The steel industrially employed for the transport of liquid hydrocarburics in the presence of hydrogen at high temperatures is ASTM A335 P11, a steel for seamless pipes for use at high temperature. Three different kinds of tests have been performed: tests of prenatured fracture, tests of thermal analysis and infrared analysis. The present work has deeply studied the mechanisms of interaction between hydrogen and steel, trying to integrate and motivate the industrial data, commonly used for plant planning and sizing (A.P.I. Diagrams), proving that the cause for the damaging of this material is mainly due to the harmful interaction of carbides and inclusion with hydrogen.