What is galling?
Galling is also sometimes referred to as cold welding and refers to a form of severe adhesive wear. It occurs when two metal surfaces are in relative motion and there is sufficient load for the transfer of material from one surface to the other to take place. The transferred material causes surface protrusions and these are called galls. In order for galling to happen, the load must be enough to disrupt the oxide layer covering surface asperities of the metal so that metal to metal contact can take place. Galling is most frequently associated with high load, slow speed situations, but it can also occur in high speed situations where there is little load.
Galling can be quite common in applications such as engine bearings and pistons, hydraulic cylinders, screw threads and valves. It can be a serious problem as severe galling can cause metal components to seize. When galling occurs in parts which have been screwed together it can damage the thread to an extent that makes it impossible to disassemble the unit.
How can galling be prevented?
Some materials are more prone to galling than others, so careful selection of materials can reduce the risk of galling taking place. Galling relies on cohesion through metallic-bonding attractions and the ability to change shape without breaking (plasticity). These properties are found in most metals, but hardened metals are more resistant to galling than softer materials. For example, austenitic and precipitation hardening stainless steels have a low resistance to galling.
Another way to avoid galling is to use a lubricant between the two surfaces. Alternatively, you could use two dissimilar materials when manufacturing parts as this makes them less likely to suffer from adhesive wear.
It may not be possible to manufacture the parts from different materials, but strategic surface engineering using an anti-galling surface coating, such as those available from www.poeton.co.uk, can make the two surfaces sufficiently different to prevent galling.
In addition to their role in preventing galling, surface coatings can also provide resistance to wear and corrosion. There are many applications where the risk of galling is combined with abrasive or corrosive media, so surface coatings can be particularly useful. The combination of factors can act in synergy to speed up surface damage.