Identification of Intergranular Phases in Precipitation Hardening Ni Alloy UNS N07725
Precipitation hardening (PH) nickel-based alloys, such as UNS N07725 (Alloy 725), are typically selected for oil and gas (O&G) applications that require high strength levels as well as corrosion and environmentally assistant cracking resistance. However, unexpected cleavage failures of subsea components have been associated with hydrogen embrittlement (HE), calling into question the assumed superior properties of these alloys. Recent findings have linked Alloy 725 HE susceptibility to the full coverage of the grain boundaries by a nanoscale and brittle precipitate. However, none of the known phases expected in Alloy 725 heat treated as per API 6ACRA can explain the morphology, extent, and deleterious nature of the observed intergranular precipitates.
The research herein clarifies the observed grain boundary decoration utilizing a detailed characterization of the intergranular phases in Alloy 725. Two commercial batches of Alloy 725 were studied, including a sample deemed susceptible to HE during service. In particular, the (i) crystal structure, (ii) orientation relationship with the matrix, (iii) composition, and (iv) extent of the intergranular precipitates were determined. The microstructures were studied by transmission electron microscopy (TEM), Electron Backscatter Diffraction (EBSD), and X-ray powder diffraction (XRD). The results suggested that a combination of carbides and a—not previously reported—topologically-close packed (TCP) phase were present in the grain boundaries of commercial Alloy 725 samples. Both precipitates formed semi-coherently with the matrix. Finally, the TCP phase and carbide ratio notably differed in the two commercial Alloy 725 batches studied, being higher in the batch deemed susceptible to HE in the field.