Influence of force parameters of rotary friction welding on the microstructure and mechanical properties of welded joints of high-strength drill pipes

The present investigation provides an assessment of the influence of the force parameters of computer numerically controlled (CNC) rotary friction welding (RFW), namely the friction force and the forging force during welding tubular samples with a diameter of 73 mm and a wall thickness of 9 mm from Cr-Mn-Mo steel G105 according to API 5DP in connection with AISI 4340. The microstructure, microhardness, tensile mechanical properties and Charpy V-notch impact toughness of welded joints after welding and after post-weld stress-relieving tempering were studied. The evolution of the microstructure was studied using optical and scanning electron microscopy. The length of microstructural features of the joint such as thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were measured using computational image analyzing procedure (ImageJ software). The 3D modeling software “CATIA” was employed for designing the tubular samples of welded joint and scheme of specimen extraction. The welded joint of G105 and AISI4340 steels under the studied conditions has equal tensile strength compared to the base G105 steel, both in the initial state after welding and after post-weld tempering. It has been shown that carbide particles of various morphologies, separated during welding and subsequent tempering, play a major role in the processes of strengthening in TMAZ. The tensile failure location is fixed in the G105 base steel. It has been established that welding power parameters influence the impact toughness of the interface zone: with an increase in welding force parameters, an increase in impact toughness and fracture ductility occurs. The dominant influence is exerted by the friction force. The highest impact strength occurs after the following welding parameters: friction force 145 kN, forging force 280 kN, rotation speed 600 rpm, burn-off length 7 mm and post-weld tempering at 550 ºC. Fractographic studies have established that the fracture surface after this treatment consists entirely of small dimples of ductile fracture. This mode can be recommended as optimal for obtaining an equal-strength connection with high impact strength.