Role of Grain Boundaries and Microstructure on the Environment Assisted Cracking of Pipeline Steels

In this research, two common types of environment assisted cracking (EAC) of pipeline steels, namely the intergranular stress corrosion cracking (IGSCC) and the hydrogen induced cracking (HIC), have been studied, and computer models have been developed to simulate the intergranular stress corrosion crack propagation behaviour in pipeline steel as well as to predict the intergranular fracture susceptibility, due to mechanical loading in non-corrosive environment, of polycrystalline materials.;Subsequently, a novel integrated modeling approach, combining Voronoi Algorithm, Markov Chain theory, and Monte Carlo simulations, has been developed in order to predict the IGSCC behaviour of pipeline steels. The model takes both the physical microstructural features, such as the grain shape and grain size distribution, as well as the grain boundary characters and their orientations with respect to the external stress axis into account. The predicted crack propagation behaviour has been found to be in excellent agreement with the experimental crack-propagation and arrest data in API X65 pipeline steel.;In addition, a texture based grain boundary character reconstruction model has been developed that can reproduce the experimentally determined grain boundary character distribution (GBCD) from the simple texture and overall GBCD descriptions. This model has been coupled with the intergranular crack propagation model that can take into account the crystallographic orientations of the grains and the resulting grain boundary character, individual grain boundary fracture strength, and projected local stress onto the grain boundary plane based on applied stress magnitude and in-situ crack propagation length. The predicted threshold fracture stress has been compared with the experimental fracture stress data of various textured and random Mo polycrystals obtained from the literature, and good agreement was observed.;Besides, hydrogen induced cracking (HIC) behaviour of two relatively new high strength pipeline steels, API X80 and API X100, in high pH carbonate-bicarbonate environment has been studied using slow strain rate test (SSRT) technique. While both the steels are found to be highly susceptible to HIC at cathodic potentials, the bainitic lath type microstructure (API X100) is more susceptible to HIC compared to the ferritic/granular bainitic steel (API X80) at high cathodic potential. This can be primarily attributed to the bainitic lath boundary separation phenomenon in the API X100 steel. This study also shows that applying cathodic protection can lead to significant hydrogen embrittlement in these steels.;First, a new understanding of the IGSCC resistance of pipeline steel has been obtained by studying the grain boundary character and crystallographic orientation in both cracked and non-cracked pipeline steel samples using electron backscattered diffraction (EBSD) and X-ray texture measurements. It has been found that the low-angle and certain types of special boundaries, known as the coincident site lattice (CSL) boundaries (S5, S11, and S13b types), are crack-resistant while the random high angle boundaries are prone to cracking. However, it has been also observed that the grain boundaries associated with {110} and {111} neighbour grain orientations having <110> and <111> rotation axis, respectively, are crack-resistant, while the cracked boundaries are mainly linked to the {100} orientation with <100> rotation axis.