Investigation on fatigue crack growth and hydrogen embrittlement on pipeline steels

In this thesis, fracture properties in the presence of hydrogen in two pipeline steels, are studied. In particular this research exploits a non-hazardous charging technique, developed at Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, able to “soak” the iron lattice of atomic hydrogen in order to study the variation of the main fracture-mechanical properties (Charpy impact energy, JIC and crack growth rate) in the presence and absence of hydrogen and at different temperatures. Additionally other important testing variables in fatigue crack growth such as temperature and frequency are considered. The steels under investigation are widely used in “Oil & Gas” industry and are commercially known with the names: API 5L X65 and F22. Mechanical tests are conducted on both steels in a temperature range between -120°C and room temperature onto CV and CT specimens extracted directly from the bulk pipeline. The well-known phenomenon that occurs in this circumstance is hydrogen embrittlement HE, and this appears to be enhanced in slow-rate tests and it has been related to low temperature and microstructural properties. This is due to atomic hydrogen, since no hydride or blistering is observed. In particular, this research focuses on crack growth tests, and suggests superposition model, able to predict the crack growth rate versus tests parameters such as: temperature, frequency, presence of hydrogen and ΔK; the model is applied to a real case of a pipeline with an internal crack. Using SEM fractographs on the crack surfaces, a micromechanical explanation of the phenomenon is suggested and conclusions are drawn.