Safety Margin on the Ductile to Brittle Transition Temperature after Hydrogen Embrittlement on X65 Steel

Hydrogen embrittlement is a phenomenon that affects the mechanical properties of steels intended for hydrogen transportation. One affected by this embrittlement is the Ductile to Brittle Transition Temperature (DBTT), which characterizes the change in the failure mode of the steel from ductile to brittle. This temperature is conventionally defined and compared to the operating temperature as an acceptability criterion for codes. Transition temperature does not depend only on the material, but also on specimen geometry particularly the thickness. Generally, the transition temperature is defined for the conservative reason by Charpy impact test. Standard Charpy specimens are straight beams with a thickness of 10 mm. For thin pipes, it is impossible to extract these standard specimens. One uses in this case Mini-Charpy specimens with a reduced thickness due to pipe curvature. This paper aims to quantify the effect of hydrogen embrittlement on the transition temperature of pipe steel (API 5L X65) using two types of Charpy specimens.