Fatigue failure analysis and heat treatment strategy optimization of torsion bar spring for automobile suspension system used in light vehicles

The failure analysis was conducted on unqualified torsion bar spring in automobile suspension system used for light vehicles during engine test. The effects of through hardening, surface induction hardening, quenching and tempering, and tempering temperature on the microstructure and fatigue life of 45CrNiMoVA steel torsion bars were also investigated. Results showed that only the torsion bar spring after through quenching and tempering is subjected to surface induction quenching and tempering to achieve the fatigue life of the qualified torsion bar. The fatigue life of torsion bar spring reaches 3 × 10⁵ cycles more than the required 2 × 10⁵ cycles. This is because the distribution of gradient microstructure was helpful to relieve the applied stress during the fatigue process. The microstructure of the non-hardened region, which consists of tempered sorbite regardless of whether it is tempered at 330 or 430 °C, contributes to minimizing the impact of temper brittleness on the fatigue life of the torsion bar. Consequently, the fatigue life of the torsion bar is relatively unaffected by temper brittleness due to the presence of tempered sorbite in its non-hardened regions. And the reason for the unqualified fatigue life was that the depth and hardness of the hardened region did not meet the standard requirements of 5–7 mm and 47–52 HRC, respectively.