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The role of the strengthening phases on the HAZ liquation cracking in a cast Ni‑based superalloy used in industrial gas turbines

Rakoczy Łukasz, Grudzień-Rakoczy Małgorzata, Rutkowski Bogdan, Cygan Rafał, Hanning Fabian, Cios Grzegorz, Habisch Stefan, Andersson Joel, Mayr Peter, Zielińska-Lipiec Anna

Archives of Civil and Mechanical Engineering

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2023

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Vol. 23, no. 2

art. no. e119, 2023

EN

This work presents the influence of microstructural constituents on liquation crack formation in the cast Ni-based superalloy, Rene 108. The investigation was divided into three parts: characterisation of the material's microstructure in pre-weld condition, hot ductility studies and analysis of liquation cracking induced by the gas tungsten arc welding process. Using advanced electron microscopy techniques it is shown that the base material in pre-weld condition is characterised by a complex microstructure. The phases identified in Rene 108 include γ matrix, γ' precipitates, MC and M23C6 carbides, and M5B3 borides. Based on Gleeble testing, it was found that Rene 108 is characterised by high strength at elevated temperatures with a maximum of 1107 MPa at 975 °C. As a result of constitutional liquation, the superalloy’s strength and ductility were significantly reduced. The nil strength temperature was equal to 1292 °C, while the nil ductility temperature was 1225 °C. The low ductility recovery rate (32.1), ratio of ductility recovery (36.2) and hot cracking factor (Rf = 0.05) values confirmed the low weldability of Renѐ 108. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The thin non-equilibrium liquid film, which formed along high-angle grain boundaries, led to crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products are visible on the crack edges.

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Publisher Correction to: Characterization of the as-cast microstructure and selected properties of the X-40 Co-based superalloy produced via lost-wax casting

Rakoczy Łukasz, Grudzień-Rakoczy Małgorzata, Cygan Rafał, Rutkowski Bogdan, Kargul Tomasz, Dudziak Tomasz, Rząd Ewa, Milkovič Ondrej, Zielińska-Lipiec Anna

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 4

art. no. e170, 2022

EN

The thermodynamical simulation predicts the phase transformation of M7C3 to M23C6, proven previously via electron microscopy. Some other reported experimental works suggest that this can also take place also during heating [22, 45, 46]. Considering this, the melting process of the primary M7C3 carbide can be that the M7C3 first undergoes a phase transformation into M23C6 and then melts, instead of directly melting. A similar conclusion was given by Gui et al. [47-49] based on experiments on the Co-based superalloy strengthened (in as-cast condition) by M7C3 and MC carbides. It was suggested that the creation of the liquid phase follows the reaction M23C6 + α→L. The reaction was initiated on the M23C6/α interface and proceeded towards the center in the range of 1280 - 1348 ˚C. Before melting, the MC eutectic carbide degenerated, and its morphology changes to a well-rounded shape. Exceeding 1400 ˚C leads to the melting reaction of MC + α→L in the X-40 Co-based superalloy.

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Characterization of the as‑cast microstructure and selected properties of the X‑40 Co‑based superalloy produced via lost‑wax casting

Rakoczy Łukasz, Grudzień-Rakoczy Małgorzata, Cygan Rafał, Rutkowski Bogdan, Kargul Tomasz, Dudziak Tomasz, Rząd Ewa, Milkovič Ondrej, Zielińska-Lipiec Anna

Archives of Civil and Mechanical Engineering

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2022

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Vol. 22, no. 3

art. no. e143

EN

The X-40 Co-based superalloy is often used in the aerospace industry directly in as-cast condition and its analysis in this state is essential to understand further possible phase transformations during service. With this in mind, this work focuses on characterizing the material’s as-cast microstructure, phase transformation temperatures and oxidation resistance. Observations and analyses were performed via thermodynamic simulations, X-ray diffraction (XRD), light microscopy (LM), scanning electron microscopy (SEM), scanning-transmission electron microscopy (STEM-HAADF), energy-dispersive X-ray spectroscopy (EDX), dilatometry (DIL) and differential scanning calorimetry (DSC). The microstructure of the dendritic regions consisted of the α matrix, with MC, M7C3 and M23C6 carbides being present in the interdendritic spaces. Based on DIL, it was found that precipitation of the Cr-rich carbides from the saturated α matrix may occur in the range 650-750 °C. DSC determined the incipient melting and liquidus temperatures of the X-40 superalloy during heating to be 1405 °C and 1421 °C, respectively. Based on oxidation resistance tests carried out at 860 °C, it was found that the mass gain after 500 h exposure was 3 times higher in the air than in steam.

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Characterization of the microstructure, microsegregation, and phase composition of ex-situ Fe–Ni–Cr–Al–Mo–TiCp composites fabricated by three-dimensional plasma metal deposition on 10CrMo9–10 steel

Rakoczy Łukasz, Hoefer Kevin, Grudzień-Rakoczy Małgorzata, Rutkowski Bogdan, Goły Marcin, Auerbach Torsten, Cygan Rafał, Abstoss Kevin Gordon, Zielińska-Lipiec Anna, Mayr Peter

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 4

437--455

EN

Quaternary powder mixtures yNi–20Cr–1.5Al–xTiCp (y = 78.5, 73.5, 68.5; x = 0, 5, 10) were deposited on ferritic 10CrMo9–10 steel to form on plates ex-situ composite coatings with austenitic-based matrix. Plasma deposition was carried out with various parameters to obtain eight variants. The microstructure, chemical composition, phase constitution, phase transformation temperatures, and microhardness of the two reference TiCp-free coatings and six ex-situ composites were investigated by X-ray diffraction, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, thermodynamic simulation, and Vickers microhardness measurements. All composites had an austenite matrix with lattice parameter a = 3.5891–3.6062 Å, calculated according to the Nelson–Riley extrapolation. Microstructural observations revealed irregular distribution of TiCp in the composites. Large particles generally occurred near the external surface due to the acting buoyancy effect, whereas in the interior smaller particles, with an equivalent radius around 0.2–0.6 μm, were present. Due to initial differences in the chemical composition of powder mixtures and also subsequent intensive mixing with the low-alloy steel in the liquid pool, the matrix of the composites was characterized by various chemical compositions with a dominating iron concentration. Interaction of TiCp with matrix during deposition led to the formation of nano-precipitates of M23C6 carbides at the interfaces. Based on the ThermoCalc simulation, the highest solidus and liquidus temperatures of the matrix were calculated to be for the composite fabricated by deposition of 73.5Ni–20Cr–1.5Al–5TiCp powder mixture at I = 130 A. The mean microhardness of the TiCp-free coatings was in the range 138–146 μHV0.1, whereas composites had hardnesses at least 50% higher, depending on the initial content of TiCp.