Comparison between slow strain rate test and constant load test on notched specimens for HSLA steels HE susceptibility evaluation

The assessment of hydrogen embrittlement (HE) susceptibility in high-strength steels, intended for use in subsea components that are exposed to cathodic protection for corrosion control, is of great importance. Therefore, the present study investigates the behavior of AISI 4340 steel with hardness of 32, 36 and 40 HRC. The investigation involves employing slow strain rate test (nSSRT) and constant load test (CLT) on notched specimens with three different notch root radii. These tests were carried out under two different levels of cathodic protection (-1100 and -950 mV Ag|AgCl ) in a 3.5 % wt NaCl aqueous solution. The outcomes from the tests on notched specimens were compared with the hydrostatic stress profile obtained through finite element analysis (FEA). Additionally, the post -test fracture surfaces were analyzed using scanning electron microscopy (SEM). The test results revealed a higher hydrogen embrittlement index (HEI) with increasing material hardness and the applied cathodic potential. Moreover, a smaller notch radius was associated with a higher HEI. A clear correlation was observed between HEI and the extent of intergranular fracture in the test samples. Furthermore, the results from the nSSRT tests were correlated with the threshold stress ( sigma th ) obtained from CLT. It was evident that a relationship of sigma th = 90 % sigma nSSRT_min could be employed for a faster determination of this parameter.