Study on Time Dependence of Trenches on SSC Initiation Limits for Low-Alloy Carbon Steels (C2026-00328)

Taishi Fujishiro, Taro Muraki, Daichi Izumi, Nobuyuki Ishikawa, Takuya Hara, Eiji Tada, Koji Fushimi, Mitsuo Kimura

In the oil and gas industries, pipelines are exposed to wet H2S environments (sour environments) that cause sulfide stress cracking (SSC) in low-alloy carbon steels in some cases. Recently, a judgment flowchart was proposed to determine the SSC susceptibility based on the corrosion pit morphologies or stress groove geometries on the steel surface observed in the early stages of SSC. However, whether the crack-like corrosion morphologies, called trench or fissure, that are formed in low-alloy carbon steels containing Ni cause failure remains unclear.
In this study, the effects of trench on SSC initiation limits in the Ni-containing steel were investigated by comparing fracture toughness (KISSC) and stress intensity factors (K). KISSC values of the steels with and without 1 mass% Ni were evaluated by in-situ double cantilever beam (DCB) tests. K at the tips of various trenches were calculated by finite element analysis (FEA) applying the actual trench geometry at each test duration. The results suggest that the estimated K values vary with time, however K values do not exceed the KISSC values and SSC does not occur at certain H2S partial pressures and steel hardness levels even when trenches are generated and grown.