The purpose of this work is to study low cycle fatigue life of engine cases.A welded test piece has been designed according to reality of cases

using material of 1Cr18Ni9Ti in order to get fatigue function survey of seam welded joints.From tests S-N curve and P-S-N curve are obtained.Then the reduetion factor of fatigue strength and dispersion coefficient of fatigue life for the seam welded joints are achieved with statistics analysis methods.Compared with a smooth test piece

the fatigue strength of the seam welded joints is obviously less than smooth test piece

but dispersion coefficient of fatigue life is better.