Slurry aluminide coatings on boiler tubes for overheater/reheater (I): Preparation and microstructure

The work aims to prepare the high-temperature resistant steam oxidation coating on the inner surface of austenitic stainless steel boiler tube. Al-Si slurry deposition process was employed to prepare aluminide coatings on the inner surface of Super304H boiler tubes used on overheater and reheater. The effects of process parameters (time and temperature) on the structure, composition, phase composition and atomic diffusion of the coatings were studied within the scope of the heat-treatment on steel tube. Meanwhile, a prediction model of coating microstructure was established based on the Fick’s diffusion law. The aluminide coating had a thickness of about 30~150 μm and showed good metallurgical bonding properties with the substrate. The coating had a two-layer structure, with an outer layer of (Fe,Ni)Al(Cr) phase, CrxSiy phases and a little Fe3Al and IDZ of (Fe,Ni)Al phase, CrxSiy phase, spotted M6C phase and needle-like TCP phase. The thickness of IDZ was significantly affected by the temperature and holding time. The thickness variation of this layer followed parabolic law with the holding time and the temperature. Besides, an excessive temperature within the heat treatment regime of tubes could easily reduce the thickness of outer layer. Aluminized coatings are successfully prepared on the inner surface of Super304H austenitic stainless steel boiler tube depending Al-Si slurry deposition process on the basis of matching Super304H boiler tube heat treatment process. A prediction model of coating microstructure control is established based on the Fick's second diffusion law. The prediction results are in good accordance with the experimentation results and can provide a reference for preparing the coatings with different microstructures. © 2020, Chongqing Wujiu Periodicals Press. All rights reserved.