Cyclic stress response and fatigue behavior of Cu added ferritic steels

Steels in which Cu content was altered from 0 to 1.5 mass% were subjected to various heat treatments to change the state of Cu. Concerning these respective steels, fatigue ratio by a stress controlled fatigue test and fatigue resistance by a strain controlled fatigue test thereof were compared. Furthermore dislocation substructure and surface defect during and after cyclic straining were investigated to clarify the effect of Cu on fatigue properties. The fatigue ratios at 2.0 × 106 loading cycles of the Cu added steels after aged at 450 and 750 degrees C are 0.7, remarkably high as compared with those of the Cu added steels after aged at 550 and 650 degrees C, the Cu free steels and any other conventional steels whose fatigue ratio are approximately between 0.5 and 0.6. The fatigue resistance of the as-rolled Cu added steel maintains steady cyclic hardening until fracture. To the contrary the Cu added steel after heat treatment at 550 degrees C shows cyclic hardening to the peak stress and then shows a cyclic softening until fracture. The surface roughness of the Cu added steels after cyclic straining are relatively shallow compared with those of the Cu free steel. The internal substructure of the Cu free steel shows typical cell structure but those of the Cu added steels exhibit vein structure.