Fracture surface characterisation

Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20 - St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials.

Hydrogen embrittlement of austenitic steels is of high interest because of the potential use of these materials in hydrogen-energy related infrastructures. In order to elucidate the associated hydrogen embrittlement mechanisms, the mapping of heterogeneities in strain, damage (crack/void), and hydrogen and their relation to the underlying microstructures is a key assignment in this field.
Specifically mapping the connection between microstructure heterogeneity
and the associated hydrogen trapping at similar spatial resolution opens a novel pathway to identify hydrogen embrittlement mechanisms in complex alloys.
One of the materials classes that is expected to be applied for energy-related structure parts are austenitic steels with high Mn content. In particular, twinning-induced plasticity (TWIP) high Mn austenitic steels are well known for an exceptional balance of ductility and strength with less hydrogen susceptibility compared to ferritic steels with a similar strength.
The hydrogen embrittlement phenomenon has been observed under severe mechanical deformation and hydrogen charging conditions such as delayed fracture testing in a deep drawn cup or tensile testing during hydrogen charging at a high current density. Fracture was in such cases caused by various metallurgical factors e.g. by the formation and failure of deformation twins. The importance of deformation twins on hydrogen embrittlement of austenitic steels has been recently studied. It was found that they can act as crack initiation sites and enable crack propagation. Microstructure-sensitive hydrogen mapping in a TWIP steel has been conducted by using microprinting experiments, which enable visualization of hydrogen emission from a sample through reduction of silver ions in gelatin-based AgBr emulsion. The microprinting technique demonstrated that hydrogen is indeed localized at deformation twins, hence, promoting the initiation of hydrogen embrittlement. Also, from our previous experiments we suggested that the hydrogen localization at/near deformation twins requires local plastic straining at/before deformation twins. These facts indicate that the hydrogen-assisted twin boundary cracking of TWIP steels is a complex phenomenon including both, local plastic straining and local hydrogenation. To better understand the influence of deformation twins on the hydrogen embrittlement phenomenon, it is thus essential to map the spatial hydrogen distribution through a high resolution detection approach that is sufficiently microstructure and concentration sensitive. It should be noted here that the microprinting technique is only sensitive to hydrogen at relatively
high concentration levels, or more precisely at relatively high emission rates. It has, therefore, to be applied directly after a sample is charged with hydrogen. It will be shown here that hydrogen is critical for prolonged times after charging. Hence, the findings obtained by microprinting needed to be confirmed also for longer times after charging by a more sensitive technique. In the last years several promising novel approaches have been reported for the localized resolved and sensitive detection of hydrogen.
One is a direct electrochemical detection via a capillary cell, developed by Suter et al. which, however, does not provide the resolution required here. Another approach is the use of Kelvin probe techniques, which allow detection of hydrogen in a material by means of the change ofwork function caused by the hydrogen entering the oxide at the surface. Studies at quite high resolution have been carried out with Scanning Kelvin Probe ForceMicroscopy (SKPFM), where diffusion profiles of hydrogen have been mapped successfully at relatively high
resolution at cross section of samples after hydrogen charging.
However, a direct quantification is not possible by this method, due to the complex dependence of the work function of oxide on different defect states in the oxides. For the same reason this approach is not suitable to provide reliable information on hydrogen at different features of the microstructure, because different oxides show a different dependence on hydrogen. A new approach by applying a thin palladium layer eliminates this problem. More specifically, the SKPFM with a thin palladium layer is a novel and efficient method to very sensitively analyze local hydrogen concentrations down to levels well below 0.01 atom ppm at spatial resolutions as small as several tens of nanometers. The SKPFM can hence be used for detecting the hydrogen distribution with high sensitivity, since the potential measured by the SKPFM on the palladium that has been deposited as a thin layer on the hydrogen charged sample surface correlates logarithmically with the hydrogen content in the palladium. Because of this Nernstian-like behavior, which is similar to a hydrogen electrode, as also reported for immersed palladium metal, we also refer to the work function as electrode potential. In fact, as discussed previously, the surface of the palladium is, even in dry nitrogen atmosphere, still covered by an ultrathin water layer, which together with the hydrogen in the palladium leads to the formation of an hydrogen electrode “in the dry”. By exploiting this effect, SKPFM was successfully applied to the spatially resolved detection of hydrogen in ferrite/austenite duplex stainless steels. More specific in this project the hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1–2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations
of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption.

""Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20 - St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials.