Environmentally Assisted Cracking Evaluation of Alloy IN686 (UNS N06686) Using Constant Extension Rate Testing

Submarine and ship designers desire a marine grade, shock qualified material of a higher strength level (150 ksi) that is galvanically compatible with titanium and Alloy 625 components.  IN686 is a corrosion resistant Ni alloy which has been suggested as a possible candidate material for fracture critical applications.  It can be cold worked to a yield strength in excess of 150 ksi while retaining significant ductility and cracking resistance.   Alloy producer testing has indicated good environmentally assisted cracking (EAC) performance at lower strength levels (115 ksi).  However, EAC susceptibility typically increases with strength, and the EAC behavior has not been well documented at the higher, desired strength levels.  As a part of the 2010 Bowman Scholar Program at the United States Naval Academy, constant, slow extension rate tests in air and ASTM D1141 seawater were conducted on two cold worked bars of Grade 2 IN686 (125 min yield strength) where the cold work was imparted by two different methods:  conventional drawing and rotary forging.  Baseline tests were also conducted on Alloy K-500, which is a marine grade material currently in use.  Specimens were cathodically polarized to simulate shipboard 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 EAC susceptibility.  In addition, qualitative fractographic assessment to identify EAC fracture modes and crack path 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 IN686 at -1.0V_SCE via test data at a 9x10^-7 in/s elongation rate and the presence of secondary cracking.  IN686 ductility losses were 20% at most, whereas Alloy K-500 measured a ductility loss of more than 60% under the same conditions.  The 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.