As-Cast Spot Segregation of Gear Steel and Its Evolution in the Rolled Products

As a crucial transmission component and basic component in mechanical equipment, gears have extremely high requirements for homogeneity of elements. However, the banded segregation defects in a rolled bar will reduce the hardenability of the finished gear and cause uneven deformation in heat treatment. Generally, the formation of banded defects is believed to inseparable from semimacro segregation in the bloom castings. Spot segregation in gear steel bloom and banded segregation defects in a rolled bar were experimentally studied to explore its evolution. The correlation between the spot segregation and banded defects was clarified from the aspects of morphology characteristics, position distribution, element segregation, and spacing statistics using hot acid etching, dendritic etching, electron probe microanalysis (EPMA), and statistical calculations. The evolution law of spot segregation and the possibility of elimination of banded defects in the heat treatment stage were also discussed. Experimental results show that spot segregation mainly exists in the central equiaxed crystal area of the castings, and solute elements such as C, Mn, and Cr in the segregated spot show positive segregation. The wide banded defects are consistent with the spot segregation in their geometrical location and element distribution pattern. Morphologically, there are two types of banded defects in which the dendrite microsegregation and the independently existing spot segregation are changed into separate band structures during the hot rolling process, while the interconnected spot segregations convert into a type of converged band defect through the hot deformation. Thus, more fine and coarse banded defects have been observed in the final rolled bar products than the numbers of as-cast spot segregation. Heat treatment through heating and insulation at 1200 degrees C can reduce the degree of solute segregation in the banded defects with a width less than 40 mu m, but it shows little beneficial effect on the more coarse-sized banded defects, which suggests that they should be controlled from the original as-cast dendrite morphology during the casting process.