Abstract: Hard metal particles having hardness which is lower than the hardness of a nitrided outermost layer of a spring and in the range of Hv 500 to 800 and a diameter of 500 to 900 &mgr;m are protected against the nitrided surface of the spring at a velocity of 40 to 90 m/sec to prevent generation of a microcrack in the surface layer and provide a compression residual stress comparatively deep inside the spring. Against the resultant spring surface, a number of fine metal particles having a mean diameter of all particles of 80 &mgr;m or less, a mean diameter of each particle in the range between 10 &mgr;m inclusive and less than 100 &mgr;m, a spherical or near spherical shape with no square portions, a specific gravity of 7.0 to 9.0, and hardness which falls in the range between Hv 600 and Hv 1100 inclusive and is equal to or less than the hardness of the outermost surface layer of the spring after nitriding, at a velocity of 50 to 190 m/sec.

Abstract: Fatigue-resistance is significantly increased by performing shot peening using fine shots. A plurality of particles of hard metal having a diameter ranging from 20 to 100 micrometers are ejected onto a surface of a steel workpiece at an impinging speed greater than 80 m/second. The impinging speed is controlled so that the upper limit of temperature rising of the surface of the workpiece is maintained at a temperature more than 150.degree. C. but less than the temperature at which recovery recrystallization and austentizing of steel occurs.