A Study On Stress Relaxation Accelerated Test Of 60Si2MnA Steel

The tensile and stress relaxation tests of 60Si2MnA steel were researched in this paper. Optical microscope, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation of the 60Si2MnA steel were taken. According to the test parameters, the stress relaxation properties of materials has been predicted.As the temperature rises, yield strength, tensile strength, breaking strength of 60Si2MnA steel show a decline. The total fracture elongation, fracture elongation and area reduction rate increase with increasing temperature. The fracture surface of 60Si2MnA steel is cup cone fracture and a typical dimple fracture. Distribution of tensile tissues along the tensile direction is observed by metallographic observation and the higher the temperature, the more apparent. TEM observation showes that under the action of an external load, dislocation multiplication appears in the ferrite grain boundary, carbide interface also produces a proliferation of dislocations; the interaction between the two near the carbide produces dislocation tangles. The emergence of dislocation tangles makes the plastic deformation resistance of the alloy increase. As the temperature increases, atomic thermal activation increases, dislocation movement becomes easier, so the deformation increased, but the dislocation is also easier to pass the carbide, thereby the dislocation tangles reduce and deformation resistance declines, so the strength of material decreases.Because of the disadvantages of the traditional stress relaxation test devices, a stress relaxation test device was designed, and then a series of stress relaxation test was taken by this device. Under different conditions of stress relaxation, the curves are of the same shape which shows that there are two distinct phases. Metallographic analysis showes there is a clear distribution of carbide along the load direction. TEM analysis showes that dislocations appear along the ferrite grain boundary in the process of stress relaxation. Its interaction with the proliferation of dislocations near carbide forms dislocation tangles leading to the increase of plastic deformation resistance. As the temperature rises, due to the role of atomic thermal activation dislocation movement becomes easier, so the load declines faster, but the dislocation tangles will decline, so deformation resistance will be lower which means lower stress relaxation limit. The rise of tht initial load can increase the movement of dislocations too, which lead to the reduction of dislocation tangles, also lead to lower deformation resistance and lower stress relaxation limit rate.Using Arrhenius model, inverse power law model to predict the stress rexalation of the material at the temperature of 24℃of initial load of 20kN, the results are verified and the applicability of the prediction model is given.