2.5 Seizure risk with stainless steel threaded couplings

The stainless steels generally used for fasteners are:

• A2 (similar to AISI.304 steel)
• A4 (similar to AISI.316 steel)

An indelible marking always identifies the steel type and the mechanical strength class.

The tightening torque is dependent upon:

• The nominal diameter of the threading
• The mechanical strength class of stainless steel (50-70-90)
• The friction coefficient.

A high friction leads to the dissipation of a large amount of energy. The stainless steel thermal conductivity is about half that of carbon steels, therefore the tightening of the screw and nut, both made out of stainless steel, increases the heat generated towards the plastic deformation of the material thus creating a potential locking condition (seizure) of the coupling. In the case of disassembly and reassembly of the coupling, the seizure risk increases considerably. In practice, to avoid this risk, it is recommended to lightly lubricate both the threading and the nut under head with MoS2 paste or simply use some anticorrosive grease.