What Causes Metal Galling & How to Prevent It?

What is Metal Galling?

Let us begin by establishing a definition: what is galling? Galling is a severe kind of adhesive wear on metal surfaces that slide against each other under high pressure and temperature. The resultant friction leads to issues like material transfer (cold welding), surface tears, and seizure in extreme cases.

Metal galling is a huge problem in applications with metal-metal contacts, such as in fasteners, bearings, or pistons. Nut-bolt joints sometimes lock up under the action of tightening torque. This is a well-known type of metal galling known as thread galling.

Mechanics of Galling

Metal galling is essentially an undesirable material transfer mechanism. When two surfaces slide against each other under pressure, material transfers from one surface to the other due to adhesive wear.

The trigger for this metal-to-metal adhesion is often the breakdown of protective oxide layers on the contact surfaces. Without this layer, the atoms from both surfaces become free to bond together, forming microscopic welds.

Thereafter, the relative surface motion, in combination with friction and pressure, tears apart these welds. This tearing action creates surface asperities like rough spots, craters, and lumps. These asperities further accelerate the galling action as they increase the surface roughness, increasing friction and local pressure points.

Generally, metals with high ductility are more prone to galling due to their plasticity. They deform easily under lower stress levels.

Different Factors Causing Galling

Galling is the result of several factors. This section discusses the main factors contributing to galled metal.

High Pressure

Pressure is the main factor causing metal galling. The surface pressure translates to high contact stresses, which in turn crush the protective oxide layers more effectively.

Once this happens, the pressure also contributes to increasing the materials’ plasticity in the local contact zone, further contributing to the galling action.

Relative Motion

Relative motion between the mating surfaces is the other leading factor for metal galling besides pressure. Relative motion generates friction and adhesion between the contact surfaces and removes and drags particles from the bonded areas.

This promotes the erosion of protective oxide layers, surface tearing due to debris rubbing against the exposed surfaces, and heat generation. All these factors cause galling.

A common example of this is galled metal in bearings, which house several components in relative motion to each other under high-speed conditions.

Pairing of Similar Metals

Metals with similar atomic structures carry a greater risk of galling. This is the reason that experts recommend lubrication or anti-seize when using stainless steel nuts and bolts, since stainless steel galling is a common issue.

On the other hand, dissimilar metal pairs like stainless steel-brass are averse to galling due to their incompatible atomic structures.

Lack of Lubrication

Lubrication on the mating surfaces is another factor in metal galling. Lubricants form a layer between the contact surfaces, preventing direct metal-to-metal contact. Thereby, they prevent the removal of protective oxides and physically separate surface irregularities.

Debris

Debris particles from the environment, machining operations, or surface wear can get stuck between metal surfaces. These particles accelerate metal galling as they increase the abrasiveness of surfaces and scratch the protective oxide layers.

How to Prevent Galling?

Engineers use several techniques to optimize their designs for minimal metal galling.

Lubrication

Lubrication is generally the most effective method to minimize the chances of metal galling. Lubricants like oils or anti-seize serve to create a protective layer between the mating surfaces. This decreases direct metal-to-metal contact and friction, both of which are core contributors to galling.

Moreover, the presence of lubricants also decreases heat generation, which prevents plastic behavior.

Load Management

Metal galling has a direct relationship with pressure and contact stress. Thus, reducing external loads that minimize the pressure on the contact surfaces also helps prevent galling.

Load management components like washers or tightening torque guidelines are typically useful in avoiding galling.

Surface Coating

Since metal galling is specifically a surface-related issue, using surface coatings to alter surface properties is a highly effective method of preventing galling.

Coating techniques like nitriding, anodizing, or Teflon coating help the material surface resist adhesive wear and cold welding. Furthermore, these surface coatings also improve qualities like lubricant retention.

Material Selection

The choice of material pairs can significantly reduce the risk of metal galling. As discussed previously, similar metals gall more easily than dissimilar metal pairs. For example, brass or zinc coating is are common material choice to go with stainless steel surfaces to prevent stainless steel galling.

Moreover, within a material group, there are certain grades that are more galling-averse than others. In the case of stainless steel, SS 304/316 is more resistant to galling than SS 410/420.

Increased Contact Area

A common technique to prevent metal galling is to increase the surface area at the contact interface. This decreases the pressure on the surfaces (as Pressure = Force/Area), indirectly leading to better load management and lower chances of galling.

Optimize Surface Finish

Galling is a surface phenomenon, so the surface finish plays a major role in how galling-prone a pair of surfaces is. Very smooth surfaces have no surface irregularities, so there is a lot of direct contact between both sides, which promotes chemical bonding and galling. Rough surfaces, on the other hand, cause surface tearing and destruction of protective oxide layers.

Common Occurrences of Galling

Let us take a look at some common examples of metal galling from different applications.

• Threaded fasteners: Thread galling is quite common in poorly designed or installed fasteners. Galling is the leading cause of stainless steel bolts jamming.
• Valve Stems/Seats: Valves in automotive engines slide within a valve seat, creating a contact surface with surface pressure and temperature. Galling is a part of the regular wear and tear in these components.
• Die and Molds: Dies/molds in applications like injection molding go through thousands of cycles each day. In each cycle, sliding metal pairs in parts like ejection pins rub against each other, causing galling to occur.
• Bearings: The rolling elements in bearings slide against the bearing rings under high-pressure loading and high-temperature conditions. This often leads to galling, reducing the bearing’s remaining useful life and functionality.

Tips to Repair Galled Metal Surfaces

We discussed the main causes of metal galling and common occurrences in industrial settings. However, galling, being a microstructural phenomenon, is still impossible to completely eliminate from mechanical designs despite the necessary precautions. This section shares some tips from experts for repairing galled metal surfaces.

The preliminary step before repairing, however, is a quick severity check. Galling is a surface defect that may be on the surface or exist deeper in the material. It is good engineering practice to assess the level of damage before deciding between repairing or scraping the galled metal part.

If a part is deemed suitable for repair, the following techniques can be employed:

• Grinding/Machining: A simple approach is to just remove material from the surface with galling defects via grinding or metal cutting operations. This, however, depends on whether material removal is viable or not. If a part requires dimensional accuracy and tight tolerancing, material removal would not be an option.
• Fine Abrasives/Lapping: Minor metal galling is reparable with polishing operations, like using fine-grit sandpaper or lapping pastes.
• Re-Tapping/Re-Threading: In the case of thread galling, a viable repair method is to re-tap the holes or re-thread the fasteners. This is only feasible for minor thread damage and requires precautions like proper lubrication.