Kinetics of Austenite Grain Growth in Low Alloy Pipeline Steels: An Empirical Study Using Nigerian Crude Oil Processing Conditions

This study investigates the kinetics of austenite grain growth in low alloy API 5L X65 pipeline steel under thermal cycles representative of girth weld heat-affected zone conditions encountered in pipeline construction for Nigerian crude oil transport. Grain growth in the HAZ directly influences local toughness and susceptibility to hydrogen-induced cracking, which are critical concerns for sour service pipelines in the Niger Delta. Specimens are subjected to single-pass Gleeble thermal simulation cycles spanning peak temperatures of 900, 1100, 1200, 1300, and 1400 degrees Celsius with controlled heating and cooling rates. Prior austenite grain size is measured from quenched specimens by optical microscopy using the McQuaid-Ehn method. The Sellars and Whiteman grain growth model is calibrated to the experimental dataset by non-linear regression. The activation energy for grain coarsening is determined, and a constitutive grain growth equation is developed for the specific steel chemistry studied. Results reveal a marked onset of abnormal grain growth above 1200 degrees Celsius, attributed to dissolution of NbC pinning particles, confirmed by TEM analysis of the precipitate population at each peak temperature. The calibrated grain growth model shows good predictive accuracy (R-squared 0.94) across the experimental temperature range. The study provides a validated thermal-microstructure model applicable to welding procedure qualification simulations for Nigerian pipeline projects. Keywords: austenite grain growth, pipeline steel, Gleeble simulation, heat-affected zone, kinetics.