Friction stir welding of high-strength steel

The examination of microstructural and mechanical properties of friction stir welded XPF800 steel

“…35 Fine and equiaxed grains are formed as a result of DRX. 36 In addition, a deformation-induced ferrite transformation (DIFT) mechanism takes place, resulting in an increase in the number of ferrite grains per unit area due to high plastic deformation near A 3 temperatures and thereby causing grain refinement. 1 The microstructure of SZ was composed of polygonal ferrite, acicular ferrite, and perlite (Figure 5(b)).…”

“…However, at medium and high carbon steels, not only ferrite, perlite but also martensite, and bainite may occur in the microstructure of the SZ depending on the carbon ratios and the highest temperatures reached. 36…”

“…39 It is thought that, although the grains may have elongated due to plastic deformation during FSW, the grains that undergo dynamic recrystallization during cooling become fine and equiaxed grains. 36…”

“…However, at medium and high carbon steels, not only ferrite, perlite but also martensite, and bainite may occur in the microstructure of the SZ depending on the carbon ratios and the highest temperatures reached. 36 In low carbon steels, the temperature rising above A 3 during FSW causes the formation of single-phase austenite in the microstructure. After recrystallization, coarse grains may form in the SZ due to the absence of secondary phases to prevent grain coarsening due to high temperatures.…”

“…39 It is thought that, although the grains may have elongated due to plastic deformation during FSW, the grains that undergo dynamic recrystallization during cooling become fine and equiaxed grains. 36 Figures 5(d) and 6(c)-(c1) show the images of the OM and SEM of the advancing side microstructure of HAZ, respectively. During the FSW, the microstructure of the HAZ is seen to be composed of ferrite and a small number of pearlite.…”