RTJ

It is suitable for high-pressure and temperature applications and require high bolt load to seal.
That allows it to resist pressures as high as 10,000 psi but also means it has virtually no compression. Very high bolt loads are needed to create enough deformation for joint sealing.

RTJs are available in oval and octagonal cross sections and are available in different metals from soft iron, low carbon steel, and a variety of stainless steel.  RTJs are available in ring No. R11 to R105.