Abstract: High temperature tensile tests of 40CrNiMo steel at different temperatures (950, 1050 and 1150 ℃) and different strain rates (0.5, 1.0 and 5.0 s^-1) were carried out by using smooth and notched tensile specimens. The mechanical properties, microstructure evolution and plastic damage formation mechanism of 40CrNiMo steel during high temperature tensile were investigated, and the influence of different stress triaxiality on high temperature plastic damage was analyzed. The results show that increasing deformation temperature or decreasing strain rate reduce peak stress. When the strain rate increases from 0.5 s^-1 to 5 s^-1, the unevenness of grain size increases, and the material is more prone to plastic damage. When the deformation temperature increases from 950 ℃ to 1150 ℃, the grain size increases by nearly three times. The damage undergoes three steps: nucleation, growth and formation of microcracks. The stress triaxiality is negatively correlated with the notch radius. The increase of the stress triaxiality aggravates the occurrence of ductile damage and reduces the fracture strain value of tensile specimens. In the actual rolling process of the axle, under the premise of ensuring a certain production efficiency, the forming angle of the cross wedge rolling die should be reduced as far as possible, and the widening angle should be appropriately increased to reduce the dynamic stress triaxiality value of the material during plastic deformation and reduce the probability of damage.

Key words: 40CrNiMo steel, ductile damage, high temperature tensile, microstructure evolution