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High-strength Nickel Low Alloy Steels for Oil and Gas Equipment: ASTM A508 Grade 4N under cathodic protection and simulated sour environments.

Low alloy steels (LASs) combine relatively low cost with exceptional mechanical properties, making LASs commonplace in Oil and Gas equipment. However, the strength and hardness of LASs for sour environments and for applications that generate atomic hydrogen at the surface, e.g., cathodic protection, is limited to prevent different forms of hydrogen embrittlement (HE) 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 (80 ksi), while the most common pipeline steels are API(1) X65 to X70, with a SMYS of 450 MPa (65 ksi) and 482 MPa (70 ksi), respectively. Moreover, ISO(2) 15156-2 restricts LASs to a maximum of 1.0 wt% Ni due to SSC concerns. The LASs that exceed the ISO 15156-2 limit have to be qualified for service, lowering their commercial appeal.  

In this work, the HSC resistance of the high-nickel (3.41 wt%), quenched and tempered (Q&T), nuclear-grade ASTM(3) A508 Gr.4N LAS was investigated using slow strain rate testing (SSRT) as a function of applied cathodic potential. Results showed that the yield strength (YS) and ultimate tensile strength (UTS) were unaffected by hydrogen, even at a high negative potential of -2.0 VAg/AgCl. HE effects were observed once the material started necking, manifested by a loss in ductility with increasing applied cathodic potentials. Indeed, A508 Gr.4N was less affected by H at high cathodic potentials than a low-strength (YS = 340 MPa) ferritic-pearlitic LAS of similar nickel content. SSRT results were linked to microstructure features, which were characterized by light optical microscopy (LOM), scanning electron microscopy (SEM) coupled to electron backscatter diffraction (EBSD). 

Product Number: 51320-14706-SG
Author: Andreas Viereckl, Esteban Rodoni, Zakaria Quadir, Garry Leadbeater and Mariano Iannuzzi, Yuta Honma
Publication Date: 2020