Coatings

Are you tired of continuously replacing your process components time and time again? Do you find yourself “caught between a rock and a hard place” of juggling your short-term budget limitations vs maintaining your equipment with a long-term “cost of ownership” approach? Do yourself a favor – don’t replace your high-wear components before FIRST speaking with Palmer Tool about the potential benefits of HVOF or PTA to EXTEND the service life of your equipment.

Hard-Facing – Plasma Transfer Arc Welding (PTA)

Customer applications which frequently benefit from PTA welding include chemical processing, dry powders, mining, power generation, oil and gas, cement/asphalt, steel mills, pulp and paper mills, particle reduction or recycling, rotary airlocks or other rotating components, and many more. The Plasma Transferred Arc process (also known as PTA) is a melting welding process to apply wear and corrosion resistant layers (hard-facing) onto metal surfaces. The hard-facing is added with a superalloy metal (in wire or powder form), such as Stellite 6 or Colmonoy 88.  In addition to hard-facing, softer alloys or carbide composites can be used to achieve diverse properties such as improved mechanical strength, wear resistance, galling resistance, and corrosion resistance.

The PTA process is characterized by a high density Plasma Arc, obtained by ionization of Argon gas passing through an electric arc to serve as a shield. A second electric arc (Transferred Arc) is produced simultaneously to provide the energy needed to melt both the base metal and the filler metal/powder. The transferred arc is “throttled” to further increase the temperature, obtaining a plasma column temperature between 14,000°F (8,000°C) and 32,000°F (18,000°C). After the plasma column passes and the base and filler metals re-solidify, a substance-to-substance bond between the filler material and the base material is created. Because PTA utilizes a constricted, columnar welding arc, the heat input is highly localized. This allows full fusion of the overlay material, while introducing minimum heat into the part.

PTA especially excels applying powder alloys that are too hard to draw into wire, such as various cobalt and metal matrix composites. PTA is normally more economical than laser (lower operating and equipment cost), GTAW or TIG (lower heat input allows the process to achieve the required overlay properties in a thinner layer, and alloys are often less expensive in powder form vs wire). The capability of producing a low dilution, porosity free, thin overlay allows the PTA process to achieve a higher quality product using much less material and with drastically less heat input into the base material.

Advantages of Plasma Transfer Arc Welding over other hard-facing processes include…

• Limited heat addition to base metal (due to constricted and columnar welding arc combined with high working speed)
• Maintain structural quality and metallurgic homogeneity of base metal
• Semi-automated control ensures constant results (controlled penetration, with precise positioning of weld deposits on a single pass)
• High working speed (often 2X – 3X faster than MIG or TIG) reduces heat addition to base metal and lowers operating cost
• Better wear resistance due to harder, tougher coatings
• Improved corrosion protection due to less porosity

Hard Facing – HVOF Thermal Spray

Customer applications which frequently benefit from HVOF include chemical processing, dry powders, mining, power generation, oil and gas, cement/asphalt, steel mills, pulp and paper mills, particle reduction or recycling, rotary airlocks or other rotating components, and many more. High-Velocity Oxygen Fuel (HVOF) coating is a thermal spray coating process that improves a component’s surface properties, and extends equipment life by protecting against wear and corrosion.

The HVOF process involves feeding a mixture of fuel and oxygen into a combustion chamber, where they are ignited and continuously combusted. The resultant hot gas flows through a converging–diverging nozzle and enters a straight-walled barrel section. From the barrel perimeter, a powder feed stock is continuously injected toward the center-line into the gas stream, further accelerating the gas/powder mixture. The stream of hot gas and powder exits the barrel with a jet velocity (1000 m/s) exceeding Mach 3.

The resulting jet stream of partially melted powder feed stock and hot gas is sprayed directly onto the substrate, where it begins to deposit as the powder simultaneously is embedded into both the underlying substrate and into other embedded particles, creating a hard-faced coating. The tremendous kinetic energy of the powder impacting the surface results in low porosity, high bond strength, and relatively low processing temperatures (which reduces thermal stress/deflection/warpage of the coated part).

HVOF coatings range in thickness from 0.1mm – 12 mm (1/2″), and offer the benefits of extreme wear and corrosion resistance. Common coating powders include Stellite, Colmonoy, Tungsten-Carbide, Stainless Steels, Inconel, and other non-ferrous alloys with hardness values ranging from 35 RC (380HV Vickers) to 64 RC (850 HV Vickers).

Advantages of HVOF spraying over other thermal spray processes include…

• Higher density (lower porosity) due to greater particle impact velocities
  
• Higher strength bond (both to the underlying substrate and within the coating)
  
• Lower oxide content due to less in-flight exposure time
  
• Retention of powder chemistry due to reduced time at temperature
  
• Smoother as-sprayed surface due to higher impact velocities and smaller powder sizes
  
• Better wear resistance due to harder, tougher coatings
  
• Higher hardness due to less degradation of carbide phases
  
• Improved corrosion protection due to less porosity
  
• Thicker coatings (up to 1/2”) are possible due to less residual stresses
  

Palmer Tool has over 20 years’ experience applying HVOF coatings and PTA welds to components used by Fortune 500 companies in high temperature, high wear, high pressure, and highly corrosive environments.  We are happy to discuss our capabilities to help you SAVE MONEY by EXTENDING THE LIFE of your equipment while reducing your frequency of maintenance shut-downs. At zero obligation or cost to you, our experienced staff can determine whether HVOF or PTA hard-facing is suitable for your application, and can recommend material options for YOUR unique application.

Contact us (731) 584-4681 To See How We Can Help!