Introduction
Are your metal components experiencing sudden wear and tear or material deformation? This may be a sign of galling, a significant concern in the field of metalworking that can ultimately lead to a complete breakdown of your equipment if not attended to.
Understanding galling and the conditions that cause it is vital to protecting your metal components and equipment.
What Is Metal Galling?
Galling refers to a form of wear resulting from metal-to-metal friction, involving material adhesion and considerable surface damage, often leading to surface expansion and increased roughness. Galling occurs when two metal surfaces rub against each under high pressure, inducing adhesion, surface roughening, and material transfer.
Galling is highly prevalent in applications where there is continuous metal-to-metal contact, such as the manufacturing, automotive, and machinery industries. This type of wear is commonly found in fasteners, valves, and hydraulic systems, among other areas where metals need to move or slide against each other under a load. Galling can lead to significant operational problems, such as malfunctioning equipment, increased friction, and damage that necessitates costly repairs and downtime.
How Does Galling Work?
Galling is a result of a specific sequence of stages that are all detrimental to the condition of the metal components. In the beginning, two metal surfaces start sliding against each other under high pressure, creating relative motion. The friction generated by this motion also generates a significant amount of heat. This heat will result in the material of the metal surfaces becoming softer. As the temperature increases, parts of the microscopic points on the metal surfaces will start bonding. These bonded sections are known as cold welds, as the material becomes malleable due to the high pressure.
When the cold welds are created, the movement of one metal surface compared to the other will lead to a forceful separation. However, the material will not separate cleanly and will instead tear into the metal surface, resulting in a rugged and crude form. These loose pieces of metal will then stick to the other surface once the sliding motion is continued. Over time, the friction will result in the torn rough patches to grow, escalating the wear process. Eventually, the galling will cause a significant amount of damage to the metal surface, and the equipment or machinery will malfunction and fail.
Real-World Example of Galling
Consider two steel bolts being tightened against each other. As the metal surfaces press against each other under high torque, the friction between the two surfaces also increases. In the absence of proper lubrication or material compatibility, the bolt threads would begin to gall. With galling, microscopic material transfer occurs where the tiny pieces of metal from one bolt will stick to the other, roughening the surface and making it difficult to unscrew the bolts manually, without causing further damage.
Factors That Trigger Galling
Several factors, including direct contact between materials, can accelerate the onset of galling, so it is important to recognize and address those factors to minimize galling-related damage:
Exposed Surfaces
Metallic surfaces that are not protected by lubricants, imitation, or other protective coatings (i.e., surface treatments that improve durability and functionality), are at a higher risk for galling. This is due to the increased friction and direct surface contact to trigger the onset of galling. In instances of environmental exposure or high-pressure applications, uncoated metal surfaces will be far quicker to cold weld to one another.
Debris
Debris caught between the contacting surfaces can lead to potential galling. More abrasive particles such as dust and swarf or processing chips can increase local friction. This added friction expertise provokes metal transfer/cold welding to occur.
High Stresses
High-stress applications can also lead to galling. The increased pressure or stress will generate more frictional heat and lead the contacting surfaces to adhesive bond more quickly under friction than under less stressed applications.
This effect is compounded when galvanic and atmospheric corrosion considerations are frequent, as in the case of many industrial and hydraulic systems. Further tightening force can exacerbate this issue, leading to the shearing off of the bolt head or stripping of the threads, especially in stainless steel and aluminium bolts.
Similar Metals
Galling is exacerbated when the metal surfaces in question share similar properties, as the atoms more readily bond into a single surface under friction. This is why stainless steel, which contains high resistance to atmospheric corrosion rates, is so quick to undergo galling when rubbing against other stainless steel components.
Stainless steel and aluminium are particularly susceptible to galling, especially in threaded applications where the passive oxide layer can be damaged, leading to issues such as freezing of bolt threads and potential severe mechanical failures during metalworking processes.
High Temperatures
Heat typically has a large impact on the potential for galling to occur. As the metal surfaces heat up during friction due to thermal effects, they will soften and become dead set on bonding under increased pressure earlier and more frequently. The risk rises even more in high-temperature environments such as steam-based applications for other operational temperatures and mechanical loadings.
How Galling Affects Stainless Steel Thread Connections
Galling is a significant concern in stainless steel thread connections, as it can lead to costly repairs, equipment failure, and production delays. Stainless steel is particularly susceptible to galling due to its high corrosion resistance, which can make it more prone to adhesive wear. When galling occurs in stainless steel thread connections, it can cause the threads to seize, making it difficult or impossible to remove the bolt or screw.
