Mechanical properties of nuclear 18Ni(300) steel tie rod under service conditions

The microstructure, room temperature cyclic loading mechanical properties and fracture morphology of the tie rod made of 18Ni (300) steel after aging heat treatment were studied, and the fracture mode of the pull rod was analyzed. The results show that the 18Ni(300) steel after solution treatment exhibited quenched martensite structure, the original austenite grain size was about 24 μm, and the area fraction of retained austenite was about 0.26%. After aging treatment, the original austenite grain size was about 26 μm, and no retained austenite was observed. Martensite recovery of 18Ni (300) steel occurred,the (Ni,Co,Fe)₃Ti phase was precipitated on the dislocation. Under the condition of simulated installation and use, the number of cyclic fracture cycles of the tie rod sample after cyclic loading at room temperature was greater than 480 times. It was found that the fracture site was mainly distributed at the root of the tie rod thread. The fracture was composed of plastic deformation zone and brittle deformation zone, and the plastic deformation zone was in the middle. The fracture section was mainly characterized by tensile fracture and supplemented by fatigue fracture. It was concluded that the pull rod was mainly subjected to axial tension.