Galling Resistance of Materion's Materials when Self Mated and Mated with Various Metals including Alloy 25 and ToughMet Alloy

Galling occurs when metals adhere to each other during sliding contact. This is especially important when considering threaded connections. If the surfaces adhere during connection, they will be damaged when they are disassembled leading to costly repair or replacement. The ability to disassemble and reassemble a connection is a valuable attribute to many applications. One example where this is especially important is in a drill string. Tubular components in a drill string need to be connected quickly, securely and be assembled rapidly and repetitively.

How Galling Occurs 

No metal is perfectly smooth. All materials have valleys and ridges forming asperities. The surface areas of these asperities are extremely small creating a large amount of local pressure when they come together, much larger than the overall pressure. Some material combinations are able to form a bond under these conditions. In certain materials, this can be controlled and done purposely to form a strong weld without a heat affected zone. In cases such as threaded connections, this undesirable effect is called galling which leads to thread damage and in extreme cases complete seizure. 

Material Interactions In order for galling to occur, there has to be pressure and direct metal to metal contact with heat acting as a catalyst. Different material structures have different tendencies to gall. Some materials have protective oxides that act as a barrier preventing galling. Materials also create different amounts of heat and dissipate that heat at different rates. Another phenomenon is one material might initially gall onto the other with that material acting as a protective layer. What initially seems like a few straight forward factors quickly compounds into an extremely complex interaction. 

General Rules for Galling: 

• Softer materials have a higher tendency to gall 
• Materials that are 100% miscible in their liquid state including self mated materials have high tendency to gall 
• A smooth surface creates more contact areas creating more heat leading to higher galling tendencies 
• A rough surface has a high occurrence of asperities interfering and deforming also creating heat 
• Materials that have a high work hardening rate have a lower tendency to gall 
• Materials that have a high strength to elastic modulus ratio have a lower tendency to gall 

Strategies to Prevent Galling: 

• Use lubrication to act as a barrier between the surfaces while reducing frictional heating 
• Make the connection slower reducing the heat produced 
• Use a thread locking compound to avoid applying stress that would lead to galling 
• Apply a surface hardening treatment (for steel) 
• Hard face the mating surface with a coating 
• Mate two different materials 

All these strategies are effective to varying extents but every one of them contains a compromise. The use of lubricating or locking compounds and slowing the speed all negatively affect productivity. Surface hardening steels sacrifice corrosion resistance. Coatings add cost and will have tendency to rub off. Mating two different materials can be effective but there will be an extra set of material properties to consider.