Formation of nanograin in IF steels by friction stir processing

An IF steel was subjected to friction stir processing using a carbide-tungsten pin with the diameter of 16 mm. The rotating speed of pin was 1600 rpm, whereas, the applied traverse speed was 31.5 mm/min. The specimens were water quenched immediately after the FSP. The microstructural evolutions were then characterized using atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), XRD and scanning electron microscopy (SEM) techniques. The results show that the FSP of IF steel resulted in the formation of a 30–35 μm thick surface layer with the nanograins of 50–125 nm. There was a continuous decrease in the hardness from the nanograin surface layer towards the center of specimen. There was almost a three-fold increase in the hardness, reaching to 310 Hv pertaining to the nanograin stirred zone compared to the 100 Hv related to the base material. The restoration mechanisms responsible for the formation of nanograins are then discussed.