What is thread galling? Let’s answer first with a picture:
Fig. 1 – Thread Galling (American Fastener Journal, 10/18/02)
In the above image, we can see obvious damage to this bolt’s threads, which at first glance appear to be from some sort of striking impact. In fact, the condition we’re looking at is thread galling, a type of metallic wear caused by the forceful separation and transfer of material from one object to another. Galling is a common problem with metallic fasteners, though luckily this can be avoided with some easy preventative action.
For a better understanding of the problem, we need to zoom down to the molecular level. Certain metal materials can experience plastic deformation and metallic-bonding attraction simultaneously, leading to galling. Plastic deformation is when small areas of an overall object distort while the rest of the object remains intact. Metallic-bonding attraction is the tendency for metallic objects to attract other free, similar objects with enough external force that the two objects merge into one mass.
With enough friction between two separate metallic parts (such as a nut and bolt during high-torque tightening), a small portion of one part can break free and become fused to the second part, depositing on that second part in the form of a tiny bead known as a gall.
Some resources will refer to galling as ‘cold welding’, inferring that the materials bind together similar to a welding process. Since galling is not a heat-driven connection, the materials are not experiencing the phase-change that occurs in actual welding. The ‘fusing’ that occurs here is simply the effect of the very strong metallic-bonding attraction.
Galls are self-perpetuating in a way, as they present hard clash points that further deform and strip away material as they’re rotated together. As galls accumulate on a threaded object, the added interference causes the threads to experience more friction, become more difficult to tighten and eventually seize up.
Galling is prevalent in softer metals such as stainless steel, aluminum, and titanium, given their lower relative surface hardness and higher tendency for plastic deformation. Metals that undergo a heat treatment hardening process are less likely to gall, standing up much better to increased friction and heat during assembly.
Do you have a Galling-Sensitive application but are unsure of which fastener meets your project’s requirements? E&T Fasteners offers expert support and product selection between the above options and more! Contact us to discuss your application, or to see our entire fastener catalog, visit our product selection here.
Ways to Prevent Galling
Material science has long studied plastic deformation in metals, specifically where it applies to machining, rotary power transmission, mechanical linkage, reciprocating engines, and structural fasteners. Galling cannot be directly reverted – lightly galled fasteners may be ran through with a tap and die set or re-machined to remove excess gall beads, but the underlying deformation and missing materials can’t easily be repaired. Research tells us that the best solution to galling is to stop it before it occurs, using techniques such as:
- Select compatible hardware pieces that will resist galling, such as using bronze nuts with stainless steel bolts.
- Perform heat treatment or other surface hardening processes.
- Apply a surface coating that will provide a smoother, non-bonding surface upon which the metallic parts can slide.
- Apply lubrication to the parts prior to contact.
Lubrication is perhaps the most common solution utilized today. This allows buyers to select lower cost commodity hardware of any type that suits their application, and then to apply a lubricant separately before installation at a marginal additional cost. Compared to heat treatment and advanced alloys, lubrication tends to be more economical in practice.
Today, anti-seize compounds are widely available in many different formulations addressing various applications. For example, zinc-based anti-seize compounds are often used on aluminum materials, whereas NSF certified mineral oil complex compounds are preferable for stainless steel use in food-contact applications. In some cases, anti-seize formulations are specifically undesirable due to potential hazards, such as in the use of graphite-containing compounds on aluminum materials which will promote corrosion. Always check compatibility with your applications, especially regarding temperature, pressure, environmental, material, dynamic vibration / movement, and UV exposure.
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