Microstructural evaluation of APIX80 steels in different thermal cycles

In this work, a microstructural evaluation of the influence of thermal cycling was carried out on three API5L X80 grade steels: Nb-Cr, Nb-Cr-Mo and Nb-Cr-Mo-V manufactured in Brazil. These steels were manufactured using a controlled thermo-mechanical process without undergoing a subsequent accelerated cooling stage. Microstructural characterisations were carried out under two conditions, post-rolling and austenitising at 900oC followed by three (3) thermal cycles: cooling in air (1.5oC/s), in oil (30oC/s) and in water (115oC/s). Data such as TG, phase distribution: ferrite, bainite and AM, hardness and micro-hardness were analysed, as well as checking for differences in their substructures: subgrains, cells and fine precipitation.