Effect of Deoxidation Process on Inclusion and Fatigue Performance of Spring Steel for Automobile Suspension

Abstract

55SiCrA spring steel was smelted in a vacuum induction levitation furnace. The liquid steel was treated by Si deoxidation, Al modification with Ca treatment and Al modification, and the steel samples were obtained with deformable Al₂O₃-SiO₂-CaO-MgO inclusions closely contacted with steel matrix, Al₂O₃-CaO-CaS-SiO₂-MgO inclusions surrounded by small voids or Al₂O₃(> 80 pct)-SiO₂-CaO-MgO inclusions surrounded by big voids, respectively. Effect of three types of inclusions on steel fatigue cracks was studied. The perpendicular and transverse fatigue cracks around the three types of inclusions leading to fracture were found to vary in behavior. Under the applied stress amplitude of 775 MPa, the fatigue lives of the three spring steels decreased from 4.0 × 10⁷ to 3.8 × 10⁷, and to 3.1 × 10⁷ cycles. For the applied stress amplitude of 750 MPa, the fatigue lives of the three spring steels decreased from 5.2 × 10⁷ to 4.1 × 10⁷, and to 3.4 × 10⁷ cycles. Based on the voids around inclusions, the equivalent size of initial fatigue crack has been newly defined as $\sqrt {\frac{{{\text{area}}_{\text{inclusion}} }}{{(1 - {\text{CC}})}}}$, where the contraction coefficient CC of inclusion was introduced. A reliable forecast model of the critical size of inclusion leading to fracture was established by the incorporation of actual width b_inclusion or diameter d_inclusion of internal inclusion; the model prediction was found to be in agreement with experimental results.