Galling
Galling turns up in 3 fields: tribology, fasteners, valves. Here is what it means to the people in each one.
Galling in tribology
Severe adhesive wear where sliding metals seize and tear, raising lumps of transferred metal.
A form of adhesive wear in which sliding surfaces, often of similar metals, bond and rip apart, leaving raised lumps and plucked craters instead of a smooth wear scar. Stainless steel, aluminum and titanium threads and slides are notorious for it, and galling can seize parts within a few strokes. Anti-seize compounds, dissimilar material pairs, hard coatings and lower contact stress prevent it.
"Stainless nut galled on the stud halfway down, next time put anti-seize on the threads first."
Galling in fasteners
Adhesive wear where thread surfaces cold-weld and tear under load, seizing a nut partway down a bolt.
Severe adhesive wear in which the protective oxide films on mating threads break down under pressure and sliding, so the metals bond and tear. Stainless steel, aluminum and titanium are the usual victims, and fast installation with power tools makes it worse by heating the threads. Prevention includes anti-seize, slower rundown speed, coarse threads and pairing different alloys or hardnesses.
"Nut galled halfway down the stainless rod, it's welded on, cut it off and get anti-seize on the next one."
Galling in valves
Severe adhesive wear where loaded metal surfaces slide, cold-weld together and tear material out.
Stainless steel on stainless steel is notorious: stems seize in stem nuts, threads lock up, and seats or wedges tear when they slide under load. Once it starts, galling usually gets worse every cycle. Prevention means pairing dissimilar or hardfaced materials, keeping threads lubricated, and controlling seating loads, which is why valve trim specs pay so much attention to hardness differences.
"Stem nut is galled to the stem, it took both of us on the wheel to move it and now it's chewed."