The effect of brazing on microstructure of honeycomb liner materials Hastelloy X and Haynes 214

The efficiency of a turbine is closely associated with minimal clearance between the rotor and the casing. Therefore, rub-in tolerant honeycomb liners, are used as a casing avoiding severe damage of the rotor in case of rub-in events. Hollow structures of honeycomb liners lead to fast heating of the thin walled structure and to yield against the rotor fin.

Based on microstructure examinations of industrial honeycomb liners we observed that the brazing procedure changes the microstructure of the thin nickel-base alloys drastically. In this study we compare the effect of braze alloy BNi-5 on functionality of honeycomb liners, manufactured from the two nickel-base alloys Hastelloy X and Haynes 214.

For Hastelloy X, a nickel-chromium-iron-molybdenum alloy, the braze increases the rub-in volume due to the capillarity path all the way up to the rub-in surface. Furthermore, exceeding of critical gap width, the braze/substrate mixture forms brittle hard phases between foil nodes. Also, the reduction of the braze melting point by silicon to leads to silicon diffusion into the thin honeycomb foil and to a strong reduction of initial foil microstructure. The effect of brazing on Haynes 214, an alloy with a high content of aluminium, lead to formation of hard and uniform inter-diffusion layer on the surfaces of the thin foil. According to EDX measurements, the inter-diffusion layer consists of elongated γ', α-Cr and Cr₂₃C₆. The high brazing temperature result a grain coarsening in both systems. The microstructure examinations of the brazing effect we carried out using SEM, EDX and EBSD, the micro hardness we measured with Fischerscope H100.