Papers by Keyword: Nickel-Based Alloy

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Authors: D.W. Walsh, Mark L. Bright, Trevor L. Jackson, D.B. Gibbs
Abstract: Incoloy 903 overlays have been used to provide hydrogen environment embrittlement (HEE) resistance to welds in nickel alloy 718 structures. This is problematic because application of the required overlays has a history of high rejection and rework due to interpass microfissuring. Kovar has been identified as a potential hydrogen resistant replacement for Incoloy 903. A weldability study was initiated to compare the hot crack (microfissure) resistance of the two alloys to determine if substitution of Kovar for Incoloy 903 has the potential to improve the fabricability of HEE overlays. Varestraint testing indicates that Kovar has much higher crack initiation strains for both HAZ and weld metal cracking. Crack initiation strains were approximately 2% for Kovar while Incoloy 903 crack initiation strains were only 0.25% . Maximum crack lengths (MCL) observed on Kovar Varestraint tests were 0.12mm and 0.58mm for base and weld metal respectively, while 903 MCLs were 0.56mm and 2.3mm. Gleeble hot ductility testing indicates that Kovar has a nil ductility range of 7 degrees C while Incoloy 903 has a range of approximately 45 degrees C. The larger range observed for 903 is an indication of its greater crack susceptibility. Fabricability was correlated to material microstructure using optical microscopy, scanning electron microscopy and microprobe analysis.
Authors: Zhong De Shi, Amr Elfizy, Helmi Attia
Abstract: An experimental study was undertaken to explore the conditions and performance on rough and finish grinding fir-tree root forms of turbine blades made of a nickel-based alloy using vitrified CBN wheels and water-based grinding fluid. This work was motivated by switching the grinding of fir-tree root forms from grinding with conventional abrasive wheels to vitrified CBN wheels for reducing overall production cost and enhancing productivity. Grinding experiments were conducted to measure grinding forces, power, surface roughness, and stress near the blade roots under various dressing and grinding conditions. Wheel re-dressing life in terms of the total number of good parts ground between dressing was tested with the condition producing the maximum material removal rate while satisfying preset part quality and process requirements. It was found that the maximum material removal rate achievable in rough grinding was restricted by the stress limit and the wheel re-dressing life was dominated by the radial wheel wear limit. The targeting part quality and process requirements were achieved. It was proved that vitrified CBN grinding process is feasible and very promising to machine fir-tree root forms.
Authors: P. Szakàlos
Abstract: This paper gives an overview of the different processes of metal dusting (MD) that operate on low and high alloyed iron and nickel base alloys exposed in CO+H2–containing environments with or without water vapour. MD of pure metals like iron and nickel occur with a solely carbon-induced corrosion mechanism. However, in high alloyed materials with strong oxide formers such as Cr and Al, a more complex MD-process takes place which involves both carbon and oxygen in close collaboration. The “alloyed” carbides, i.e normally Cr-containing carbides, formed in such materials are thermodynamically stable with respect to the carbon activity. However, in the reaction front of a MD-pit with non-protective spinel oxide, they destabilise and dissolve due to the influence of the low oxygen activity determined by CO-dissociation. Based on recent results in the field of MD a chart with tentative MD mechanisms is presented as a function of alloy composition and temperature.
Authors: M. Watanabe, Z. Horita, Takayoshi Fujinami, T. Sano, Minoru Nemoto
Authors: Masaomi Mitsutake, Yoshihiro Terada, Takashi Matsuo
Abstract: The features of the creep deformation of γ-single phase single crystals with the composition of Ni-20mass%Cr are characterized by the extended transient stage, which consists of Stage I and Stage II. In the Stage I, the creep rate just after loading remains unchanged, while the creep rate decreases continuously in Stage II. In the single crystals except for the single crystals with the stress axis of [001] and [1, – 11], the predominant creep deformation using the primary slip plane continues. By this deformation, the cross section of specimen turns to elliptical in shape. However, in the single crystals with the angle between stress axis and primary slip plane (111) is more than 45°, the deformation using the primary slip plane does not continue, as a result, the duration of Stage II turn to shorter one. The single crystal with the stress axis of [011] has the largest angle of 55°. In this study, the deformation manner during transient stage of single crystal with the stress axis of [011] orientation is investigated from the two viewpoints. The first one is to clarify the change in deformation manner with decreasing the stress. As a result, with decreasing the stress, the Stage I become clear and strain during Stage I and Stage II become small, furthermore, the decreasing ratio of creep rate with definite strain becomes larger. While, the second viewpoint is to investigate the change in crystallographic orientation of the [011] single crystals with creep deformation using the inverse pole figure obtained by the EBSD method. As a result, at the stress of 29.4 MPa, the spot of stress axis turns from the [011]-[1, – 11] line to the <1, – 01> direction. While, at the stress of 19.6 MPa, the stress axis moves for the [1, – 11] pole along the [011]-[1, – 11] line from the [011] pole. And, it is noteworthy that the spot widely spread from the [011] pole during transient stage. This indicates the large distortion in the primary slip plane and the evidence of heterogeneous deformation.
