Mechanism and Kinetics of Solid-State Transformation in High-Temperature Processed Linepipe Steel

A relatively new class of linepipe steels with yield strength greater than 500 MPa created for thermomechanical processing at temperatures in excess of 1473 K (1200 °C) has established a firm foothold in the market for modern, large diameter, and high-pressure gas transmission systems. The design concept for the steels takes advantage of the enhanced role which higher levels of niobium can play in very low carbon steels, during the plate manufacturing process. The transformation products observed after cooling have been interpreted in conflicting ways in the literature, using ambiguous terms which are not established rigorously. Revealing characterization experiments have therefore been conducted to establish that the principal transformation product grows by a displacive transformation mechanism, and that it is properly identified as bainite. The implications of this, on both the interpretation of microstructure and on the processing of the steel, are discussed.