Study on the influence of residual temperature normalizing process on the performance of automotive hub bearing forgings

55 steel is a commonly used material for flanged parts of passenger vehicle hub bearings in China. Generally, it is used after normalizing treatment following hot forging forming. To address issues such as high energy consumption, coarse grain structure, and low yield strength associated with the traditional normalizing process, this study systematically evaluated the feasibility of the residual temperature normalizing process for Grade 55 steel hub bearing forgings. Through comparative analysis of the mechanical properties, riveting port deformation capacity, and bending fatigue life of hub bearing units between the residual temperature normalized parts and conventional normalized parts, it was found that the residual temperature normalized parts showed a 5 % increase in tensile strength (from 740 MPa to 856 MPa) and a 3 %–5 % increase in hardness (HBW 215–230). The austenite grain size reached Grade 9 (Grade 7 for conventional normalized parts). The riveting port showed no cracks after 75 % compressive deformation, and the qualification rate for the 50 riveted products was 100 %. In the bending fatigue test, the residual temperature normalized parts fractured after 429,000 cycles under a strengthened load of 3225 Nm, significantly outperforming the conventionally normalized parts, which fractured after only 51,000 cycles. The research indicates that residual temperature normalizing can achieve the dual goals of energy conservation, consumption reduction, and performance improvement by controlling the high cooling rate (10 °C/s ± 2 °C/s) at the initial stage of forging stop and the ferrite precipitation amount (F = 8 %–15 %), providing a theoretical basis for the optimization of hub bearing unit manufacturing processes.