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In general, hydrogen assisted cracking is a result of a critical combination of local microstructure, mechanical load and hydrogen concentration. In that connection, welded microstructures of low-alloyed creep-resistant steels can show different hydrogen trapping kinetics. That influences the adsorbed hydrogen concentration as well as the diffusion itself in terms of moderate or strong trapping. A common approach to describe trapping is by the activation energy that is necessary to release hydrogen from a specific trap site. In the present study, T24 base material and weld metal were investigated. For that purpose, electrochemically hydrogen charged specimens were analyzed by thermal desorption analysis(TDA) with linear heating using a mass spectrometer. The results showed a microstructure effect on hydrogen trapping kinetics at elevated temperatures. Additionally, it is necessary to monitor the specimen temperature. A comparison between idealized temperature profile and real specimen temperature showed that the calculated activation energy varied up to a factor of two. Thus, the assigned trap character(moderate or strong) changed. In case of high temperature peaks, this effect could be more important compared to the microstructure effect itself.
The aluminum alloy 2618A is an Al-Cu-Mg alloy with additions of Fe and Ni, which was designed for long-term operation at elevated temperature in transportation and aerospace industries. Typical applications include aircraft parts and structures (sheet material) or engine components such as turbo charger centrifugal compressor wheels (forged material). Such components are subjected to prolonged aging during service, (e.g. 50 000 h) at temperatures which are close to their age hardening temperature (ca. 190 °C).
The microstructural evolution during creep exposure is studied.
The aluminum alloy 2618A is an Al-Cu-Mg alloy with additions of Fe and Ni, which was designed for long-term operation at elevated temperature in transportation and aerospace industries. Typical applications include aircraft parts and structures (sheet material) or engine components such as turbo charger centrifugal compressor wheels (forged material). Such components are subjected to prolonged aging during service, (e.g. 50 000 h) at temperatures which are close to their age hardening temperature (ca. 190 °C). The microstructural evolution was investigated.
The current study reports on the effect of the sintering temperature and secondary carbide (VC, Mo2C and TiC) additions on the microstructure and concommitant Vickers hardness (HV30) and fracture toughness of Ni-bonded NbC cermets. All cermets were prepared by pressureless sintering in vacuum. Detailed microstructural investigation was performed by electron probe microanalysis (EPMA) and X-ray diffraction (XRD) analysis. Sintering results indicated that both the sintering temperature and secondary carbide additions had a significant effect on the properties of NbC-Ni cermets. Nickel pools and residual pores were observed in the cermets sintered at temperatures ≤ 1340 °C. Increasing of the sintering temperature up to 1420 or 1480 °C resulted in fully densified NbC-Ni based cermets composed of homogeneous contrast cubic NbC grains for the single carbide (VC or Mo2C) modyfied system, whereas core-rim structured NbC grains were observed with the additon of TiC + VC or TiC+Mo2C. The secondary carbide doped cermets with 5–10 vol.% VC/Mo2C and 10 vol.% TiC showed a significantly improved hardness and fracture toughness, as compared to the plain NbC-Ni cermets.
Additive Manufacturing (AM) by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts in comparison to the traditional subtractive manufacturing strategies. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
Spray drying based granulation processes aim for flowable granules neither containing voids nor hard shells thus leading to a homogenous microstructure in the green and sinter bodies without strength reducing large pores. The increase of the specific surface area due to the utilization of finer powders as raw materials makes the production of granules of demanded quality more sophisticated. Innovations regarding additives as well as process engineering are therefore required.
While conventional spray granulation processes of ceramic materials are based on rotational, one stream or two stream nozzles for nebulization, the investigations in the ZIM project concentrate on the applicability of an ultra sound atomizer unit. A spray dryer comprised of the aforementioned ultra sound atomization unit implemented in a commercial spray dryer (Niro, Denmark) was used as test system. Potential advantages of the ultra sound nebulization are investigated for model systems of alumina, zirconia and a ZTA composite while focusing on solids content, yield, pressability and granule properties (size, size distribution, flowability, shape and microstructure) as well as the final sinter body properties (density, microstructure and flexural strength).
First ultra sound spray drying experiments yielded granules with excellent processability. Spray drying of identical slurries, as before tested and optimized for a two stream nozzle atomization process, resulted in a more suitable size distribution for dry pressing (less particles below 20 µm) and a higher yield. Furthermore, sinter bodies produced of ultra sound granules seem to have less large pores and a more homogenous microstructure
Three high strength Nb-, Ti- and Ti+V- bearing S690QL steels were welded to investigate and compare the effects of microalloy addition on 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 the brittle fracture mode and impaired toughness of Nb bearing weld at high heat input. In contrast to this reduced toughness, Ti bearing welds exhibits satisfied toughness regardless of at a fast or slow cooling rate as a result of limited austenite grain and refined favourable intragranular acicular ferrite structure. Moreover, in the case of such refined structure as matrix, TiN particles are found to be irrelevant to the fracture process and crystallographic misorientation results also confirms that high angle boundaries between fine ferrites plates provide much effective barrier for crack propagation and contribute to improved toughness.
