Boundary characterisation of X65 pipeline steel using analytical electron microscopy

Analytical electron microscopy was used to characterise grain boundaries (G Bs) and interphase boundaries (IBs) of X65 pipeline steel. There was no seg regation of P or S at the proeutectoid ferrite GBs. This indicates that con trary to literature expectations, P and S are unlikely to be involved in th e mechanism of SCC of pipeline steels. There was Mn segregation at IBs betw een pro-eutectoid ferrite and pearlitic cementite, and desegregated from th e IBs between pro-eutectoid ferrite and pearlitic ferrite. This pattern of Mn segregation is attributed to diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in t he IBs between the proeutectoid ferrite and pearlite. A new mechanism was p roposed for pearlite formation. A GB carbide first forms at an a! : a GB, a nd then grows along the alpha/gamma interface. Subsequently pearlite initia tes from this interface carbide. (C) 1999 Kluwer Academic Publishers.