Galling is a severe form of adhesive wear that results in materials transfer and localized frictional heating, manifesting as a lump of metal on one surface. It frequently occurs when using stainless steel fasteners in conjunction with soft, ductile materials such as aluminum or unhardened austenitic stainless steels.
Preventing galling requires proper lubrication, and careful installation. Tightening bolts slowly helps reduce friction and heat, and avoiding power tools that generate excessive heat is beneficial.
Abrasive wear
In metalworking that involves cutting, galling is a type of wear that affects the friction between two surfaces. The metals transfer heat to each other, creating a layer of hard oxide that concentrates the frictional energy in a small area. This concentration of heat creates more adhesion and resistance to further cutting. It also causes a vibration in the metal that can be heard as a distinct sound.
Abrasive wear is a more general term that describes any type of damage to a surface caused by the presence of hard particles or hard surfaces. It can occur when one surface is rubbed against another or even when one surface is slid across the other.
Several studies have examined the effects of different conditions on galling and other types of wear. However, there have been few attempts to actively monitor the force and acoustic emission (AE) signals during galling and understand how they change with the progression of the process.
Cohesive attraction
Cohesive attraction is the force that causes comparable molecules to adhere to each other. It is exemplified by water, where each molecule makes hydrogen bonds with the adjacent molecule. Similarly, adhesives and substrates form ionic or covalent bonds when they touch each other. These bonds give the adhesive a sticky surface, which allows it to spread and cover the substrate surface. For maximum adhesion, the adhesive must fully wet the substrate surface.
This unique form of wear differs from abrasive or frictional formats in that it causes metals to connect to one another rather than eroding them. However, it does not eliminate metals from their usefulness, especially in cases where galling is microscopic and occurs on components with tight tolerances.
The underlying cause of galling is the difference in the ductility of different metals. Some metals, such as aluminium, are prone to galling, while others, like annealed stainless steel, are less prone to it. This difference is due to the atomic structure of the metals.
Friction
Galling occurs when strong adhesion turns into slipping and shear, tearing the crystal structure of one surface while leaving the other undisturbed. Certain metals are more prone to galling than others. Work hardened austenitic stainless steels and wrought aluminum are examples of common materials to experience galling. Martensitic stainless steel and tool steels, with their lower hardness, tend to be less susceptible to galling wear.
While liquid and solid lubricants can mitigate the occurrence of galling, they require frequent application. Altering geometry, modifying surface finishes, and selecting material combinations with low adhesion are other possible solutions. In addition to alloy selection, changing tribosystem conditions such as sliding speed and load can decrease the onset of galling. Lastly, the occurrence of galling may be controlled by volume changes or debris between the contact surfaces. These factors can also impact the scratch morphology, which is characterized by the appearance and evolution of peaks and troughs across the scratch length.
Heat
Galling is caused by the friction and heat generated between two contacting surfaces that are rubbing together. This is not a problem unique to stainless steel fasteners and can occur in any metals that rub against each other with poor lubrication.
The most important factor in preventing galling is the choice of materials and surface conditions for mating threads. Using different alloys in mating components or lubricating the mating threads with an anti-seize compound can lower the risk of galling.
Axial misalignment between clamped components or extraneous loads that are introduced during assembly due to operator error can also increase the friction and lead to galling. To prevent this, the holes of clamped components should be properly aligned and any extra load should be avoided if possible. Deep cryogenic heat treatment (DCT) can also significantly improve the resistance of HSS to galling. The improved galling resistance is believed to be related to the effect of DCT on reducing residual tensile stresses in the surface.