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. [1]
The key to increased life is reducing the coating material degradation during the application process.
Our HVAF Coating Features
Kermetico High Velocity Air Fuel (HVAF) Coatings have been shown to be very competitive in protecting steel rolls in annealing and continuous galvanizing lines.
The Kermetico HVAF AK7 System Spraying a Roll with a Cr3C2-NiCr Coating
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.
The Main Features of Kermetico HVAF and HVOF Tungsten Carbide Coatings
| WCCoCr 86/10/4 Coating Feature | Typical HVOF | Kermetico HVAF Mode | ||
| Economy | Balanced | Ultra | ||
| Deposition efficiency, % | 40-55 | 60-70 | 48-58 | 36-42 |
| Hardness, HV300 | 1,050-1,250 | 1,050-1,250 | 1,250-1,350 | 1,450-1,600+ |
| Young’s Modulus E, GPa | 300 | 400 | 450 | 450 |
| Apparent metallographic porosity | <0.8 | <0.8 | <0.5 | <0.1 |
| Bond strength, MPa (PSI) | 80+ (12,000 +) | |||
| Range of as-sprayed roughness, µm (µ in.) | 3.5-4.5 (150-180) | 1-3.2 (40-130) | ||
Numerous studies of HVAF sprayed MCrAlY coatings have shown their high hot corrosion and wear resistance, high-quality microstructures with low oxide content, high retention of the powder chemistry and low porosity.
Notes on Kermetico HVAF Equipment for 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.
We heat the feedstock material to near its liquid phase temperature without exceeding it.
Then we accelerate the particles 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.
Spraying a Tension Lever Shell used in the Steel Industry with our HVAF AK6
Blast and Spray Steel Rolls with Kermetico High Velocity Equipment
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.
Kermetico HVAF Thermal Spray Coating Equipment
Kermetico designs and manufactures three families of HVAF thermal spray equipment.
- Convertible HVOF and HVAF equipment:
- Multi-purpose HVAF AK systems, with a variety of spray guns:
- AK7 – a high-power gun for the thermal spray of large parts – recommended for steel rolls
- AK6 – a carbide gun to spray a balanced mix of parts in a job shop
- AK5 – a compact gun to spray smaller parts, thin walls and complex surfaces
- AK-ID – an 80 mm (3.15”) and larger internal diameter spray gun
- AK-IDR – a rotating ID gun to spray 4” (100 mm) and larger bores
- AK-HH – a handheld gun for on-site coating
- Specialized HVAF S equipment:
- Additional equipment:
- An HVAF grit feeder for the grit blast preparation of parts to reduce coating costs
- A Gas Permeability Tester for an instant check of coating through porosity
Kermetico, Inc. – HVAF and HVOF Material Science Excellence, Equipment Manufacturing and Thermal Spray Coating Since 2006
We create equipment that helps material scientists, engineers and business managers achieve their goals.
We have installed more than 60 Kermetico HVAF & HVOF systems in the USA, Europe, Japan and China.
Some of the systems are at work in Universities and National Labs, but most of them are used in production thermal spray shops.
We proudly design and produce our thermal spray equipment in California and install it all over the world.
You can visit our R&D center in Benicia to meet our designers and see our HVAF and HVOF equipment in action.
We also deposit HVAF and HVOF thermal spray coatings for customers in North America – ourselves and through our network of partners
Literature
- “Thermal Spray Applications in the Steel Industry,” Charles M. Kay, ASM Handbook, Volume 5A, 2013 Thermal Spray Technology
-
- Kermetico HVAF Coated Steel Roll
