Influence of CDFW Process Parameters on Microstructure and Mechanical Properties of U75V Rail Steel Welded Joint

Abstract

In the present paper, the continuous-drive friction welding (CDFW) technology has been successfully applied to join the U75V rail steel. The base metal (BM) of U75V rail steel is lamellar pearlite, and the weld zone could be clearly divided into three subzones (i.e., heat affected zone, thermo-mechanical affected zone (TMAZ), and central weld zone (CWZ)). Electron back-scattered diffraction examinations revealed the martensitic evolution in TMAZ and CWZ, suggesting that the experienced high temperature, severe plastic deformation, and fast cooling rate induce the microstructure transition during the CDFW process. The hard and brittle martensite structure explains the raised microhardness profiles and the reduced impact absorption energy of the as-welded joints. The CDFW process parameters govern the joint properties via influencing the welding heat input and plastic deformation by spindle speed and friction pressure at the friction stage, and the plastic deformation layer (flash) extrusion by upsetting pressure at the upsetting stage. More favorable results could be obtained at small set values of spindle speed (1800 rpm) and friction pressure (75 MPa) with less heat input and plastic deformation, and a large set value of upsetting pressure (175 MPa) with more flash extrusion, whose tensile strength reached 94.3% of that of the BM.

Quasi-equal strength and toughness matching rail steel joint obtained via friction welding below Ac1 temperature

“…The KU 2 for Trial I is 12.0 ± 0.6 J, close to that of the BM (12.94 J). 22 The results far exceed the KU 2 of Trials II and III and other welding methods (e.g. FBW, GPW) and the KU 2 standard in TB/T 1632.1-2014 (≥6.5 J).…”

“…Within these subzones, the peak temperature surpasses the A c1 temperature threshold, forming brittle and hard martensite and RA during the ensuing cooling stage, as validated through microhardness measurements. 22 WPZ has a sizeable relative friction linear velocity, which translates into increased welding heat input, thus resulting in the generation of additional austenitic microstructures, which, in turn, leads to heightened microhardness levels. On the other hand, the WCZ of Trial I displays BCC structures with smaller grain sizes and a microhardness comparable to that of the U75V BM.…”