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A common approach to describe hydrogen traps is by their activation energy that is necessary to release hydrogen from the trap. In the present study, Cr-Mo-V steel T24 (7CrMoVTiB10-10) base material and TIG weld metal were investigated. Electrochemically hydrogen charged specimens were analyzed by thermal desorption analysis (TDA) with different linear heating rates. The results show two different effects. At first, the microstructure effect on trapping is evident in terms of higher hydrogen concentrations in the weld metal and increased activation energy for hydrogen release. Secondly, it is necessary to monitor the real specimen temperature. A comparison between the adjusted heating rate and the real specimen temperature shows that the calculated activation energy varies by factor two. Thus, the trap character in case of the base material changes to irreversible at decreased temperature. Hence, the effect of the experimental procedure must be considered as well if evaluating TDA results.
Hydrogen can have an extreme degradation effects in steels, particularly concerning the mechanical properties. These effects can lead to hydrogen-assisted cracking in microalloyed high-strength steels during fabrication and/or operation in industrial applications. In order to study these effects, electrochemically charged tensile specimens were tested to elucidate the degradation of their properties. The carrier gas hot extraction (CGHE) method, which functionally combines a mass spectrometer with a thermal desorption analysis (TDA) process, was used for the detection of ultra-low diffusible hydrogen concentrations in the material specimens. The mass spectrometer provided rapid and automatic determination of hydrogen concentration, whereas the TDA presented the activation energy within the respective test specimen at the specific temperature. Additionally, specimen temperature was carefully monitored to reduce the evaluation error for local effusion peaks. A quenching and deformation dilatometer was used for the analysis of typical heat-affected zones during the welding process for a high reproducibility of the homogenous microstructures that were studied. The present work shows the interaction between hydrogen and lattice defects in different microalloyed materials and heat-affected zones of weldable fine-grained steels. These steels were prepared in a quenched and tempered condition and in a thermo-mechanically rolled condition. These preparations were made according to German standard DIN EN 10025-6 and to DIN EN 10149-2, respectively. The trapping characteristics of two steel grades, S690QL and S700MC, were studied with respect to the activation energy dependent on carbon content and microalloying elements such as Ti, Nb, Mo, Cr, and V. The two steel grades exhibited several types of traps: carbide formations, dislocations, and/or grain boundaries were common, which can influence activation energy and hydrogen solubility. The type and dimension of inclusions or particles also affected the hydrogen trapping behavior. A decrease of carbon and specific alloying elements in thermo-mechanically hot rolled steels led to a change in the activation energy binding the trapped hydrogen. This thermo-mechanically hot rolled steel revealed an increased interaction between hydrogen and precipitations. The higher carbon content in the quenched and tempered steel led to a higher interaction between hydrogen and iron carbide, specifically in the martensitic phase. Furthermore, the trapping behavior in heat-affected zones showed a significant increase in activation energy, especially in the coarse-grained microstructure. These previously mentioned various effects were studied to better understand the degradation of mechanical properties in these two steels.
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.
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.
Martensitic 9% Cr steels like P91 and P92 can show an increased susceptibility to delayed hydrogen-assisted cracking. The focus of this study was the microstructure and heat treatment efect on the mechanical properties of P92 base material and P91 multi-layer weld metal in both as-welded and post weld heat treated (PWHT) condition. Tensile tests with hydrogen free reference samples and electrochemically hydrogen charged samples were carried out; the mechanical properties were assessed and supported by detailed fractographic analysis. Finally, a hydrogen and microstructure-dependent fracture criterion is established. All investigated microstructures showed a hydrogen-infuenced degradation of the mechanical properties compared to the hydrogen-free reference samples. The as-welded martensitic P91 weld metal had the highest degree of degradation in the presence of hydrogen. The P91 PWHT weld metal and the P92 base material had comparable properties. From that point of view, a signifcantly increased risk for hydrogen-assisted cold cracking during welding fabrication of P91 weld joints must be considered before any heat treatment is conducted.