Critical and complex pump and valve components can now be engineered for high wear resistance through the application of cobalt-chrome-based coatings and tungsten carbides via a process that results in a strong metallurgical bond that is free of voids and with minimal dilution. Of importance, the process can also reach non-line-of-sight features that may encounter consistency problems with traditional thermal spray or weld overlay, according to its supplier, Kennametal.
With the ability to customize the coating thickness and treat complex geometries, the UltraFlex technology results in significantly extended component life, more predictable wear and maintenance requirements, higher productivity, and reduced operating costs, the company says. The UltraFlex portfolio is said to provide a wide range of material solutions to protect against abrasive, erosive, corrosive, and galling wear for base materials including carbon and alloy steels, stainless steels, and cobalt and nickel alloys. The portfolio includes tungsten carbide, as well as cobalt-chrome-tungsten-carbon, cobalt-chrome-molybdenum alloy, and cobalt-chrome-molybdenum-silicon alloys that come under Kennametal’s new Stellite product range.
Parts with complex geometries, inner diameters, and other non-line-of-sight features may encounter consistency problems with traditional thermal spray or weld overlay. Thermal spraying forms a mechanical bond, while weld overlays typically have large heat-affected zones that can impact properties of the surface treatment.
Kennametal says UltraFlex readily conforms to intricate shapes and small internal diameters and reaches the remote locations that other hardfacing technologies cannot. The surface treatment can be applied in a range of thicknesses, from approximately 0.254 to 2.030 mm (0.010 to 0.080 in). Depending on the application, surface can be used “as applied” or machined to the desired finish.
Surface treatments with mechanical bonding may be susceptible in harsh environments due to porosity and cracking. The company, based in Latrobe, Pa., near Pittsburgh, says its proprietary process metallurgically bonds the wear system directly to the substrate, providing a surface with consistent thickness and minimal dilution. Metallurgical bonding results in a fusion between the cladding and the substrate. For components such as severe service valves, Kennametal’s Stellite cobalt-chrome alloy-based grade ensures that there is no cracking or delamination of the surface to protect the substrate.
For components such as the bottoms pumps in fluid catalytic cracking units, harsh abrasion and erosion can destroy them within a matter of months. Kennametal’s cobalt-chrome-tungsten-carbide based solution — applied to the wetted surface of the component — improves component life by up to four times, it has been reported.
Pump Impeller Protected
A U.S. Gulf Coast refinery had been experiencing accelerated internal wear of oil pumps used as the bottoms pumps in a fluidized catalytic cracking unit (FCCU) and often required entire case replacements. Compared to typical industry pump performance, run lengths were about a month shorter and maintenance-related costs higher. The application requires pumps capable of withstanding significant catalyst fines.
The refinery investigated several wear-resistant products for its pumps but found limited suitability. Thermal spray coatings and weld overlays could be used in line-of-sight applications but not for small interior surfaces. The refinery found that thermal sprays also suffer from mechanical bond failure and there was preferential erosion of the soft matrix between the hard particles in weld overlays. Hard diffusion coatings (boronizing), meanwhile, are thin and can be punctured by large abrasive particles.
Working with Kennametal, the refinery used infiltration-brazed tungsten carbide cladding (Kennametal’s Conforma Clad process for surfaces that can be reached by hand) for line-of-sight components and UltraFlex to coat the surfaces on the inside of the vanes of the impellers as well as the eye of the sealed pump impeller, which was unreachable by any other means.
The company particularly found suitable the true metallurgical bond and the dense, uniform distribution of tungsten carbide particles throughout the coating, which created excellent erosion resistance and toughness in complex geometries. In general tungsten carbide grades perform better in abrasive or erosive environments, while Stellite alloy grades perform better in high-temperature, metal-on-metal, or corrosive environments.
After 13 months in service, a treated oil pump at the refinery showed no significant signs of internal erosion. The Kennametal technology also has been applied to FCCU fractionator bottoms and debutanizer reboiler services, with no signs to date of increased vibration or loss of hydraulic performance.
Severe Service Valves
Modern valves control flow of all types, including thin gases, highly corrosive chemicals, superheated steam, abrasive slurries, toxic gases, and radioactive materials. Valves can handle temperatures from subfreezing to those of molten metal, and pressures up to thousands of pounds per square centimeter.
The severe service metal-seated ball valves used in harsh services are made of exotic metal alloys and require coatings to protect the surfaces against a combination of corrosion and wear. Thermal spray and Stellite weld overlay coatings are widely used to protect balls and seats. But as customers require improved performance for longer component life, new material solutions are needed.
Kennametal has developed surface treatment technologies that metallurgically fuse advanced metal compositions onto balls and seats, resulting in “exceptional” wear and corrosion resistance in environments where chemicals are transported, often in very high temperatures or containing erosive particulates. UltraFlex technology reportedly enables Kennametal to select the most suitable engineered-material coating that will resist specific conditions. Velan, a leading valve manufacturer, has recently embraced UltraFlex technology, applying it on its severe service metal-seated ball valves used in the petrochemical industry.
For components that fit into mechanical systems requiring tight clearances (e.g., pump impellers), it is especially important for pump OEMs to work closely with Kennametal so that the components can be designed to incorporate our claddings. Kennametal says the end user usually must specify the cladding to the component OEM to begin the application discussion, and adds that OEMs often wind up offering the claddings as standard.
Kennametal carries out coating services at facilities throughout North America. The UltraFlex technology is currently offered at facilities in New Albany, Ind., and Belleville, Ont. There are also existing resources in place for enabling coating production at other sites around the world as the need arises.