The Influence of δ-Phase on the Environmentally Assisted Cracking Resistance of an Additively Manufactured Nickel-Based Superalloy

IN625 is an attractive alloy for additive manufacturing (AM) applications, but the processing conditions in AM generate considerable residual stresses, compositional gradients, and microstructural heterogeneities that degrade properties and performance. Resistance to environmentally induced cracking is of great concern for many applications within the petroleum and natural gas (PNG) industries. To evaluate the influence that AM fabrication may have on this property, AM and wrought-processed samples were given similar heat treatments and tested in tension at slow strain rates in an acidified chloride environment under both freely corroding conditions and controlled hydrogen fugacity. The results revealed that the AM solidification microstructure had a deleterious influence on the mechanical properties and significantly increased the severity of the cracking when absorbed hydrogen was present in the sample. An increase in the volume fraction of the δ-phase intensified the strength of that influence. This evaluation also demonstrated that additional thermal processing eliminated the AM solidification microstructure and the microsegregation resulting in a more homogeneous microstructure, which reduced the density and severity of the interactions between the absorbed hydrogen, interfacial precipitates (δ-phase), grain boundaries, and tri-axial stresses. It is concluded that effective post-build heat treatments can be designed that will enable AM components to perform as well as wrought components under the same environmental conditions.