The Repair Process: Thermal Spray Coatings
Thermal spray coatings involve spraying heated powder materials onto a surface substrate. Even with minimal wear, to restore a product’s ability to seal properly and put it back into service, it’s essential to strip and re-coat the ball, as well as remanufacture and coat the seats. It's uncommon for a damaged valve to be simply cleaned and lapped; in most cases, the coating must be removed and reapplied. Two common thermal spray coating processes are used: High-Velocity Oxygen Fuel (HVOF) and Air Plasma Spray (APS).
Tungsten carbide (WC) is one type of coating applied using High-Velocity Oxygen Fuel (HVOF) technology. Unlike spray and fuse or weld overlays, which form a metallurgical bond, HVOF carbide coatings create a mechanical bond. Tungsten carbide coatings are known for their ability to withstand high temperatures and offer toughness, making them effective against abrasion, erosion, and corrosion. Depending on the specific conditions of the valve's service, different coatings may be more suitable. While you might not start with the hardest tungsten carbide coating matrix, you may find that the corrosion chemistry of the service demands a different type of coating. The matrix can be adjusted to change the corrosion resistance without compromising abrasion resistance or hardness, as tungsten carbide remains present.