Microstructure Of Hastelloy Research Articles

Microstructure formation in micron-scale thin-walled Hastelloy X samples fabricated with laser powder bed fusion

Laser powder bed fusion (LPBF) has great potential for the fabricating complex geometries with improved functionality. In combination with nickel alloys such as Hastelloy X, parts produced with this technology find usage in high-temperature applications. Many studies have focused on the microstructure of Hastelloy X fabricated via LPBF technology, but most have been performed on simple cubic geometries. Considering the aforementioned applications, the parts are often characterized by fine features, like very thin-walled structured, often in the sub-1 mm range. However, little is known about the LPBF fabrication of such structures. The study presented in this paper shows that wall thickness plays an important role in microstructure formation. In situ temperature measurements and thermal simulations showed a difference in thermal history. Local heat accumulation in the vicinity of the scanned tracks was observed for the sample consisting of multiple tracks. Moreover, this effect was enhanced with the number of tracks, leading to different melt pool morphologies. Significantly, coarser solidification cells were found near the sample edges. In addition, larger grains oriented parallel to the build direction were observed for the samples consisting of 3 and more tracks, while for the thinner samples, a very fine microstructure with random orientation was found.

An investigation into the effect of process parameters on melt pool geometry, cell spacing, and grain refinement during laser powder bed fusion

An applied energy density approach defined as the ratio of laser power and scanning velocity is often used as a guideline for selecting appropriate process parameters in laser powder bed fusion (LPBF). In this study, amongst the many variables related to input energy, we investigate the effectiveness of laser energy density (LED) on the melt pool geometry and microstructure of Hastelloy X single tracks produced by fixed LED values at different laser powers and scanning velocities. The results reveal that for a fixed LED, the higher laser power has a higher effect on the melt pool depth. In addition, compared to the scanning velocity, laser power has a higher influence on the melt pool geometry. Moreover, it is proposed that the finer cell structure observed in the melt pool of high laser power is due to the higher cooling rates. Finally, the higher number of new grains observed in melt pools created with higher laser power and fixed LED are likely due to the grain detachment caused by the increase in the partially melted particles.

The Effect of Brazing on Microstructure of Honeycomb Liner Material Hastelloy X

Labyrinth seals with abradable metallic honeycomb structures are common in gas turbines to ensure a tight sealing between rotor and stator. These seals have a big impact on the turbine efficiency. The honeycombs made from nickel-based superalloys are brazed onto backing segments in order to form liners. In this work, the microstructure of Hastelloy X honeycombs after brazing with a nickel-chromium-silicon braze filler alloy is investigated. It was found that the braze filler can wet the honeycomb foils over their entire height up to the contact surface with the rotor. After solidification, the braze filler alloy forms γ and silicides (NiSi and Niy(Mo, Cr)xSi). The intermetallic silicides give high hardness to the honeycomb liners. Between the braze filler alloy and the honeycomb foils, interdiffusion zones are observed. For the rub-in behavior of the honeycomb liners, not only the foil material but also the comportment of the composite made up of the foil material, the braze filler material and the interdiffusion zones has to be considered.

Currentless Deposition of Niobium-Based Protective Coatings for Application in Molten Salts

Molten salts, particularly alkali halides, can be used for developing new methods of rare and refractory metals production and pyrochemical reprocessing spent nuclear fuels. Molten salt nuclear reactor now again attracts attention as one of the prospective power generating systems. Wide scale application of fused salts in various areas of technology is hampered by the problem of finding suitable corrosion-resistant materials. Corrosion of metals in molten salts is electrochemical in nature, and therefore the most suitable constructive materials for these media are electropositive refractory metals, for example, niobium. High cost of these materials and the difficulties associated with the mechanical processing and welding limit their application. A way to overcome this problem is forming a protective layer of a refractory metal on the base material. In the present work a novel method of forming niobium-based coatings by currentless deposition in molten salts is proposed. Nicrofer 6020, Nicrofer 6616, Hasteloy N, and KhN65MVU nickel based alloys and metallic nickel were chosen as the base materials for the present investigation. Corrosion tests were carried out in NaCl–KCl–NbCln (n=3.5 or 5 wt. % niobium) and NaCl–KCl–UCl3 (1 wt. % uranium) melts at 750 oC. Duration of the corrosion tests was 30 h. Contacting nickel based alloys with NaCl–KCl–NbCln melt leads to increase of the sample mass due to the formation of an alloy between nickel (the base constituent of the studied alloys) and niobium on the sample surface, Figure. Nickel-niobium alloy is formed as a result of disproportionation of niobium (III) ions, 4Nb3+ + 3Nialloy → Ni3Nb + 3Nb4+. Nb(IV) ions produced in the same reaction can be reduced to Nb(III) by niobium metal placed in the melt. Formation of a Ni-Nb alloy layer on the sample surface decreases the rate of the base alloy corrosion. This Ni-Nb phase forms a continuous layer on the sample’s surface and, therefore, this phase can protect the studied alloy from further corrosion. Such protection would be very useful for construction materials used for niobium electrorefining in molten chlorides; especially since currently the problem of finding suitable materials is one of the limiting factors in producing high-purity niobium. The samples of Nicrofer 6616 and 6020 alloys with the formed niobium coatings were tested in NaCl–KCl–UCl3 (around 1 wt.% U) melts (30 h exposure at 750 ºС). Here the weight of the samples covered with Ni3Nb phase decreased and the corrosion rates were comparable with those obtained earlier for uncoated samples in uranium-containing melts. Figure. Microstructure of Hastelloy N (a), KhN65MVU (b), Nicrofer 6616 (с) and Nicrofer 6020 (d) alloy samples after 30 h contact with NaCl–KCl–NbCln melts (n=3.5, 5 wt. % Nb) at 750 ºС. Figure 1

The Effect of Repair Welding Number on Microstructure of Hastelloy X Fabricated via TIG Process

The effect of repair welding number on microstructure of the hastelloy X fabricated via TIG process was investigated. The SEM, EDS and OM were used to determine mechanical properties and the microstructure of HAZ zone, respectively. Results showed that the grain size of base metal determined by OM and SEM was 64.11 μm with M6C and M23C6 carbides (6.16, 18.71 μm respectively). Also, using welding for three times caused increase of grain size (15%, 22%, 26% respectively) and the heat input made some carbides dissolve. The grain growth through HAZ zone venially affected the strength of alloy. The results of tensile test demonstrated that UTS increased by repair welding, 1%, 2% and 3% respectively. As the UTS of base metal was 727 MPa, the fracture phenomenon occurred. Furthermore, repair welding of mentioned alloy did not exceed more than three times.