A Study On Thermal-Mechanical Damage Behaviour Of 60Si2Mn Steel

The tensile, creep properties of 60Si2Mn steel in medium temperature tempering were performed at temperature of 23℃,100℃,150℃,200℃,300℃and 400℃by 810 Material Test System and RD-50 Creep Eequipment. The damage behaviour of 60Si2Mn steel after thermal-mechanical cycling between 100 and 400℃or between -50 and 250℃were also investigated. The microstructure and fracture pattern of crept and thermal-mechanical cycled specimens were observed by Optical Microscope, Scanning Election Microscope (SEM) and Transmission Electron Microscope (TEM). The distribution of stress and strain during thermal-mechanical cycling in 60Si2Mn steel lock hook was analyzed by means of the MSC.Marc finite element analysis software.The tensile experimental results showed that with increasing temperature, the tensile strength and yield strength of 60Si2Mn steel decreased, while the plasticity increased. The yield strength decreased by 24.6% from 1374MPa to 1036MPa, and the elongation increased by 150% from 9.5% to 23.4% at 400℃comparing with at 23℃. The fracture patterns of specimens exhibited more apparent characteristic of ductile fracture with increasing temperature.The creep experimental results showed that between 100℃and 300℃, the creep curve of 60Si2Mn displayed classical creep characteristic. But at 400℃, the creep curve mainly consisted of the third stage and the fracture of specimen would be take place in a short time. According to the Norton formula, the stress index of the creep specimens at each testing temperature could be obtained as 2.312 at 100℃, 2.179 at 150℃, 2.401 at 200℃, 2.921 at 300℃and 1.089 at 400℃, respectively. Therefore, it could be concluded that the creep mechanism at temperature from100℃to 300℃was mainly viscous slip of dislocation and then changed to diffusion mechanism at 400℃. This conclusion was proved by the observation of dislocation configuration by TEM. Obvious necking-down was observed on the fracture surface of the failure specimen, with disguisable dimple and tear ridges, indicating the intergranular fracture feature.Comparing 100 cycles of thermal-mechanical and pure creep at invariable temperature and load indicated that the thermal-mechanical damage was much more severe. Stress and strain curves appeared to be opened hysteretic loop during cyclic thermal-mechanical load, resulting in accumulate strain. However, the degradation rate of strain was very low for the thermal-mechanical cycling from -50℃to 250℃.The finite element analysis results of the lock hook made by 60Si2Mn steel showed that there was a maximum of stress concentration near the bending angle, corresponding that the node 130 was exerted the most equivalent stress, and was seen as the weak spot. The stress-strain curve of node 130 followed the same law of development with the experimental results.