Ductility-Dip Cracking Susceptibility of Commercially Pure Ni and Ni-Base Alloys Utilizing the Strain-to-Fracture Test

Abstract

The ductility dip cracking (DDC) response of commercially pure Ni and two solid-solution strengthened Ni-base alloys was conducted using the strain-to-fracture (STF) test method that was developed at Ohio State University by Nissley and Lippold (Welding J 82(12):355–364, 2003 [1]). Alloys 200, 600, and 625 were tested over a range of strain at 950 °C to determine the threshold strain for fracture (emin) and the cracking response as a function of strain. Using established procedures, samples were prepared with an autogenous gas-tungsten arc (GTA) spot weld in order to provide consistent grain boundary character among samples and then tested using a Gleeble® thermo-mechanical simulator. In order to evaluate the effect of oxygen on DDC susceptibility, spot welds were made using Ar-2 % O2 shielding gas. It was found that Ni-200 was most susceptible to DDC due to the large grain size and non-tortuous migrated grain boundaries (MGB’s). Alloy 625 was extremely resistant to DDC, as reported by Zhang et al. (Trans JWRI 14(2):325–334, 1985 [2]), exhibiting a strain threshold greater than 15 %. The addition of oxygen reduced DDC resistance in alloys 600 and 625. Metallographic analysis showed that DDC initiates at grain boundary triple points and then propagates along straight migrated grain boundaries. SEM fractography revealed that crack surfaces exhibited smooth intergranular features with little evidence of ductile rupture.