Avaliação da fragilização por hidrogênio em aços ARBL utilizando ensaios trativos em corpos de prova entalhados

Abstract

The evaluation of hydrogen embrittlement susceptibility in high strength steels for application in subsea components subjected to cathodic protection for corrosion control is of fundamental importance. This assessment can help decisions and materials qualification to avoid failures that cause environmental, economic and human life damages. Therefore, this study evaluates an AISI 4340 steel with hardness of 32, 36 and 40 HRC using three types of methodologies: slow strain rate tests in smooth (SSRT) and notched (nSSRT) specimens and constant load tests (CLT). The tests were carried out at two levels of cathodic protection (-950 mV and -1100 mV using Ag|AgCl reference electrode) in saline solution with 3.5 % wt of NaCl. Three different notch radiuses were used. The results obtained in the notched specimens tests were compared with the hydrostatic stress profiles obtained using finite element simulation. The post-test fracture surfaces were also evaluated via scanning electron microscopy. The test results indicated a higher hydrogen index embrittlement (HI) the higher the material hardness, level of cathodic protection applied and smaller notch radius, as well as a relationship between HI and the degree of intergranular fracture in the specimens. The results obtained by the nBTD tests were compared with the stress threshold (σth) obtained in the CLT test and a relation of σth = 90% x σnBTD_min can be used for a faster assess of this parameter. Additionally, fracture toughness test evaluation in hydrogen environment (KIc-H) was done by analyzing the fracture surface of the nBTD and CLT specimens: 90,2 ± 5,2, 88,1 ± 7,1 e 79,8 ± 6,4 MPa.m0,5 was obtained for the hardeness of 32, 36 e 40 HRC, respectively, these values were compared with pre-cracked CT specimens for 40 HRC and -1100 mV condition, the results were the same using the highest displacement rate.