Enhancing the High-Cycle Fatigue Property of 316 Austenitic Stainless Steels Through Introduction of Mechanical Twins by Cold-Drawing

The strain-controlled fatigue tests of cold-drawn 316 austenitic stainless steels used in nuclear reactors were conducted at room temperature. The fractography of the specimens after fatigue fracture was observed by scanning electron microscope. The interaction between mechanical twins activated by prior cold drawing and dislocation structures during fatigue of cold-drawn 316 steels was investigated by transmission electron microscope. The results reveal that the high-cycle fatigue life of 316 stainless steels can be raised by prior cold drawing. The high-cycle fatigue life obviously increases with the level of prior cold drawing. The fatigue crack propagation rate of 316 stainless steels was decreased by the prior cold drawing. The complex dislocation structures, like walls, channels and cells, were generated during fatigue. The mechanical twins can segment the austenitic grains and hinder the dislocations motion between two mechanical twins. The dislocations pile up and slip along the mechanical twin boundaries. The extrusion on austenitic grain boundaries resulting from dislocations motion can be reduced by mechanical twin boundaries, leading to effectively improve deformation homogeneity and delay fatigue micro-crack initiations. Also, the high-cycle fatigue resistance of cold-drawn 316 stainless steels can be improved by the grain refinement resulting from mechanical twins. Thus, the high-cycle fatigue property of 316 steels is enhanced through the mechanical twins activated by cold drawing.