How Does Galling Affect Metals?

There are a few ways to avoid galling. One is to lubricate the threads (preferably with anti-seize) and tighten them slowly. This will reduce friction heating and give the lubricant time to dissipate.

Another way is to use materials that are less prone to galling, like aluminum or stainless steel. Also, rolled threads tend to be harder and more galling resistant than cut threads.

Ductility is the measure of a metal’s ability to endure stress without fracturing. It is determined by putting a sample of the material under tensile stress and watching its behavior. A ductile metal will elongate, and it will do so in an elastic way that doesn’t cause the material to rupture or crack.

A metal’s ductility depends on its metallic character and its valence electrons. It also depends on its crystal structure. Materials with a face-centered cubic crystal structure have high ductility. Other structures, like body-centered cubic or hexagonal close packed, tend to be less ductile.

Ductility can affect galling because when a ductile material is subjected to friction, it can stretch and deform, creating microscopic excrescences on the surface. These can trap debris and heat between bulk surfaces, enhancing adhesion. The resulting localized heat, combined with friction, promotes galling. This is the main reason that avoiding galling requires the use of dissimilar and hard materials with low ductility, good lubrication, and smooth surfaces.

The friction between the metals is an important factor that must be present for galling to occur. Friction provides the energy needed for adhesive wear and also changes the ductility and crystal structure of the material. This change makes the metal more prone to cohesive attraction. While this can occur on hard materials, it is more likely to affect softer metals.

The scratch test data used to categorize the stages of galling also reveals that the amount of material transfer is proportional to the coefficient of friction. Using this data, it is possible to split the galling stages into four groups. A scratch test segment can be characterized as exhibiting stage 1 only, only stage 2, or both stages 1, 2 and 3.

The use of dissimilar materials that have low ductility, a lubricant that is chemically inert to the tribopair, and rough surfaces will help prevent galling. Altering the geometry of a part, using an oxide layer, adjusting surface roughness, or finding material combinations that have low adhesion are other ways to reduce galling wear.
Cohesive Attraction

The tendency of molecules of alike substances to stick together is referred to as cohesion. The water molecule, for example, exhibits cohesion due to its polarity. Its oxygen atom has a slightly negative pole and its hydrogens have a positively charged pole. The force of polarity causes water to adhere to itself and other materials such as glass.

In some cases, metals will form ionic or covalent bonds at their interface. This is a form of adhesive wear called chemical adhesion. This is different from galling which occurs when dissimilar surfaces come in contact with each other.

This type of metal wear is common in applications where there is not enough lubrication to allow smooth sliding contact between two metals. Some metals are more prone to galling than others because of their atomic structure. For instance, aluminium is prone to galling, whereas steel that has been annealed or fully hardened is resistant to it. This is why manufacturers need to choose the right metal for their products and use appropriate lubrication for them to slide smoothly together.

While galling does create a localised area of heat, this never comes close to melting the materials. The heat is generated from plastic deformation, not friction, and is focused into the area of adhesion.

Galling can occur between different types of stainless steel, and even between different alloys within a single type. However, this can be minimised by using a properly formulated anti-seize compound. Many of these are available in varieties that are suited to specific alloys or environments, and they should be used where possible.

Generally speaking, using dissimilar grades of stainless steel reduces the likelihood of galling. However, if the grades involved in a design are too similar, then using a specialised coating or plating that provides lubrication can greatly reduce the chance of galling. Regular cleaning of threads can also help. Rough or damaged surfaces are more prone to galling, so deburring and electropolishing can help prevent this from happening. Lastly, reducing the torque during installation or removal can reduce the amount of friction generated and help to minimise galling.

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