Research on the Influence of the Isothermal Normalizing Cooling Rate on the Mechanically Polished Surface Roughness of Wind Power Gear Blanks

This study takes 18CrNiMo7-6 wind power gear steel as the object. Following the first holding stage of isothermal normalizing, the 18CrNiMo7-6 wind power gear blanks were cooled to the isothermal temperature via air cooling (AC) and forced-air cooling (FA), respectively. The influence of cooling rate on the roughness of the mechanically polished surface of wind power gear blanks was comprehensively studied by means of white light interference, EBSD, TEM, DSC and other technical characterization methods. The results show that a difference in cooling rate leads to a variation in the morphology and distribution of Cr-rich carbides (mainly Cr7C3), which affects the roughness of the mechanically polished surface. During air cooling (slow cooling), atoms diffuse fully. Owing to the relatively low cooling rate in the inner ring of the blank, C and Cr segregate, and abundant Cr-rich carbides precipitated and accumulated at grain boundaries, forming coarse blocky structures. This resulted in uneven mechanically polished surfaces and bright spot defects. The average roughness of the inner and outer ring is 2.648 nm and 2.096 nm, respectively. Forced-air cooling (fast cooling) eliminates surface quality defects by inhibiting long-range atomic diffusion. Meanwhile, radial elemental segregation in the original cast blanks was inherited in subsequent processes, which affected the uniformity of carbide precipitation during cooling. In addition, the differences in cooling rates will also cause variations in the precipitation temperatures of carbides in steel, which in turn further affects the homogenization distribution of carbides in steel. This research provides a theoretical basis and an optimization method for the microstructural regulation and surface quality enhancement of wind power gear steel.