Mechanical behavior of x15CrNi12-2 structural steel under biaxial low-cycle fatigue at normal and elevated temperatures

The results of the experimental studies of the low-cycle fatigue characteristics of heat-resistant structural X15CrNi12-2 steel for aircraft purposes (chemistry: C - 0.13%; Cr - 12.5%; Si - 0.05%; Ni - 2.05%; Mo - 1.50%, W - 0.70%; Nb - 0.20%; V - 0.20%) under biaxial cyclic loading are presented. For cyclic tests a specialized Instron 8850 two-axes testing system was used which allows the planning of cyclic and static tests with an arbitrary stress sequence under the conditions of tension and torsion. The Epsilon 3550-010M dual-axis dynamic strain sensors for testing at normal temperatures and the Epsilon 3550HT-025M for testing at high temperatures were used to determine the values of axial and shear strains during the experiments. The test methods for biaxial cyclic loading under normal and elevated temperatures are described which allows to analyze the mechanical behavior and structural steel destruction processes under plane stress conditions. The tests results of X15CrNi12-2 heat-resistant alloy under low-cycle fatigue at different temperatures and cyclic strain paths with proportional and non-proportional changes in axial and shear deformations are presented. For different types of tests hysteresis loops are represented in the form of dependences of normal and shear stresses on axial and shear deformations, respectively. It is shown that the durability of X15CrNi12-2 steel in these parameters significantly depends on the cyclic strain path, the shape of the cycle and the test temperature. In the case of non-proportional deformation, the fatigue life of X15CrNi12-2 steel decreases 1.5-2 times as compared with the proportional loading at different test temperatures. Depending on the strain path, a significant decrease in fatigue life at a temperature of 600 ° C by 17-44% in comparison with the room temperature was observed.