PSI - Issue 43

Vít Horník et al. / Procedia Structural Integrity 43 (2023) 136–141 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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is predestinated for use at high temperatures and/or significant thermal gradients, high-frequency vibrations, and centrifugal forces loading, e.g. Reed (2008), Pineau and Antolovich (2009). A disadvantage of Ni-based superalloys is most frequently processed by casting, which inevitably leads to the occurrence of casting defects (pores). The effect of casting defects as a stress concentrator on the fatigue behavior of superalloys was investigated in many papers, e.g. Pollock and Tin (2006), Kunz et al. (2012), and Šmíd et al. (20 20), and is generally accepted that the fatigue lifetime decreases with increasing casting defect size. The mechanism of fatigue crack initiation and propagation is strongly influenced by the operating temperature. The crystallographic propagation stage I regime vs non-crystallographic stage II regime of crack propagation and their mixture are characteristic for the fracture surfaces of failed specimens (facets). The transition temperatures between crystallographic and non-crystallographic crack propagation for Ni-based superalloys are in a range from 650 to 1000 °C as presented in MacLachlan and Knowles (2001), Pollock and Tin (2006), Pineau and Antolovich (2009), Šmíd et al. (2016), Šmíd et al. (b) (2016). The transition temperature range depends on chemical composition of an alloy and volume fraction of strengthening precipitates and on the occurrence of secondary phases, such as carbides and borides. The aim of this work is to determine the high-cycle fatigue properties of the IN 738LC superalloy at temperatures of 800, 900, and 950 °C in fully reverse loading. Detailed fractographic analysis with the aim to identify fatigue crack initiation and to describe fatigue crack propagation was performed. The character of the fatigue crack propagation with regards to test temperature was analyzed with close relation to potential change from the crystallographic to the non-crystalographic fatigue crack propagation. The obtained fatigue life data were compared with IN 713LC and MAR-M 247 superalloys. 2. Material The cast polycrystalline IN 738LC superalloy in a form of pre- cast rods was provided by PBS Velká Bíteš company. The chemical composition of the studied superalloy was following (in wt. %): 0.09 C, 15.83 Cr, 8.46 Co, 3.42 Ti, 3.32 Al, 2.46 W, 1.73 Ta, 1.71 Mo, 0.78 Nb, 0.03 Zr, 0.008 B, balance Ni. The casting temperature was 1385 ± 15 °C. After the casting, there was no hot isostatic pressing treatment applied. The pre-cast rods were processed by two-step heat treatment consisting of solution annealing at 1120 ± 15 °C for 2 hours with a cooling rate higher than 25 °C per minute by argon and precipitation annealing at 843 ± 15 °C for 24 hours with cooling by argon cooling fan. The final structure of the processed IN 738LC superalloy is a heterogeneous coarse dendritic with an average grain size of about 1.5 ± 0.3 mm (measured by the linear intercept method on 10 different areas of the microstructure). The material structure consists of γ matrix and γ’ precipitates (approx. 57 % of the volume fraction). Moreover, γ / γ’ eutectics, fine carbides, and borides are present along grain boundaries and in interdendritic areas, as shown in Fig. 1. Numerous casting defects with a size range from 220 to 700 μm were observed in the cast structure.

Fig. 1. Microstructure of IN 738LC.

Fig. 2. Fatigue specimen geometry.