Surface recrystallisation and oxidation of polycrystalline superalloy castings during isothermal exposure at 1100°C

Abstract

The surface recrystallisation and oxidation of polycrystalline superalloy castings with different casting moduli were studied. Specimens exposed at 1100°C for 2 h in hydrogen, and at 1100°C for 2, 20 and 60 h in air were examined by optical and scanning electron microscopy and energy dispersive spectroscopy. No surface recrystallisation was observed on the surfaces of heavy section castings with moduli of 2·8 or 5·2 mm after exposure at 1100°C, and the oxidation scale was only 50 μm in thickness after exposure in air for 60 h. Surface recrystallisation was observed in the thin wall casting of 0·28 mm modulus; recrystallisation produced higher oxidation rates, owing to enhanced diffusion, and the oxidation scale thickness was 120 μm after 60 h. The mechanism of surface recrystallisation and its effect in accelerating surface oxidation are discussed.

Keywords:

• Superalloy
• Investment casting
• Modulus
• Recrystallisation
• Residual stress
• Oxidation

The authors would like to thank Professor Pingli Mao for the helpful discussion.