Low-heat-input friction-stir welding of creep-resistant martensitic steel

This work was undertaken to develop an effective approach for friction-stir welding (FSW) of a typical high-chromium martensitic steel. Under low heat input conditions, FSW is carried out in the two-phase (ferrite-austenite) field. In this case, a highly dispersed ferrite-martensite microstructure forms in the weld. Moreover, the superposition of processes of very large plastic deformations with phase transformations during FSW of these steels makes microstructural studies in this area interesting also from a fundamental point of view. Therefore, the present study aims to shed light on this phenomenon. To this end, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the microstructure formed during FSW of creep-resistant steel containing 10 % Cr. From experimental results, it was deduced that the peak FSW temperature slightly exceeded the A1 point. Accordingly, microstructural evolution was mainly governed by the continuous dynamic recrystallization in the ferrite phase during FSW and contributed to by a partial austenite-to-martensite transformation during the weld cooling cycle. Both these processes resulted in drastic microstructural refinement and a high dislocation density within the stir zone. Accordingly, a significant material strengthening was observed within the weld zone, which ensured a 100 % joint efficiency of produced welds. Given the outstanding advantages of FSW, the adaptation of this technology for welding steels, which are one of the most widely used structural materials, could be of great practical importance, in particular for the manufacture of reactor claddings.

1. Russian Science Foundation - 24-79-00138