Metal galling is surface wear that occurs when one metal comes into contact with another. Metal galling can cause metal pieces to separate from their original component. Metal galling is a quick process that, once started, can lead to component failure in subsequent operating cycles.
What is Metal Galling?
Metal Galling, also known as cold welding or adhesion, is a phenomenon in which two sliding metal surfaces adhere to one another due to friction and welding. This effect is caused by high stress on the metals and usually occurs when two dissimilar metals are locked together for an extended period of time. The resulting weld can create havoc with finely machined parts, reducing tolerances and affecting manufacturing processes. In addition to using lubricants like oils and greases that contain anti-galling agents such as zinc dithiophosphates, it may be helpful to use galling preventive alloys (e.g., stainless steel) or coating methods (e.g., hard-chrome plating) to prevent metal galls from forming in critical components exposed to extreme temperatures or environmental conditions.
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Causes of Metal Galling
Galling occurs in metals due to high frictional forces that cause adhesive wear. This is common when two or more metals come into contact with one another under extremely high loads. Metal galling is ideal for ductile materials with a crystal structure that promotes cohesive attraction. Many industrial metals exhibit both of these characteristics, which is why galling is such a common occurrence in most metals. The frictional force provides the energy required for adhesion to occur. So, for galling to occur, all three of the following factors must be present:
- High coefficient of friction
- Material ductility (one is softer than the other), and
- Adherent attraction
Mechanism of Metal Galling
When one metal surface is compressed over another, they merge at the contact zone. The contact zone is the strongest point. When these compressive forces are strong enough, friction occurs, and the pressure in the contact zone rises. The friction and pressure energy cause adhesion between the two surfaces, welding the two materials together and plastically deforming them. If protruding points are present on any of the surfaces, they can easily penetrate the surface oxide layer of the opposing material, damaging the metal beneath.
When sliding friction occurs, the strong adhesion becomes galling. The amount of stress increases in the contact zone, allowing the two surfaces to separate and dissolve the contact zone. As a result, one material loses particles to the other.
Metals Susceptible to Galling
The most common metals that suffer from galling are austenitic stainless steel, aluminium, and titanium. Metal galling is less likely in martensitic stainless steels, tool steels, and other hardened materials.
When Does Galling Occur?
Galling can occur in applications involving two or more ductile materials imposing high frictional forces on one another. The most common types of galling are found on stainless steel and aluminium fasteners. All such fasteners generate high frictional forces while being torqued, which is the primary cause of metal galling. Machinery that is subjected to excessive friction is also prone to galling.
How Can You Avoid Galling?
Metal galling must be avoided because annoying can lead to component failIt is choosing hazards. The factors that impact metal galling are:
- Applied Material Load
- Coating and lubrication
- Hardness and surface finish
- Contact Area and Degree of Movement
In the metal industry, there are several methods for preventing galling. Some examples of galling prevention methods include:
- Choosing a metal not prone to annoying, such as hardened tool steels, work-hardened austenitic stainless steels, and so on.
- Lubricants are used to reduce friction between surfaces. Anti-galling lubricants containing molybdenum compounds and anti-galling coatings have recently become available.
- I am choosing a coarser thread to reduce friction in threaded fasteners.
- Thread tightening speed reduction reduces frictional heating and the risk of galling. In such cases, manual tightening will be beneficial.
- We maintain clean, debris-free metal contact surfaces.
- Load values between contacting metallic surfaces are reduced.
- Using dissimilar materials with low ductility or different grade can help reduce the irritating effect.
- Galling is more likely in bolts with dented or damaged threads. As a result, always choose bolts that are clean and free of damage.
- It increases the contact area to reduce stresses below the galling stress threshold.
- Increasing the contact area to spread wear volume over a larger area results in less wear depth.