Influence of temperature and stress during setting on the mechanical properties of SAE9254 steel for automotive suspension springs

This study investigates the mechanism by which setting (preloading) suppresses spring relaxation in SAE9254 steel for automotive suspension springs. Tensile tests simulating setting were conducted at room temperature (25 °C), 200 °C, and 300 °C, followed by tensile and creep tests to evaluate mechanical properties. Higher setting temperatures and preload stresses increased yield stress and induced the yield-point drop phenomenon. Creep tests at 80 °C after setting showed that higher preload stress during setting suppressed strain increase, improving spring relaxation resistance. When the preload stress during hot setting exceeded the 0.2% proof stress at the setting temperature, dynamic strain aging occurred, leading to the yield-point drop phenomenon. Improved spring relaxation resistance was observed even at 200 °C, where hardness does not increase due to low-temperature annealing. In contrast, room temperature setting did not cause carbon segregation to mobile dislocations, even with high preload stress, resulting in only limited improvement in spring relaxation resistance, mainly due to work hardening. Utilizing dynamic strain aging from carbon segregation led to a dramatic enhancement in spring relaxation resistance. Furthermore, a linear relationship was found between the 0.2% proof stress after setting and creep strain, indicating its effectiveness as a predictive indicator for spring relaxation resistance.