Abstract
ISO 11114-4 standard specifies the disc test (method A) for selecting metallic materials resistant to hydrogen embrittlement. The test consists of a disc-shaped specimen subjected to an increased gas pressure at a constant rate until it bursts or cracks. The Hydrogen Embrittlement Index (HEI) is the ratio between the rupture pressure under helium and the rupture pressure under hydrogen. The standard proposes to carry out the test on a disc with a thickness of 0.75 mm. Using this geometry, failure very often occurs in the clamping area, making the test analysis difficult. This study aims to test two new sample geometries allowing for disc rupture outside the clamping area. The first geometry consists in a 3 mm thick disc in which a truncated spherical cap is machined in the center, obtaining a minimum thickness of 0.75 mm (Cap Disc). The second geometry is a 2 mm thick disc with an axisymmetric machined notch with a radius of 1.25 mm, obtaining the same minimum thickness of 0.75 mm (Notched Disc). Tests using the geometry proposed by the standard were also carried out for the sake of comparison. Two different steel grades were studied: a vintage X52 steel for pipeline applications and a modern E355 steel. After machining, metrology assured that all specimens had the specified roughness in the region of interest (i.e., the center of the cap and the bottom of the notch). During the test, the pressure applied to the disk was recorded together with the displacement of the center point at the outer surface of the disk.