Preparation and Characterization of Slurry Aluminide Coating on TP347H FG Stainless Steel
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UCTION
AUSTENITIC steels have been extensively utilized in supercritical boilers due to the corresponding excellent high temperature mechanical properties and corrosion resistance, especially as tube materials in superheaters and reheaters.[1,2] For an ultra-supercritical (USC) boiler, the operating steam temperature is expected to increase from the current temperature to about 630 C, even to 650 to 700 C in the near future. The currently utilized austenitic steels, such as TP347H (both coarse grained and fine grained) and Super304H, however, are not preferred due to poor oxidation resistance to steam at a higher steam temperature.[3–5] Researches have indicated[3,5,6] that two-layered oxides, consisting mainly of Fe-Cr spinels or/and chromium oxide in the inner layer and iron oxides in the outer layer form on the top surface and spalled due to the significant differences of the thermal expansion coefficient between the oxides and the metal substrate. The spalled oxides might cause the
JINYANG HUANG, JINTAO LU, XINGXING ZHANG, ZHENG YANG, ZHEN YANG, YINGYING DANG, and YONG YUAN are with the Xi’an Thermal Power Research Institute Co. Ltd., No. 136, Xingqing Road, Xi’an 710032, P.R. China. Contact e-mail: [email protected] YONGLI ZHOU is with the School of Energy and Power Engineering, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi an 710049, P.R. China. Manuscript submitted December 18, 2018. Article published online May 29, 2019 3776—VOLUME 50A, AUGUST 2019
boiler tube to be blocked or be washed into the steam turbine. Although the higher level nickel-based or nickel-cobalt-based superalloys, such as the Inconel740H and the Haynes282, will well perform under the demanding conditions, the high costs still limit the ability of widespread use in USC power plants.[7–9] Therefore, austenitic steels are still the preferred materials for ultra-supercritical boilers operating beyond 600 C. In order to sufficiently achieve the performance of austenitic steels, high temperature coatings have been proposed to increase the oxidation resistance of these steels, among which, the aluminide coatings are the most widely used.[10] Until recently, various aluminide coatings deposited through well-known methods have been successfully tested, such as powder packing cementation, Chemical Vapor Deposition (CVD), slurry deposition processes, and High Power Impulse Magnetron Sputtering (HIPIMS).[11–14] For the utility boiler tube, however, decided by the size characteristic with a low diameter and a high length, the slurry process was a better choice to prepare aluminide coatings on the inside surface of boiler tubes. Currently, slurry deposition aluminizing has been proven to improve steam oxidation resistance in certain steels by different researchers.[11,15–17] Nevertheless, the single-aluminide coating has presented certain weaknesses in situations such as brittleness, easy spallation, and relatively weak heat corrosion resistance.[18,19] One effective way to simultaneously improve the mechanical properties, high temperature s
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