Answer
Thread galling is a common, yet seldom understood problem with threaded fasteners. Galling is a cold welding process which can occur when the surfaces of male and female threads come in contact with heavy pressure.
Stainless steel fasteners are particularly susceptible to thread galling, although it also occurs in other alloys which self generate an oxide film for corrosion purposes, such as aluminum and titanium. During the tightening of the fastener a pressure builds between the contacting thread surfaces and breaks down the protective oxides. With the absence of the oxide coating, the metal high points can shear and lock together.
Minor galling may cause only slight damage to the thread surface and the fastener may still be removed. However, in severe cases galling can completely weld the nut and bolt together and prevent removal of the fastener. Often times, once galling begins, if the tightening process is continued, the fastener may be twisted off or its threads stripped out.
Unfortunately, even with the understanding of the mechanism of galling, little is known on how to successfully control it. However, probability of galling occurring can be minimized with the following measures:
Thread lubrication is one of the most effective measures to lessen the potential for galling. The lubricant reduces friction which is the key element in thread galling. Certain material environments, such as stainless steel fasteners used in the food processing equipment, preclude the use of lubricants. Also, attention must be given to the torque-tension relationship which will be altered with the use of lubrication.
Use coarse threads with a 2A-2B fit instead of fine threads. Coarse threads have a larger thread allowance and are more tolerant to abuse during handling.
Heat contributes significantly to thread galling. Installing a fastener generates heat and high speed installation generates significantly more heat. Lowering the wrench speed during installation and removal can be helpful.
Mating parts of the same alloy have a greater tendency to gall that parts of dissimilar alloys having different degrees of hardness. Most stainless steels are more susceptible to galling than carbon alloy steels. However, not all combinations of stainless steels act the same. For instance, a 400 series stainless steel nut can work well on 316 series bolts, but with a reduction in corrosion resistance.
A smoother surface texture will lead to less frictional resistance. As previously mentioned, friction increases the possibility of galling. Rolled threads provide a relatively smooth surface. Unfortunately, an easy method of rolling internal threads has yet to be found.