The evaluation of hydrogen embrittlement threshold force using the Small punch test

Abstract

This study aims at developing a methodology for assessing the hydrogen embrittlement threshold force (Pth) in high strength steels using the Small Punch Test (SPT) technique and the ASTM F1624 standard recommendations. This methodology is based on material specimen stiffness and aims to test steels with hardness lower than 33HRC. In the methodology development phase, non-notched specimens made of four high strength steels (AISI 4137 M − 32HRC and AISI 4130 M − 25, 40, 44HRC), widely used in the oil and gas offshore industry, were tested according to the ASTM F1624 standard. Based on the literature, hold times of 20 min for the first 10 and 40 min for the last 10 steps were adopted. The tests were performed in a cathodic protection environment, consisting of a 3.5 wt% NaCl aqueous solution for hydrogen embrittlement (HE). The threshold force values (Pth-SPT) for the Incremental Small Punch Test (ISPT) were measured based on the specimen stiffness. In order to validate the proposed methodology, the ISPT and the incremental step loading (ISL) techniques were applied to test notched specimens manufactured from AISI 4130 M − 25 HRC and AISI 4137 M − 32HRC. The Pth-SPT values showed a systematic error of -4.0 % and 2.2 %, respectively, in relation to the ISL method, indicating an excellent accuracy. The Pth-SPT values provided by the ISPT under the experimental conditions investigated are adequate and quite promising. These values corroborate that the proposed methodology for the measuring of the Pth-SPT, based on material specimen stiffness, proved to be robust, efficient, and of easy implementation.