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 degrees 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 degrees 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 degrees C) has a similar microstructure to the base material and has the best impact energy of 336.33 J. When Tp = 950 degrees C, LB disappears, and the grains become refined due to complete recrystallization, resulting in a higher toughness property than that at Tp = 850 degrees C. The lowest microhardness occurs at Tp = 850 degrees C because of the coarsening of precipitates and some PF.