Realizing the outstanding strength-toughness combination of API X65 steel by constructing lamellar grain structure

Abstract

The development of pipeline steels with exceptional cryogenic toughness is deemed imperative for their applications in various engineering fields. In this work, API X65 steel with lamellar grain (LG) structure is constructed through warm deformation techniques. Comparative analysis with raw API X65 steel revealed notable enhancements in both yield strength (YS) and ultimate strength (UTS), exhibiting increments of 315 MPa and 164 MPa, respectively, while retaining substantial fracture toughness (KIc = 251.14 MPa m0.5). Remarkably, the LG steel exhibits a distinct absence of a ductile-brittle transition behavior over a wide temperature range, maintaining an exceptionally high impact energy from room temperature (RT) to liquid nitrogen temperature (LNT), and the Charpy impact energy exceeding 330 J at LNT, nearly 16 times higher than the raw API X65 steel. This performance can be primarily attributed to the superior plastic deformation capability, coupled with the delamination toughening mechanism. The resultant tortuous path of crack propagation effectively facilitates the absorption of substantial energy, decreases notch tip sensitivity depending on the temperature, stress triaxiality, and strain rate, thereby fostering excellent cryogenic toughness.

Keywords

• API X65 pipeline steels
• Warm deformation
• Lamellar grain structure
• Ductile-brittle transition temperature
• Delamination toughening mechanism