Investigation of the fracture resistance of high strength ferritic steel welds in gaseous hydrogen environment

Hydrogen is expected to play a major role in the decarbonization of the energy grid. As a fuel, it possesses an elevated energy density per unit mass, about twice as much as natural gas but also a very low mass density; for this reason, it is preferably stored and transported at elevated pressures in the gas phase to achieve a comparable volumetric energy density. High strength quenched and tempered ferritic steels are widely used for gaseous hydrogen storage despite the fact that hydrogen affects material properties and result in higher fatigue crack growth rate and lower fracture toughness.This work presents the results of a comprehensive fracture toughness test program in gaseous hydrogen for the characterization of base metal, heat affected zone and weld metal on a high strength, quenched and tempered steel, suitable for the construction of hydrogen pressure equipment.Full penetration, butt welded joints were made using partly mechanized gas metal arc welding technique on a weldable, high strength steel, namely EN 101216-3 grade P690 QL2. Two different welding procedures were evaluated, and fracture toughness tests were conducted according to ASTM E1820 requirements, in high purity (99.9995%) hydrogen gas at a pressure of 200 bar on specimens extracted from base metal, heat affected zone and weld metal. While base metal and weld metal exhibited stable crack growth behavior, heat affected zone showed variable behavior as a function of the post-weld heat treatment conditions. It was found that local micro-hardness spots are a concern and should be controlled through dedicated post weld heat treatment operation to limit hardness values for use of welds in H2 gas conditions.