In base material, the fatigue strength increases with increasing static strength. In weld joints the base material strength has in most cases no influence.
Improvement methods can increase the fatigue strength. Machining or surface grinding locally of the weld toe or critical area is one method. Local melting with TIG, Plasma or laser also improve the local geometry.
Local compression or overloading might introduce compressive residual stresses. Cold hammering, peening and Ultrasonic Impact treatment are other methods.
HFMI (High Frequency Mechanical Impact) has positive effect on the fatigue life of weld joint and the FAT-value can be increased with increased of material strength, picture 1.
Picture 1: HFMI, high frequency mechanical impact.