Let’s talk about galling
This blog post is centered on an issue that continues to plague the energy industry – the galling of threaded fasteners. So first things first, what is galling?
The technical definition is that it is a form of material wear during sliding in which macroscopic transfer of material is driven by adhesion. In practice, however, galling is something virtually everyone has encountered. It’s stuck nuts and seized bolts. It normally happens when you go to break them out and they won’t budge. It’s more than just friction or debris, they act as one part. It’s something that happens every day, in every plant. However, for something so common, there are a number of misconceptions about galling.
The common explanation of the galling process is that friction causes heat, which in turn melts the metal to create miniature welds. When, in actuality, the heat generation is driven by deformation of the material (you’ve seen this effect if you’ve ever bent a piece of metal back and forth and felt it heat up).
Also, fusion of the two parts is due to adhesion rather than phase change. Meaning that the bonding mechanism here isn’t that the parts melt, it’s that they get close enough together and are at a high enough energy state that they form bonds and become one part.
How does galling occur? Let’s take a look at the galling process itself:
Here we have a representation of two pieces of metal sliding past each other. To the naked eye, this is a reasonable representation, but under a microscope, none of the materials we are dealing with have a perfectly smooth surface. There are high and low spots.
Naturally, the initial contact is at the high points or asperities. From a macroscopic viewpoint, the load is distributed over a large area resulting in a low apparent stress. But at the microscopic level this small contact area sees a tremendous local stress, exceeding the strength of the contact material (which would be the oxide layer) allowing it to penetrate the opposing surface. The resulting plastic deformation increases the local heat, and since adhesion scales with temperature, the adhesive attraction increases as well.
As the materials start to move past each other, the embedded asperity begins to move and build up material, due to both geometry and adhesion, and a lump beings to form.
The lump then continues to grow and eventually is large enough to penetrate the protective oxide layer and damage the underlying bulk material.
In the earlier stages, most of the interaction is limited to the brittle oxide layers, but brittle fracture rarely produces the kinds of conditions conducive to galling. The fact that we are now deforming the ductile base metal is not a trivial one; we are scraping away those protective oxide layers.
As the lump grows and material has to plastically flow around it, heat is generated, again think of bending a piece of metal back and forth. Heat generation along with a small cross-sectional area for conductance means that more energy is going into this small region than is going out, thus the energy storage increases.
Eventually, we reach a level such that there is a clear change in the adhesive properties of the materials and the contact and plastic behavior change.
The parts are now in such intimate contact and at a high enough energy state that they start forming metallic bonds. That is, they share a common electron cloud. These are the same bonds that permeate the entire metallic structure. These atoms have no way of knowing that they are from different parts. So, the parts are fused together, not similarly to them being one part, they ARE one part. Just as any other region of the metal is the same part. And there has been no liquid phase – no melting. They were simply close enough, with enough stress and energy, to start sharing electrons.
By looking at the process, we begin to understand the factors that affect galling:
Heat certainly promotes galling. During movement the adhesion scales with heat and in-service, heat increases the creep penetration of those high points. This leads to additional potential initiation sites for galling. Whether it’s during movement or in-service, nothing ever gets hot enough to actually melt the metal. There’s never any liquid phase.
Ductility also promotes galling. It takes energy to create a new surface because at some level you have break the internal bonds. This is related to a material’s surface energy. So, in brittle fracture, energy goes to creating the new surface. In ductile fracture, however, energy also goes into deforming the material and is stored as heat. And, again, that higher localized heat increases adhesion.
Oxide layers are a little trickier. They certainly inhibit galling and cold welding, by physically getting in the way of the underlying bulk metals. They are also typically brittle, thereby providing an energy storage mechanism. However, in some cases the volume change and debris can effectively promote galling by reducing clearances and increasing initiation sites.
Now that we know more about the galling phenomenon and process, we can begin to look for solution paths.
One solution that we all employ is lubrication. Liquid and grease-based lubricants form a film thicker than the asperities, preventing metal-to-metal contact. However, these lubricants evaporate slowly over time, reducing their effectiveness.
Solid lubricants work well even when dry, but unlike liquids, they cannot reflow into areas where they have been scraped away. After sliding between two surfaces, the solid lubricant will be worn away to expose bare metal asperities. They can also be mildly reactive, so over longer times scales, they can degrade.
As for other solution paths, we can alter the geometry to lower the stress, change the surface finish to limit asperities, change the friction to reduce contact, limit the ductility, look into ideal oxide layers, find material combinations that result in low adhesion – or, to eliminate galling altogether, consider INTEGRA Technologies Pop-Washer™, as there are technologies to prevent this common but complex issue.
INTEGRA Technologies delivers targeted and practical solutions for bolted assemblies in the energy industry. We draw on over 30 years of field experience and combine it with our engineering expertise to provide products that deliver certainty of outcome. Contact us today to learn how INTEGRA Technologies can help you reach your goals.