Thermal Spray Roll Coatings for High Strength Steel Applications
William Jarosinsky - Praxair Surface Technologies

Praxair Surface Technologies is affiliated with the Industrial Gas part of Union Carbide. It has about 10% of the employees and its main customer are in the Aerospace or Energy sectors. That being said, I work primarily on projects involved with high volume manufacturers who need high performance components for their processing equipment. It is worth investing in high performance coatings if an unscheduled disruption in production can be avoided in many high volume industries (such as automotive).

Tonight's talk will focus on the treatment for rolls used in the production of galvanized sheet metal. Bill serves on the American Institute of Steel Technology Galvanizing Technology Committee. Coating rolls to prevent downtime (and maintain saleable quality) in galvanized steel production is a main focus of this committee.

The condition of rolls greatly impacts strip quality. As the thickness of strip has decreased (to reduce weight), a more uniform roll surface quality is required. Much of this talk will deal with the way roll coatings are applied to deal with this issue.

First, we will discuss the two processes used to apply coating to rolls. The Thermal Spray process is a continuous application process in which a combustion burner provides heat and velocity to powders to form molten droplets which stick to the surface they are applied to by a sort of mechanical bonding with an overlapping structure. High velocities for powder application are achieved by a HVOF (High Velocity Oxygen Fuel) variant of the process. The D-Gun process provides much higher velocity to the powder which gives coatings applied this way the advantage of higher hardness, better bonding, and higher density. Unlike Thermal Spray, the D-Gun process provides intermittent bursts of coating rather than a continuous spray. For some rolling lines the D-Gun process (applying cermets) is preferred because of superior corrosion resistance and texture obtained.

In Galvanized strip processing there are basically three types of rolls to be treated. Process Rolls are typically carbide coated to promote wear resistance. Furnace rolls demand a coating that takes high temperatures, and resists pickup from the metal stock being annealed. Pot rolls (which are exposed to the molten zinc) have both a temperature requirement and must provide resistance to dross (various oxides) which are formed during the dip coating process.

For furnace rolls there are two types of annealing equipment to be considered: Vertical and Horizontal. Horizontal furnace lines tend to use established aircraft type coating which are slightly modified for this purpose. Vertical furnace lines are designed for higher speed and oxidation resistance becomes a much more serious consideration due to less control of atmosphere. The addition of manganese in strips has made the Oxide pick-up problem much more complicated. In addition to the predominant FeO, now there is potential for pick-up of other oxides such as Cr2O3, MnO, SiO2, TiO2, Al2O3 and CaO. Carbide coatings are good against FeO up to about 850o. Cermets extend the temperature capabilities up to about 1100o. Other ceramic based coatings extend to pick-up resistance at 1100 to MnO.

A major difficulty with the ceramic based coatings is the difference in CTE (coefficient of Thermal Expansion) between it and the roll base materials. This limits the temperature range which they can survive without spalling off of the rolls. Another difficulty (particularly) in Asian furnaces is the use of Aluminosilicate fiber insulation. This crystallizes at about 982 degrees and rains from the furnace interior onto the rolls causing blemishes in product.

Finally there are applications for coatings on pot rolls. Here the main problem is dross which can get onto the rolls causing blemishes in the product. There are two different types of dross, top dross floats on the zinc bath, and bottom dross sinks. Solubility of Iron in zinc is dependent on the Aluminum content, so pots which alternate between regular galvanizing and galvanealing (aluminum containing) pose very difficult challenges to preventing dross pickup. Aluminum can penetrate and react with the coating's binder to produce intermetallics in the coatings. There has been some success with using barrier coatings to prevent this. It is important to know what is going to be in the pot to accurately predict the length of a rolling campaign, but often the mill operators do not have a good handle on this information in advance.

Praxair produces and applies its coatings all over the world. Test pads are used for qualification and detailed written recipes are followed. Rolls typically come from either Japan, France or Brazil.

In conclusion, process rolls (outside the furnace or pot) are the easiest to protect with coatings. Furnace Rolls employ coatings to prevent oxide transfer, but frequently furnaces are not operated as the design would suggest and things like insulation raining can create major problems. Coatings for Pot Rolls are used primarily to fight dross pick-up. They can only delay the pick-up, not actually prevent it. The advantage of the coating lies in giving better predictability to the duration of a galvanizing campaign, making maintenance more predictable. The application of coatings as protection from HS Steel applications is practice that experiences constant change.