Intergranular Cracking of Ni-16Cr-9Fe Alloys in High Temperature Water

The objective of this work is to provide a better understanding of the role of microstructure in intergranular stress corrosion cracking (SCC) of controlled-purity, Inconel 600-type alloys in high temperature, high purity water. High purity heats Ni-16Cr-9Fe alloys with controlled additions of carbon, phosphorus, and boron were heat treated to produce chromium depletion, grain boundary impurity segregation, and inter- and intragranular precipitation of chromium carbides. Constant extension rate tensile (CERT) tests were conducted at 36CPC in deaerated (<10 ppb O₂) high purity water containing a hydrogen overpressure, and in 360 to 400°C Ar atmosphere at an initial strain rate of 3 × 10⁻⁷ sec⁻¹. Results showed that (1) the high purity Ni-Cr-Fe alloy exhibited inherent IG cracking susceptibility in water and Ar, (2) prior cold work introduced by compression significantly increased IG cracking susceptibility, (3) carbon in solution enhanced IG cracking resistance, (4) phosphorus and boron segregation retarded IG cracking, and (5) chromium depletion did not affect IG cracking. The results also suggest that creep plays a role in IG cracking in 36ơC water and in Ar at 360 and 400°C.