Imagine you’re in the middle of an important project, and as you tighten a stainless steel fastener, something feels off. The threads are binding, the movement is jerky, and before you know it, the bolt is stuck. No matter how much force you apply, it refuses to budge—or worse, it breaks.
If this scenario sounds familiar, you’ve likely encountered galling, a common problem in stainless steel fasteners. Galling can turn a simple task into a frustrating one, damaging components of your fasteners and delaying work progress.
But here’s the good news: galling is preventable. In this article, we’ll explore what galling is, why it happens, and how to stop it from happening. Learning these techniques will save you time, money, and headaches. Read on; let’s tackle this problem together.
What is Galling, and Why Does it Occur in Stainless Steel?
Closeup view of two threaded bolts in contact, illustrating potential friction
Galling or cold welding is a form of surface damage due to adhesive wear or excessive friction when two metal surfaces, such as a bolt and nut, slide against each other under pressure. This can happen when installing a bolt into a threaded hole. It is usually prevalent with stainless steel parts but can occur with titanium, aluminum, and other soft metal types.
Galling can happen regardless if the metals in contact are different or the same. The friction from the two metal surfaces sliding against each other generates heat, which can cause them to stick together. In severe cases, galling leads to freezing, making the fasteners impossible to loosen. Applying further force may break them, shear off the head of the bolt, or strip the threads.
Metals made of stainless steel possess a self-repairing chromium oxide layer that provides excellent corrosion resistance. However, this property creates a highly reactive surface under friction, rendering it susceptible to galling. Galling of stainless steel fasteners occur when this oxide layer is damaged or stripped off under high contact forces, and their bare reactive material is exposed for material transfer between the surfaces.
How Can You Identify Signs of Galling During Assembly?
Initial signs of galling in your stainless steel fasteners are often subtle. Identifying these galling signs early can save you from costly repairs and replacements. During assembly, pay attention to the following warning signs:
1. Sudden Resistance
If you notice unexpected resistance while tightening a fastener, this could be a sign of galling. It may become difficult or impossible to turn the fastener without applying excessive force, which can cause further damage to the thread.
2. Jerky Movements
Galling often causes uneven interaction between bolt threads and nut threads. Instead of a smooth, continuous motion during tightening, the threads stick and then release unpredictably, causing a stop-and-go effect. This jerky motion indicates that the metal surfaces are seizing due to the excessive friction and heat generated during the process. Left unaddressed, this can escalate, damaging the threads or making the fastener unusable.
3. Visible Damage
After disassembling the fastener, carefully inspect it for any signs of wear or damage. Common indicators of galling include the presence of metal shavings, rough or uneven patches, and visible distortion on the threads. In severe cases, the threads may appear stripped, flattened, or rusty due to the material transfer caused by galling. These visible signs confirm that the fastener surfaces have experienced excessive friction and heat, leading to material adhesion and tearing. Identifying these issues early can help prevent reusing damaged fasteners and avoid further damage to connected components.
What Lubrication Options Reduce Galling in Fasteners?
Lubrication is one of the most effective ways to prevent galling in fasteners. Lubricants create a barrier between the metal surfaces by reducing friction and heat generated, allowing for a smoother assembly. Here are some of the lubricant options you can use:
1. Anti-Seize Compounds
Anti-seize lubricants are specifically designed to combat galling. They mostly contain solid particles like graphite, molybdenum disulfide, or nickel, which minimize adhesive wear. Examples include Permatex® Anti-Seize Lubricant and Finish Line™ Anti-Seize Assembly Lube. To use products containing anti-seize compounds, apply only a thin, even layer to both the male and female threads. Avoid over-lubricating, as excess lubricant can attract dirt and debris.
2. Thread Lubricants for Stainless Steel
Specialized thread lubricants are another excellent option for preventing galling in stainless steel fasteners. These are specifically formulated for stainless steel applications to provide long-lasting protection against cold welding.
3. Dry Film Lubricants (DFLs)
Dry film lubricants are made of fine particles of lubricating agents, binders, and other additives such as tungsten disulfide, molybdenum disulfide, and corrosion-inhibiting pigments that bond to the surface of a part as a solid film. They are applied to parts where liquid lubricants aren’t suitable to create a thin, durable film that adheres to the surface of your nuts and bolts. DFLs are also less prone to creating a mess by attracting soil particles than greases and oils.
4. Wax-Based Lubricants
Wax-based products are easy to apply and offer long-lasting protection. They’re particularly useful for outdoor applications where water resistance is essential.
How Does Proper Thread Design Minimize Galling?
The type of thread design you choose is critical in preventing galling. You can significantly reduce friction and improve performance by improving the interaction between fastener threads. Here are some ways you can minimize thread galling:
1. Avoid Cut Threads
Cut bolt threads have rougher threads, which create more friction and heat when coupled with nuts that usually have tapped threads. In comparison, it’s better to choose rolled threads as they are smoother and more uniform. The rolling process hardens the thread surface, making it more wear-resistant.
2. Thread Coatings and Plating
Stainless steel fasteners with coatings like Teflon, zinc, or nickel experience less friction during assembly. These coatings serve as a barrier, preventing direct metal-to-metal contact. For example, a stainless steel bolt with a Teflon coating will slide more smoothly into a nut, reducing its chances of galling.
3. Choose Coarse Thread Over Smooth Threads
Coarser threads have less surface area when in contact with other surfaces, decreasing friction and heat generation. For components prone to galling, always opt for a coarser thread instead, as fine threads have more surface area easily subjected to friction.
4. Use Dissimilar Materials
Use slightly different stainless steel grades for the bolt and nut when possible. This reduces the tendency for the materials to adhere to each other.
Metal grades have different hardnesses; when mixed, you can reduce galling. For example, you can pair a type 304 stainless steel nut with a type 316 bolt.
5. Slow Down Tightening Speed
The slower the tool speed used during assembly or removal, the less heat generated and the less likely galling will occur. So, control speed when using powered equipment, or opt for hand tools for areas where galling may occur.
Tips for Preventing Galling in High-friction Environments
High-friction environments, such as industrial or high-temperature settings, require extra precautions to prevent galling. Here are some practical tips:
1. Tighten/Loosen Slowly and Evenly
Rapid tightening generates excessive heat, hence increasing the risk of galling. For assembly or removal, you can always use a calibrated torque wrench within the manufacturer’s recommended specifications to ensure even and controlled tightening.
2. Avoid Over-Tightening
Applying too much torque risks galling and can weaken the stainless steel fastener or joint. If unsure, refer to the manufacturer’s guidelines to determine the optimal torque levels.
3. Pre-Coated Fasteners
Many manufacturers offer pre-coated stainless steel fasteners with anti-galling finishes. When necessary, always for those products as they are ideal for high-friction or high-stress environments.
4. Pre-Test Assembly
Before starting a large project, test your assembly process on a sample component. This will help you identify potential galling issues and address them immediately.
5. Proper Storage and Handling
Keep your fasteners clean and dry. Contaminants like dirt, moisture, or rust can worsen galling, especially in outdoor or industrial settings.
Conclusion
Galling is a common but preventable issue in stainless steel fasteners. The key to preventing galling or cold welding lies in understanding its causes, identifying its signs, and implementing effective strategies to avoid the frustration and expense of damaged components.
Always choose the best lubricants, as they are one of the most effective ways to prevent galling in your fasteners and avoid over-lubricating. Use anti-seize compounds, opt for rolled threads, and carefully handle your assembly.
For high-friction environments, consider pre-coated fasteners and test your process. With these tips in your toolkit, your stainless steel fasteners will perform better and last longer—you’ll never have to worry about galling again.