Direct drive friction welding effect on mechanical and electrochemical characteristics of titanium stabilized austenitic stainless steel (AISI 321) research reactor thick tube

Abstract

Well known for its good weldability by conventional welding techniques and also for its strain hardening ability, titanium stabilized austenitic stainless steel (AISI 321) thick tube in a nuclear research reactor was subjected to direct drive friction welding. The mechanical and electrochemical behavior of the welded material was investigated. Four heat diffusion times’ effects on mechanical properties were experimentally investigated by hardness tests and tensile tests using wide and narrow gauge length extensometers for local and global measures. A generally hardened structure was produced in all heat conditions, and the micro-hardness profiles showed increased hardness variation at the weld zone. Best results, with regard to base metal ductility, were found at medium heat input (friction time 8.5 s), over which the ductility relapsed. The recrystallized zone, the thermo-mechanical affected zone and the heat affected zone X-Ray diffraction examinations revealed martensitic evolution occurrence, suggesting induced martensite formation during the welding operation. This can partially explain the hardness profile obtained. Light optical microscopy and scanning electron microscopy were used to analyze welded zones microstructures and tensile fractures morphologies. Electrochemical tests in 3.5 wt% NaCl solutions revealed improved resistance of the weld joint which can be explained by the fine structure observed in this region of the welded material. Nearby the welding area, the material exhibited straight selective inclined paths of pitting attacks.