Microstructures and mechanical properties of flash butt welded high strength steel joints

Weld thermal cycles, microstructures and mechanical properties of flash butt welded RS590CL steel joints have been investigated. The results indicated that the weld thermal cycles are characterized by high peak temperatures and rapid heating and cooling rates, and the joints included the weld interface zone (WZ), coarse grain zone (CZ), fine grain zone (FZ) and partially recrystallized zone (PZ). The WZ and CZ consist mainly of coarsened upper bainite while the FZ has a fine equiaxed grain structure containing ferrite and pearlite with NbC precipitates. The WZ and CZ have higher hardness compared with the FZ, PZ and base metal. The joint strength (584-611 MPa) can match the base metal strength (575-595 MPa). The impact toughness of weld interface zone in the joints depends on welding parameters. The excessive flash allowance results in reducing the joint toughness. The reasons for the reduced joint toughness are related to the coarsened upper bainite, retained oxides and joining defects in the weld interface zone. The suitable combination of flash allowance (8-9 mm), upset allowance (5.5-7.0 mm) and upset pressure (123-160 MPa) is beneficial for the improvement of joint toughness. (C) 2016 Published by Elsevier Ltd.