Structural Integrity of HSLA Steels under Hydrogen Embrittlement Condition

Abstract

High strength steels have large applicability in the gas and oil industry and are often used in aggressive environments, requiring the use of cathodic protection, with the consequent generation of atomic hydrogen in the cathode that may lead to hydrogen embrittlement, HE. The aim of this work was to evaluate the fracture toughness and HE susceptibility of two high strength steels, named as 38CrMo4 and 30CrMo6, used as components for oil extraction from the seabed. J-Δa curves for JIC evaluation were carried out following the ASTM E1820 [1] at room temperature in laboratorial air using C(T) specimen. For KTH evaluation the tests were based on NACE TM0177 standard [2], using sharply notched DCB specimens. For this test a 3.5 wt% NaCl aqueous solution with −1.2 V cathodic protection for the in-situ hydrogen charging were used. From the results it was observed that both steels presented predominantly martensitic/bainitic microstructures, with 30CrMo6 steel exhibiting lower mechanical strength and higher total elongation. The great microstructural differences rely mainly on the prior austenite grain size and in the S and Ca contents. 30CrMo6 steel presented a very fine prior austenite grain and mostly spherical CaS inclusions instead of the elongated MnS. These microstructural features lead to much higher KJIC value and it did not exhibit HE in the conditions analyzed here.