PSI - Issue 30

S.P. Yakovleva et al. / Procedia Structural Integrity 30 (2020) 201–208 Yakovleva S. P. et al. / Structural Integrity Procedia 00 (2020) 000–000

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which slows down the diffusion and largely provides the alloys with high-temperature strength and heat resistance. In addition, the samples of a heat insert showed a decrease in the carbon content to <0.0005% (mass) which indicates decarburization due to overheating during operation. The material of a heat insert is chemically closer to low-alloyed nickel alloys, the softening process of which is controlled by the development of recrystallization under prolonged exposure to high temperatures as pointed by Bernshtein and Rakhshtadt (1985), Haessner (1978). Indeed, the structure of a heat insert is characterized by heterogeneity (Fig. 2, a ). Table 1. С hemical composition of the metal of a heat insert and the standard for the HN60VT alloy (base - nickel) Material C Si Mn Cr Mo Fe W Ti Al Conventionally 0.003 0.28 0.49 26.5 0.082 2.75 0.050 0.68 0.023 initial Heat insert <0.0005 0.28 0.46 26.5 0.081 1.49 0.037 0.63 0.17 HN60VT ≤ 0.10 ≤ 0.80 ≤ 0.50 23.5-26.5 - ≤ 4.00 13.0-16.0 0.3-0.7 ≤ 0.5 The grain size varies from 5 microns to 50 microns and higher due to the recrystallization processes (Fig. 2, b ) which reduce the heat-resistant characteristics. At the most thermally stressed sections we can observe the intensive dissolution of the strengthening γ '-phase in a solid solution, followed by the formation of new larger particles of this phase, their coagulation, as well as coarsening of grain-boundary formations which sharply reduces the heat resistance as it described by Kleshev et al. (1995), Haessner (1978). A significant amount of the γ '-phase of the plate modification is observed (Fig. 2, c ) which is characterized by incoherency with a matrix (a factor that promotes softening at high temperatures) as it shown by Sims et al. (1987), Nazarov and Maslenkov (1970), Kolachev et al. (1981), Reed (2008). The identified features of metal degradation of a heat insert indicate insufficient thermal stability of the hardening phases and the matrix, on which the service properties of nickel-based alloys depend. a b

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d

Fig. 2. Features of the metal microstructure of a heat insert: nonhomogeneity ( a ), recrystallization ( b ), sheet-like phases ( c ), porosity ( d ). The arrows show recrystallized grains