear primarily brittle.

The discussion on the various mechanisms is, however, still controversial and requires further experimental and theoretical investigations.

The theory of hydride formation on surfaces developed in the past research period shows a markedly reduced hydrogen solubility (solvus line) caused by the relaxation of coherent distortions between the metal and the hydride. The resulting surface phase-diagram, which predicts a solubility limit of surfaces and cracks in the room temperature range, which is reduced by up to two orders of magnitude for austenite, suggests a bridge between the descriptions of hydride-forming and non-hydride-forming systems. It provides a quantitative explanation approach for the significantly increased hydrogen concentrations on free surfaces, which can lead to embrittlement in the sense of the HEDE image.

 

Integration von mehrskaligen Simulationen und Experimenten auf verschiedenen Längenskalen
Skalenübergang

 

Mikro-Makro-Skalenübergang

 

Experimentelle Charakterisierung auf verschiedenen Längenskalen

 

Skalenübergang
ab initio - Kontinuumsmodellierung

Mikro-Makro-Skalenübergang für das chemo-mechanische Modell der Wasserstoffversprödung Experimentelle Charakterisierung auf verschiedenen Längenskalen

For this purpose, individual subprojects from project areas A, B and C will co-operate in the "Hydrogen Management" cloud. The central scientific question in Cloud III is:

What is the cause-and-effect relationship for the delayed cracking in high manganese steels and for the avoidance of this embrittlement due to aluminum addition?

The aim of the investigations is in general the development of a multi-scale model approach. In the field of characterization (C) it is planned to observe the crack formation in a locally resolved manner by means of in situ experiments (SEM, 3D X-Ray microcomputer tomography analysis) and, at the same time, to derive requirements for hydrogen embrittlement resistance on the basis of various applications of sheet metal forming. The investigations are accompanied by a simulation at various levels for the quantification of the boundary conditions of hydrogen loading and -diffusion, of the deformation and the damage and crack formation. The development is supported by deepening the understanding of hydrogen in the high manganese system, by adapting the material models and mechanism maps from area A as well as by an intensive analysis of the influence of the process route in area B with respect to hydrogen sources.