Properties of Coatings for Continuous Galvanizing Line Heat Treatment Hearth Rolls

Abstract

The surfaces of rolls used in the hearths of continuous galvanizing lines (CGL) need to maintain a specific roughness and to be able to combat against the formation and growth of oxide build-ups and metallic pick-ups. Thermally sprayed metallic, ceramic and cermet coatings have been utilized in this application, and these coatings need to have adequate thermal shock resistance as well. In the experimental part of this thesis, eight different coating types were subjected to wear resistance, thermal shock and build-up formation tests. Very high quality MCrAlY coatings were successfully sprayed with the high velocity air fuel (HVAF) method. The addition of alumina increased the wear resistances of the high velocity oxygen fuel- (HVOF) and HVAF-sprayed MCrAlY coatings without noticeable drawbacks. The metallic and cermet MCrAlY coatings sprayed with HVOF and HVAF methods formed large build-ups in the iron oxide build-up test probably due to not having pre-oxidized oxide layers. The MCrAlY coatings sprayed with the atmosphere plasma spraying (APS) method absorbed more manganese in the manganese oxide build-up test probably due to the greater surface area. The increasing thickness of the formed oxide layers on top of the coatings did not have an increasing effect on the amount of formed manganese oxide compounds.