An Alternative Method for the Invariant Threshold Force Evaluation in Incremental Step Loading Tests

Abstract Hydrogen embrittlement (HE) affects the main groups of metal alloys used in the industry, varying from steels to nickel-based corrosion-resistant alloys. It is estimated that 25 % of failures that occur in the oil and gas industry is caused by fracture associated with hydrogen. The incremental step loading (ISL) technique, put forward by ASTM F1624-18(2018), Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique, has been widely used for HE susceptibility evaluation. However, testing metallic materials, other than steels with hardness lower than 33 HRC, following the ASTM F1624 method proved that the invariant threshold force (Pth) determination is not possible. It was observed that the Inconel 718 under certain experimental conditions did not reach the advised 5 % of the fast fracture strength, PFFS, force drop as this material has a low crack growth rate because of its high ductility and low hydrogen diffusivity. Although another force drop value could be adopted, it would still be an arbitrarily defined value that could not be applied to other materials without further study. To overcome this problem, a robust and efficient method of easy implementation was developed and validated. The new approach considers the relative variation, VΔF¯, associated with the difference between the input force (set point value) and the measured force as an alternative to a notch fracture strength force drop percentage. This method does not contradict the ASTM F1624 determinations as it too is based on the force drop values. The Pth values obtained for the AISI 4140 steel using the ASTM F1624 standard corroborate the values obtained based on the proposed method. This method allows the assessment of the Pth values in Inconel 718, as they were corroborated by crack nucleation observations at the notch root using a high-resolution scanning electron microscopy after the ISL tests. The obtained results show that the proposed method proved to be robust, efficient, and of easy implementation. Thus, it can be used as an alternative to the Pth evaluation of steels with a hardness less than 33 HRC and for metallic materials not covered by the ASTM F1624 standard.