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, wear- resistant, 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℃. In order to achieve the required properties in such a crucial operation environment, hearth rolls are usually thermal sprayed with cermet coatings. 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 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. However, due to the development of high strength steel for the automobile industry, these coatings become vulnerable to buildup forming again. High strength steel contains large amount of Mn, 1-2% in many cases, as a solid-solution strengthening additive. Unfortunately, it was found that Mn or Mn oxide is easy to react with the conventional MCrAlY-ceramic- type cermet coatings used on hearth rolls, resulting in the buildups of MnAl2O4 and Mn1.5Cr1.5O4. Manufacturing the high strength steel, however, becomes a challenge since the steel sheet is harder and is easier to develop buildups on the rolls.
In order to understand the effect of Mn on the buildup formation upon the MCrAlY-ceramic-type cermet coatings, accelerating experiments with and without Mn oxides have been conducted in the laboratory to simulate the reaction between steel sheets and coated hearth rolls in a CAL. In addition to the laboratory results, actual buildups from a used hearth roll were also examined and discussed.
Effect of Mn on the buildup formation is apparent from this study. Large Mn buildups were duplicated in the laboratory, while the buildups without Mn were very small. It is suggested that Fe oxide does not react with MCrAlY-ceramic-type cermet coatings too much. However, Mn oxide is active and will react with this type of coating. Once a buildup is initiated by Mn oxide, Fe oxide is not inactive anymore. It will start to accumulate on the existing buildups and even accelerate the growing process afterward. Mn is shown to enhance the formation of buildups.
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