Study on Stress Corrosion Cracking Behavior of Incoloy825/X65 Bimetallic Composite Pipe Welded Joint in Wet Hydrogen Sulfide Environment

Abstract

The stress corrosion cracking behavior of an Incoloy825/X65 bimetallic composite pipe welded joint in wet hydrogen sulfide (H2S) environment was investigated by means of the creviced bent beam (CBB) test in this study. The microstructure, element distribution and crack propagation behavior of the welded joint were analyzed by optical microscope (OM), scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The results showed that two types of cracks were observed in the Incoloy825/X65 bimetallic composite pipe welded joint in wet H2S environment, they initiated from the notch and the intersection of the three zones (cladding Incoloy825, base X65 and weld), respectively, and propagated along the fusion boundary(FB) and the Type-II-like grain boundary. The mechanisms of the two types of cracks are due to the combination of anodic dissolution, stress and hydrogen. Near the FB, there are high angle grain boundaries, Type-I, Type-II and the Type-II-like grain boundaries, which have high SCC sensitivity. The element distribution in the intersection of the three zones and the crack tip is complex, with element diffusion, Cr loss and large residual strain. All these provide the conditions for cracks initiation and propagation.