Microstructure and Impact Toughness Property of API X90 Heat-Affected Zones at Different Peak Temperatures

The objective of this paper is to study the effect of peak temperature on the microstructure and impact toughness property in the simulated heat-affected zones (HAZs) of American Petroleum Institute (API) X90 pipeline steel. Simulated HAZs were produced at different peak temperatures (Tp) from 650 to 1350 °C with a heat input of 25 kJ/cm. With a decreasing Tp, the grains become significantly refined, and the granular bainite (GB) increases at the expense of lath bainite (LB). When the peak temperature is 1350 °C, the crack initiator martensite/austenite (M/A) constituents are coarse and distribute along the prior austenite grain boundaries with a short rod-shaped morphology, while the coarse grain heat-affected zone (CGHAZ) has the lowest impact energy of 269.67 J. The subcritical HAZ (Tp=650 °C) has a similar microstructure to the base material and has the best impact energy of 336.33 J. When Tp = 950 °C, LB disappears, and the grains become refined due to complete recrystallization, resulting in a higher toughness property than that at Tp = 850 °C. The lowest microhardness occurs at Tp = 850 °C because of the coarsening of precipitates and some PF.