HVAF Wear Resistant Coatings for Steel Rolls
There are a significant and growing number of steel rolls where high velocity thermal spray coatings are feasible.
Annealing Line Rolls are being protected mainly with coatings using oxidation-resistant MCrAlY cermets for extremely high furnace temperatures. For lower-temperature heat treatment of low-manganese steels, Cr3C2-NiCr coatings are used.
For high-temperature annealing of low-manganese steels, oxides, borides or cermets of MCrAlYs and alumina have been used in production. High-manganese steels result in more severe problems for rolls.
Corrosion and oxidation products, typically in the form of oxides of chromium and aluminum on MCrAlY coatings, react with manganese from the steel reducing the service life of the coatings and the quality of the steel sheets. We provide HVAF coating solutions that optimize MCrAlY coating properties.
Continuous Galvanizing Lines
Degradation of sink rolls and other rolls associated with continuous galvanizing lines is due to zinc or aluminum reactions with iron from the steel rolls. These reaction products degrade the rolls and affect the surface of the steel products. Some of these reaction products are called dross. The most commonly used zinc baths are galvanized zinc with minor concentrations of aluminum and minor amounts of silicon.
Currently, coatings use tungsten carbide/cobalt powders applied via high velocity thermal spray technology. The success of these coatings depends on the spray parameters, powder manufacturing method, and sealant system. The key to increased life is reducing the coating material degradation during the application process.
HVAF Coatings
Kermetico High Velocity Air Fuel (HVAF) Coatings have been shown to be very competitive in protecting steel rolls in annealing and continuous galvanizing lines.
Kermetico HVAF carbide coatings are superior to HVOF and Detonation rivals regarding wear resistance, corrosion protection and production cost. The high velocity of the in-flight particles (higher than 1,000 m/s | 3,300 ft./sec.) in the Kermetico HVAF process enables the production of very dense coatings with high bond strength.
Moreover, the low combustion spraying temperature (1,960-2,010°C | 3,560-3,650°F depending on fuel gas) and gentle particle heating lead to minimal feedstock phase transformation and almost nonexistent elemental depletion or decomposition of the in-flight particles.
Furthermore, the replacement of pure oxygen in the HVOF coating process with air in the Kermetico HVAF process significantly reduces the oxide content in the coatings, which is desirable for high-performance coatings.
Steel Roll Coatings
The traditional thermal spray approach is to melt and atomize the feedstock, propel it to the surface of the target part whereupon contact ‘splat cooling’ builds up a coating.
The Kermetico HVAF process operates differently.
The feedstock heats the material to near its liquid phase temperature without exceeding it.
The particles are then accelerated to an optimized high velocity, and when the particles impact the substrate, there is a rapid conversion of kinetic to thermal energy that allows for the plastic deformation of the particle and a bond which we cannot accurately measure.
In the ASTM 633C bond test, the only result we get is broken glue, even with 0.040“ (1 mm) of WCCo 88/12.
Blasting
Usually, we deposit coatings using robotic blast and spray operations.
We blast a surface with a Kermetico HVAF gun (it is extremely fast and uniform) and spray with the same gun after switching the powder feed hose and perhaps changing the nozzle.
It is much faster, more accurate and needs much less grit than manual blasting.
It also provides very even surface preparation and induces less stress into the base metal.
