Mechanical Properties of Intercritically Annealed X80 Line Pipe Steels

Microalloyed low-carbon steels are used for line pipe applications as they combine high strength and acceptable fracture toughness with good weldability. During multi-pass welding, the strength and impact toughness of the material in the heat-affected zone (HAZ) is potentially degraded, in particular the regions where the thermal fields from multi-pass welds overlap (for example: the intercritically reheated coarse grain heat-affected zone, ICCGHAZ). Using a Gleeble thermomechanical simulator, bulk microstructures were produced that are representative for the ICCGHAZ for two high-strength X80 line pipe steels. Here, the first thermal cycle produces a bainitic microstructure that is characteristic of the coarse grain heat-affected zone (CGHAZ) and the second cycle involves intercritical annealing of this region to form microstructures representative of the ICCGHAZ. The effect of the intercritical austenite fraction and the resulting martensite–austenite (M/A) constituents on the tensile properties and the ductile-brittle transition temperature (DBTT) has been quantified for two steels with different carbon contents, i.e., 0.063 and 0.028 wt. pct. Detailed fractography studies have been conducted to evaluate the fracture mechanisms with respect to the microstructural features. Upon intercritical annealing (relevant to ICCGHAZ), the ductile–brittle transition temperature was above room temperature when a nearly continuous necklace of M/A formed on the prior austenite grain boundaries (for M/A ≈ 10 pct). Finally, the role of carbon content on the yield strength and plasticity of martensite has been considered for the tensile fracture behavior and ductile–brittle transition temperature. It is proposed that as the average carbon content of the M/A decreases (both due to (i) a decrease in the bulk carbon content of the steel and (ii) an increase in the volume fraction of austenite formed during intercritical annealing), martensite plasticity was possible which reduced nucleation of voids or cracks at the M/A–bainite interface.