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Environmentally Assisted Cracking Evaluation of UNS N06686 Using Constant Extension Rate Testing

Picture for 11296 Environmentally Assisted Cracking Evaluation of UNS N06686
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Product Number: 51300-11296-SG
ISBN: 2011 11296 CP
Author: Michelle G. Koul and Peggy S. LeGrand
Publication Date: 2011
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$20.00
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Constant, slow extension rate tests in air and ASTM D1141 seawater were conducted on two cold worked bars of Grade 2 IN686 (UNS N06686, 125 min. yield strength) where the cold work was imparted using two different methods: conventional drawing and rotary forging. Baseline tests were also conducted on Alloy K-500 (UNS N05500), which is a marine grade material currently in use. Specimens were cathodically polarized to simulate catholic protection conditions and evaluate the alloys’ resistance to hydrogen-assisted EAC (HEAC). Ductility measurements using elongation at fracture and reduction in area were used to quantitatively evaluate the HEAC susceptibility. In addition, qualitative fractographic assessments to identify EAC fracture modes were conducted using optical and scanning electron microscopy. The strength and fracture morphology was found to vary with the method of cold working employed. Susceptibility to HEAC was indicated for smooth specimens of IN686 at -1.0 VSCE<.sub> and 9x10-7 in/sec elongation rate via test data and the presence of secondary cracking. IN686 ductility losses were 25% at most, whereas Alloy K-500 measured a ductility loss of more than 60% under the same conditions. The measured effect of a thread-like notch was to reduce the elongation at fracture of IN686 to less than 10% for all conditions, and exacerbate the less ductile fracture modes that were observed.

Key words: environmentally assisted cracking, hydrogen embrittlement, nickel alloys, cathodic protection, marine corrosion
Constant, slow extension rate tests in air and ASTM D1141 seawater were conducted on two cold worked bars of Grade 2 IN686 (UNS N06686, 125 min. yield strength) where the cold work was imparted using two different methods: conventional drawing and rotary forging. Baseline tests were also conducted on Alloy K-500 (UNS N05500), which is a marine grade material currently in use. Specimens were cathodically polarized to simulate catholic protection conditions and evaluate the alloys’ resistance to hydrogen-assisted EAC (HEAC). Ductility measurements using elongation at fracture and reduction in area were used to quantitatively evaluate the HEAC susceptibility. In addition, qualitative fractographic assessments to identify EAC fracture modes were conducted using optical and scanning electron microscopy. The strength and fracture morphology was found to vary with the method of cold working employed. Susceptibility to HEAC was indicated for smooth specimens of IN686 at -1.0 VSCE<.sub> and 9x10-7 in/sec elongation rate via test data and the presence of secondary cracking. IN686 ductility losses were 25% at most, whereas Alloy K-500 measured a ductility loss of more than 60% under the same conditions. The measured effect of a thread-like notch was to reduce the elongation at fracture of IN686 to less than 10% for all conditions, and exacerbate the less ductile fracture modes that were observed.

Key words: environmentally assisted cracking, hydrogen embrittlement, nickel alloys, cathodic protection, marine corrosion
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