Role of Hydrogen and Stresses in the Failure of Heat Exchanger Pipes

Material analyses — including micromechanical tests, optical and scanning microscopy, fractography, and chemical analysis of nitrogen and hydrogen content — were conducted for pipe material from the different parts of a failed heat exchanger (steam generator). Hydrogen-induced metal degradation accelerated by thermal stresses and stresses from vibration was proposed as a cause of the failure, despite the use of special hydrogen-resistant steel. Computation of the stress level and distribution confirmed the possibility of local buildup of stresses, causing the fracture of material weakened by hydrogen action. Modification of heat exchanger construction to suppress pipe vibration was suggested.