The phenomenon wherein unexpected adhesion occurs in stainless steel thread connections is often triggered by factors such as high load, temperature, and speed. When two metal surfaces are in sliding contact, the friction between them can cause the formation of microscopic high points, or asperities, which can lead to cold welding and galling. In stainless steel thread connections, the threads can become damaged, leading to a rough surface finish, which can exacerbate the galling effect.
To prevent galling in stainless steel thread connections, it is essential to use the right materials, surface treatments, and lubricants. For example, using a lubricant with anti-galling properties can help reduce the friction between the threads and prevent galling. Additionally, applying a surface treatment, such as nitriding or carburizing, can help harden the surface of the stainless steel, making it more resistant to galling.
How to Prevent Galling
Preventing galling requires a combination of strategies that aim to reduce friction, minimize heat generation, and inhibit material adhesion. Here are the most effective methods in industrial and manufacturing settings:
Lubrication / Coating
One of the most successful methods to prevent galling is to use anti galling lubricants or to coat the metal surfaces with a protective layer. Lubrication is achieved through a variety of oils or greases, paraffinic hydrocarbons with anti-weld additives, or even a molybdenum disulfide-based compound.
These lubricants work to reduce the amount of friction between the sliding parts, which in turn reduces the material transfer and adhesion due to the pressure and heat generation. Alternatively, specialized nitrided or plated coatings have also been successful at shielding the underlying metal from adhesion to another surface.
Lower Load, Temperature, and Speed
Decrease the level of applied operational loading, material temperatures, and other mechanical rotational kinematics. Lowering these variables will reduce the amount of relative friction and heat available to bond in the desired high-pressure environment. If need be, use cooling systems to lessen the heat generation in the first place.
Material Selection
Choosing the right materials for metal components is essential to prohibiting galling. By using dissimilar metals or a combination of metal and nonmetal materials, you can inhibit the process of galling from occurring in the first place. An excellent example is gauging stainless steel against prospect bronze and brass metal integral to a system, which is subsequently not subject to galling due to a more diverse material makeup.
Use Clean & Undamaged Parts
Keep parts free from contaminants and damage to ensure a more frictionless and non-galling metal interface. Contaminants and surface damage increase the localized abrasive point regions and make the surface more likely to form a cold weld as the metal removes the contaminant in bonding or adhesive fashion.
Part Design
Increase the contact area between metal parts to help negate the likelihood of galling. By creating a larger surface area to conduct frictional heat, you can modestly displace the galling process by increasing the force and surface area distribution over more points.
Surface Finish
Using a finer surface finish with fewer points of hard contact diminishes the risk that galling may take place. Refining the part’s surface finish to become less likely to hold onto another cladding takes away the high points and instead results in a smooth surface.
Mitigation and Repair
Mitigating and repairing galling in stainless steel thread connections requires a combination of techniques and strategies. Here are some methods that can be used to prevent and repair galling:
Lubrication: Using a lubricant with anti-galling properties can help reduce the friction between the threads and prevent galling. This is particularly important in high-load and high-speed applications where the risk of galling is higher.
Surface Treatment: Applying a surface treatment, such as nitriding or carburizing, can harden the surface of the stainless steel, making it more resistant to galling. These treatments create a protective oxide layer that reduces the likelihood of cold welding.
Material Selection: Choosing materials that are less susceptible to galling, such as brass or bronze, can help prevent galling in stainless steel thread connections. Using dissimilar metals can reduce the tendency for adhesive wear.
Thread Design: Designing threads with a larger contact area can help reduce the pressure between the threads, making it less likely for galling to occur. This can be achieved by optimizing the thread profile and pitch.
Cleaning and Inspection: Regularly cleaning and inspecting the threads can help identify and prevent galling before it becomes a major problem. Removing debris and contaminants from the contact zone can reduce the risk of adhesive wear.
Repair: In cases where galling has already occurred, repair methods such as thread repair kits or replacement of the damaged threads may be necessary. These kits can restore the functionality of the threads and prevent further damage.
What Is the Difference Between Welding and Galling?
Welding and galling are distinctly different processes, despite both involving the bonding of metal.
Galling is a form of unintentional adhesive wear due to the friction and pressure between two contacting metal surfaces. This causes tiny pieces of metal to transfer from one surface to another. Subsequently, galling leads to the freezing of bolt threads, potentially resulting in severe damage such as shearing off bolt heads or stripping threads, which is exacerbated by the loss of the protective oxide layer under high contact forces.
Galling will cause the surface to roughen, increase material transfer, and ultimately lead to failure of the two components. Galling is a detrimental process that does not contribute to the value or functionality of the product.
Welding, however, is an intentional process to join metals together permanently. It involves utilizing heat, pressure or both to fuse metals at their contact points. Unlike galling, welding is a controlled process meant to produce a strong and lasting bond between multiple metal parts for structural or functional purposes.