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- Hydrogen embrittlement (4) (entfernen)
In the present work, the influence of deuterium on the microstructure of a duplex stainless steel type EN 1.4462 has been characterized by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) supported by scanning electron microscopy (SEM), focused ion beam (FIB), electron back scattered diffraction(EBSD) and energy dispersive x-ray (EDX) investigations. Characterization has been carried out before and after electrochemical charging with deuterium which has been used as a tracer, due to its similar behavior to hydrogen in the steel microstructure. In a first approach, the distribution of the deuterium occurring at temperatures above 58 °C has been visualized. Further it turned out that sub-surface micro blisters are formed in the ferrite-austenite interface, followed by the formation of needle shaped plates and subsequent cracking at the ferrite surface. In the austenite phase, parallel cracking alongside twins and hexagonal close packed (martensitic) regions has been observed. In both phases and even in the apparent interface, cracking has been associated with high deuterium concentrations, as compared to the surrounding undamaged microstructure. Sub-surface blistering in the ferrite has to be attributed to the accumulation and recombination of deuterium at the ferrite-austenite interface underneath the respective ferrite grains and after fast diffusing through this phase. Generally, the present application of chemometric imaging and structural analyses allows characterization of hydrogen assisted degradation at a sub-micron lateral resolution.
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high-resolution scanning electron microscopy are well-acknowledged tools in materials characterization. The ability to map chemical species on the surface of an investigated sample with often low mass detection limits makes ToF-SIMS an essential tool in fields where many question marks concerning Degradation processes and damage mechanisms exist. The aim of this paper is to describe the power of data fusion of ToF-SIMS and high-resolution scanning electron microscopy results employing computational methods for multivariate data Analysis such as principal component analysis. As a case study the investigation of hydrogen distribution in an artificially charged Duplex stainless steel microstructure is presented aiming on a better understanding of hydrogen embrittlement.
The steel grade 20MnMoNi5-5 (according to German DIN standard or 16MND5 according to French AFNOR standard) is widely applied in (weld) fabrication of reactor pressure vessel components. Thus, a wide range of welding technologies (like submerged arc welding (SAW) or tungsten inert gas (TIG)) is used resulting in different heat affected zone (HAZ) microstructures. During weld fabrication, the weld joints may take up hydrogen. Especially, the HAZ shows an increased susceptibility for a degradation of the mechanical properties in presence of hydrogen. In addition, the hydrogen-assisted degradation of mechanical properties is influenced by three main local factors: hydrogen concentration, microstructure, and load condition. Hence, the base material (BM) and two different simulated non-tempered as-quenched HAZ microstructures were examined using hydrogen-free and hydrogen-charged tensile specimens. The results indicate that the effect of hydrogen on the degradation is significantly increased in case of the HAZ compared to the BM. In addition, hydrogen has remarkable effect in terms of reduction of ductility. It was ascertained that the degradation of the mechanical properties increases in the order of BM, bainitic HAZ, and the martensitic HAZ. Scanning electron microscope (SEM) investigation showed a distinct change of the fracture topography depended on the microstructure with increasing hydrogen concentration in case of the as-quenched HAZ microstructures.
Spectacular failure cases of fossil power stations in the recent years exhibited severe cracking in T24 welds. The results show that hydrogen-assisted cracking up to 200 °C cannot be excluded. Hence, it is important to gain a basic understanding on how hydrogen might affect the basic material properties in the respective weld microstructures. The present study focuses on hydrogen degradation of the respective weld microstructures, i.e., the weld metal and the coarse grained heat affected zone, where actually cracking appeared in practice. Tensile tests were carried out for coarse grain heataffected zone (CGHAZ) and the weld metal in uncharged and electrochemically hydrogen-charged condition. It turned out that both microstructures show distinct tendency for gradual degradation of mechanical properties in the presence of increasing hydrogen concentration. Already for a hydrogen concentration about and above 2 ml/100 g Fe, a significant ductility reduction has been observed. SEM investigations revealed that the fracture topography changes from ductile topography in uncharged condition to intergranular topography for the CGHAZ and to ductile-brittle mix for the weld metal (WM) in hydrogen charged condition. Ti-rich inclusions were identified as central regions of quasi-cleavage fracture areas in the WM. An approximation procedure is applied to quantify the degradation intensity.