Heat treatment of codeposited aluminium-silicon coatings on ferritic steels by CVD-FBR technology

Ferritic steels are often used in boiler or supercritical steam turbines which operate at temperatures between 600-650 °C under pressure. Nowadays generation energy industries are interested in increasing the efficiency of the turbines. Increase of the operation temperature should meet this objective. In this study Al/Si protective coatings are developed by CVD-FBR at moderate temperature to respect substrates mechanical properties. Al-Si codeposition had been formed on three ferritic steels (P-91, P-92 and HCM-12 A). The initial parameters of the process were optimized by thermodynamic calculations using Thermo-Calc software. Then, those parameters were used in the experiments to obtain Al-Si coatings at low temperature and atmospheric pressure. Fe-Al intermetallic coatings containing Cr and Si were obtained. The effect of diffusion heat treatment was studied in order to allow the phase transformation from Fe2Al5 (brittle phase) to lower Al content phases such as FeAl (more fracture toughness).