Abstract: It is studied to improve the fatigue strength of bull and cruciform welded joins of steel Q235B and 16Mn by peening the welded toes with ultrasonic equipment. The σ-N curves and fatigue limits (2×10{sup}6) of steel Q235B and 16Mn of both joints are obtained for both as welded and peened conditions. The results show that the fatigue strength of butt and cruciform joints of steel Q235B have been increased by 57% (tension) and 64% (4-points bending test), the fatigue life of the peened joints is 40~60 times as long as the as – welded joints. The fatigue strength of butt and cruciform joints of steel 16Mn are increased by 90% (tension) and 67% (tension), the fatigue life of the peened joints is 30~100 times as long as the as – welded joints, respectively. The quantity of improving the fatigue strength and life of 16Mn steel welded joint is higher than that of Q235Bander ultrasonic peening condition.

Key words: Butt joint, Cruciform joint, Fatigue strength, Ultrasonic peening, Weld toe, Welded joints