Metal galling is a type of surface wear that causes a metal unintentionally engage with another metal in contact. Galling can result separation of metal pieces from the original component. The process of metal galling is quite rapid and once initiated can lead to component failure with subsequent operating cycles.
Galling mechanism in metals is quite different from other wear mechanisms due to the adhesion it creates. Contrary to other types of wear processes, metal galling causes the metal to adhere to the other material and is unique. Whereas, other wear forms involve erosion of one metal when in contact with another metal.
The effect of galling in metals can be at varying levels; Large or microscopic level. Microscopic galling is not easily visible. Large metal galling can easily be seen on metal surfaces by naked eye. Even though microscopic galling does not make the component completely useless, but still it can cause performance failure for components with tight tolerances.
Causes of Galling
Galling in metals occurs because of large frictional forces that cause adhesive wear. This usually happens when two or more metals are in contact with one another under the influence of very high loads. Ductile materials having crystal structure that promotes cohesive attraction are ideal for metal galling. Many of the industrial metals exhibits both of these characteristics and that is why galling is quite common phenomena among the most metals. The frictional force gives the required energy for adhesion to occur. So for galling to occur all the following three factors need to be present:
- High frictional force
- Ductility of material (One softer than the other), and
- Cohesive attraction
Mechanism of Galling
Upon compressing one metal surface over another, they merge at contact zone. The contact zone is the highest force points. When this compressive forces are large enough, friction is created and the pressure in the contact zone is also increased. The energy generated by this friction and pressure, cause adhesion between the two surfaces, welding the two materials together and plastically deforming them. The protruding points if available in any one of the surfaces can easily penetrate the surface oxide layer of the opposing material, causing damage to metal beneath.
Now, when sliding friction occurs, the strong adhesion turns into galling. In the contact zone, the amount of stress increases which enables the two surfaces to break away from each other and dissolving the contact zone. This causes one material to lose particles to the other.
Metals Susceptible to Galling
Austenitic Stainless Steel, Aluminum, and Titanium are the most prominent metals that frequently encounter galling issues. Martensitic stainless steels, Tool steels or other similar hardened materials are less likely to have metal galling occur.
When Does Galling Occur?
Applications involving two or more ductile materials imposing high frictional forces to one another are susceptible to galling. The most frequent instances of galling is found on fasteners made of Stainless steel and aluminum. While torquing, all such fasteners results is high frictional forces that is the prime component for metal galling to occur. Machineries subjected to excess friction are also at risk for galling.
How to Prevent Galling?
As metal galling can result on component failure leading to hazard, It must be prevented to occur. The factors that impact metal galling are
- Applied Load
- Lubrication and Coating
- Surface finish and Hardness
- Contact Area and Degree of Movement
A number of ways are prevalent in metal industries to prevent galling. Some of such methods for galling prevention are:
- Selecting a metal not prone to galling like hardened tool steels, work-hardened austenitic stainless steels, etc.
- Applying lubricants to reduce the friction between surfaces. In recent times, anti-galling lubricants containing molybdenum compounds and anti-galling coatings are also available.
- Selecting coarser thread to reduce the amount of friction in threaded fasteners.
- Reduction of thread tightening speed reduces frictional heating and risk of galling. Manual tightening will help in such cases.
- Keeping the metal contact surfaces clean, free from debris.
- Reducing the load values between contacting metallic surfaces.
- Use of dissimilar materials with low ductility or different grades of materials can reduce galling effect.
- Bolts with dented or damaged threads increase the potential for galling. So, always clean and damage-free bolts need to be selected.
- Increasing contact area to lower the stresses below the threshold galling stress
- Increasing contact area to spread wear volume over a greater area and thus producing less depth of wear.