A STUDY OF HIGH TEMPERATURE HYDROGEN ATTACK IN SA516 GRADE 70 CARBON STEEL AND WELDMENT

Increasing operation conditions (temperature and pressure) of the methane reforming process is crucial for improving hydrogen production efficiency, which unavoidably induces more severe high temperature hydrogen attack (HTHA) damages in pressure vessel components fabricated with carbon steels and low-alloy steels. In this work, multiple designed experiments were conducted to further investigate HTHA mechanisms. SA516 Grade 70 carbon steel and its weldment were exposed to low-pressure 100% pure hydrogen at elevated temperature for different periods (up to 1000 hours). HTHA-induced microstructure and mechanical property degradation were comparably characterized and evaluated. Creep performance of the as-welded steel weldment under a low-pressure 4% hydrogen-96% Ar gas exposure was also tested with in-situ digital image correlation (DIC). The characterization results show the HTHA exhibits a progressive breaking and dissolution of cementite in pearlite bands of this carbon steel, but no visible voids or cavities were formed in the steel under low-pressure hydrogen and without applying external stresses. There was also no obvious matrix grain growth associated with the carbide dissolution. The creep test shows the creep strength across the weld was highly sensitive to microstructure of weld sub-regions. The in-situ DIC strain measurements reveal a highly localized creep deformation between the base metal (BM), heat affected zone (HAZ) and weld metal (WM). The WM with a microstructure of acicular ferrite and blocky ferrite exhibits the minimal creep strength reduction and deformation. The BM with pearlite bands was more susceptible to creep deformation under 4% H₂ exposure.