Trapping in T24 steel weld joints – Effects on activation energy for hydrogen diffusion during TDA
(2017)
Failure cases in the past decade exhibited severe cracking in T24 welds and showed that generally hydrogen-assisted cracking (HAC) occurring up to 200°C cannot be excluded. A basic understanding is necessary on how hydrogen diffusion is influenced by the weld process. In this regard, both weld microstructures HAZ and weld metal have particular influence on hydrogen diffusion compared to the base material. In general, hydrogen diffusion at a certain temperature is described by diffusion coefficients representing an effective value of combined lattice diffusion and effects of reversible hydrogen traps. Those traps are typically precipitates, interstitials, grain boundaries and so on. A common approach to describe the trap character and its effect on diffusion is the determination of so-called activation energy. This can be done by respective thermal desorption analysis (TDA) with linear heating. In the present study, different T24 as-welded microstructures (BM, HAZ, WM) were investigated. For that purpose, electrochemically hydrogen charged specimens were analyzed by TDA with linear heating using a mass spectrometer for detection of ultra-low hydrogen amounts. The results showed that typically the as-welded HAZ had higher energy traps than the tempered base material. Nonetheless two important effects were ascertained: (1) it is strictly necessary to monitor the sample temperature due to its great impact on the hydrogen desorption peak temperature and (2) the real heating rate in the specimen vs. the applied heating rate has to be considered. Both influence the calculated activation energy, i.e. the assigned hydrogen trap character (moderate or strong trap), which changed up to a factor of two in terms of the calculated activation energy. This effect can be much more important compared to the microstructure effect itself. Hence, suitable experimental boundary conditions should be mandatory for the determination of hydrogen trap kinetics.
Additive Manufacturing by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts as compared to the traditional subtractive manufacturing strategies. However, the residual stresses (RS) developed during the processing can reduce the load bearing capacity as well as induce unwanted distortion, limiting the application of SLM parts. In the present work, residual stresses in additivly manufactured IN718 part were analised by means of neutron diffraction and synchrotron X-ray diffraction.
Prospects and Restrictions of the Metal Magnetic Memory Technique
Since the BSE signal depends on many factors, like the chemistry of the phase and the acceleration voltage, the size and position of the detector array is (slightly) different from phase to phase so that an (iterative) post-processing of the stored patterns is highly recommended. The derived BSE signal can be used for phase assignment in high resolution and high speed maps when EBSD fails and/or EDS (energy dispersive spectroscopy) needs too much time for a suitable and parallel signal acquisition.
The thermal stability of the microstructure of a heat-resistant cobalt alloy, which consists of a γ solid solution strengthened with γ'-phase precipitates, has been studied. The temperature behavior of the dissolution of the hardening γ' phase and the kinetics of its coarsening at 700 and 800°C have been determined. It is found that, during prolonged annealing at 800°C, the γ' → β phase transformation occurs.
Pre-Inca civilizations like the coastal cultures Moche and Nazca (Early Intermediate) and the inland culture Wari (Middle Horizon) were agrarian societies which supported indigenous elites of impressive wealth, power, and organization. With the expansion of the Wari Empire, the polychrome style and technique of Nazca propagated to the other cultures. High status burials, most of the Late Moche Fine Line ceramics and a large corpus of ceramics with Wari-derived decoration have been recovered in San José de Moro since 1991. The degree of transfer of procedures in this highly interactive scenario is of special interest: is there a limitation to decoration or is it adopted by the local potters also regarding the formulation of the ceramic bodies? In this context the relative amount, size and type of incorporated non-plastic inclusions as temper are important.
The roles of microalloying niobium, titanium and vanadium for controlling austenite grain growth, microstructure evolution and hardness were investigated at different simulated heat affected zones (HAZ) for high strength low alloy (HSLA) S690QL steel. High resolution FEG-SEM has been used to characterize fine bainitic ferrite, martensite and nanosized second phases at simulated coarse and fine grain HAZs. It was found that for Ti bearing steel (Ti/N ratio is 2) austenite grain had the slowest growth rate due to the presence of most stable TiN. The fine cuboidal particles promoted intragranular acicular ferrite (IGF) formation. Nb bearing steel exhibited relatively weaker grain growth retardation compared with titanium bearing steels and a mixed microstructure of bainite and martensite was present for all simulated HAZs. IGF existed at coarse grain HAZ of Ti+V bearing steel but it was totally replaced by bainite at fine grain HAZs. Hardness result was closely related to the morphology of bainitic ferrite, intragranular ferrite and second phases within ferrite. The microstructure and hardness results of different simulated HAZs were in good agreement with welded experimental results.