Effects Of Temperature And Stress Accelerated Testing On Stress Relaxation Behavior Of Helical Compress Spring

The stress relaxation is one of the most important form of failures for the spring in service. It affects the efficiency of the machine and the precision, sensitivity, stability and reliability of the instruments. Therefore, it is significant to research the stress relaxation, investigate the mechanism of the relaxation and forecast the service life of spring at room temperature.This paper investigated the stress relaxation of the quenched-tempered spring and the normalized spring made of 60Si2Mn at different temperatures, different loads and different test durations under static loads. The stress relaxation testing was carried out with a dynamic spring testing machine and the stresses relaxation data at different temperatures and under different loads were collected automatically. The service lives of the two different springs at room temperature were predicted according to the theory of temperature accelerated stress relaxation.The effects of the temperature, load and test duration on the microhardness, macroscopic residual stress, lattice distortion and microstructure of the quenched tempered spring and the normalized spring were investigated using hardness tester, residual stress instrument, XRD, OM, SEM and TEM. Meanwhile, the mechanism of stress relaxation was discussed.The results show that the stress relaxation curves of spring were composed of two stages with different relaxation rates. The relaxation rate of the first stage was larger than the second stage, however the time of duration of the first stage was short than the second one. The load loss rates of ten years for the quenched-tempered spring and the normalized spring at room temperature are predicted as 0.8% and 2.97%, respectively, indicating that the spring with quenched-tempered microstructure presents better anti-relaxation performance. The microhardness of springs changed a little during the stress relaxation at low temperature. The macroscopic residual stress relaxation of springs plays an important role in the stress relaxation of spring. XRD analysis shows that the micro-stress which induced lattice distortion does not play a decisive role in the whole relaxation process, but the macroscopic internal stress in the springs plays an important role. Microstructure observation and analysis shows that a strip structure will be generated in the quenched-tempered spring in relaxation process due to the movement of dislocations. However the lamellar structure of the normalized spring will be refined, widened, wrapped and broken. With the relaxation continued, the strip structure and the lamellar structure will be dissolved and the microstructure homogenized.