What Causes Galling in Inconel 625 Fasteners and How Can Buyers and Engineers Prevent It?

Galling is one of the most common and most misunderstood failure modes with Inconel 625 fasteners. 

What Causes Galling

1. Self mating and material similarity

Galling occurs when two metal surfaces slide against each other under load and the friction generates enough heat to cause micro welding at asperity contact points. Inconel 625 is especially prone to this because nickel based alloys have inherently high coefficients of friction and a strong tendency to cold weld to themselves. Running an Inconel 625 bolt into an Inconel 625 nut is the worst case scenario, same material, same hardness, same surface chemistry.

2. Passive oxide film breakdown

Like stainless steel, Inconel 625 relies on a thin chromium/nickel oxide passive layer for corrosion resistance. Under the high contact pressure of thread engagement, this film shears away, exposing bare, reactive metal. Once that happens, adjacent asperities weld together instead of sliding smoothly.

3. Low thermal conductivity

Nickel alloys don't dissipate frictional heat well compared to carbon or even stainless steel. Heat stays concentrated at the contact points, accelerating micro welding.

4. High ductility and work hardening

Inconel 625 work hardens significantly during deformation. As torque is applied, the material at the thread flanks hardens and roughens rather than smoothing out, which increases friction on subsequent turns a self reinforcing effect.

5. Lack of lubrication or improper lubricant

Dry threading, or using a lubricant not rated for high nickel alloys, removes the boundary layer that would otherwise separate the mating surfaces.

6. Speed and thread engagement

Fast installation speeds and long thread engagement lengths increase heat generation and the surface area at risk of galling.

Prevention Strategies

• Avoid self mating pairs. Specify a different alloy for the nut than the bolt where possible e.g., Inconel 625 bolts with silicon bronze, Nitronic 60, or a coated steel nut.

• Request anti-galling surface treatments at the point of manufacture: silver plating, gold flash, or specialized coatings like tungsten disulfide or Dicronite.

• Specify coarser threads rather than fine threads when the application allows, coarse threads have lower thread engagement friction per turn.

• Buy pre-lubricated fasteners with an anti seize compound already applied and specified.

• Ask for surface finish specs: smoother finishes reduce asperity contact.