Producing nanostructured super-austenitic steels by friction stir processing

Research highlights: → FSP was successfully used to produce nanostructured super-austenitic steel. → FSP led to breaking down the coarse sigma precipitates into the nanosize ones. → Determinable effect of sigma can be minimized by its fragmentation to nanoparticles. → FSP is a unique approach to break coarse particles to attain a uniform microstructure. - Abstract: In the present work, friction stir processing (FSP) was used to produce the nanostructured super-austenitic steel. After preheating, the specimens were subjected to FSP using the rotation and traverse speed of 2600 rpm and 30 mm min-1, respectively. The specimen temperature during FSP was about 950 ± 2 deg. C. The results show that a nanostructured layer of about 91 μm thick was produced on the specimen surface. The formed nanograins ranged from 50 to 90 nm. Besides, the hot severe deformation applied during FSP led to significant fragmentation of the coarse sigma particles to nanosize ones. The produced nanostructured layer was then characterized using field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The formed nanostructure led to a twofold increase in the hardness. The formation of nanostructure resulted in an increase in hardness up to 350 Hv, comparing to 185 Hv pertaining to base structure of super austenitic steel.