Rapid precipitation of intermetallic phases during isothermal treatment of duplex stainless steel joints produced by friction stir welding

Abstract

The propensity of duplex stainless steels to form secondary phases that harm their main properties is a metallurgical problem that continues to be studied. Friction stir welding has shown excellent results when applied to duplex stainless steels, enabling the formation of joints that maintain the austenite-ferrite balance, without the presence of secondary phases. Welding processes cause significant microstructural reformulation in the joint microstructure, so the properties and metallurgical behavior of welded joints differ significantly from those of the corresponding base metals. The formation of secondary phases in duplex stainless steels occurs by diffusional transformation processes that are dependent on the transformation time, with the kinetics being influenced by the previous microstructure. Although friction stir welding makes it possible to obtain joints free from secondary phases, it is important to study the propensity of welded joints to subsequently form secondary phases, in order to be able to understand the behavior of these joints when they are subjected to hot operations, such as weld repairs. In this work, friction stir welded joints were treated isothermally at 850 °C and were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, Ferritoscope measurements, and X-ray diffractometry. The results revealed that the friction stir welded joints presented much faster formation of intermetallic phases, compared to the base metals, with the appearance of regions showing advanced stages of intermetallic phase precipitation after only 5 min exposure at 850 °C.