Numerical and experimental investigation of helical rolling process for producing steel balls with large diameter

Helical rolling (HR) with high temperature is an efficient forming process to produce bearing steel balls with diameter more than 25 mm. This article discusses the advantages and disadvantages of two HR roller designing methods: single side variable lead method and double side variable lead method. Finite element (FE) simulations of HR processes based on the two methods indicate that the double side variable lead method has higher forming accuracy and is more suitable for forming bearing steel balls. Experiments based on the double side variable lead method are carried out to verify the validity of FE model. The distributions of strain and stress, temperature evolution and rolling force parameters are analyzed based on the FE method. Axial motion analysis shows that the axial velocity of the rolled piece is closely related to axial push velocity of roller convex rib. Microstructure evolution indicates that grains near ball connector area and ball surface get refined due to drastic plastic deformation. The forming accuracy of HR process including radius deviations and ovalities of three cross sections are measured. The statistical results indicate that radius deviation of rolled balls can be controlled within 0.1 mm and ovality can be as small as within 1%. Effects of roller parameters on forming accuracy are analyzed including volume coefficient at groove closed position, forming zone length and starting height of convex rib. Forming accuracy gets better with the increase of volume coefficient, the increase of forming zone length, and the decrease of staring height of convex rib.