Kinetics of stress relaxation in high carbon spring steel at low temperatures under bending, tension and torsional stress

The objective of this research is to investigate the effects of variable initial applied stress, temperature, heat treatment and pre-stressing on the stress relaxation behavior of high carbon spring steel and to determine whether kinetics determined for one stress system could be used to predict stress relaxation for a different stress system. The materials studied were; AISI 1095 spring steel strip; ASTM grade A228:- spring steel wire and commercial grade compression coil springs.; Data were obtained by measuring elastic springback of mill received steel and normalized steel strip in 100-250 C temperature range up to 1000 hrs; at 23{dollar}spcirc{dollar}C up to 10,000 hrs for mill received strip and up to 1000 hrs for normalized strip. Results indicate that stress relaxation rate is lower in high yield strength than in low yield strength of this material. Stress relaxation of spring steel wire was studied in the range of 23-140{dollar}spcirc{dollar}C up to 100 hrs by Frequency Test(resonance frequency) and Loop Test (elastic springback measurement) methods. Slower stress relaxation rate in wire by Loop Test than by Frequency Test may be due to residual stress already present in spool of steel wire. Stress relaxation in coil springs were obtained by measuring elastic springback of coil in 23-250{dollar}spcirc{dollar}C temperature range up to 300 hrs and at 23{dollar}spcirc{dollar}C up to 1000 hrs. Negative stress relaxation has been observed in coil springs stress relaxed at an initial stress near maximum design stress and st 250{dollar}spcirc{dollar}C. Results indicate that to improve the reliability of coil spring, it is sufficient to apply pre-stress and allows stress relaxation to take place for 25-100 hrs at maximum working stresses and service temperature.; By using empirical relations obtained, it is possible to estimate kinetics of stress relaxation, only at low temperature and short period of time interval, under some state of stress e.g., bending stress if the stress relaxation behavior is known under different state of stress e.g., tension or torsion and vice versa.