A significant toughness enhancement, and microstructural evolution of an electric resistance welded (ERW) microalloyed steel

Abstract

In this investigation, microstructural evolution and mechanical properties of an X52 microalloyed steel processed by a full scale industrial electric resistance welding (ERW) equipment was studied. The need for adequate level of mechanical properties, in particular toughness, in the welded pipes is discussed due to its industrial prominence. Optical microscopy (OM) and scanning electron microscopy (SEM) were used for microstructural evolution and fractography analysis. Results showed that employing a special annealing treatment of the welded pipe could enhance the mechanical properties and alleviate low toughness problem. Results also showed a significant variation in the microstructure of the pipe as a result of welding and its subsequent heat treatment processes. Electron backscatter diffraction (EBSD) results for the initial plate and the ERW processed cases showed a significant grain size refinement for the case of as weld and after the two-stage heat treatment. A fully random texture component was observed in {111} pole figure of the initial plate. A textured structure, with the components of Copper, Brass, S and Cube, was seen in the as weld condition. These texture characteristics remained in the system, though with lower intensity, after the normalizing stage heat treatment. Hence for its unpredictable toughness behavior. A completely weakened undesirable texture components was detected after the second stage of heat treatment together with smaller amount of second phase particles and smaller grain sizes. Finally, it was concluded that a proper mechanical properties combinations, particularly toughness value, could be attained as a result of this microstructural evolution.