Effect of Mn on the Formation of Oxide Buildups Upon HVOF-Sprayed MCrAlY-Ceramic-Type Cermet Coatings
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sai-Shang Huang (Submitted March 30, 2010; in revised form June 6, 2010) Thermal spray coatings have been widely used on hearth rolls in a continuous annealing line to improve steel sheet quality and to prolong the roll service life. One of the common defects formed on a working hearth roll is the oxide buildup. HVOF-sprayed CoCrAlY-CrB2-Y2O3 coating was used in this study to duplicate buildups by reacting with Fe and Mn oxides. The reaction was performed in a furnace at 900 °C with inert gases flowing through. After reacting for 8 days, large Mn-rich buildups were formed on the coating while the buildups without Mn were very small. Mn was shown to enhance the formation of buildups. Buildups from a hearth roll were also examined and compared with the laboratory ones.
Keywords
buildup, cermet, CoCrAlY, hearth roll, HVOF
1. Introduction Surface engineering and coating technology are more and more important for enhancing the performance of many facilities, not only to shorten the maintenance time, but also to cut down cost and save energy. Thermal spray technology is a widely used surface treatment to provide functional coatings for different working environments. Depending on the demands, thermal spray coatings can be designed to be porous, dense, corrosion-resistant, wearresistant, and other different properties. In a steel mill, many components are operated in harsh working conditions such as high temperature, high velocity, or heavy load. For example, hearth rolls in a continuous annealing line (CAL) are operated at temperatures varying from 600 to 1300 °C (Ref 1). In order to achieve the required properties in such a crucial operation environment, hearth rolls are usually thermal sprayed with cermet coatings (Ref 1-3). These coatings not only provide good thermal resistance, but also have a relative high hardness, good wear resistance, and good spalling resistance. One of the common problems on hearth rolls is the formation of oxide buildups. Steel sheets usually have more or less oxide particles formed on the surface even in a reduced atmosphere. When they are continuously annealed in a CAL, these oxide particles could be transferred and built up on the surface of the rolls. At such an elevated temperature, these oxides not just build up on the rolls; they could react with the coating of the rolls. As a Tsai-Shang Huang, China Steel Corporation, Kaohsiung, Taiwan. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
result, buildups usually have high bonding strengths and are not easy to be removed. They tend to generate scars on the surface of the steel sheets being processed and lower the sheet quality. In order to suppress the formation of buildups on hearth rolls, it is desirable to coat the rolls with materials chemically inert to steel sheet and oxides thereupon, especially at elevated temperatures. Many coatings, such as MCrAlY-ceramic-type cermet, have been developed to provide anti-buildup properties for this application (Ref 4-8). However, due to the development of high streng
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