Which joining method is not permissible for stainless tube?

Soldering is not a permissible joining method for stainless steel tubing, primarily due to the lower melting temperatures of solder compared to the melting point of stainless steel. Soldering involves the use of a filler metal that has a melting point below that of stainless steel; however, this can lead to various issues such as the risk of contamination and loss of structural integrity at the joint.

In contrast, welding is a widely accepted method for joining stainless steel because it creates strong, permanent connections by melting the base metal and allowing for a seamless integration. Threading is acceptable for stainless steel applications as well, typically seen in pipe connections, provided that proper thread engagement and sealing methods are employed. Brazing is also an appropriate method for stainless steel, as it allows the use of higher melting point filler metals that can form robust joints without compromising the material properties of the base metal.

Therefore, soldering stands out as the inappropriate choice for joining stainless steel tubing due to its inherent limitations with respect to temperature and strength.