Fracture reason of cold drawn spring wire for nuclear fuel assembly

Aiming at the fracture problem of XH77TЮP alloy spring wire for nuclear fuel assembly in the winding process, the fracture causes of spring wire were analyzed by means of macroscopic observation, scanning electron microscope analysis, metallographic examination, chemical composition analysis, X-ray diffraction phase analysis, winding performance test and electron backscatter diffraction. The results show that the fracture mode of cold drawn spring wire was brittle fracture, the microstructure near the fracture was rough, and there were many fine cracks. There was a work hardening layer on the surface of the spring wire, the grain size in the edge region was small, and the internal and external microstructure distribution was uneven. There were surface defects such as micro cracks and scratches on the surface of the spring wire. Under the tensile stress of the spring winding, the surface hardening layer and the defects formed cracks and expanded rapidly, resulting in brittle fracture of the spring.