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 Hard Chrome Plating vs. HVOF Process

Why the Technology?

  • Used for fluid power hydraulic repairs in cylinders and corner groups
  • Provides long-term robust repair for hydraulic and fluid power components
  • Cutting edge environmentally friendly technology
  • Provides a safe working environment for associates to do repairs


Hard Chrome Plating

  • Chrome Discovered 1798, first successful electrolytic plating 1856, commercial utilization late 1920’s
     
  • Advantages
    • Resists most chemicals, oxygen, and moisture in air
    • Does not tend to seize, gall, or cold weld
    • Hard surface typical 69 Rc, 0.003-0.015” typical thickness
    • Simple technology
    • Excellent wear resistance
    • Historically relatively inexpensive 
  • Issues with hard chrome plating
    • The process produces large amounts of hydrogen gas in the gas bubbles burst throwing hexavalent chrome solution into the air as fine mist. Hexavalent chrome is a know carcinogen.
    • The process produces large volumes of toxic waste.
    • EPA and OSHA have mandated more stringent stack emission levels and lowering of permissible exposure limits for workers.
    • This results in increased costs and business risks.
Chrome Plating Solution Sources HVOF

 Alternatives to Hard Chrome Plating Sought

  • The Hard Chrome Alternatives Team (HCAT) formed during the 1990’s
  • HCAT is a bi-national team comprised of Dept of Defense, US airlines, Canadian military and airline industries, support manufacturers
  • Methods evaluated were Arc Spraying, Plasma Spraying, Flame Spraying, and High Velocity Oxy-Fuel (HVOF)

The Best Alternative

  • The best clean alternative method determined by HCAT was HVOF thermal spray for Hydraulic
    Actuators (Cylinder Rods)
  • The HVOF process utilizes a material in powder form injected into a flame of supersonic gas
  • The material softens in the flame and forms a dense coating on the substrate.

HVOF

  • The fuel for the flame is a gas such as hydrogen, acetylene, propylene, or a liquid such as kerosene
  • The coating material is usually a metal alloy such as chrome carbide
  • Typical deposit coating layers range from 0.003-0.015” in thickness

HVOF Spray Gun Cutaway

HVOF Spray Gun Cutaway

HVOF

  • The two primary companies that make the HVOF spray equipment are Sulzer-Metcoand Praxair/TAFA
     
 Sulzer-MetcoPraxair/TAFA
FuelHydrogen, Propane, Propylene, Natural GasKerosene
Carrier GasNitrogenNitorgen, Argon
OxidizerOxygenOxygen
Particle VelocityMach1Mach2+ (3300-3900 feet/second)
Temperature5000 F5600 F


 

HVOF

 HVOF

  • The part to be coated must have chrome removed by stripping electrolytically or grinding
  • If the surface has been previously HVOF coated, the surface must be ground to remove any
  • imperfections
  • Any area not to be coated must be masked off
  • The surface to be coated must be thoroughly grit blasted using aluminum oxide material-36 grit-the prepared
    surface must be kept clean from this point on
  • The work piece is put into the HVOF thermal spray cabinet.
  • During the spray process the work piece rotates at 300 surface feet per minute
  • Each pass applies 0.001”of coating
  • The work piece temperature is not allowed to exceed 350 F, the spray process can be stopped and started to
    limit this temperature HVOF
  • Final surface finishing requires use of either diamond belts or diamond grinding wheels
  • It is necessary to not only grind but also
    superfinish
  • The finished surface is measured and documented
  • The bond strength is >10,000 PSI

Comparison of Hard Chrome Plating to HVOF 

  Hard Chrome PlatingHVOF 
 Surface Hardness 69 Rc 71Rc
 Time required to coat Komatsu 830E first stage 7 hours (0.002" per hour to get 0.007"/side) 1 hour (12 lbs at 12 lbs/hr to get 0.007"/side)
 Work Piece Temp 145 F 300-350 F
 Post Plate H2 Relief 275-425 F for 2-4 hours Not required
 Desired Surface Finish 0.4-0.8 um
16-32 u in
 2-6 u in

Comparison of Hard Chrome Plating to HVOF

  • At the bottom of each photo is the substrate material that is being coated. 
Hard Chrome PlatingHVOF Plating

 Benefits of HVOF Coatings

  • High density, low porosity
  • Improved corrosion barrier
  • Higher hardness ratings
  • Improved wear resistance
  • High bond strength
  • Thick coatings
  • Smoother as-sprayed surfaces


Summation

  • Hard chrome has the advantage that it is a single material and deposition method that can be used for a wide variety of applications
  • HVOF is a single technology, with a wide variety of materials that can be used to achieve the right combination of properties for many purposes.

Hazardous Environment Changes

  • Provides a safer work environment for our people
  • 80% reduction in hazardous waste
  • Waste produced is considerably less hazardous than Hard Chrome Plating

 

HVOF Spray Booth with Stage Being Sprayed - Pre-heat Pass
 

HVOF Spray Booth with Stage Being Sprayed Pre-heat Pass


HVOF Spray Booth with Stage Being Sprayed. Actual Spray Pattern
 

HVOF Spray Booth with Stage Being Sprayed. Actual Spray Pattern

Grinder/Super Finisher

HVOF Grinder Super Finisher

 


 



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