Study On The Bending Stress Relaxation Behavior Of 65Mn Steel And Stress Relaxation Induced Failure Prediction Of Disk Spring

Stress relaxation behavior of disk spring and bending stress relaxation behavior of oil quenched-tempered 65Mn steel were tested using two sets of self-designed spring stress relaxation measurement apparatus.Bending stress relaxation tests were carried out under different temperatures and initial load. It showed that the stress relaxation process is thermally activated and the apparent activation energy increases with time, ranging from 0.24eV to about 0.63eV. Microstructure of the specimens before and after the stress relaxation tests was examined using TEM and it could be observed that a kind of microband structure was formed during stress relaxation and tangled and piled up dislocations could also be observed. Bending stress relaxation tests of the specimens which were tempered at different temperatures were carried out. It showed that the oil quenched 65Mn steel gains its optimum stress relaxation resistance when it was tempered at 400℃.Stress relaxation behavior of the disk spring could only be tested continuously due to the small size and large rigidity of the disk spring. According to the stress relaxation curves, stress relaxation induced failure could be predicted. Stress relaxation tests of disk spring were carried out in the temperature range of 25℃(working temperature) to 200℃and the ratio of the relaxed stress to the initial stress of disk spring after 12 years of working was calculated to be 2.38% according to the results of the tests carried out in the temperature range of 80℃to 200℃, while it was calculated to be 2.50% according to the results of the test carried out under working temperature. These two calculation results were consistent. Thus it could be assumed that the prior method, i.e. predicting the relaxed stress from the results of the tests carried out under series of relatively high temperatures was to some extent reliable. The error of the stress relaxation test data was less than 0.182%. It showed that the accuracy of the stress relaxation test data was relatively high when the test was carried out using the self-designed spring stress relaxation measurement apparatus.