Hydrogen related challenges for the steelmaker
Dr Richard Thiessen, Thyssenkrupp Steel Europe, Germany
The modern steelmaker of advanced high-strength steels has always been challenged with the conflicting targets of increased strength while maintaining or improving ductility. These new steels help the transportation sector, including that of automotive, achieve goals of increased passenger safety and reduced emissions. With increasing tensile strengths, certain steels exhibited an increased sensitivity towards hydrogen embrittlement. Characterizing the material’s sensitivity in as-delivered condition has been developed and accepted (SEP1970), but the complexity of the stress-states that can induce an embrittlement together with the wide range of applications for high strength steels make the development of a standardized test for hydrogen embrittlement under in-service conditions extremely challenging. Some proposals for evaluating the material’s sensitivity give an advantage to materials with a low starting ductility. In spite of this, newly developed materials can have a higher original elongation while suffering only a moderate reduction in elongation due to hydrogen. This work presents a characterization of new materials and their sensitivity towards hydrogen embrittlement.
Hydrogen embrittlement in structural steels
Professor Norman Fleck FRS, University of Cambridge, UK
Hydrogen embrittlement reduces both the ductility and toughness of steels, and such degradation of performance is important in a range of applications from energy storage in transport (hydrogen tanks in automobiles) to the energy supply industry (such as subsea pipelines that are exposed to hydrogen as a consequence of cathodic protection measures).
In the first part of the presentation, it is argued that the reduction in strength and toughness by hydrogen is associated with the embrittlement of grain boundaries and other trapping sites for hydrogen. Elementary kinetic theory suggests that embrittlement is associated with trap binding energies in the range -20 to -30 kJ/mol at room temperature. In order to predict the reduction in macroscopic tensile strength due to the presence of hydrogen at grain boundaries, it is argued that the cohesive strength of the grain boundaries is reduced by hydrogen. This can be modelled in two ways:
(i) macro-level: no elevation in local tensile stress at the grain boundary and the presence of hydrogen reduces the macroscopic cohesive strength to the order of the yield strength;
(ii) meso-level: a stress raising defect (such as a short crack) exists at the grain boundary such that the local stress level much exceeds the yield strength; the presence of hydrogen reduces the cohesive strength but it remains much above the yield strength.
In the second part of the talk, an analytical and numerical analysis is given of the electro-permeation test. This test is commonly used to measure the diffusion behaviour of hydrogen in engineering steels (and other alloys). There is no consensus in the literature on the values of trap binding energy and trap density for particular classes of trap, and this is in-part due to misinterpretation of permeation data, and by not varying the initial concentration of hydrogen over a sufficiently wide range. Our analysis reveals regimes of behaviour, and the resulting permeation map can be used to obtain a clear and unique interpretation of the data.
Hydrogen embrittlement investigated by novel critical experiments
Tarlan Hajilou, Norwegian University of Science and Technology, Norway
Among the experimental approaches to the hydrogen induced degradation, small scale testing has the capability to resolve the hydrogen interaction with microstructure and the crystal defects in the same length scale. However, small scale testing inquiries in situ examination to avoid hydrogen gradient or depletion on the testing materials. In this approach, in situ nanoindentation experiment capable of registering the onset of plasticity in a sub micro meter scale showed a reduction in dislocation nucleation energy in the presence of hydrogen. Going one step forward, in this study, we used the in situ electrochemical cantilever bending test method to probe the effect of hydrogen on the crack propagation in the micron sized notched beams. The experimantal setup is the integration of a miniaturized three electrode electrochemical cell inside a nanoindenter. This experimental method has the advantage of providing a complete overview of the plasticity and dislocations on the entire sample by post-mortem analyses. For this study Fe- 3wt% Si single and bi-crystal microcantilevers have been investigated. Mechanical behavior of the beams bent under hydrogen charging condition are compared with the air condition. The load-displacement curves reveal a continuous decrease in the flow stress for the cantilevers bent within the presence of hydrogen. Crack initiation and propagation are examined in the presence of hydrogen while the notch blunting occurs for the beams bent< in the air. Post-mortem cross-sectional EBSD analyses of the beams showed a localized plastic region for the hydrogen condition comparing with the air one.
Hydrogen induced stress cracking in steels – examples of failures and numerical modelling
Dr Vigdis Olden, SINTEF Materials and Nanotechnology, Trondheim, Norway
The occurrence of cracks in offshore structures and pipelines is an environmental and safety risk that should be eliminated. Hydrogen Induced Stress Cracking (HISC) has been a challenge in the Norwegian oil & gas industry since the late 1990s. The main sources of hydrogen are cathodic protection and to some extent also hydrogen from welding. Hydrogen induced stress cracking from cathodic protection is a result of interconnected mechanisms involving electrochemistry, diffusion, metallurgy and hydrogen degradation at different length scales from the nano- to the macro-scale.
Safe service requires predictive tools for assessing the structural integrity under CP conditions. For several years our group at SINTEF has worked with numerical models applying hydrogen informed cohesive zone elements to model HISC fracture as well as hydrogen-induced fracture in general.
Numerical simulation of hydrogen embrittlement requires a coupled approach; on one side, the models describing hydrogen transport must account for local mechanical fields, while on the other side, the effect of hydrogen on the accelerated material damage must be implemented into the model describing crack initiation and growth.
The talk will include examples of HISC fractures from the oil and gas industry as well as a review of numerical cohesive zone approaches for the prediction of hydrogen embrittlement.
Effects of hydrogen on fatigue crack growth in steel
Professor Hisao Matsunaga, Kyushu University, Japan
In the context of the fatigue life design of components, particularly those destined for use in hydrogen refueling stations and fuel cell vehicles, it is important to understand the hydrogen-induced, fatigue crack growth (FCG) acceleration in steels. In the presentation, existing studies on the hydrogen-induced, FCG acceleration in various steels are first briefly reviewed, together with the acceleration mechanism and some of its influencing factors. The focus is then placed on the peculiar frequency dependence of the hydrogen-induced, FCG acceleration in steels. In a high-frequency regime (e.g., 10 ~ 0.1 Hz), the ratio of hydrogen-induced, FCG acceleration is seen to gradually increase with a decrease in test frequency, later reaching a peak. To justify the interpretation of the mechanism based on the hydrogen-enhanced successive fatigue crack growth (HESFCG) model, using both “internal” and “external” hydrogen, some critical experiments were performed on two types of material: Type 304 stainless steel and ductile cast iron.