Effects of post-drawing treatments on the fatigue strength of eutectoid steel wires

The effects of bluing and shot peening on the S-N curve of eutectoid steel wires, drawn up to a strain of 3.5, were studied. Vickers hardness tests and measurements of residual stresses in the direction of the wire axis using Heyn's method were also performed. The fatigue limit of the wires, together with the hardness and the residual stresses in the surface layer of the wires, increased with the drawing strain. The bluing increased the hardness at low temperatures up to 300°C and then decreased at higher temperatures. The residual stress decreased monotonically to zero with the temperature rise up to 500°C. Bluing increased the fatigue limit, being highest at 400°C, through competitive effects of strain ageing and recovery of the structure. Shot peening slightly increased the surface hardness and drastically changed the residual stress from 1000 MPa to -300 MPa, which resulted in the highest fatigue limit of 1200 MPa in the wire drawn to a strain of 3.55. It is concluded that both the hardness and the residual stress are the controlling factors for the fatigue limit of the wires examined, where the crack initiation is observed to occur from the surface flaw.