Authors: Kentaro Yuge, Yoshihiro Terada, Takashi Matsuo
Abstract: The creep deformations of γ-single phase Ni-20mass%Cr single crystals with stress axes within standard stereographic triangle and at the three pole positions have been investigated. The most of the creep life is occupied by the transient stage, which consists of Stage I and Stage II. In Stage I, the creep rate just after loading remains constant. In Stage II, the creep rate decreases continuously. Except for the single crystals with stress axes of [001] and [1,–11] poles, the single crystals make the creep deformation using the primary slip plane of (111). As a result, the cross section of the specimens turns from circular to elliptical in shape. However, there are marked difference in deformation manner among single crystals with the stress axes within standard stereographic triangle. The single crystals whose angle between stress axis and primary slip plane of (111), θ. is more than 45° shows the heterogeneous deformation during creep. While, the homogeneous deformation will be expected in the single crystals with θ less than 45°. In this study, by using the four single crystals with θ less than 45°, the change in the stress axis with the creep deformation at 1173K-29.4MPa, is investigated and the deformation manner due to the primary slip plane of (111) is estimated by conducting the creep interrupting tests. In the two single crystals with stress axes in the standard stereographic triangle where the moving range of θ is narrow, comparing to the others, the spot of the stress axis in the inverse pole figure moves for <1,– 01> direction by using (111)<1,–01> slip system, and after arriving at the [001]-[1,–11] line, the spot turns to its direction for [1,–11] pole using (111)<1,–10> slip system. While, in the other two single crystals whose stress axes located in the area with wider moving range of θ, the spot of stress axis only move for <1,–01> direction. And, the widely spread spot of the stress axis is confirmed after subjecting the small strain.
Authors: K. Khenfer, S. Lebaili, S. Hamar-Thibault
Abstract: The mechanical engineering industry is always in the search of new hardfacing alloys which nowadays make it possible to solve considerable delicate problems of abrasive wear and corrosion. These alloys are heterogeneous materials and are generally composed of a ductile matrix and a significant proportion in hard phases (carbides, borides or silicides), which give a good wear resistance and corrosion. The following studies made on alloys containing Ni, rich in Silicon and tungsten. We noted that addition of these elements (Si and W) got interesting mechanical properties. The alloys that we chose for this study contain the elements C, B, Si, Cr and W with additions in Ti and Mo. The aim of this work is the characterisation of the various phases formed during solidification, to study chemical affinities in this system poly-constitutes and to make a correlation between the chemical composition, the conditions of development and the microstructures obtained.
Authors: Mohamed Sennour, Loïc Marchetti, Stéphane Perrin, Régine Molins, Michèle Pijolat, Olivier Raquet
Abstract: The oxide film formed on nickel-based alloys in Pressurized Water Reactors (PWR) primary coolant conditions (325°C, aqueous media) has been investigated by Transmission Electron Microscopy (TEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni(1-x)FexCr2O4). Moreover, nodules of Cr2O3 were present at the interface between this spinel and the alloy. The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni(1-z)Fe(2+z)O4) resulting from precipitation phenomena. The influence of alloy surface defects was also studied underlining two main consequences on the formation of the passive film e.g. the internal layer. On one hand, the growth kinetics of the internal spinel rich in chromium increased with the surface defect density. Besides that, when the defect density increased, the oxide scale became more finely crystallized. This result agrees with a growth mechanism due to a rate limiting process of diffusion through the grain boundaries of the oxide. On the other hand, the quantity of Cr2O3 nodules increased with the number of surface defects, revealing that the nodules nucleated preferentially at defect location.
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