High-strength Nickel Low Alloy Steels for Oil and Gas Equipment: Hydrogen stress cracking resistance of ASTM A508 Grade 4N under cathodic charging
Low Alloy steels (LASs) are, by volume, the most widely used alloy family in critical oil & gas (O&G) components. LASs combine relatively low cost with exceptional mechanical properties, achievable through correct processing and heat treatments. However, the strength and hardness of LASs for sour environments and for applications where atomic hydrogen is generated at the surface, e.g., cathodic protection, is limited to prevent different forms of hydrogen embrittlement such as hydrogen stress cracking (HSC) and sulfide stress cracking (SSC). As a result, the specified minimum yield strength (SMYS) of forged LASs for, e.g., subsea components, rarely exceeds 550 MPa, while the most common pipeline steels are API X65 to X70, with a SMYS of 450 MPa and 482 MPa, respectively. Moreover, ISO 15156-2 restricts LAS to a maximum of 1 wt% Ni due to SSC concerns. The LASs that exceed the ISO 15156-2 limit have to be qualified for service, lowering the appeal of alloys that do not meet ISO 15156-2 prerequisites.
Due to the growing demand for developing unconventional O&G reservoirs, LASs with improved hardenability, excellent toughness, fatigue life and yield strengths above 690 MPa could be essential to overcome design challenges in extreme O&G environments. Thus, pushing the boundaries of LASs is critical to the safe and profitable exploitation of, e.g., arctic and high-pressure and high-temperature fields. High-strength low alloy steels (HSLAs) containing >1 wt% Ni and with outstanding track records in hydrogen-bearing environments in, e.g., the nuclear, defense and pressure vessel industries have yet to be evaluated for O&G environments, possibly due to ISO 15156-2 restrictions.
In this work, the HSC resistance of the high-nickel (i.e., 2.8 to 3.9 wt%), quenched and tempered (Q&T), nuclear-grade ASTM A508 Gr.4N HSLA with a SMYS of 690 MPa was investigated in the first round of tests. Herein, tensile samples were cathodically polarized at three different potentials, i.e., -1.05 V(SSC), -1.4 V(SSC) and -2.0 V(SSC), and the HSC resistance evaluated using the slow strain rate test. Additionally, a set of specimens was tested under constant load, applying a cathodic potential of -1.4 V(SSC), to determine the threshold stress at which the alloy shows no fracture due to hydrogen embrittlement. Results are linked to microstructure features, which were characterized by light optical microscopy, scanning electron microscopy and electron backscatter diffraction.