Abstract:
A spring wire (20) is subjected to a first shot peening process (S6) and a second shot peening process (S7). In the first shot peening process (S6), a first shot is projected on the spring wire (20) at a first projectile speed. High kinetic energy of the first shot produces compressive residual stress in a region ranging from the surface of the spring wire (20) to a deep position. In the second spring wire process (S7), a second shot is projected at a second projectile speed lower than the speed of the first shot. The kinetic energy of the second shot is lower than that of the first shot. The low kinetic energy of the second shot increases the compressive residual stress in a region near the surface of the spring wire (20).