Phase transformation and impact toughness in HAZ of micro alloyed X80 line pipe steel

Abstract

In the present work, phase transformation behavior and structure-property correlation in X-80 steel was analyzed at three distinct austenitizing peak temperatures (1250 °C, 1030 °C and 950 °C). Dilation studies and simulations of Charpy size samples were carried out using Gleeble 3800 thermal simulator. Dilation studies indicated that the dissolution of Nb rich carbides occurred at 1020 °C. During cooling from 1250 °C, mixed microstructure of bainitic ferrite and martensite formed at fast (200 °C s−1 −80 °C s−1) and medium (50 °C s−1–40 °C s−1) cooling rates. For slow cooling rates (20 °C s−1–0.5 °C s−1), ferrite/pearlite microstructure was obtained. For lower peak temperatures 1030 °C and 950 °C, mixed microstructure of bainitic ferrite and martensite was observed for fast cooling rates. Whereas, ferrite/pearlite with small fraction of bainitic ferrite nucleated in the microstructure at medium and slow cooling rates. Highest area fraction of M-A (8%) was observed at medium cooling rates (50 °C s−1–40 °C s−1) for 1250 °C peak temperature. Charpy size samples simulated for a 1250 °C peak temperature, exhibited the minimum notch toughness at 12.5 °C s−1, due to deleterious effect of granular bainite and M-A constituents. Simulated samples at lower peak temperatures 1030 °C and 950 °C exhibited higher notch toughness due to a mixed microstructure of bainitic ferrite and ferrite-pearlite.