Study of Microstructural and Mechanical Evolutions of API X80 Heavy Gauge Large Diameter (OD1,219 mm×22 mm) Pipes and Their Weldability With Customised Niobium Microalloying

Abstract To explore the optimum application of niobium (Nb) element and study the influences of Nb in 22 mm-thick API X80 heavy gauge pipes with a large diameter, the investigation regarding 0.026 wt.% Nb difference, i.e. middle Nb (0.058 wt.%) and high Nb (0.084 wt.%), focuses on the correlations between microstructure, mechanical properties and weldability of the Nb-microalloyed X80 pipe steels. The Thermo-Calc software was used to predict the precipitation behaviours of equilibrium phases. Afterwards, the microstructures (grain size, precipitation and dislocation) and mechanical properties (strength and Charpy V-notch-/shear area-based ductile brittle transition temperatures), were analysed through multiple characterisation methods to verify the designed solid solution and precipitation of Nb during hot rolling, aiming at optimised performance of X80 pipes and their subsequent weldability. The weldability comparison of two Nb-level steels was conducted through welding thermal simulation. The positive factors that Nb has enable the improvement in weldability from the points regarding the combined controlling of prior austenite grain size, bainitic microstructure, and martensite-austenite constituent during single-pass welding thermal simulation. And these facts have led to the conclusions that the higher Nb X80 has a better performance after welding simulation. This study evidently clarifies the advantage of Nb addition in designing base material and its weldability for API X80 pipes with a heavy gauge and a large diameter, which can contribute to the safety during pipeline operation.