Torsion performance of pearlitic steel wires: Effects of morphology and crystallinity of cementite

The torsion behavior of pearlitic steel wires is investigated and the conditions responsible for torsion delamination are determined. Experimental results reveal that the morphology and crystallinity of cementite regulate the torsion response of pearlitic steel wires. Specifically, both the torsion strength and torsion fracture strain of the wires increase with cold drawing strain up to epsilon < 0.68. After annealing treatment at 250 degrees C, the torsion strength increases at the expense of torsion fracture strain irrespective of drawing strains. Moreover, the torsion delamination occurs in the wires prepared at epsilon > 1.76 and also subjected to annealing treatment between 150 degrees C and 350 degrees C. It was found that annealing treatment turns the amorphous cementite to nanocrystalline state, which prevents dislocations from bowing out. Consequently, localized and severe plastic deformation takes place, resulting in the torsion delamination. By contrast, the cementite with a mix of amorphous and nanocrystalline phases is capable of withstanding plastic deformation during torsion tests. Moreover, annealing-induced spheroidization of cementite is effective in preventing the pearlitic steel wires from torsion delamination.