The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Corrosion resistant coating for rock bit bearing races

Roller cone rock bit that are used for drilling in earth have roller bearing races of case carburised steel like 8720 and 9313 with surface hardness of 58-60 Hrc. The races are hard turned (not ground). While working the 3" dia race is subjected to loads of 30,000 lbs. Compressed air is used as a bearing cooling and flushing medium. However, since water is also injected long with the air, for dust suppression, it causes corrosion and pitting / flaking of the loaded side of the bearing surface.
Is there a thin film surface coating that can take the load and also prevent corrosion for some time atleast? Does hard chrome flash coating stand up to the loads and rolling members like the rollers?

Srinivas Prasan
product designer - Hyderabad, AP, India
July 19, 2008

Hard chrome by itself is harder (+70Rc) and theoretically could take that wear. The problem is that its adhesion to steel has been recorded to be on the order of 20,000-30,000 PSI and is quite fragile so it may suffer at high loads if thicker than a few thousands of an inch. It is also not a very good candidate for corrosion resistance because of its superficial microcracks and low resistance to acidic environments that contain chlorides. Low phos electroless nickel with its hardness of about 60Rc as plated could be a better choice. It is also more corrosion resistant, impervious and uniform in thickness than chrome. Its adhesion to steel could also be better.

Guillermo Marrufo
Monterrey, NL, Mexico
July 23, 2008