Low Alloy steels (LASs) are, by volume, the most widely used alloy family in critical oil & gas (O&G) components. However, the strength and hardness of LASs for sour environments are limited to prevent different forms of hydrogen embrittlement, such as hydrogen stress cracking (HSC) and sulfide stress cracking (SSC). Moreover, ISO 15156-2 (1) restricts LASs to a maximum of 1 wt% Ni due to SSC concerns.
In the present work, the hydrogen diffusivity of the nuclear-grade ASTM(2) A508 Gr.4N LAS was measured using the hydrogen permeation method. Results are linked to quenched and tempered (Q&T) microstructure features characterized by transmission electron microscopy (TEM). Additionally, a comparison was made between the A508 Gr.4N and a ferritic-pearlitic steel with similar Ni content. This work is connected with the HSC evaluation of the same alloy by slow strain rate testing (SSRT) described in a separate publication.
Due to the progressive depletion of conventional oil and gas (O&G) reserves, the energy demand drifts its attention to unconventional reservoirs.1 These new O&G fields pose challenges to the materials used that are often associated with high-pressure (>103 MPa) and high temperature (>177 °C) or, in arctic service, temperatures as low as -60°C.1-3 Moreover, the presence of atomic hydrogen produced either from corrosion reactions or by cathodic protection can lead to hydrogen stress cracking (HSC). In sour environments, the cracking mechanism is referred to as sulfide stress cracking (SSC) in which H2S acts as a hydrogen recombination poison and increases severity. Materials susceptible to hydrogen embrittlement (HE)—by either HSC or SSC—characterized by ductile behavior under normal circumstances, can fracture in a brittle mode in the presence of atomic hydrogen. Therefore, materials selection is paramount to reduce possible catastrophic failures.
Low alloy steels (LASs) are widely used because of their low cost and good mechanical properties.1 By appropriate heat treatment processing, a high yield strength (e.g., 690 MPa (100 ksi)) can be achieved while retaining adequate toughness. The hardenability can be enhanced by the addition of alloying elements such as molybdenum, chromium, and nickel.4
Nickel additions produce an increase in yield strength (YS) due to solid solution strengthening and subgrain size reduction.1, 5 Hardenability is improved with Ni as it stabilizes the austenite phase, delaying the austenite to martensite transformation.1 Additionally, Ni increases the low-temperature fracture toughness due to grain refinement and modification of the ferrite properties.6 Furthermore, Ni has a low impact on weldability as reflected by its carbon equivalent coefficient, which is the lowest compared with other alloying elements.1
