We have discussed this in the past but it is a universal problem worth the re-visit. Many years ago, I had the rare opportunity to visit a maintenance crew at a major refinery. The purpose of the meeting was for them to identify, in order of importance, their major maintenance problems and concerns. The response was singular, almost orchestrated. Number one problem; “Galling”, or cold welding of mating pieces of metal. This affected plant performance in daily production, during maintenance shutdowns, and at nearly every instance of inspection.
Galling is a Product of Similar Metals Similar Hardness
That ought to give you a hint as to the quickest way to approach resolution to your galling issues; Male part = one metal at one hardness. Female part = different metal at a different hardness. But you can even do better with a little effort.
1. Material Selection: There are metals made specifically to resist galling. Armco Steel has historically addressed resolution of this persistent maintenance issue. You might want to look into their series of Nitronic products. Here at Associated Steel we have two unique stainless shaft grades that have been engineered to resist galling and provide a host of additional advantages, such as added strength, straightness and machinability. Those grades are Mir 50, and ASC 2250 LDX, Lean Duplex.
2. Use a “non-galling”, or “galling-resistant” grade to replace one of the matting parts.
3. Change the hardness also: Give some thought to which of the parts (male or female) you wish to be sacrificial; i.e. which one can you most easily replace. Then, approach the difference in hardness accordingly. Lower hardness piece will most likely become the sacrificial part. Avoid mating similar metals (don’t make one part from 304 stainless and then make the mating part from 304 stainless). Even if you are unable to secure a non-galling grade, at least try some diverse materials, that happen to be within the scope of materials you would consider in the application. If throughout hardening is not feasible; consider surface hardening one of the parts in the wear area. Start with something gentle like Nitrating or Carburizing. With caution, more dramatic surface hardening techniques may be employed.
4. Prep mating surface areas. Harder and smoother finishes will resist cold welding. Avoid “cut” threads. Use rolled threaded products whenever possible. Spend a little time investigating some of the new lubricants. And…
5. Slow the speed when you tighten and install the assembly.
-Howard Thomas, February 1st 2019