Why are stainless steel screws and nuts not recommended to be used together under certain conditions?

In industrial assembly and equipment maintenance, the saying that "stainless steel screws should not be used with stainless steel nuts" has been around for a long time. It's important to emphasize that this is not an absolute taboo, but rather a summary of engineering experience regarding failure risks under specific working conditions. The core issue is the "gagging phenomenon" of stainless steel fasteners.

 

In applications such as cryogenic pipeline flanges, pressure vessels, and chemical equipment, when stainless steel bolts and nuts have been in service for a long time under high preload conditions, severe rotational resistance or even complete inability to remove them often occurs during disassembly.

 

This problem is particularly prominent under static load, no lubrication, and same material mating conditions.

 

 

From a materials science perspective, stainless steel has good ductility and low surface hardness. When stainless steel screws and nuts are tightened, the passivation film on the threaded contact surface is easily destroyed under the combined action of high pressure and frictional heat, and the exposed metal matrix then undergoes microscopic adhesion. As rotation continues, this adhesion repeatedly occurs and is sheared, eventually leading to localized "cold welding" of the threads.

 

Furthermore, during long-term service, fasteners undergo slight plastic deformation under load, making the threaded joints even tighter and further amplifying the frictional effect. When attempting to disassemble in reverse, it is necessary to overcome not only the conventional frictional force but also the additional resistance generated by material adhesion and structural deformation, thus significantly increasing the difficulty of disassembly.

 

 

To reduce the risk of seizing in stainless steel fasteners, the following methods are commonly used in engineering practice:

 

First, the probability of adhesion can be reduced by using dissimilar materials, such as using stainless steel bolts with copper nuts or surface-treated nuts, breaking the adhesion conditions of the same material at the material level.

 

Second, lubricating the thread surface before assembly is one of the most effective methods. Common lubricants include grease, graphite, molybdenum disulfide, anti-seize agents, or wax impregnation. These measures can significantly reduce the coefficient of friction and suppress temperature rise.

 

Secondly, in high-temperature or high-load environments, specialized anti-seize agents should be used preferentially to prevent sintering or adhesion failure under elevated temperature conditions.

 

 

Compared to carbon steel fasteners, stainless steel fasteners are more prone to locking, primarily due to differences in material properties. Taking common stainless steel as an example, its hardness is generally lower than that of 8.8 grade carbon steel, but its ductility is better. When the thread surface is damaged, stainless steel quickly forms a new oxide film to resist corrosion. However, during tightening, this oxide film is continuously destroyed and regenerated, forming a chain reaction of "destruction-adhesion-shear-re-adhesion," ultimately leading to locking.

 

This phenomenon is particularly common in stainless steel, aluminum alloy, and titanium alloy fasteners, while its probability is relatively low in carbon steel fasteners with higher hardness and lower ductility.

 

 

In practical applications, locking problems are often not caused by a single factor, but rather the result of multiple operational errors. For example, mismatched mechanical property grades of screws and nuts, the presence of burrs or metal shavings on the thread surface, or excessive force or excessive tightening speed during installation can significantly increase the risk of seizing.

 

When using an electric wrench for high-speed tightening, the temperature of the threaded pair rises sharply in a short period, which is one of the typical causes of instantaneous seizing of stainless steel fasteners. Furthermore, failure to keep the nut perpendicular to the bolt axis or the failure to use a flat washer or spring washer can also lead to localized stress concentration, thus inducing seizing.