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Eingeladener Vortrag
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Hydrogen-assisted cracking is a critical combination of local microstructure, mechanical load and hydrogen concentration. Welded microstructures of low-alloyed creep-resistant Cr-Mo-V steels show different hydrogen trapping kinetics. This influences the adsorbed hydrogen concentration as well as the diffusion by moderate or strong trapping. 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. Finally, realistic temperature assessment is mandatory for calculation of activation energy via TDA.
A study was made on the effect of creep loading on the precipitate radii evolution of the aluminum alloy 2618A.
The overageing process of the alloy was investigated under load at a temperature of 190 °C with stresses between 79 and 181 MPa and compared to stress free isothermal ageing. The precipitates responsible for strength were characterized using dark-field transmission electron microscopy (DFTEM). This allows the experimental Determination of radii distributions of the rod-shaped Al2CuMg precipitates and the evaluation regarding their mean precipitate radius. It was found that the mean precipitate radius enables the comparison of the different microstructural conditions of crept and uncrept samples. The mean precipitate radii of the samples experiencing creep are significantly higher than those of undeformed samples. It was shown that the acquired radii distributions are viable to determine averaged particle radii for comparison of the aged samples. A ripening process including pipe diffusion along dislocations describes the data on coarsening very well for the creep samples.
Low heat input gas metal arc welding for dissimilar metal weld overlays part II: the transition zone
(2018)
Dissimilar metal weld overlays (DMWOL) of nickel base alloys on low alloy steel components are commonly used in the oil and gas, petrochemical, and power generation industries to provide corrosion and oxidation resistance in a wide range of service environments and temperatures. Traditionally, dissimilar weld overlays are produced using cold or hot wire gas tungsten arc welding. This study aims to identify and evaluate potential advantages of low heat input gas metal arc welding processes over the conventional gas tungsten arc welding in the production of dissimilar weld overlays. In order to evaluate the quality of these overlays regarding resistance against hydrogen-assisted cracking, their transition zone region is investigated in this part of the publication series. Metallurgical characterization, including energy-dispersive x-ray spectroscopy, is performed on Alloy 625/grade 22 steel overlays. The transition zone is characterized by a narrow planar growth zone and steep compositional gradients from the fusion boundary towards the weld metal. Evidence of low carbon contents in the planar growth zone, as well as for carbide precipitation in the cellular growth zone was found. The microstructure in the transition zone region of the fusion zone shows characteristics known to be suitable for good resistance against hydrogen embrittlement.
Three high-strength Nb-,.Ti- and Ti+ V-bearing S690QL steels were welded to investigate and compare the effects of microalloy addition on heat-affected zone (HAZ) toughness. Charpy V notch impact tests from three microalloyed welds under different cooling rates have been performed. Fractographic examination shows that several factors, including large-sized grain, upper bainite or hard second phase, interact to determine brittle fracture and impaired toughness in Nb-bearing weld with high heat input. In contrast to this reduced toughness, Ti-bearing welds exhibits satisfied toughness regardless of at fast or slow cooling. This is attributed to its limited austenite grain and refines favorable intragranular acicular ferrite structure. Moreover, in the case of such refined structure as matrix, TiN particles are found to be irrelevant to the facture process. The crystallographic results also confirm that high-angle boundaries between fine ferrites plates provide effective barriers for crack propagation and contribute to improved toughness.
From simple homopolymers to functionalized, 3-dimensional structured copolymers, the complexity of polymeric materials has become more and more sophisticated. With new applications for instance in the semiconductor or pharmaceutical industry, the requirements for the characterization have risen with the complexity of the used polymers. For each additional distribution, an additional dimension in analysis is needed. Small, often isomeric heterogeneities in topology or microstructure can usually not be simply separated chromatographically or distinguished by any common detector, but affect the properties of materials significantly. For a drug delivery system for example, the degree of branching and branching distribution is crucial for the formation of micelles. Instead of a complicated, time consuming and/or expensive 2d-chromatography or ion mobility spectrometry (IMS) method, that also has its limitations, in this work a simple approach using size exclusion chromatography (SEC) coupled with electrospray ionization mass spectrometry (ESI) is proposed. The online coupling allows the analysis of reconstructed ion chromatograms (RIC) of each degree of polymerization. While a complete separation often cannot be achieved, the derived retention times and peak widths lead to information on the existence and dispersity of heterogeneities. Although some microstructural heterogeneities like short chain branching can for large polymers be characterized with methods such as light scattering, for oligomers where the heterogeneities just start to form and their influence is at the maximum, they are inaccessible with these methods. It is also shown, that with a proper calibration even quantitative information can be obtained. This method is suitable to detect small differences in e. g. branching, 3d-structure, monomer sequence or tacticity and could potentially be used in routine analysis to quickly determine deviations.
This study examines the relationship between the magnetic mesostructure with the microstructure of low carbon steel tungsten inert gas welds. Optical microscopy revealed variation in the microstructure of the parent material, in the heat affected and fusion zones, correlating with distinctive changes in the local magnetic stray fields measured with high spatial resolution giant magneto resistance sensors. In the vicinity of the heat affected zone high residual stresses were found using neutron diffraction.
Notably, the gradients of von Mises stress and triaxial magnetic stray field modulus follow the same tendency transverse to the weld. In contrast, micro-X-ray fluorescence characterization indicated that local changes in element composition had no independent effect on magnetic stray fields.
Niobium carbide is a ceramic material which can be used with excellent results to replace tungsten carbide with cobalt binder. Furthermore, it might be manufactured by classic ceramic technologies and even colloidal processing. To achieve high toughness and strength, it is necessary to have a perfect mix of hard phase and binder, which is mainly achieved by ball milling. Mechanical and physical properties as well as results on different tribological and application-oriented machining tests are presented. The results have been compared with results obtained with NbC grades with Co and Fe3Al binders as well as with pure binderless hot-pressed NbC and SPS sintered Nb2O5.
Ni-base superalloy single crystals have been used in turbine blades for hot sections of gas turbines for over four decades. In order to increase the efficiency of the turbines, a continuous increase in the inlet temperature of combustion gases into the turbine has driven the design of turbine blades to complicated shapes and the presence of a complex pattern of cooling channels. These three-dimensional shapes, together with the inhomogeneous distribution of stresses along the blade, induce an also complicated triaxial stress state, which does not compare to uniaxial tests that are performed to characterize high temperature properties such as creep. A round notch on a test piece represents a simple configuration that generates a quasi-isostatic stress state across the notch. In the present contribution, the effect of a sharp round notch on the microstructural micromechanisms within the notched region cylindrical bars, loaded along [001] at 1020 °C and 160 MPa net stress, is studied. To this end, a series of interrupted creep tests is conducted on plain and notched bars and the microstructure is compared. Results are discussed in terms of degree microstructural coarsening, and dislocation activity. The effect of notch generation via grinding is also discussed in these terms. The presence of carbides evolving in from residual carbon is also shown and discussed. .Funding by the German Research Association (DFG) [grant number AG 191/1] is acknowledge
Materials subjected to high-temperature service conditions will change their microstructure with time. Associated with this aging process is a change of mechanical properties as well as a change of damage mechanisms. Within the scope of the FVV project Aging and Lifetime, Fraunhofer IWM in Freiburg and BAM in Berlin (both Germany) experimentally characterized the widespread high-temperature aluminum alloy EN AW-2618A in different overaging states. Based on the experimental findings, models for numerical lifetime assessment with the finite-element method were implemented.