Slow Strain Rate Testing for Hydrogen Embrittlement Susceptibility of Alloy 718 in Substitute Ocean Water
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JMEPEG (2017) 26:2337–2345 DOI: 10.1007/s11665-017-2675-x
Slow Strain Rate Testing for Hydrogen Embrittlement Susceptibility of Alloy 718 in Substitute Ocean Water M.P. LaCoursiere, D.K. Aidun, and D.J. Morrison (Submitted January 29, 2017; in revised form April 2, 2017; published online April 12, 2017) The hydrogen embrittlement susceptibility of near-peak-aged UNS N07718 (Alloy 718) was evaluated by performing slow strain rate tests at room temperature in air and substitute ocean water. Tests in substitute ocean water were accomplished in an environmental cell that enabled in situ cathodic charging under an applied potential of 21.1 V versus SCE. Some specimens were cathodically precharged for 4 or 16 weeks at the same potential in a 3.5 wt.% NaCl-distilled water solution at 50 °C. Unprecharged specimens tested in substitute ocean water exhibited only moderate embrittlement with plastic strain to failure decreasing by about 20% compared to unprecharged specimens tested in air. However, precharged specimens exhibited significant embrittlement with plastic strain to failure decreasing by about 70%. Test environment (air or substitute ocean water with in situ charging) and precharge time (4 or 16 weeks) had little effect on the results of the precharged specimens. Fracture surfaces of precharged specimens were typical of hydrogen embrittlement and consisted of an outer brittle ring related to the region in which hydrogen infused during precharging, a finely dimpled transition zone probably related to the region where hydrogen was drawn in by dislocation transport, and a central highly dimpled ductile region. Fracture surfaces of unprecharged specimens tested in substitute ocean water consisted of a finely dimpled outer ring and heavily dimpled central region typical of ductile fracture. Keywords
hydrogen embrittlement, nickel-based superalloy, slow strain rate testing
1. Introduction The nickel-based alloy UNS N07718 (Alloy 718) has been used for many years in aircraft turbine engine applications such as compressor blades, compressor disks, turbine disks, and fasteners because of the alloyÕs superior corrosion resistance and high-temperature strength (Ref 1). Strength is achieved through solid solution strengthening and the controlled precipitation of c0 (Ni3Al—fcc-type structure) and c00 (Ni3Nb—bcttype structure). The c00 is the primary contributor to the strengthening (Ref 2). In addition, the microstructure frequently contains d precipitates (Ni3Nb—orthorhombic-type structure) and various metal carbide particles (Ref 3). More recently, the alloy has been used in oil and gas applications such as subsurface valves, blowout preventers, and fasteners (Ref 4, 5). In many oil field applications, Alloy 718 is used in systems that use cathodic protection to enhance corrosion resistance (Ref 6, 7). Unfortunately, the resulting electrochemical reaction causes hydrogen to be produced at the surface of the metal. This hydrogen diffuses into the metal and can cause hydrogen embrittlement. The phenomenon of hydrogen embrittleme
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